Ive read some where a while back that if you use a larger lobed quick cam that you can reduce the actual engine compression. I think is was a professor overdire artical or something. Is this true, If so how much compression can I bleed of an engine running at least 12:1? I could just swap heads (running 58cc with 194 valves) but these heads have alot of work in them and have proven themselves capible of 7k+.
The rest of the engine is as follows 355, 4vr flattop's ZZ4 hot cam clone.
Thanks!
SSC

 

huh, I haven't heard anything about losing compression when you change to a bigger cam. What you do lose is vacuum though. Thats why if you switch to a cam to big its usually necessary to run a vacuum storage tank into the system...

Mike

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1989 RS.. 355, Hypereutectic pistons, double roller cam, double roller timing chain, accel wires, blue streak cap and rotor, rapidfire plugs, chevy caprice 350 chip, 200* fan switch, 3 angle valve job, 3 inch cat-back exhaust, 3 core radiator, 3.73 gearing with POSI.

 

Actually I've read that too, in some Chevy mag. I don't know the specific numbers, but the general idea was this: when a cam has high overlap (when both intake and exhaust valves are open) it'll bleed off cylinder pressure. But this only works for slow engine speeds. At higher speeds when airflow is increased the c.r. won't be lowered as much.

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91 Z28
Come see The Vicious...

[This message has been edited by ViciousZ (edited December 05, 2001).]

 

What was said above about overlap is where you bleed off dynamic compression, where and how much depends alot on the cam and where its timed at. The static compression ratio wont be affected at all, but once the engine is running there isnt much purpose in calculating static compression anyways.

 

Your talking about effective cylinder pressure. Anything more than about 200psi cranking compression will detonate on pump gas. Too many variables come into play to conclusively determine what will be sufficent. Gearing, ambient temps and such are factors.

with something in the range of 11.5 or more to 1, you will need about 245 degrees duration at 050" on a Lobe separation angle of 108 or less to run on pump gas.

[This message has been edited by jcb999 (edited December 06, 2001).]

 

It's true that a really big overlap cam will bleed off some cylinder pressure at low engine speeds.When you spin it with a load on it, though, it's gonna need better gas.
I know a guy that runs 12:1 CR on pump gas, but he has a really loose converter (5,000RPM
Stall) and a cam that has .050 duration numbers in the 250's and 108 degree lobe centers.While lugging it around town he can run on pump gas.When he's gonna race somebody, he switches over to fuel cell in trunk full of 114.He's got a completely seperate fuel delivery sytem for the good gas.(up to the carb, anyway)
-Rich-

 

Thanks for the reply's!
Yea the availability of proper octane fuel is my problem. I'm stuck with 91 as Premium fuel. 91+-octane boost isn't allowing me to advance to the full potential of the engine .
To bad there isn't any crooked guys at the airport anymore
Thanks.
SSC

 

You've got too far to go. If you were at 10.5-10.8:1 you might get there with a bigger cam but you're gonna need a MONSTER to get you down where you want to be.

 

IMHO you're out of luck. A cam's "leak" effect helps you out at low RPM only, you're still in trouble once you get into the cam's intended operating RPM range.

I'd suggest trying to find somebody with a set of comparably worked-over 64cc heads that would take a trade. Just for giggles, what has been done to the heads you've got?

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"So many Mustangs, so little time..."
ICON Motorsports

 

Yea thats what I was worrying about. I cant recall the casting numbers of hand but they are the only good 58cc heads GM made.
(Good being used loosly)
They are the open runner with closed chamber.
They have been extreemly ported and polished combustion chambers have been re-shaped and have 1.94in which are in case anyone is wondering a PITA and dangerous considering the amnount on materal that neeeds to be taken out.

A local braket racer was interested in swapping me heads last summer but instead got a blower and 58's werent do able for that application.
SSC



[This message has been edited by SSC (edited December 06, 2001).]

 

SSC: Instead of wasting money on usless octane booster,(there is no way a few ounces
of tolene+xolene+mineral spirits added to a tank of gas is going to signifigantly raise octane). Just mix about 25% 110 to 114 octane
unleaded race gas with your 91 octane pump gas. This should be enough actual octane for your 11.0:1 motor. You'll save a lot of money too.