Yeah, I know - it's a little knit-picky - but I'm currently doing a mild build on the ol' L98 for 88 IROC. But the more I research pistons, the more confused I get - stupid, yes, but get this.

The noted replacement piston from Federal Mogul is the STL-423NP (STL-H423NP for Hypers) ... specs say 8.4:1 compression but with what? Doesn't say.

The Flat-Top piston, STL-H345NP states 9.24:1 w/ 64cc head chambers. Some advertise (other sites) that these will give you 10:1 w/ 64cc head chambers - I would probably believe Summit.

I get yay's and nay's regarding getting flat-tops and here's what I think the confusion is.

Stock L98 compression ratio for 1988 is 9.5:1 - others argue 9.3:1, nevertheless, a flat-top piston would theoretically give you more compression, right? So why is the FM 345's spec out at 9.24:1?

Am I missing something here? I do realize that there is "advertised" compression and "actual" compression, based on the cam that the engine is running. I'm just talking advertised.

My goal is to stay at 9.5:1 compression since I am using the cast-iron L98 heads (with some work, porting, bowl work, 3-angle valve job and milled to approx. 62cc). I'm guessing that for the compression ratio noted for 64cc, add 2.33 (i.e. 9.3:1 would be about 9.53:1 w/ the 62cc chambers).

I don't want to risk detonation. The cam I plan on putting in is the XE269HR Comp Cams at 114 LSA (custom grind). Specs are 218/224 dur @ 0.50 and .495/.503 lift. I am also running ZZ9 code on the PROM for an 89.

In summary, where I'm confused is this. If I were to just go by the specs from Summit, I'd opt for the FM Hyper flat-tops since they say 9.24:1 w/ 64cc (estimating 9.47:1 w/ 62cc). But if stock are dished and getting 9.5:1 (for sake of arguement) - why would flat-tops provide "less" compression?!

Unless I'm looking at Vette L98 compression stats w/ aluminum heads at 58cc .... that would explain everything.

 

Oh, I'm also getting headers and using the stock TPI induction system at first, then upgrade to the Holley Stealth Ram. Was thinking forced induction, but I think I'll stick with NA since I won't be able to keep the compression low enough with the stock heads.

 

Boo yah, realized it while I was typing it so I dug a little deeper. L98 w/ iron heads @ 64cc is 9.0:1 and L98 w/ aluminum heads @ 58cc is 9.5:1.

Okay so is 9.5:1 too much compression for cast iroc L98 heads on pump gas?

 

You should be able to get away with a 9.5:1 on cast iron and 10:1 with aluminum on high octane.

The things that will impact your compression ratio is piston, head gasket, combustion chamber size, and I guess deck height would also be a factor.

To do a true piston compression comparison make sure they are advertised at the same combustion chamber size and same thickness of gasket.

 

I've seen 10.5:1 engine live fine on the street with 89 Octane WITH iron heads, and I've seen a 8.5:1 motor detonate like crazy with 93 Octane. Timing advance NOT being a factor.

Turns out the difference was the quench area. Too much can do weird things, so definitely take the depth of the piston in the hole in to account.


HTH

 

Quench and cam profile have alot to do with it. Dynamic compression is a whole new ball of wax thats mostly a black art to normal people like us though. All we can do is make guesses.

I cant see 10:1 creating any problems on that setup. Shoot for 9.5 or 9.75, it'll be fine. If the manufacturer doesnt list all the specs, including compression ratio with a specific chamber size, call them.

 

Go with flattops for sure.