Highest compress ratio w/ 92 octane
 

Okay i know this question has been asked before but i can't seem to find it in old posts, im trying to build a 396 out of my lt1 and the local machine shops are telling me not to go over 10.5:1 compression on pump 92 octane. Yet i know ive seen many others with 11:1, 11.5:1, up to 12.5:1 here on the board. I know that you all can't be running racing fuel. So what im wondering is with fully ported aluminum heads and JE forged aluminum pistons what is the highest compression ratio i can go on 92 octane premium fuel. Thanks for any help you can give me,
Dan Roach

Some of it depends on the size of the cam. The larger the cam the lower the dynamic CR.

You should be able to get away with 11.5-12:1cr on pump gas.

I am at 11.7:1 with my current setup without problems. I have a huge cam though.

DaveW

considering LT1's start out higher than 10.5:1 I don't think its a problem :)

One thing that your machine shop probably doesn't know is that LT1's have reverse flow cooling and I believe sodium filled valves. That being said, you may want to consider finding a machine shop a bit more familiar with the LT1 setup...

i would say try to stick in the neighborhood of 11.5-12.0 myself. that is, if your looking for the highest CR you can go with out race gas. you could prolly get away with 12.0-.5 with a MIX of pump and race, but not just pump. anything higher than 12.5 and you WILL need race gas.

if it were me, id prolly stay around 11.0-.5. but thats me:)



jon

lookup dynamic compression ratio and read about how intake vavle closing time affects it. the later the vavle closes (usually means bigger cam) the more the dcr drops which is why when you look at cams they recommend a higher compression ratio for all the larger ones. that being said i am fairly sure that as long as your DCR doesnt exceed 8.4 or so then i think it will be fine on pump gas. i'm also pretty sure that a tuner will set the static compression ratio due to what cam he wants to run in the engine but thats just mho. there are others here who know more about this than i do. i hope they chime in.

I'm at 11.3:1 compression on 91 octane.

How can you guys get away with higher compression and 91-92 octane when so many guys with stock LT1s with stock programming end up seeing knock retard on a scan tool when using these octanes? (I've seen it on my own car, which is bone stock)

Is it due to the heads/cam being more efficient perhaps?

No problems here..i had to install a better starter my stock unit didn't have enuff power...:D

If you do a search over in "Advanced Tech" you can read a number of long, informative threads on dynamic compression ratio. You sure can't run a stock cam with 11.5-12.0:1, but you can run that high a static CR with a big cam. Some people with solid rollers are running in the 12.5:1 range on pump unleaded!

Rich Krause

i ran 11:1 on 91 no problem

i'm going to mill the heads and use thinner gaskets
see if i can get to 12:1

11 to 1 with aluminum heads is fairly safe for 92-93 octane. My 69 road runner was a measured 10.9 to 1 compression, 440 motor, with iron heads and ran great, has a 585 lift 305 duration solid cam, not huge but big for a street car. Sure you could run say 12 to 1 and a big cam, but compression isn't everything. Matched componets will be much better at 11 to 1, than a 12 to 1 motor with a cam too big for the rest of the setup.

I'd stick around 11 to 1, get a good converter if its an auto, cam and gears matched to the rest and you'll be set. 8 to 1 and 10 to 1 comp is a big difference, but 11 to 1 and 12 to 1, isn't as much as a gain in hp. It definelty adds some with the right setup, but probally not worth it.

Trying to make too much can sneak up on ya, you might not know it, but it might be detonating, and next thing you know you burnt up a piston. It might be fine when its cooler, but when it gets up to 90s-100s, then it might catch up with ya. If you go more than 11 to 1, I'd reconemend mixing some race gas, just to be safe!

Compression isn't everything, don't forget this. Theres alot more to hp and 1/4 mile times than high compression.

Thanks guys for all your help. Its nice to be able to get some advice from people who know what they are doing. Im sick of these machine shops who think they know everything that run out of the back of a Napa parts store telling me stuff about motors they have no experience. I guess its my fault in the fact that i asked them and expected them to give me decent advice. I plan to run a TPIS ZZX camshaft with afr heads probably 195s or so. TPIS as well as you guys are afirming that with premium fuel i should stay around 11.5:1 to max out my power. Guys in the machine shop told me that to even come close to making 500 hp out of a small block id have to be running 13:1 or higher compression. Does anyone on here have experience with this combo (TPIS ZZX cam, AFR 195 non ported heads, and a 396 setup) Im hoping that with all the other mods to back it up, 58mm TB, Long tube 1 3/4 headers, 30 lb injectors, MSD ignition, 3 inch b&b exhaust 2800 vig stall along with other things can push my nearly full weight 96 vette into the tens. Does that sound reasonable? To those of you not familiar with the cam duration at .50 is 239/239 lift is .595/.595(w/ 1.6 rollers) Lob sep at 112. Any advice is greatly appreciated.
Dan

do yourself a favor
TPIS is lost when it comes to the LT1
call combo motorsports for advice. ALL they is pretty LT1 and some LS1.

I'm shooting for 500hp and i have relativly tame car on 11:1 CR

Most LT1 cams have a "split pattern" with the exhaust duration>intake duration. This is desirable due to the relatively good intake flow vs. exhaust flow on a typical LT1 combo. I do not recommend a cam with the specs you posted.

Rich Krause

I'm with Rich, but then again we usually agree on things.

a 239/239 is WAY TO BIG, esp with what you are tring to accomplish.

Bret