When I rebuilt the L98 engine in my '89 IROC, I didn't know anything about compression ratios (among other things). I don't know a whole lot more now beyond the simple fact that it can affect power.

When I rebuilt engine, I aimed to keep it as close to stock spec as possible to avoid having to make other modifications that I didn't know how to make/couldn't afford at the time (i.e. needing to burn a PROM). That means that I re-used the stock iron heads, same rods, same crank, GM-replacement head gaskets, and a new cam of stock specification. I did, however, use hyperkineutic pistons (as opposed to the stock cast pistons).

I couldn't tell you much about the pistons since my machinist ordered them for me. I can tell you that each has a raised rim about the outer circumference of the piston, and each has four small divits cut out (supposedly for valve clearance--although not likely necessary for stock heads with a stock cam).

Any ideas if this change has significantly affected my compression ratio?

And, just for the record, compression ratio is the ratio of the volume of uncompressed air in the cylinder at BDC to the volume of compressed air in the cylinder at TDC, correct?

Thanks

 

changed it yes, but not enought to make any real differance

 

If they are flat tops, then i wouldn't worry about it. My L69 had pistons like those with 58cc heads and the CR was 9.5:1, which is more-or-less the same as an L98 I think.

 

Air_Adam: If, by "flat-tops", you mean that the top of the pistons are completely flat, they aren't. Another way of saying that "there is a raised rim about the circumference" is to say that there is a depressed region within the raised rim. I wouldn't call them domed (because there is nothing hemispherical about the depression), but they are recessed compared to the raised outer rims; thus, they are not completely flat. (I could just be splitting-hairs, too. )

The pistons are definately not the kind that "bulge out" to match the shape of the combustion chambers in the heads.

Hopefully, you're still right to say that it didn't change it by much, though. Stock CR for an L98 is 9.3:1; hopefully a fraction of a point of change won't make a difference.

Thanks for the replies.

 

Sancho,

Bad news, bud. It's time to fire your machinist. It sounds like he/she installed dished pistons in the engine. Did they look like this:

 

Vader: Yup, pretty much...

 

That looks like a typical 10cc dish rebuilder piston, your CR with a standard bore,.041 compressed thickness head gasket and 64cc heads should be in the 8.9-9.0:1 range.

 

boogie: Standard bore is boring out to 355 ci, right?

If that's the case (i.e. the CR lowered by .3 to .4 points), what will that mean in terms of power losses, etc. ?

 

Standard bore is 4.00, your bore is 4.030. the power loss would be slight. That looks like a felpro rebuilder head gasket, you could go to a thinner gasket such as Victors nitroseal .026 compressed thickness from NAPA and gain back your CR. Part # is 5746

 

RE: the head gasket... That is a FelPro gasket, but it's not what's on there now. I replaced it with a GM replacement gasket, which should be the same thing it came from the factory with. How thick was the stock one?