Good CR for FI
 

Rebuilding a motor after breaking a piston.
Looking at either 9.8-1 or 8.8-1
Running 93 octane

What CR is everyone using?

Most builds these days seem to be higher compression

The lower compression you run the lazier the car will feel out of boost. What are your goals for the build and what type of fuel do you plan to run?

If those are your options i would go with the 9.8-1 the only time your going to want to go to the 8.8 would be if your going to always be stuck on 93 and want to run high boost. on 93 with meth i ran boost in the high teens with around 9.8-1 cr. now im going to be swapping to e85 and wish i had a little more compression.

I'll go 9.8:1 of those two. 8.8 sounds like a turd on the street. that was only acceptable back in 1998 when people didnt know better.

9:8:1 personally I’d go 10:1

I’m at 10.8:1 on 18psi with 93 and 50/50 meth.

What’s the stock cr? 10.25 to 10.52 ? Keep it close to this since you have a AA a 10.1 Would prob be ideal ! What size combustion chamber is in your head ? 64-70cc Those are the approximate cr values with your heads ! 9.8 9.7 I think you gonna see lag on the low end unless you gonna speed up cam to a 122 atleast and not 114-115 lsa you have to compensate somewhere for loss ! The more you can your variables close to 0 the better ! Since your running a boosted car you will want to break atmospheric pressure ! Without blowing the head bolts off . 12-1 might do it not 10-1 with 10 psi of boost . 4 cylinder engines handle 9-25 psi on a stock turbo with a higher then normal cr. Upgrade your bolts for the heads to block!

Im currently building my ls6 to 11.2:1.

10.8 to 1 here ! With 12 psi plus meth ! Purrs like kitten! Def working on running a higher cr myself !

How did you achieve that ratio?

Switch the heads to a 243 ! on a normal ls1 you start at 10.2 with a 853 with a 70cc chamber and I was running a stock gasket. As you know podium what’s already in the car ! So a 243 head swap is prob gonna raise it from 10.2 to a 10.5 and then it’s just a a matter of getting your gasket right .40 .30 .51 which ever you like but if you wanted to bump it more you can mill the head down a tad that would make your 11-1 your looking for. Many ways to get it this is just one on a stock bore !

Iwas asking how you achieved that. 853’s are 66cc or so. Ls6 heads are 64. What gasket did you run to get the 10.8 you’re running.

I am achieving 11.2:1 with stock unmilled heads and stock ls9 gasket.

.40 with a 64 cc chamber with new 243 heads.

853 in 00 have a 70cc chamber


I assume your gasket is less then .30 because that only gets to to about a 10.9 anyways your running a .25 gasket in my mind for quick math! With a dome piston !

No. They are 66cc.

I have after market pistons.

maybe your right and I’m thinking exhaust ports this morning !

https://www.corvetteforum.com/forums...853-heads.html


There is a 2.22cc difference between the 853 and 243 heads. Intake and exhaust ports are 10 and 5cc larger, respectively, on the 243 heads.

He asked a simple question. "looking for either 9.8:1 or 8.8:1"
Don't go trying to recommend camshafts when you don't understand how they truly work.
"Running a boosted car you want to break atmospheric pressure"? It's called boost. That's how you "break" atmospheric pressure.
Whether it's a 4 cylinder or an 8 cylinder has absolutely nothing to do with how much boost an engine can handle.
You clearly haven't any clue what you're talking about. This isn't any matter or difference of opinion. This is fact. You clearly have no legitimate knowledge or experience to justify 99% of what you post and therefore your advice is reckless, baseless and downright dangerous.


OP. Between the two options, 9.8:1 is the better choice. I understand why anyone with your experience would be gun shy and begin considering lowering compression to mitigate possible issues down the road, but this isn't one that you want to sacrifice.

Well I Gave my opinion and then some so what ? Your there to debate it slander whatever you wish to call it !


when we spoke you wanted 700-900 bucks to start tuning he can get it done where I did mine at Colorado speed for much less because again I have no idea some people got the nerve your good but not a god sir !


what your saying doesn’t add up if you even know the value of atmospheric pressure ! When your lowering your cr there more to compensate for end of story on that one ! I shouldn’t have to explain to someone so called smart as your self I believe but if I must break it down I can and I will !

my suggestion would had obvious been the higher ratio if you read keeping it close to stock with all his forged internals which I dunno if you kept track on his other thread ! He has more then enough to support my opinion so maybe you should well understand all what he is doing and if I gave a opinion it could be higher without harm what’s the big deal.


By the way your about to have a video on verything I’ve done and invested in because I have no idea wtf I am doing yet I’m dusting all your 6’s and 7’s common really ! I don’t know it all really I’m just a weekend warrior sometimes ! All a hobby to fiddle and waste time ! Since I was young at 6 I was beefing up a lawn mowers nothing more then whatever my old man had laying and my own intuition to fix shit not even knowing a thing about it . My friend some IQ ‘s for exceed some others dont believe you can speak for what in my brain! Virtually telling me to shut to say the you can get bent ! Buddy !

Just to be a bit more concise and save you some time, I get the part where you say that you got quoted and went somewhere else and I'm no god, cool. I also get that you're trying to excuse yourself for quoting higher or lower compression ratios. The rest I don't understand.
Just in case you're saying that he has good enough parts to justify a higher compression ratio, let me help you out a little. Having aftermarket pistons and rods (or not) doesn't dictate recommended compression ratios. That is not even close to reasonable logic. You started talking about camshaft LSA. what does that have to do with it? It's as if you're trying to recommend a 122 LSA for a lower compression ratio.

With your lack of proper punctuation and run on sentences, I'm going to admit that I'm too stupid to understand half of what you're even trying to say. I can promise you that I'm not the only one, so how about you do me/us all a favor and break it down. Tell us how camshaft LSA recommendations correlate to static compression ratios.
Explain how 4 cylinder engines handle boost differently than 8 cylinder engines do.I have customers with stock V8 motors that are running 23psi boost and running great so tell me/ua what you're trying get at?