Inliners International Forums General Discussion Hi Performance 302 jimmy

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