Inliners International Forums General Discussion Hi Performance Deciding on cam for my 250

Hello all. I've been researching for a while on here and other sites, and I figure I'd finally tell about my build and get some advice from the pros.

Overview:
Vehicle - 1947 Chevy truck, M2 front suspension, dropped S-10 leafs w/8.5" 10 bolt w/ 3.42 posi in rear. Going to use an S-10 T5 (haven't gotten yet) and around a 28" tall tire (225/75 or 235/70 on 15x7)

Engine:
1972 250 inline 6, which is at the machine shop getting freshened up:
- .040 over flat tops (~9.3-9.5:1)
- new bearings/rings
- lump ports installed in the head with 1.94 valves
- new oil pump, timing gear...
-Offy 4bbl intake, Holley 600 carb, Flow Tech headers (1.5" primary with 2.5" collectors

But I'm torn between two different cams for this combo...
-Crower 03242 Level 3
lift intake - 0.478
lift exhaust - 0.490
adv dur int - 262
adv dur exh - 272
dur @ .050 - 204
dur @ .050 - 212
lobe center - 112
rpm range - 1800-4500, peak of 5000

Comp 268H -
lift intake - 0.499
lift exhaust - 0.499
adv dur int - 268
adv dur exh - 268
dur @ .050 - 218
dur @ .050 - 218
lobe center - 110
rpm range - 1200-5200, peak of about 5500

I'm leaning towards the Comp cam, especially since it comes in a kit with everything that's needed, for good price too. I'm shooting for about 250hp and about 275ft lbs of torque. Will that get me there?
Thanks,
Wayne

ps, here's a link to some pics from last fall when I was test fitting some body parts after the suspension was finished and engine was in it.

My '47 Chevy truck pics

Hey thrift! Your build looks almost identical to mine. I went with Tom's 268 cam and Clifford intake for my 292. Here's my thread.
http://67-72chevytrucks.com/vboard/showthread.php?t=558819

Greetings . . . I am not a student of the 'more modern' Chevy I6. But as a student of the earlier I6 I prefer the dual pattern grinds that extend the duration of the exhaust valve - and avoid unnecessary overlap on the intake valve events. In the early design Chevy's this part of the Volumetric Efficiency equation was typically neglected by the average hot-rodder who was more concerned with getting fuel/air into the cylinder then assessing the cleanliness of the cylinder when that charge arrived. If exhaust gases remain they asphyxiate/dilute the incoming charge - reducing VE. Moreover, with the right exhaust system treatments I think that there are perhaps scavenging effects to be had by having the exhaust valve open late too . . .

regards,
stock49

Call Schneider cams in San Diego and get a cam that is ground just for your motor, no compromises. You get to talk to the owner, tell him what you have motor wise, and what you want from the cam, and they will grind up a cam. They have done my last couple of cams and I was very happy with the customer service.

Nice Build! And nice garage too.

What carb are you going to go with? I wanted to do the popular 390 or 465, but they're soooo expensive. I found an elec choke 600 at a swap meet for $20...plus another $26 for a rebuild kit. I hope it'll work ok. I should have enough airflow for it, between the intake, lump ports, bigger valves, bigger cam, higher compression, and headers.

I went ahead and got the Comp 268H kit, with all the recommended parts...springs, lifters, rockers, pushrods, etc. The 1200-5200 rpm band is more appealing. Plus I want that "choppy" idle. lol

Wayne,
Sounds like you have a good plan.
I think IF you have good exhaust ports, headers, and a good exhaust, a single pattern cam can actually be better.
Here is what the DeuceCoupe Gonkulator computer said for your combo. (I used the Gonkulator to compare a to a bunch of dyno runs, most notably Tom Lowe's dyno series similar to your build).

With the Crower 204-212
torq 288 at 3600
Powr 248 at 5000

With the Comp 218-218
Torq 288 at 3800
Powr 264 at 5200
With a careful build you should indeed meet your goals.
I agree w/u on carbs, I like swap meet specials plus a rebuild.
I did a lot of carb shootouts on the clock a couple years ago with a pretty stock 250-4bbl and 292-4bbl, ~170-180hp level. The CFM didn't matter as much as whether the engine just "liked" the carb. You should be fine with a Holley 600 if its set up right. But even on those, my best 600's are back from the 1950s-60s era, not the newer "reverse idle" style.

Another carb suggestion is the Ford Autolite 4100 carb, its 440-500cfm depending on which one you get, and they fit just like a Holley. And, you can always re-sell em to the original Ford guys.

I did find though, the 390cfm and 450cfm Holleys clocked the fastest on my otherwise stock sixes. A 600 Holley should be fine for yours.

Thanks Deuce, that's good info to know. I went with the 268H, and when it arrived, the cam card actually showed 219 dur @ .050. I ran all the specs on the Comp Cams CamQuest simulator, and it showed 259hp at 5000, and 297tq at 4000. So that's really close to what you came up with. Thanks for running the numbers on your end.
One other decision I'm wondering about now is between the Offy intake and the Clifford intake. Any thoughts on those?

If you want your car to pull cleanly with authority, the Offy is much better, especially on a 250 & smaller engine.

It's all about port velocity, & the Offy has much more of that going for it.

The Cliffords intake runners are so huge, the port velocity is horrendous.

The Clifford intake works as if you removed the intake boss on the cylinder head, opened up the port window as large as possible & installed a 1.94" intake valve, = poor low port velocity, which absolutely kills low end engine response.

The Offy also bolts right on w/no mods needed, the Clifford, grinding, modding brackets to make it work.

DeuceCoupes tests showed the Offy was the quickest accelerating..

And so goes on the debate.

It would be a different story, if you had low rear gears a manual trans or high stall converter w/a cam & compression.
The Clifford will make more power later in the RPM range.

BTW,, I like both manifolds, they each have there own purpose.
They have very different distinctive characters for each manifold.

It just depends on your combo in the engine & how the gearing is in the vehicle.

MBHD

Hank you may have overstated my testing a little bit-
In what I did, it was too close to call between the Offy and the Clifford - GTECH clock times were much more sensitive to the carb than the intake. The Clifford even won a few of the 60ft times.

BUT-
For living in WV, unless we can get Al Gore to bring back Global Warming, it gets kinda cold there. I DID find the Offy to have better cold driveability - and that was here in Cali where a "cold" morning means 40F or so. Then again both my intakes were unheated.

And regarding port velocity, you are right but I suspect it may not have mattered in my case. With the stock cam, "ram tuning" is pretty much zilch, so the engine might not have cared so much about port velocity, good or bad, it wasn't going to have the valves open long enough to use it.

With a bigger cam, ironically the smaller offy ports might work better in the "mid-build" range, where ram tuning matters but you haven't run out of port size yet.

Bigger still and the Clifford likely wins out.

I think I commented at the time, if I found both intakes at a swap meet for a good deal, I'd buy em both but if could only afford one, I'd buy whichever one was cheaper- for a warm climate that is.

But for a WV driver, a heated intake would be a must and I'd favor the offy.

The "offy vs Clifford" has been one of the more lively discussions here, at least it keeps folks awake!

Thanks Hank and Deuce!
That's good info. I will probably not be spending much time in the upper rpm range anyway. And with this combo, the Offy probably sounds like the better choice. I plan on using a T-5, just not sure which OD yet, .72, .76, or .86. Any one of them would work. I guess it's just which decent one I find first. I have a 8.5" 10 bolt with 3.42 posi already installed in the truck. I'll be using about a 28-29" tall tire...225/75-15 or 235/75-15. We have cold winters here but we have pretty hot summers too. Even still, seems like for mid range punch, the Offy might have it.

I just got back from the machine shop to drop off all my parts and talk with the machinist. We decided on 1.84" int valves and 1.50" exh valves. He's going to unshroud around the sides of the valves a bit for better flow, but when we put both a 1.94 valve in and a 1.60 valve in to see how much unshrouding would be needed, he wouldn't be able to do much without getting into the head gasket area. Since he's been doing heads for like 40 years, we both thought that the slight increase in valve size and limited amount of unshrouding wouldn't be anymore beneficial than just doing unshrouding and using the slightly smaller valves. There would be more area around the valves for more airflow with the 1.84/1.50 combo. He can do some bowl blending and smooth out the radiuses and get a great flow in conjunction with the lump ports.

Should be a strong midrange with this setup I think.

DeuceCoupe,

I was simply stating that the fastest time you recorded was with the Offy intake, @ least that is what you stated before.
Is this correct?

Also, if you get too big a cam ("not enough motor")
The Offy will out pull the Clifford down low & w/the Cliffford it will be playing catch up.
I ran that combo also, too big a cam & not enough compression = bad/wrong combo.

47Thriftmaster250,

good choice on the intake!
It would be a good idea to help w/the exhaust flow & install a 1.6" exhaust valve.

As far as unshrouding the valves, IIRC, you do not need to open the chamber up on the sides of the valves so much, it is more along the long turn radius side of the chambers on the spark plug side.
Good pic of this in Santucci's second edition pg 112..
This is where most of the airflows entering the combustion chamber.

It flows this way in a lot of cyl heads.

Here is my aftermarket cyl head that has a very nice port design on the short turn side & it still benefited & picked up CFM by opening up the chamber on the long turn radius side.

Not a good comparison of the heads, I know, but, just saying most cyl heads airflow seems to be on the long turn radius side.



MBHD

I just looked back through the book and you're right.

I think Harry (my machinist) just didn't want to remove too much metal so he wouldn't increase the chamber size and lower the compression too much. He told me of times where customers wanted oversized valves (larger than his recommendation) and the car actually slowed down (drag racing). But maybe I'll just take the book up to his shop and let him look through it some.

Thanks,

Wayne

Wayne,
no need to worry about slowing down your vehicle by installing a 1.6" valve, the chambers can handle that. Not too close.

Good luck on your build.

MBHD

Hank,
Great comments and pics on head porting. I'm porting a set of Brand X right now and it got me thinking about stuff.

On the Offy vs Clifford, here are the 2 best runs with each intake. Results are corrected for car weight (down to a gallon of gas) and weather (Temp, Barom, RH altho it was all pretty constant)

Intake....Carb....60foot....0-60mph.......1/8 et/mph
Clif.....600AFB.....2.580...10.127.......11.266 at 62.383
Clif.....390Hol.....2.585...10.469.......11.318 at 62.870

Offy.....450Hol.....2.582...10.208.......11.238 at 62.506
Offy.....390Hol.....2.574...10.487.......11.284 at 62.277

That's why I called it a "tie", to me all these are SO close, how would you pick? So I'd go by price $$$ in a nice climate like ours, or go with the Offy for its cold weather driveability for a cold-weather capable car.

EDIT, FYI these tests were done with weight~3050lb (includes me), 250cid stock + 4bbl about 170hp, 1.82 powerglide, 3.08 open rear (no traction problems for sure)

I printed out some lump port info and took that and both of my Leo Santucci Books Vol. 1 & 2 up to my machinist so he can look over all of the inline info. He agreed that the 1.60 exh valve was the way to go. So it's gonna have 1.84-1.60 valves with porting and unshrouding.
He's got my short block about finished, then he's going to work on the head. I guess I should order my Offy intake and headers soon.

Stan,
Great thread on your 292 (and truck too, lots of craftsmanship!)
Did you ever get the 292 finished and dyno'd? Guessing about 300ftlb and 260hp?

Hey Deuce. Looking at your ET's, I'm guessing that you went through the traps while still in first gear. That's good for basically running the whole track like you did it in second gear.

Also, do the Holleys mount front to back on the Offy intake or can they be mounted where the front of the carb is facing the driver side front tire?

Thanks

Thrift,
It shifts from "LO" to "HI" right after the 330ft mark at about 4500rpm, goes thru the 1/8 mile in "HI" at 2900rpm or so depending on the run.

Doesn't exactly get air under the front tires, but my goal in the testing was consistency not low ET so the setup is ok for now.

I mount the carb sideways so the primaries are outboard, trying to make the runner lengths closer to the same while cruising. Not sure how much it matters but that's how I did it. The Offy is a little closer to the valvecover than the Cliff but both of them have plenty of clearance.

Here is a thread discussing some of the testing and a picture of the carb mounted:

https://www.inliners.org/ubbthreads/ubbth...=true#Post61158

The 4100 Autolite is the "longest" carb so sits closest to the valvecover but it does fit even with the Offy. The Holley 4150 is shorter as is the AFB carb.

To mount front-rear or side-side you just take the carb adapter plate off the intake and turn it. Hope you realize that the adapter plate is a SEPARATE PIECE from the intake so you might have to order them separately, yet another chance to help out the economy.....

Ahh, yeah, I didn't take into account that you were shifting at 4500, I was thinking 5500---too high for a stock 250.

When I was first racing my '70 Camaro back in the early 2000's (2000-2002, I had 2.73 gears, a 3000 stall converter, and a mild 350 (~350ish-hp). It seemed like I could go to half track (660ft) before I shifted into second (5500rpms), and then went through the traps (1/4 mile) around 5000 still in second. It ran 13.80's, had drag radials so it didn't spin a lick, and would run that number all day long. Then the speed bug bit me, and in went a 10.5:1 406 with AFR 210's, solid roller, big carb & exhaust, 4.10's, 4000 stall, etc, and got it down to 11.0's at 121 w/ 1.50 60' times. That was a fun street car, but now it's torn apart getting the pro-touring treatment...after the truck is finished, though. lol

I'm building my truck to be a fun cruiser with a little bit of power, but didn't need something else that was super fast. That's what the Camaro is for.

That was a very good and informative write-up. For my 250, I could probably go with either and make sufficient power. But since I'm going to drive it on the street (A LOT), the benefit of the Offy intake's heat exchanger and slightly better low end power should do well.

Thanks,
Wayne

BTW, what year Nova do you have?

Deuce Coupe,

I see what you are saying but as your tests proved the Offy showed a 2.9% gain in a roll on comparision, were most driving is done for an everyday street car, were not @ W.O.T. all the time as your test are done in. Also Tlowes dyno tests, wide open throttle.

Part throttle tests will show an advantage in the lower RPM range also towards the Offy.

That is one area that most all intake manifold comparisons are done with the throttle wide open.
That's great, but like I said, we are not @ wide open throttle all of the time, & when you have too big of port runners, port velocity suffers, meaning you lose low end torque & that is where the Offy has shown below 3000 RPM, the Offy does make better low end torque.

The Offy is better for lower RPM street everyday driving.

The Clifford is better above 3000 RPM.

It all depends on your combination also on which one will work better for your application.

They are two different manifolds for different applications,
The RPM ranges are different for both.
Just looking visually, they look to be much different in design.

I would never recommend one, of these two intake manifolds for any type of application.
I would want to know what the person is looking for power wise, type of driving, what RPM range the engine will be operating in, etc.

While others seem to think the Clifford is the only choice in a manifold between these two.
That's fine, they have an opinion, & so does everyone else.

If you ever install too big a camshaft for your engine, the Clifford manifold will fall on its face down in the lower RPM range.
Meaning, if your engine combination is off, the Offy is a more forgiving manifold.

I believe my best mileage was done with the Offy also, it gives a better signal to the carb.

And you are correct the Clifford took longer to warm up, but it did not bother me too much.
Maybe in colder climate, it just might make you mad.

IIRC, Tlowe never published the max HP & torque between both manifolds with the same camshaft/engine combo.

I heard they were pretty close.

I have not used the intakes on a 292, just 250's only.
I think a 292 engine w/the 4.120" stroke can handle the larger runners of the Clifford.

I think if the tests DeuceCoupe were done w/a 194 engine the results would had had wider gaps between the two intakes (favoring the Offy), & if your 250 was overcammed the results would have been in more favor of the Offy, which I have done with my testing, but not on purpose, I was just overcammed for the compression & gearing I had.
Later got the combo better matched.
I never tested on a completely stock engine like DeuceCoupes.

MBHD

Wayne,
Well the 250i6 will feel slow compared to the Camaro but should be a good cruiser. BTW my Nova (63 ss) was a 3-on-the-tree so I did some testing before I swapped in the Glide. With the stick in there I did turn it to 5000, it didn't complain and ran well after that. My guess is the 292 is safe to at least 5000 bone stock, and the 250-230-194 are safe to at least 5500. That's just me guessing, I don't like to break stuff so I stay under that.

The 3-on-the-tree took about 3 or 4 minutes for the 1-2 shift with its sloppy column linkage so I just did some in-gear clocking and then swapped the Glide in. The car would and did lay 2 black streaks in the driveway though if I popped the clutch on the stick. The little 194 would just stall.

I like those year cars. My cousin bought (actually traded a pulling garden tractor for) a '63 Nova wagon last fall. Someone swapped in a 250 for the 194 and it has factory 3.55 gears. It needs the correct seats, as a previous owner installed buckets from a later car and removed the rear seats. But it's a pretty decent car and should make a good cruiser once it's fixed/cleaned up.

As far as feeling slow, the Camaro has been off the road (on a rotisserie) for a while now, so I only have a 3.4L v6 powered minivan and a 2500 Suburban (albeit 8.1L, headers, 4.10's, etc) to compare it to, so, hopefully, it should feel at least as quick as those. lol

Now for a news update. The short-block is finished, but my machinist doesn't have a mill anymore to remove the bosses, so I'm going to pick up the head tomorrow and take to my dad's friend's shop. He has a big machine shop at his house. He's been collecting machines for awhile now and does machining as a hobby (he's good though...has an 8 second drag car too). He should be able to remove them for me much faster than myself and a sawzall. I'll clean the leftover bosses up with a burr grinder, and maybe gasket match the ports while I have it.

Then comes the other news. I ordered a set of headers, an Offy 5416 intake and a few other parts from Summit yesterday. Originally, the Offy intake was going to ship from the manufacturer. Today, the order says I won't get it until November. WTH?! I talked to a CS person and he called Offy while I was on the phone. He said they're receiving 80 this month (all spoken for), and the next round would be November. Jeg's has it and says it'll ship in the middle of August...15th if I order today, I think.
**Just called Jeg's. They'll definitely have them in August.

So I wonder...(and I hate to rehash an old argument, but) I checked the Clifford site again and their manifold (now a heated version) says "it works exceptionally well on the street" and is available immediately. So my question... Can my 255 with it's 9.5:1 compression, lumped and ported head with larger valves 1.84/1.60, 1 1/2" headers, and 600cfm 4 bbl handle the Clifford intake? I'm hoping to get it running sometime in August, but I don't want to wait forever for the Offy intake, especially if it means November.

Downside though...it's about $100 more with shipping...Decisions...

Thanks in advance.

Wayne

*I spoke to a sales rep at Jeg's and they'll definitely have them for mid August shipping. I didn't order yet. Just waiting to see what some of you guys think. (I'm guessing it'll be wait for the Offy though, and I guess I can wait.)

Here's what I'd do:
* I still favor the Offy for your build and CLIMATE especially
* Don't wait - put it all together and use your stock 1bbl intake and carb (jet a couple numbers richer)
* This will do fine to break in the engine anyway.
* See how it runs, maybe even some dragstrip fun.
* Then swap on the Offy and enjoy / clock the extra kick!

Sorry for the late update. I finally got my engine back home. I had my main machine shop do the long block and porting and valve work on the head, and I had another shop, friend of my dad, do the lump ports. He had a mill where he could do a better job of the cutting of the bosses and the spot facing. Turned out great. I also painted it the early Chevy truck medium gray. Looks good. My goal is to have it dressed up like an early 216/235, with stickers and air cleaner, but be a hopped up 255 underneath.

I'm going to break the cam in with the original intake/exhaust manifolds and 1 bbl, then swap the Offy intake and Holley 600 and the headers on (good idea DeuceCoupe). That way I know it should run ok for the break-in. Pics to come.