Inliners International Forums General Discussion Hi Performance GMC Camshaft Advice

I would like to upgrade my GMC's performance. I have run an Isky E2 for many years. This is a mild cam with .298 cam lift and 286 deg. duration, .22o duration at .050 lift. I am thinking about installing another cam.

I have a 3250 pound 40 Chevy Coupe with a ported head and big valves, 9:1 compression and HEI distributor. I run a Camaro T5 and a 4.11 rear end. Right now it idles well at 600 rpm, pulls well at 1500 in overdrive at 40 mph and winds up to 5500 rpm quickly. My best performance figures are: 9.9 sec. and 72 in 1/8 mile and 15.9 sec. and 89 mph in 1/4 mile. Based on drag racing calculations my engine puts out about 185 hp.

I want to improve my drag race performance while still being able to drive the car easily. I am consider both flat tappet and roller cams. Any ideas would be appreciated.

What kind of carb and intake are you running?

I use three Weber 32/36 mm DGV two barrel progressive carbs. They were used as performance replacement carbs on 2 liter foreign and some US cars. I have seen flow rate figures claiming 150 to 250 cfm for these carbs. The sit on homemade tubing bends that gradually taper down from the two barrels into the single intake port. Each runner feeds two cylinders. There is a one inch rubber hose between each runner. The setup idles fine and milage is good as the Webers run on one barrel most of the time.

Thanks for the question.

Im guessing your following the post regarding GMC rocker ratio. Finding or getting a set of roller rockers made that has all (12)rocker with the same ratio will be a good start to maximize your current camshaft. Since most production rocker arms are made with a "theoretical" ratio of some amount, they are only cast, and usually they are all (12) different to the specified ratio, so you don't really get the most potential you can get out of your camshaft. Changing cams, but not changing to better rockers will also make you fall into the same pitfall again. You can also play with your current camshaft by advancing it to see if you have any gain left in it to further tweak this profile before moving on to another one, after you get better rockers of course.

With 9:1, 290 duration is about the limit. Pull your valve covers and see how much valve lift you can get away with. Check for valve seals clearance and coli bind. I ran a 286 with 256 @50 and it ran very well. (180 in a roadster.) Any cam grinder can improve on you @50 numbers. Check thinks out and I think you will find your option will be limited, without doing additional engine work.
Hopes this helps

15.9 at 89 MPH in a 3200 pound vehicle is about 200 HP.I think your engine has the equipment to go faster but needs fine tuning..The fact that it revs to 5500 says something..

What have you done about dialing in the present cam? Is it 9-1 actual measured compression? Is the quench tight a possible? Proper ring sealing and accurate valve job? what is the ignition total timing?
And as some have found out,a ported head doesn't always mean more power...

I'll take your comments in order.
I like to make changes gradually. The rocker arm idea sounds good. I believe they come in different lift ratios so that might give me the need boost. Who makes them for a GMC?
A machine shop installed the Isky E2 cam 20 years ago. I assume they used the Isky instructions. The old Isky E3 cam has .298 cam lift, 286 deg. duration and 250 duration at .050. That might work? I don't mind doing additional engine work to make it faster as long as I don't loose too much in drivability. I plan to take the car apart this winter. I would like to drive it from San Diego to the Inliner convention in Colorado next summar.
I just found a calculation sheet from Arias Pistons which puts my compression at 8.77 The pistons have 27cc dome volume. The head has not been planed except for a slight cleanup surfacing, so I could take more off the head and raise the compression above 9:1. The head was done by Sissell and is a first class job. I think it would handle any reasonable cam grind. The HEI stays at 12 deg. up to 1300 rpm and then advances evenly up to 38 deg. at 3,600 rpm where it flattens out, with no vacuum advance connected. The engine never pings. The Sissell ported head improved performance quite a bit with times dropping from mid 16 to high 15 sec and speed going from the 85 to 89.

The change from 220° @ .050" to 250° @ .050" is huge, and will have considerably worse "manners", including higher minimum cruising RPM in O/D.
Even though the duration is similar to the E2, I would guess that the LSA is tighter and the installed ICL advanced slightly. Idle quality will suffer; this may improve by adding 2-4° to the initial spark lead (an subtracting it from the total by shortening the curve).
In my opinion, it's going to increase peak power, but raise the torque range, meaning that the added power will require higher RPM before shifting.
AFAIK there are no aftermarket rockers at all for the GMC, and not many that can be adapted because they're so long. I suspect some big truck rocker may be suitable, but no actual choices so far.
What's the exhaust like?

Thanks for the advice. What do you think about having the E2 cam reground to give say 0.235 lift at 0.050 ( half the way between .220 and .250). Also, I found out that Pat Riley makes roller rockers for his GMC. He might make me roller rockers to give me a 5 to 7 percent increase in lift across the whole profile without changing the duration. Does this make sense?

Your E2 cam is valuable, and I'm not sure you'd get enough lobe size to track accurately since the base circle gets a big whack to make a pointy lobe into a lumpy lobe.
One-off rockers are going to be very expensive in value for money. Not all heads get high benefit from lift increases, and it's not possible to do this without extending the duration.
The nominal (paper) duration will be constant, but the duration at every point of lift will be extended.
Many of the GMC cam data I've seen shows some exhaust bias (more exhaust duration than intake), do you have the entire data set for the E2 and E3?

Take advantage of the large diameter of the GMC lifter. My 286 256@ 50 HOWARD H15 CAM, 615 lift, idled at 600 rpm and made power to 6200. This is a air cooled VW profile. Isky has some nice .875 lifter diameter profiles. They are listed on their web site. Cam profiles have come a long way in the past 30 years. Don't settle for some 1 size fits all profile.

NICE CAR FRANCIS
SAW IT IN ESCONDIDO
Dennis Thornton

Francis,
This is a really cool build with good timeslips.
I would like to try it in the DeuceCoupe Gonkulator computer, then maybe see what more cam might do.

Unless I missed it can you say:
* What CID (270, 302, etc)
* What exhaust (headers, pipe diameters/lengths, mufflers, exhaust if any)
* What tires
* 60ft times (and 1/8 et/mph if you have it)
* Also I cant find the bore spacing on the GMC (assuming its same 1939-1962) would you know that too?

Thanks again. The old Isky Cam Spec. Sheet lists the cam lift, valve lift, dur, .o50 dur. lobe ctr., and lash for their early cams including E2, and E3, but not the entire profile. I called Isky yesterday and they say can regrind my E2 to E3 specs for $110, but I am rushing into that.
Engine is a military 302 bored .030. The dual headers are 1.5 in. tubes on single port and 1.625 on Siamese ports merging into 1.75 in. at the flange and length is 17.5 in.. I run aprox. 4 ft. of 1 3/4 exhaust pipes w/out mufflers or tail pipes. It sounds like a B29. Tires are 27/8 in. Hoosier's using traction bars. Best times to date are: 2.231 @60 ft. and 9.810 @ 1/8. On average it runs 2.3 sec and 9.9 sec. Looking at an old head gasket - cylinder bore spacing is 4 7/16 in cyl. 1/2 and 5/6 and 3/4. It's 4 1/2 in. between cyl 2/3 and 4/5.

That long 60 isn't all from low power, I'd try varying the launch technique. You have enough low gear to almost idle out with no spin.
Just a guess: 110° LSA is pretty common for streetable grinds, so your E2 286° may be:
IO 35°
IC 71°
XO 75°
XC 31°
Overlap 66°
Installed 2° advanced

The E3 with the same (?) duration is more likely to be 108° LSA:
IO 39°
IC 67°
XO 75°
XC 31°
Overlap 70°
Installed 4° advanced

Well the Gonkulator doesnt have too much experience with the GMC (nor do I!) so I dont trust it here as much as on the "Nova 6" or some others. But here goes. I couldnt totally match your car but after some reasonable fiddling, and assuming its a bit out of tune (or I might just be too optimistic on the intake design) I Gonkulated to
Torq 289 at 3500
Powr 248 at 5400
2.26
9.84 at 71.9
15.29 at 89.6
I couldnt see how to get 15.90 and have all the others match up so I left it there. Note those are "SAE gross" figures. NET flywheel is about 200nhp as noted already.

Before heating up the cam, I tried a hard launch and shifting at 6000 in stead of 5500. Easy for me to say, the computer wont throw a rod- and this is more fair to the bigger cam. So the 220-220 cam Gonkulated to:
2.06
9.44 at 71.7
14.86 at 89.5

So I then changed duration from 220-220 to 250-250 to compare:
Torq 285 at 4100
Powr 261 at 5900
2.09
9.42 at 72.7
14.77 at 90.7
I wouldnt put up with that wild duration for a carlength!

I then Gonkulated a 235-235 cam
Torq 287 at 3800
Powr 255 at 5600
2.07
9.42 at 72.3
14.80 at 90.2
To me it doesnt seem worth the bother if your current cam is doing good.

Thoughts based on what I've read about the GMC:
* OF course a good tune, timing and carbs. I used that Weber on my Pinto, great carb but I did fiddle with jets and air bleeds a lot.

* Is your rocker ratio really as advertised, Ive heard that is something to check from other threads

* What about swapping intakes/carbs instead, a lot easier than cams. Maybe the Nicson triple noted in Fisher's book, or a 4bbl if somebody makes one. Likely pretty easy to re-sell em after a few timeslips.

* Maybe collector length too although the Gonkulator liked your current setup a lot.
Sounds like a cool combo anyway.

I am getting the message that with some fine tuning and driver education performance could improve. I am a little hesitant about the takeoff. I doubt the rpms are ever at 5,000 when I pop the clutch. I am too busy staring at the 3rd yellow light to know rpms exactly at launch. But, the T5 trans, 11 inch clutch and 9 inch rear are built to take a lot more power than my old six puts out. I'll try higher rpms next time.

What is the Gonculator? I read in the 12 Port News that the GMC rockers have a 1.47 ratio, not the 1.5 that Isky assumes. Mine are fabricated steel rockers, not the cast iron type, but the the 1.47 applies to both. I measured a spare set and 1.47 looks correct. People have recommended a Clifford 4 barrel manifold and a 600 cfm Holley with vacuum advance. I might do that first.
Thanks again for your help.

* What I do is mount a movie camera during most runs, so it sees the tach, oil press, fuel press, maybe more. Gets real interesting but also you can watch the launch RPM easily that way. MUCH cheaper to watch the movie later (crank the home stereo too) than make all those runs again. Less fun though.

* The Gonkulator is a computer program I started writing back in the 1970s for TQ/HP and ET/MPH. It got its name over on Fordfe.com, but I also run a Nova 6 so use it over here too.

* I didnt know you run a Ford 9", they cost about 2% power, so the Gonkulator figures youre only maybe 4% out of tune. And thats a guess of course. But 4% is very possible to find with timing or jets/air bleeds. Do you know what your timing curve and jets/air bleeds are in there now?

* Good idea on the 4bbl if you can find one, to compare. Might or might not be better but at least you'd know.

* Another reason I'd rather fiddle with carbs etc is, all these engines are so old, I dont like to risk higher revs, its not like a new block is waiting at Autozone (or Gratiot Auto to go way back). I hate breaking parts but especially OLD parts.


* Post back if you get more data, GMC timeslips are very rare so I am glad to have some real data for the Gonkulator.

I think the jets are originals. The Weber book shows the primary as .140 main jet and .170 air corr. jet. The secondary has .0135 main jet and .140 air correction jet. The spark plugs always look good - a light brown color.
What do you think about planning the head about .040 to raise compression from 8.77 to say 9.5? How would the Gonkulator react?
I only race 2 to 3 time a year but I have saved all the slips since 2007. The early ones cover 1/4 mile and the later the 1/8 mile. All the 1/4 mile strips have closed in LA and SD. You are welcome to a copy if you like.

Actually the Gonkulator likes that idea:
Torq 296 at 3500
Powr 255 at 5400
2.24
9.74 at 72.6
15.15 at 90.5
Gain of .13 sec and 0.9mph without going to a bigger cam.
I dont know the ins&outs of milling a GMC head but if doable (valve clearance, thickness, cracking etc) this seems a good next step.

PM sent on the timeslips thanks!

Does anyone know how much can be taken off a GMC 302 head safely. The GMC engine manual says limit is .020 inches.

Another area where I might be able to improve what I have is lubrication. I have always run Valvoline 50 weight oil. It has done the job, but it is heavy and thick. I might be able to run a lighter oil and reduce pumping losses and still protect my engine. Any experience out there with lighter oils in a GMC 302?

California Bill's Speed Manual says that taking .0625 from a 270 head got 9:1 compression with Venolia pistons. The 302 head was milled .125 for 8.7:1 with a 4 1/8 bore. This was in his 1955 book and he talks about all 4 heads.

I have run everything from 10w30 to Valvoline 60w racing oil in my 270. I never hurt a bearing and I've been driving it hard since 1978.

The E2 Isky grind is 264 duration with .448 lift, 23-61=61-23
The E3 Isky grind is 286 duration with .448 lift. 34-72=72-34

I don't know the stock spec, but TRW list a stock replacement cam for the 302 with 252 intake duration, and 276 exhaust duration, with .279 lobe lift. Others might have the stock specs. Hope this helps.

64U,
Thanks that helps.
Did the TRW cam show a duration at .050" lift?
Or a CS number like CS-169 or something?

Deuce,
Sorry no @ .o50 numbers or CS. How about 10 degrees more on the cam (274). Hook the vacuum advance back up to manifold vacuum for better idle and fuel mileage. Mill cylinder head .060 to help performance with the larger cam. The E2 cam in a 235 Chevy is called a track grind, In a 302 GMC it is called a Full. It takes a big cam to fill a big cylinder. what does the Gonkulator say.

I discovered a typo in my first post above. My Isky E2 Cam duration at .050 is 0.228 not 0.220 as I stated. I misread the Isky spec. sheet .Sorry for the error.

Duce Coupe, Are the Gonkulator rpm figures above for torque and horsepower at the peak of the two curves? I have read that exhaust tube sizing should be based on the peak torque rpm and gear changes should be at the peak horsepower rpm.

Shift timing is determined by transmission ratio spread.

Frank,
The RPM peaks above are at peak TQ and peak HP.
*Usually*, with open headers and minimal belts, best shift is at least 500rpm past peak HP (if the engine holds together).

But, peak NET HP is somewhat below peak gross and by coincidence, that puts the best shift point near peak gross HP which is what many factory automatics did.

So in reality as Panic said, its a function of the drop to the next gear (a Powerglide will want higher shift points than a new 6speed auto), parasitic losses, and how much you want to risk your engine.

I usually Gonkulate the shift point for the best ET for my combo, then give up .1 sec or so, I'd rather be easier on the parts than get that last carlength.

As far as exhaust:
My belief is that diameter is mainly a function of how much exhaust (loosely, how much power) youre trying to send out thru it. The Lengths determine the RPMs where the exhaust will contribute to power peaks. And where to put them depends on whether youre drag, road, or oval racing. Also a function of budget, time, and ground clearance of course! Its so empirical, I think youire close already but trying different pipe lengths should be easy.

My T5 ratios are 1st 2.95, 2nd 1.94, 3rd 1.36, 4th 1.0. the rear end ratio is 4.11. The slicks are 28 inches. The engine will go quickly up to 5400 and beyond in 1st. Same for 2nd and 3rd but with less rapid acceleration. If I shift from 1st to second and second to third at 5400, by the time I get to the end of 1/8 mile I am at about 4700 in 3rd. Would shifting from first to second at 5400, then shifting at say 5000 from second to third help my overall time and speed?

You'll need to determine where your actual effective powerband is before you can speculate about shift points. A chassis dyno is going to be your best bet to help find out what you engine is actually doing, then you can make a more accurate shift point choice.

To get shift points, well as I was saying, here is my method with the Gonkulator and your car (as best I can Gonkulate it tho I cant match your 60ft, 1/8, et and MPH, the 15.90 doesnt fit with the other numbers).
But here is what I get:

6000 15.19 at 90.51 Best/Lowest ET
5500 15.29 at 89.70
5400 15.32 at 89.64
5400 15.46 at 89.06 (shift 5400-5000-5000)
5000 15.50 at 89.09
So I would give up my "tenth" and save the engine 500rpm by shifting at 5500. OF course if going for a class record you cant do that but its a lot easier on the budget.
If you shift another 500 early, at 5000, you give up TWO more tenths, now its kind of wasting the engine.

Beyond the Gonkulator, CNC's idea will help but even then you need a Gonkulator of some sort to know what to do with that info.

The most direct way I know is use an accelerometer in the car. You want to shift so you're pulling the same G's at the bottom of 2nd as you were at the top end of 1st, and do that in each gear. Not a new idea, its in old magazine road tests back as far as the 1940s!