Hi guys,
I read the whole thread about the 302 jimmy build for bonneville. Thanks for sharing the info. It mentions using Hudson rods, which sounds better than the quotes I got on billets. I was just offered a 54 Hornet long block( no- I can't have the Hi-compression head or the manifolds). I think the engine would be a 308?? Are these the correct rods?
Any thoughts on using 3 carter sidedrafts???
I also lost that thread. Where is it? thanks Wilbur

Try looking at Building a GMC 302 for LSR, https://www.inliners.org/ubbthreads/ubbthreads.php?ubb=showflat&Number=47701&fpart=1

Thanks Jalopy 45.
That is the thread. I've got it bookmarked this time.
I was a member a while ago, but hot rods had to take a back seat to some health related B.S.
I'm starting my Bonneville project up again.
A big Jimmy in a 49 chev fastback.
The tip on some usable long rods in this thread is gold for me!
Last thing I need is a sidedraft manifold for the big port head.

What are you planning for carburetor?

Wilbur,

I'm the one that built the 302 Jimmy using Hudson rods. I originally tried to buy a set of aluminum rods from Bob Johanson but got screwed out of the $600 I sent him, and finally had to get half the money thru a collection agency.

At that point a friend and I researched rods and came up with the Hudson rod. It's 1.125" longer than the Jimmy which let me run a much shorter, and lighter, piston. I had custom JE pistons made and with the lighter piston, rod and wrist pin each piston/rod/wrist pin assembly was 1.25 POUNDS lighter than the stock Jimmy assembly.

The engine is in a HAMB dragster and we hold the HAGR records for MPH & ET. The engine made 346 HP and 347 TQ on our DTS dyno. We turn the engine 6000 rpm all the time without any problems.

Lots of work, time, money to build a good Jimmy.

Ron

you may want to look into 39-54 pontiac inline 6 rods instead. i purchased a set for my proposed low-deck stroker 248 based build. they are real beefy looking compared to the hudson rods, share the same metallurgy and rifle drilling that the gmc rods have, feature a 7.565 c to c, the same 2.125 rod journal diameter that the hudson rods use (ideal for conversion to the chrysler 251/265 rod bearings that Ron used), and have a 1.5" wide big end that can be easily narrowed to the gmc's 1.25" width. i plan on using off-the-shelf 3.905" bore late model gm ls1 pistons (around 1.38" compression distance) and a 270 crank offset ground for a 4.375" stroke for 314 c.i.

for your 302 based build, you will be able to use off-the-shelf 4" bore or larger ls2 or 6.0 pistons and run a little more stroke to compensate for the higher deck

garagepunkfan -

First, welcome to another Wisconsinite!

Second, tell me more about these rods. If my math is correct, they may be a viable option for a 292...? I'm sure someone will correct me, but that's the first thing that popped into my head.

-Sam.

not much more to tell than what i have stated already as i have not mocked up my combo yet (currently unemployed and just starting as a full time cnc program student with a 14 credit load this semester). the small end bushing will need to be honed from its stock .9369-.9375 dimension for the .945 ls1 pins. i also mis-stated the compression distance, its 1.3385"

The 292 pin is 2.100".
A .030" O/S Pontiac bearing (= 2.095" ID) will need the 292 journal turned down .005" to fit.
10.875" deck with 4.12" stroke and 7.5625" (7-9/16") rod makes the new piston CD 1.2525" @ zero deck - should work.

Sam, the only issues I see with this conversion, is you will have to remove around 1/4" of an inch from each side of the rods, and this might possibly machine in to the bearing tang notches in the rod and cap that locate the bearing shells. The 292 cranks already have little journal overlap, because of their stroke and relatively small journal sizes to start with. So that could cause further crank flexing by reducing the rod journal even more. Not saying it wouldn't or couldn't work, just these things came to mind right off as potential issues for concern.

Not sure about your math.
270 crank offset ground for a 4.375" stroke
The original 270 stroke is 4.00"
The 270 pin is 2.3125" (nominal), reduced @ maximum eccentricity to 2.125" removes .1875", which moves the journal centerline outboard by .09375".
The new stroke is 4.1875", not 4.375".

. . . and with a 3.905-inch bore and a 4.1875 stroke, one should then have a 300.9 cid engine. If a 248 GMC block can be bored safely to 3.905, then that's a lot of capacity for a 248. . . . but why not start with a 302?

God's Peace to you.

d
Inliner #1450

the bearing tang notches are well inboard of the original width. i forsee no problems there, and its 1/8" material removed per side of each rod

panic, welded stroker then.

Ouch $$$!

I have no idea what the limiting factor for stroke is in the GMC (rod shoulder to cam, to bottom of cylinder wall, etc.), but if you're going to weld it you could use a longer piston, even longer stroke, and ordinary shorter aftermarket rod, like the common 7.25" BBF, BBC, BBM.

BTW: one important clearance factor is that the stovebolt and GMC have the rocker directly above the cam (vertical pushrod), which places the cam in the rod path for longer strokes.
The Gen-3 292 etc. have the entire cam tunnel moved side-ways (angled pushrod) for clearance.

To use it in your GMC application...yeah an 1/8" of each side. To use it in a 292 engine that I was answering samwise's question to, requires a 1/4" of material to be removed from each side, thats what im referencing, the stock 292 rods are just slightly over 1" wide, and to use the same rods as you mentioned will require that much more additionally to be removed.

i would begin to worry about piston skirt speed with a longer stroke and a shorter rod. selecting 1937-38 pontiac rods instead (which i could not find readily) gets you a 1.999 journal diameter which could improve the situation by netting a 0.3135 stroke increase. i purchased 9 new-old-stock federal mogul 1939-54 rods with slight water damage on e-bay for $9.99 (for the whole lot!). take-out ls1 pistons are commonly found inexpensively on late-model performance forums, i purchased new main bearings and new-old-stock cam (to regrind) very reasonably from Memphis Equipment. so far i'm into this cheap. if the welded stroker, cylinder head work, balancing, and what little machine work that i can't do myself and will have to be farmed out is the most expensive part, then i'm ok with that

The mean piston speed tracks stroke length directly: 4.500" is only 7.4% higher than 4.1875".
For 4,000 f/m it's 2,400 ÷ stroke. 4.1875 is 5,731 RPM.
The instant (max) speed does go up with shorter rod, but again only a few percent.
What does change quickly is max piston acceleration, which more or less ~ with RPM^2 (6,000 is not 20% higher than 5,000 but 44%).

3.905" is pretty big for a 248 (std. is 3.719"), almost + 3/16".

Do you make the deck height 11-3/32"?

Garage punk,
Can you post up some pics of the pontiac rods? Tom

panic, Harry Warner stated in one of the Wayne catalogs that max overbore for both the 270 and 248 blocks alike was at 3 15/16. i calculated theoretical deck height at the 248's stock 11 3/32, which puts the piston a couple of thousandths below the deck. good info on piston speed/acceleration.

let's try this for a picture

http://i.ebayimg.com/04/!BlfY11gB2k~$(KGrHqMH-EMEtrLW3Tt9BLb11T,gNw~~_12.JPG

That looks like the rod I've been looking for to make a destroked 302 using a 248 crank. I knew there was a rod some where but old rod specs are hard to come by. Thanks, Beater

With all deference to Warner, these opinions as to maximum bore size were all believed valid at the time, but largely since the wall didn't actually fail immediately and not all these engines developed enough power to hurt themselves. Jahn's etc. sold many 3/16" and 1/4" O/S pistons to people who "never cleared up that overheating, oil consumption, blowby problem".
Since perhaps 1970, the wall thickness in general and in particular at the thrust surfaces is now known to be much greater than was previously acceptable. This does not include allowance for internal corrosion, core shift, inclusions, etc.
Not enough wall permits the shape to distort under combustion pressure, and makes it difficult for the ring to seal. You get a small amount of extra displacement, but the leak-down rate, oil consumption, crankcase pressurization and ring life are worse - on balance, it's a net loss. Jenkins reported less power from a SBC 350 block at .060" than at .040" for these reasons, and recommended "only enough oversize to clean up and remove any starting chamfer".
I'd try a sonic test of the specific block you're going to use before deciding on a bore size.

I do believe there is at least one Inliner running tractor rods but I can't remember which ones. It's not the source, it's the specs. With the Jimmy rod journal at 2.311 offset grinding the crank with a minimum bore gets you the extra cubes and a block that can be used again. I wonder what the wall thickness is on a Jimmy 6 VS a SBC? My thought is, how far can you bore a Jimmy before it has the same cylinder wall thickness as a STOCK SBC. Just to make an apples to apples comparison. A 302 starts at a 4 inch bore.

yes, agreed; a sonic check is definitely in order for this level of overbore. jimmys do seem to share the robust cylinder-wall construction typical of the higher-quaility gm engines of the era, i.e. 320 buicks, 303 oldsmobiles, and the aforementioned 239 pontiacs. 302 gmc's routinely take a 1/8" overbore without sonic check and there are a quite a few currently running, driving examples of 303 oldsmobile builds over on the HAMB that sport a whopping 1/4" overbore and are driven pretty hard. an educated guess tells me that gm probably shared the same core patterns between 248 and 270 blocks and simply bored the 270 a bit larger and machined the decks to different heights. 270's are still routinely overbored to the 3 15/16" size with no ill effect.

I'm pretty sure a GMC has considerably more wall thickness than an SBC:
1. it was normal practice at the time
2. it's a sound accounting decision (fewer blocks scrapped due to core shift, etc.)
3. extra weight is harmless in a truck chassis
4. more possible re-builds = longer service life

However, no "modern" V8 seems to work as well after about .060".

However, IMHO a comparo to a Chrysler 440 is closer (despite the bore size difference), because the long unsupported wall inside the water jacket is roughly proportionate to the deck height (SBC is 9.025").
The low-deck 9.98" B is preferred to the high-deck 10.725" RB for just this reason: wall stability.

IMO the large overbore trend of the 1950s is entirely due to a few factors now irrelevant:
1. "my Ford V8 accepted a big overbore without frequent failure, therefore I'm going to bore my Cadillac 3/16" as well".
2. "stroker crank? What's that?" Biggest motor wins.
3. bad ring seal, high case pressure, etc. were normal for not only rebuilds, but even some stock engines

An "ill effect" is subjective to the application: a street rebuild with 200 hp will tolerate a thinner wall than a 500 hp blower motor.

I realize you're in a tight spot - budget concerns suggest a specific bore size to avoid a cu$tom piston, so it's a compromise like everything else in motor building.

garagepunkfan -
Not knowing what the application of your GMC will be, I share the concerns of Panic and some of the others. You need to take a hard look at the rod to cam clearance. Even with a 4" stroke, the clearance is very tight. I assume that with the smaller 2.125 rod journal, the big end of the rod will be smaller than a stock GMC, but it may not be small enough for a 3/8" stroker.

Another concern would be the amount of overbore. If this is a street engine, I would be worried about overheating due to the thin cylinders. I have run 302's with up to .210 overbore in competition engines with the water jackets filled, but I wouldn't try it in a street engine.

The last thing I would question is if the tangs on the Chrysler bearings will be compatible with the Pontiac rods (I have no idea about this). Let us know how your project is progressing.

Bob

We have some creative thinking here, and i admire it. We also have a lot of machine work to get to 300.9 cid -- that is, if a 248 block will take a 3.905-inch bore, which some of us seriously doubt. If the 248 does take that overbore, it will -- like the 4.125 in my 302 -- be the last one.

A stock-displacement 302 could be built with far less machime work and a greater tolerance for rpm than a bored and stroked 248 that does not quite match the 302 in displacement or, i daresay, in power. In the long run the 302 will be far less expensive to build and maintain, and infinitely more durable.

God's Peace to you.

d
Inliner #1450

i want to apologize to wilbur46 because i really had not intended to hijack his thread. i hope all of this is some use to him as well. thanks to all of you for your replies and your insight. i have been thinking about this concept for well over a year now, and it's still a ways from reality. i'm taking panic's advice to heart a bit. i will reduce my proposed stroke to 4.1875, deck the block to 11.004 and build a 300 incher to avoid having to weld the crankpins, but i will stand by my concept that a long-rod, short-piston-skirt longblock with somewhat reduced rod bearing surface area (less friction), a favorable rod/stroke ratio (1.81), and a modern lightweight piston with a low-friction ring package can only stand to improve durability over a stock 302 bottom end. we all know the tall, heavy piston dillemma. as far as cam clearance goes, the big end is smaller than the gmc and with deciding to limit stroke to 4.1875, its centerline will be moved only 0.09375 closer to the cam. it has to be close to a wash between the two. only a mockup will prove it out.

the cost of offset grinding the crank would be cheaper for me than to purchase a $700-$900 302 core which i do not have.


i'm shooting for around 12.25 compression with flat-top pistons by filling the chambers to 64cc's, mostly on the sparkplug side, and using extended tip plugs to reduce the cartridge-fire issue. i will try to emulate the early pontiac v8 as a model for the combustion chamber. i'm planning on building a dual-quad intake for rochester 4gc carbs, and plan to run e-85 for fuel

cam specs will be: flat tappet, 242/252 duration at .053", .316"/.316" lift at lobe (.445"/.445" calculated with a 1.41 rocker ratio), intake opens 21 deg ATDC, closes 41 deg ABDC, exhaust opens 48 deg BBDC, closes 24 deg BTDC, 100/102 installed centerlines, lash .018"/.018" i plan on using EMPI 31mm vw type 1 mushroom tappets with bronze sleeves pressed into the lifter bores, Manton 3/8 pushrods, second-hand sbc titanium retainers (they show up on e-bay frequently and pretty cheap) with buick 455 valvesprings, pontiac 350 intake valves cut down to 1.94 and pontiac 455 exhaust valves used as-is. i will make my own rockershafts and girdles for them. ignition will be a stock delco dizzy converted to dual point with a dyna-flyte kit, mallory voltmaster coil, and an msd-6

i also have an aluminum flywheel and will use the new BHJ 235/261 balancer

I agree on the shorter piston - the tensile (fatal breaking) load on the rod is pretty much a function of the recip weight, and reducing it by 1/3 or more is a big step forward.
The rod ratio is much better, but it's preferable to the strokers using the 292 rod (which is worse, and with a smaller pin).
Trying to do trig mentally is ^%#${@!!, but... since the rod angles at every point are almost the same (rod ratio determines the angle), and the pin is only bumped over by 3/32", I agree that the cam clearance should only be slightly less.
On the cam: I know that some exhaust bias was effective back in the day and (JMO) probably still true, but the flow data posted recently on the 235 cause me to question all of it.
The split between the 848 as-is and well done show much more intake improvement than exhaust. From this I conclude that a highly modified engine wants even more exhaust bias than a hot-rodded stock head.
I'd leave the event split for now, until you can have the head flowed. You may out to be on the money (can't be too far off), but the results might be better.
Is this specifically a VW Type I grind? I.e., requires a 31mm tappet?
The stock GMC rockers don't impress me, especially with higher speed and more spring, but I'm not sure what will work. The similar Slant 6 (laminated) rockers have been stiffened back-in-the-day by simply spot-welding plates to both sides across the bridge above the shaft, drill through the plates first for rosette penetration.
AFAIK the GMC is about 3-3/8" long O/A, which is huge vs. automotive stuff (except Chrysler hemi exhaust). There's probably something for a tractor or Waukesha that's close but I haven't found anything. One good thing is that the pushrod and valve are parallel, which certainly isn't common in modern engines.
BHJ: I'd ask them if your longer crank length and stroke length changes the frequency much (stock 235 estimated at 216 Hz). They may be willing to slightly mod the rubber to adjust without $$$.

the grind is a variation on the crane fireball 316-2 grind for harley evos with the lobe centers tightened up a bit. the cam calculator software i was using indicated that as i moved lobe center closer to 100 degrees on the intake side the dynamic c/r rose slightly (to 11.26:1) and the effective stroke (3.80) moved closer to the actual dimensional figure of 4.1875.

i don't claim to be a cam expert so any advice is appreciated, and i will admit that i don't have a good sense of how well a grind like this would work with the mushroom tappets, only that they would effectively exaggerate the camshaft events to some degree, but also to reduce valvetrain mass in a very cost effective way. there seems to be very little info out there on what to look for in a mushroom tappet grind, or what diameters work well. that's really why i put all this out there. Delta's really reasonable on regrinds, so a mildly experimental grind just to see what works is not out of the question

i may end up circumventing the entire intake and exhaust porting altogether on the head and raise both significantly (milling the entire upper port side of the head away and brazing in a big chunk of billet steel with new ports - similar to what they did with cleveland fords in pro-stock in the '70's . i have several small port closed chamber heads to play with.

i am not in love with the gmc rockertrain either and the stiffeners are a great idea.

Which cylinder head casting are you going to be using for your engine combo. I just got through porting a 983 casting for a guy, and that is probably the best casting # for using on a performance build-up such as what you have planned. Also, what valve sizes are you going to be running.

Sorry, no sale on the cam.
That lobe is a roller tappet only, designed for a very big roller tip (.9375" OD, not made for automotive apps TIKO). It might not actually be damaged by operating against a mushroom, but the curve area will be way down, and the accel/vel rates will be all over the map - not a good idea.
If Delta has any VW Type I (aircooled flat 4, 1600cc etc.) profiles, these are all suitable if the timing figures are bumped around. I'm not sure if their GMC profiles for the original tappets are actually based on the .990" tappet, or just a borrowed SBC .842" profile (very common), in which the VW tappet would be a lot of work for no change in valve action.
Remember to tell them your rocker ratio, since VW cams are normally made for the original (much smaller) ratio.

There's no easy fix to replace the rockers: find something, probably from a large engine.
I wonder what the later GMC 305 V6 rockers look like? Too short.
Ford 534, etc. also possible.
The 2 factors that have to be very close:
1. length almost the same (pushrod hole center to stem tip)
2. ratio > 1.5, which means the shaft has to move toward the pushrods

For inserting a port, a large-radius piece of Schedule 40 weld-ell in 2" ID allows you to make the insert at an angle and have either a side-draft or down-draft port without fabricating the curve itself.

However: Toyota F (Land Cruiser) engine has some commonality with GMC, and these parts are easy to find. However: there are several chamber types as to valve angles, etc.

panic

ok, lets try this one on for size...a Schneider 289F vw Type 1 grind, installed 6 degrees advanced...

240/264 duration at .050", .380"/.404" lift at lobe (.536"/.570" calculated with a 1.41 rocker ratio, .518"/.552 at valve, factoring .018/.018 lash), intake opens 20 deg BTDC , closes 40 deg ABDC, exhaust opens 64 deg BBDC, closes 20 deg ATDC, overlap 75 degrees, 106/106 lobe centerlines

RPM Range:
2800-6600



Static compression ratio of 12.22:1.
Effective stroke is 3.82 inches.
dynamic compression ratio is 11.06:1 .
dynamic cranking pressure is 240.44 PSI.
dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of PSI is 11.06 :1.
V/P (Volume to Pressure Index) is 198

CNC-dude, the heads that i have are the 2194819 small-port, closed chamber variety.

i saw the modified 235 head on e-bay with the downdraft intake ports and i'm aware that the Rajo's were modified somewhat similarly and i had thought about that approach as well, but it doesn't solve the crappy exhaust port issue. it's possible that the gmc heads flow worse on the exhaust side than 235 heads do. the head bolt bosses could all be counterbored to use shorty allen head capscrews and one could raise all of the ports as panic described, plunging into the head at a 30 degree angle on a horizontal mill at all port locations and brazing in the weld-ell's. a new flange would also have to be brazed on.

Should work, but I'll bet it's intended for lower static CR.
Delta or ? will have to get the last word on whether your existing lobe will take it - big cut off the base circle.
Your cylinders are also 60% larger than the subject engine (2 liter), so any cam will be much milder.
Remember to think 400 V8, not 300 L6 for the characteristics of the cam, valve size, CFM. etc. I always forget this!
Can you tolerate such high DCR?

The BHJ may not need anything at all done - I notice that the same SEMA damper is used for all Slant 6, despite 1" difference in stroke.

i think the e85 fuel will make the engine very forgiving of the high DCR (94 to 96 octane). any suggestions about ignition timing?

Nope!
Part of that is based on quench, some on rod ratio, how compact the chamber is @ TDC (surface to volume ratio), what the dome looks like, fuel burning rate.
Just a guess: 30° is probably safe as a test (i.e., shouldn't knock with E85), and more conservative will raise the temp.

Depending on the weight, you might need an adjustable advance curve to prevent knock at lower RPM: 15-20° initial advance (whatever gives a stable idle and keeps the throttle plate in the best position), remainder begins at 2,000, full advance @ 2,500, etc.

Before you get too happy with that 6,600 RPM cam, have you figured out your limiting port velocity?

I'm not confident that the usual .55 mach is all that helpful unless applied as a limiter to a well-developed existing port (where the basic design is not a variable).
The mean air speed (which is what this determines) isn't what's actually happening in the port except during a small proportion of the time. If there were no other inputs it would be useful, but there are more factors that are not considered.
An (easy) one is rod ratio: how much change in "leverage" does the descending piston have against the port. At TDC: none, at BDC: none. At maximum piston velocity (almost always between 71 and 78° ATDC): highest (lagging port and chamber volume somewhat). The rod ratio here (7.5625" ÷ 4.1875" = 1.806:1) is similar to other high-performance engines, and some comparisons will have value.
Another is static CR: very high means that the vacuum signal is not buffered by chamber volume, but acts more directly against port volume on valve opening. High CR works the port harder.
Obviously, the worst example is a badly-shaped large port, which will have very low velocity for its X-area, but also low CFM. A straight port (EFI, DOHC) with 2.5:1 "n" can be very small and work very well even above .55.
What I'm confused about is the effect of the high bowl volume in any siamese-port engine, where the charge mass "waiting" directly above the valve is very large compared to both port volume and port X-area.

that powerband is the one shown in Schneider's catalog for the under-2 liter vw type 1 engine that the grind was designed for. a 1600 type 1 displaces around 24.5 cubic inches per cylinder, and a 2-liter big-bore stroker type 1 still only displaces around 30.5 cubic inches. the engine combo i propose is better than twice the displacement per cylinder of the 1600, and, an even 20 more cubic inches larger than a 2-liter stroker. as panic stated, more cylinder displacement acts as a buffer for cams of this variety and the peak powerband will be nowhere near that 6600, probably more like 5900.

speaking intake port-wise on the small-port 4819 head; at minimum i could probably just use a 1 7/16 shell reamer to push down about 1 1/4" into the intake ports, countersink the port face to 1 3/4" do some bowl work, lots of blending, a good 3-angle valve job with 1.94 valves and still make some pretty decent power and the velocity might even surpass a stock 983 head. i'm hoping to do much more than that if i'm able to successfully mod a 4819 head into a high-port.

Armond/Panic,

I milled a hole in the deck of my 302 so I could check the cylinder wall thickness before I bored the block. The thickness was about the same as the OEM 350 SBC's I've measured; (0.250")

I did fill the block with Hard-Blok to the bottom of the water outlet before I bored it 0.125". With the stock stroke the engine is now 320.7 cid.

My rod ratio is now 2.03:1

With the Hudson rods the cam barrel had to be notched slightly in 12 places for rod clearance.

After building the 302 I was very surprised how thin the casting was in most places.

Ron

Flow figures for my #983 head after LOTS of porting and combustion chamber work.

SBC 2.02 & 1.60 valves.

Lift In Ex
.05 30 28
.100 60 56
.200 121 97
.300 166 125
.400 201 142
.500 227 144
.600 245 144
.700 255 144
.750 258 144
.800 259 144

The exhaust flow was an end port. I didn't flow the siamese exhaust because the end ports are really bad and I felt it was a waste of time.

Ron

Ron and/or panic, according to Ron's numbers; wouldn't then a larger exhaust valve than a 1.60 be more beneficial above .500" lift? or am i missing something here in oversimplification? i'd gladly trade off a little intake valve diameter to pick up a little more exhaust flow. i planned on using pontiac 455 1.66" exhausts with a 350 pontiac 1.96" intake cut down to 1.94"

Garage,

The 1.60 exhaust is plenty big enough since the restriction is in the port. When the flow is port-limited a bigger valve doesn't help except a "little" at low lifts. You can band-aid the flow somewhat with more exhaust duration.

Ron

thanks Ron

You know, a lot of this will make almost no difference whatsoever in your engine. Schneider grinds a 264-74F AMC 199-258 Solid Lifter Camshaft. It is all you can use on the street and will be more than lumpy enough to make people take notice. I've ran their 260/270 grind in a 261 punched to 270 and it was almost too much for a daily driver. Use the stock 1 inch lifters and avoid the hassle of an unknown with the VW lifters. Are they even compatible with the Jimmy cam? Check what it's made of. Use your money on a good head and block. 302 head if you can, 270 block is better than you have. Make life easy on yourself.

If exhaust flow is "really bad" with a 302 head, how much worse will it be with a 248 head?

Am i missing something?

God's Peace to you.

d
Inliner #1450

Armond X2

Ron

the only "improvement" of a stock 983 302 head over a stock 4819 head is the intake port

Armond, the cam is a virgin new-old-stock "late" style cast-iron 302 military cam from Memphis Equipment, not the early steel unit. vw cams are iron also.

maybe it's heresy to say so here; but for all the work that 235 chevy guys are encouraged amongst their peers to invest into those engines, they are actually more handicapped, in all ways plausible, in terms of true potential than a 248 gmc.

here is my take on it...most of you folks who are gmc guys tend to buy into the cult of hierarchy of the 270 and 302 engines and that's that. the 248 is a bit like the fat kid who gets picked last for battleball...but, if you think about it, for no real good reason except he has a couple of bigger older brothers. he has a decent amount of strength and he has his own virtues...to carry the analogy a bit further. the same situation has existed for pontiac 350's for years, but now scarcity is starting to take its toll on larger poncho v8's, so now you start to see rotating assembly parts development for the 350's more recently.

i hate to see a decent piece of american iron get scrapped as the result of a pecking order. isn't it enough of a shame that STILL so many gmc inlines get pulled in favor of a chevy bent-eight that we can't embrace the lowly 248?

A GMC 248 is, potentially, a much more powerful engine -- especially in low- to mid-range torque -- than a Chevrolet 235. Indeed, if you succeed with the plan you have proposed, you will after much labor exceed the capacity of a Chevrolet 261. Yet a 248 is not exactly a direct replacement for a 235 or 261. It shares the heft, in length and weight, if its "big brothers," but not their potential displacement or power. You may succeed in extracting as much power from a 248 as anyone ever has, but you will not exceed what you could do with a 302 or even a 270, almost certainly with less time, effort, and money.

In another time we used to say, "They ain't no substitute for cubic inches." That may not be as true as it once was: witness Tom Lowe's continuing experiments with the "late model" 250 and 292. We expect that the 250 will exceed the 292 in peak horsepower at maximum rpm, simply because the 250, for several reasons, is not as rev-limited as the 292. i'll be interested in the comparative torque numbers, which is where the most "power" that we perceive in a street engine is derived. Every engine design and configuration has its limitations, and every attempt to modify an engine is a compromise that attempts to overcome those limitations. Those who work with the 235 Chevrolet and the 228-248 GMC are struggling with more limitations than most. We can always hope that those struggles will teach us something that we can apply elsewhere.

God's Peace to you.

d
Inliner #1450

As a fellow who truly understands the idea of making the underdog into a winner, I totally empathize with your train of thought. Where we differ is in the reiteration of two old sayings, "Bang for the buck" and "No replacement for displacement". If you want to run a 248, by all means run it but if you're trying to turn it into a 270 by replacing the stock crank and over boring it. Are you really ahead of just using a 270 to start with? The 302 head is vastly superior to the small port head for the very reason you stated, intake port. It's needed to support the larger displacements. If you truly wish to run the 248, why not rethink your battle plan to utilize it's strong points. Shorter stroke and smaller displacement. The long rod, light piston is great. That plays into RPM. A smaller displacement also allows you to use that head as it doesn't need to flow as much. You would end up with a small "crisper" engine. As far as the cams go, have you measured the contact area diameter on the VW? If it is the same or less than 1 inch (stock Jimmy) you have gained nothing for your work and lost the bearing area of the lifter barrel. If it's larger, how much have you gained under the curve? Your analogy is a little off, we think in terms of "Why start with a Shetland Pony when a Clydesdale is needed?". You have some excellent ideas and some contradictions within them. You are absolutely correct about the Jimmy being superior to the Chevy. That's why my next engine will be a 302 using a 248 crank. Of the people who post here, Bob Corbett and Ron Golden have built the best Jimmy's out there. When they talk, I listen!

The shorter stroke looks like a way to increase RPM (and it is, in terms of wear and life expectancy).
Since all of the head castings are pretty bad by modern standards (or even 1951 standards: Chrysler 331), reducing engine size improves VE.
However, the stroke difference isn't that much (4.7%), and at the same level of tensile stress (assuming identical component weight, etc.) it's a very small advantage.
Using 90,000 f/s/s as an RPM limit, the 228/248 reaches 6,373 RPM, only 184 RPM higher than a 270/302 (3%).

When the Olympians at SCTA awake, and notice the high level of interest in vintage LSR classes (compare this to: how many people are planning to see how fast a small block V8 Camaro can go... for less than $100K?), they might actually carve out a smaller displacement class than the current 325" XO.
The current limits makes only 2 engines competitive: GMC and Buick.
I'd like to see a 250 OHV class (large enough to include 228/236/248, 216/235, de-stroked 261, Ford 215/223.
Dropping it to 200" would make for more entries, but also make the winners very expensive (de-stroked GMC: 4" × 2-5/8")

Some of us would like to see competition classes not only for the smaller engines such as 235s and 248s, but also for a larger-displacement class of engines like the Packard eights and the 413 cid Dodge truck motors that Tom the "Beater" has been collecting. In order to compete at all now, these engines must conform to regulations designed for the speeds that Hemis and big-block bent-eights generate.

When i think, as i frequently do, about the GMC 248 crank in a 302 block, i think about taking more advantage of the improved breathing capability of a 12-port head for more horsepower at a higher rpm.

God's Peace to you.

d
Inliner #1450

Perhaps I'm missing something, didn't California Bill used to put a 228 crank in a bored 248 block to get a short stroke 270?

With the drastic rework of the head it sounds like GPF is contemplating wouldn't that make more power (horsepower anyway) since with the lighter crank (than a 248, or so I read) it should spin up quicker.

IIRC aren't the 228/248 a shorter deck block? 'Should make for lighter pistons (not as light as Ron's but hey, lighter is lighter).

There are only 2 strokes: 228, 236, 248: 3.8125"; 256, 270, 302: 4.000". The 228 crank was probably lighter.
If you want a stroke below 3.8125", the cheapest way is a "reverse" offset grind - use a smaller rod journal, and remove all the metal from the outside.
The 2.125" Pontiac will give any stroke you want between -.1875" and +.1875" depending on the eccentricity of the grinding, the shortest being 3.625".
With welding, you can make anything you want, all it takes is careful math and $$$.
There are 2 decks, but the difference is too small to be anything but an irritation: 3/32".

Armond & panic...you have me re-thinking this short stroke idea. a 270 incher could be built using 4.0 jeep pistons but they are all dished!

Why spend money to make less power?

If it was totally about money and power this club wouldn't exist. It sounds like a fun build to me. It would make more power that a stock 248.

This might help.

248 block with 3.905" bore and 3.625" stroke = 260.49".
IMO this is faster than a 248 bored to the same displacement: a few more RPM, but more important the big bore un-masks the valves, and a far lighter piston.
Using the Pontiac rod, the CD is about 1.719" for zero deck.
The only thing I found (quick search) is a cast 350 Pontiac piston at 3.875" + O/S, has 1.710" CD, flat dome with small valve notches, .9805" pin, available from KB: http://tinyurl.com/yg444fh

I have a set of those pontiac V8 pistons, stock Overbore, flat top forged , aftermarket. If anyone can use them in a Jimmy, let me know.

OR (gotta throw that in) use a 312 ford piston @ 3.8 bore with a 1.760 PH and your 7.565 rods. That's a very livable over bore and it gives you 259 cubes. You could either offset grind the crank 12 thou. for a zero deck height or take it off the deck. Panics link
http://www.kb-silvolite.com/assets/09_silvolite_catalog.pdf

gives both bore and piston pin height.

panic, the 270 incher i was referring to would have 3.875 bore and the 248's stock stroke of 3.8125. the 4.0 jeep piston has a compression distance of 1.59

That 270 (the hard way) is clearly the least expensive of all your proposals so far, and it might rev marginally higher than a 270 (the easy way). You could put the money and machine time you save into the head -- preferably a 302 or 270H.

God's Peace to you.

d
Inliner #1450

Lets see, an 11.25 block height minus 1.9 (half the stroke) is 9.35 minus the connecting rod length is 1.785 for the piston compression height. If you want a 270 crank, I'll trade you the 248's for it. Just check the cylinder wall thickness before you bore that far. I went through all this when I put Jimmy rods in an over bored 261. Ended up using Buick 181 pistons.

Armond, doesn't the 248 block have a deck height of 11.156?

I have one on a shelf and simply put a tape measure to the side. It and the 302 block were too close to call. I've always heard that and to double check, I just put the veneer calipers on it. Don't know what to say, they are flat and not at an angle. Maybe you could check yours too.

Panic, yes, that's as I understand it, and I've read (here and in Clai Bil's book) that the 228 crank is lighter than the 248 crank, but I don't have 1 of each to weigh, so I have to go by what I read.

Yea, darn, I want the 248 crank and the Pontiac rods. I think with a 302 block and a McCulloch the 270H 0r 302 head isn't a big deal either.

Hi guys. I finally got a chance to check this thread again, and I am really impressed at how much info is available. Thanks to all. Don't worry about hijacking my thread as I am the one benefiting from all the new info. The thread had not been added to in a while when I found it, and now it has taken off again.
I have now secured my Hudson rods.
I'll be using 400 small block pistons with as much pop up as I can find and then milling to suit. I am aiming for 12:1, maybe 13:1 for B'ville.
I also plan to use 3 carter side draft carb (from the big rat pile in the shop) and want a manifold. I've use them before with good results.
Re: rod ratio. I might as well put in my two bits also. My intro to the subject was from Mickey Thompson, who told me to read what Smokey wrote on the subject. "make em as long as you can"
I built VWs for 20 years. The stock ratio is 1.9. I used 5.7 283" rods with great success. Ratio was 2.1!!! Awsum midrange torque. The longer dwell at TDC and piston speed discussions are always interesting, but dyno sheets tell the tale. I'll post a couple when I dig them out.
My reasoning on this engine is mostly based on reciprocating weight reduction. These Jimmy pistons are Monsterous!!
The mushroom tappets are another VW favorite, and there are pre drilled aftermarkets with an even larger diameter. They allowed major duration/lift increases without the lobe running off the edge of the lifter. (Smokey liked them too).
Lucky for me Elgin is located right here in town.
I'd better save something for my next post. Thanks again guys, and Regards,Wilbur

wilbur46, thanks for posting the vw rod/stroke ratio and tappet info. i have always understood that mushroom tappets do increase net duration....but how can it affect how much lift that can be ground into the cam?

Wilbur,

We custom designed a pistom for the Hudson rods and had JE build the pistons with a 52cc dome. They worked great in our 321 inch Jimmy. If you'll e-mail me I'll send a picture of the stock 302 rod/piston beside the Hudson/JE piston. I don't know how to post the picture here.

Ron (goldenri@aol.com)

how can it affect how much lift that can be ground into the cam

It doesn't - it raises the limit of how long the tappet can accelerate by increasing the maximum velocity: V in inches per degree of rotation = (D - .040") ÷ 114.6. The .040" is a safety margin for eccentric tappet bore etc. A blueprinted engine may be safe with .020".
SBC.842" tappet: .00700"/degree
Stovebolt/GMC .990": .00829"
VW 31mm: .01030"
VW 35mm: .01167"

When milling dommed pistons,make sure they are not hollow dommed pistons.



MBHD

For Ron, here ya go:



Snazzy, no?

panic, yes, that was my understanding, though in a somewhat more simplified sense. thanks for the detailed information as always. i was trying to clarify wilbur46's statement that a mushroom tappet could somehow affect lift.

Thanks for the picture. I sure don't want to be bench-pressing 6 of those mothers!
What is the pin size on the Hudson rods?
RE: Mushroom lifters. Much has been written, but to simplify a much larger lobe can be ground on a larger dia lifter. It is a technique first used (hot rod style) by the NASCAR boys. For a quick comparison, check a VW catalog for the differences in the grinds available for the stock/ larger diameter lifters. The big catch with the jimmy is cam cores. As those who have done it will attest, you gotta bust out some bucks or have a lathe/cnc plus heat treating quipment. Maybe CNCDUDE could whip up a few nodular iron blanks for experimental poipuses. The difference in flow rate-intake vs. exhaust is the absolute limitation anyway. Regards, Wilbur

Panic, re your last post.
I think your expertise (or terminology) may exceed mine, but I think what you are talking about is the increase in potential acceleration of the lifter on the ramp, whereas I'm still talking about a big ol lobe base circle affording a lot more lift/duration. The low lift acceleration of the lifter moving up the ramp is a newer idea of the Crower or maybe Comp Cam "High intensity" grinds. (is that right?)
I quit reading the mags 20 years ago, except maybe McFarlanes stuff in Circle Track.
What happened to tech editors with degrees in mechanical engineering????? Again, thanks to all who contribute and Regards, Wilbur

Certainly preferable to have a large base circle, but there still has to be clearance for the "foot" above the lobe under the tappet gallery. This is a slight disadvantage to the VW or any other mushroom - the base cannot almost disappear up into the block at full lift. I'll bet that since there's no oil pressure (unlike later stovebolt) the bottom of the gallery could be cut back to get clearance if needed?
Anyone have ideas as to how much spring pressure the cam will stand?

panic, the Empi 31mm lifters have a .024 oil feed hole in the center of the foot to increase spring pressure capacity

Erik, Thanks for posting the picture for me.

You'll notice that the Hudson rod isn't as "beefy" as the stock GMC. However,each JE piston/Hudson rod/pin assembly is more than 1.25 pounds lighter than the stock GMC and the net results is a stronger combination. It also takes a lot of load off the crank.

We turn the engine 6000 RPM without any problems. By the way a SB Chevy harmonic balancer fits perfectly. We use a Fluidamper balancer.

The wristpin is stock Pontiac. The small end of the Hudson rod can be honed to fit the Pontiac pin.

Ron

Thanks very much Ron.

How are you going to add oil pressure to the lifter galley? They have gaps between the lifter bores, so you can't drill a long oil passage like the 235/261's and no provision for rocker arm pressurization. External line? There are only four bearings supporting the camshaft, how will you keep it from deflecting?

I agree that pressure would be nice, but other engines with flat tappets have demonstrated longer life with just a simple hole that slowly bleeds drain oil from above (some always follows the pushrod) onto the lobe/tappet mating surface.
Is the VW tappet oiled from an annular groove (like a hydro), or is it perforated top down?

Erik & Ron, Thanks for that picture. It is clear that Ron has found something there. I'd like to see a shot of one of those Pontiac rods beside a GMC rod.
As far as oil to solid lifters goes it is a common practice in Studebaker V8s to plug the oil holes in the lifter bores and rely on the free oil slopping around. It forces the oil to go where it is needed. Those engines like to put all the oil into the valve covers.

the vw lifter has two narrow annular grooves around the stem about 5/16" apart. upon inspection, there is no galley or oiling holes to the lifter bores in my 248 block. i think the GMC must oil the lifters simply by splash & drainback, so i guess ordering the drilled lifters would be a moot point.

Increasing the drain-back volume is simple: increase the height of the existing drains, or relieve the tappet bores down onto the cam.
I'd consider drilling the VW tappet from the pushrod socket down to intersect the existing face hole.

the intersect hole is already there in the lower groove of the vw lifter, from what i can see in web images. looking at the block again, i see no reason why the bottom of the lifter bores could not be fully relieved for the lifter foot.

Garagepunk, there is nothing wrong with the mushroom tappet idea, but many times we look too far outside the box thinking we are gaining a lot, when we are actually no better off that if we used tried and true methods and concepts that have been proven to work and function well. NASCAR used the mushroom tappets in the late 70' and early 80's with good success, but changed to the conventional style flat tappet lifters afterwards with even better results. Cam technology has advanced so far and rapidly, that lobe profiles that weren't able to be had without the use of the mushroom tappets, are now way beyond what could be done with them, using standard tappets. Roller cams have been available for the GMC's since the 50's, so using this type of lobe profile instead of a flat tappet style will even take you to a much higher plateau again.
Flat tappets are more sensitive to lifter bore misalignment than a roller lifter is, so just installing a bushing into the exsisting bores can magnify this problem unless you have precision lifter truing tooling like BHJ offers to do this competently. Plus the additional machining required to install a mushroom tappet into the block, where the bottoms of the lifter bores have to be spotfaced with specialized tooling to allow the lifter foot to recess for clearance presents more challenges. I think you would be money, time and performance ahead by just upgrading to a Chrysler lifter, and having a custom lobe profile ground on your cam to get you where you want to be.

cnc-dude, those vw lifters can be had for $29.95 for a set of 8, so 2 sets would be required. that's still only $60.00; and i CAN have my existing n.o.s. iron cam ground with the vw profile for well under $100.00. that's bang for the buck that the rollers just can't hold a candle to. i have access to a CMM and we can plot the locations of the existing lifter bores and write a cnc program to bore them accurately once the block is bushed; that can't be any less accurate than the standards that GMC actually machined them to originally. i have actually seen the lifter bore bushing mod done to a vw waterboxer engine fitted with type 1 tappets, so it's not completely out of the question.

the block may or may not need spotfacing with my reduced base-circle cam. tooling would not be a real issue as i can make or modify most anything at school.

i'm also after valvetrain mass-reduction, and these fit the bill nicely.

Any tappet smaller than .990", including the "normal" Chrysler .904", requires the bore to be sleeved. The only big cam that will run on the .990" tappet is something designed for the Chrysler .970" mushroom.
AFAIK, the reason for a precision tool to spot-face the bottom of the tappet bore is tight conditions in a V8 valley area - the GMC should have room to get a bigger tool in there. However, I'm a little nervous about how much metal there is above the cam. To fully recess the VW std. 31mm tappet, the bore radius is widened by .115". On the outboard side, the skirt wall looks like only 3/16" thick.

I should have been more specific, I was meaning the 1" diameter lifter, this doesn't require bushing the GMC blocks to use them, only honing the existing bores. We have discussed this in the past and I thought everyone was on the same page, as far as which Chrysler lifters to use for the GMC's when going to a roller setup.

Maybe I'm missing a point here but, lifter oiling is almost always just oil being slung off the crank and what drains back from the valvetrain area. My GMC has flat tappets with 100# on the seat and 300# open and shows no sign of wear and never needs valve adjustment. I'm also lifting the valve more than 0.600".

I made sure the oil has lots of ZDDP to combat wear associated with flat tappets.

I don't think you need to reinvent the wheel without seeing a significant gain in performance or reliability.

Ron

Yeah Ron, that was my point also, but it sounds like he has already made his mind up on the subject....

I think your right. Maybe we can help someone that needs help.

Ron

the 1" diameter lifter

Nope, not familiar.
Replacement for the RB hemi?

Yeah, its newer Top Fuel and Funny Car technology stuff. You can pick them up used dirt cheap. I just thought this would be a simple roller lifter fix for the very hard to find application specific GMC roller lifters you can never seem to find anywhere.

Ron, Panic & CNC-Dude, all very good info and all of it is taken into consideration, as i'm not committed to anything yet. it's all in fun to talk shop with knowledgeable folks. thanks again

Ackkk - senior moment!
I already had data, but not recorded as GMC, duh.
The center-to-center tappet distance (between linked pairs) of the KB etc. hemi roller tappet is 2.000", which is (almost?) correct for the 235/261 stovebolt (the "normal" hemi and B/RB tappets are 1.800" apart). The GMC distance is probably slightly larger, so the slots etc. in the link bars will need elongating, etc.

Find the 1.00" roller lifters used if you can. We just had a custom set made (Crower)for a SB Chevy that we're building and they cost over $1200 for the set.

Ron

CNC-Dude , I will send dirt, please send lifters!!!

The big boys use larger than 1" now, for us the hot ticket is the .987 Jesel keyway lifters but way too spendy for this hillbilly.

On the VW lifter thing, my main worry would be the side load on the lifter itself. It might work just fine with the right bushing. The only way to know, is do it!

Unless there's something else, a fairly common bushing material is 660 bronze.
The stem area vs. tappet forces shouldn't be any worse than they are in the VW - it should work.
You'll have to plug the hole in the annular waist area to make a drain from the pushrod cup feed the contact surface hole.
The TF tappets can be rebuilt by the manufacturer - new axles, needles and rollers, polish the body, etc. so they need not be perfect. In a GMC (or early stovebolt with no hydraulic tappets) the tappet's oil feed area is harmless, but in a late 235 you have to measure where this is vs. the tappet oil gallery to make sure it's open front to back, and no leaks.

[quote=Armond, II#298]CNC-Dude , I will send dirt, please send lifters!!!

I wish they actually were that dirt cheap, but used they are not likely to break the bank like the Jesel stuff will. I have a guy local to me, that all he has done for the last 10 years is buy out Top Fuel and Funny Car teams for pennies, and build entry level racers for guys looking to jump into the sport. I'll stop in and talk to him about this more and see what he has to offer as far as cost. Most of the billet cranks he sells for those engines only have one pass on them, and he sell for $300-$500 bucks. He is fairly reasonable on this stuff.

Another consideration about engine size is the relative sizes of the ports, the bigger the engine, the more relatively restrictive the less than optimal porting is.
My research of the 383 Vs 440 Mopars shows that using the same magnum heads the 383 makes more Hp.
I also prefer the shorter stroke in that it should allow the engine to safely wind farther.
Roar

dual weber 38's or holley 390 4brl