cammed91

iTrader: (1)

ok my question is what is the highest compression you can go and get away with running pump gas on a stock l98 bottom end with headers, intake, cam, and ignition?

F-BIRD'88

Not near enough info. Keep it under 10:1 actual compression ratio and concentrate on improving the total airflow of the engine, including the exhaust system and you be way ahead overall on a pump gas motor.

1989GTATransAm

iTrader: (3)

Right on the info. What heads to you plan on running. Dynamic compression ratio plays a big part and that depends on the IVC of your cam.

RPOL98

I put a LT4 HOT cam into my 5.7L L98, kept the rest of the short block stock, but put angle-plug 64cc heads on (well, they might be 67, never cc'd them).

anyways, I had always run premium gas, but when high-octane went over $3.20/gal, I tried regular (87), and hooked up a data logger onto the ALDL to monitor for knock going up hills, acceleration, etc. no problem, so I've been running regular since then, it's my daily driver. wish I had done that earlier, could've saved a bunch of money, without even having to switch insurance companies.

since it's a 350, I suspect the factory heads were 78cc, they had the big bathtub combustion chambers.

Damon

From the experience of many board memebers here who have taken apart stock L98s many times I'll mention that GM switched to flattop pistons in the later speed/density engines (90-92) for an actual compression ratio of about 9.8:1 (earlier engines used dished pistons and were in the low 9s).

All factory L98 heads had nominally 64cc chambers (excluding the aluminum headed vette motor), regardless of year.

So somewhere in the high 9s is where the factory thought they could safely run with a stock setup. I assume they counted on the knock sensor to keep the motor out of detonation if the owner got some bad gas in the tank.

84z28350

It really depends on many other things...

whats the highest octane pump gas available to you?

Which cam are you running?

How much advance are you running? Been tuning the ECM?



You can actually get away with pretty high compression for pump gas. But you need a cam that will bleed off some of that cylinder pressure and you cant be running alot of spark advance.


Last year i built up my little iron head 357 and ended up with just over 10:1 static, and due to the cam it brough the cylinder pressures down enough that i was 'fine' on 91 oct, most of the time i could keep a full 36* advance but if i got a bad tank of gas i had to knock it down a tad.

cammed91

iTrader: (1)

yea my 91 l98 has the flattop pistons with the 4 valve reliefs? have the heads off right now. i can only get 91 octane here in OK. i am running the comp cams xr282hr. i believe its a 230 236 duration with 510 520 lifts on a 110 lsa. i'm not quite sure what the advance in the chip is set to because i had ed wright do it for me. i set the initial timing at 10 degrees. would i be able to safely go over 10:1 with the vette aluminum heads. i thought i read somewhere that with aluminum heads you could get away with running a little more compression.
thanks