Ok. Im picking up an 89 L98 from an Iroc, i was going to freshen the shortblock and use it in my vette.

I was under the assumption that the different C/R in vettes vs f-body was because of the 58cc heads on the vette. I think F-body is 9.3:1? and the L98 in my 90 vette is 10.25:1. Turns out the vette was 9.5:1 in 89. They went to flat top pistons in 90 and 89 had dish.

So my question is...

What type of piston is in your L98? and what year is it? Im now guessing the f-bodys got dish. In which case, i'll have to get a set of flat tops for the sob.

 

My 92 350 had flat top hypers.

 

The '92 Z28 L98 350 I picked up had OEM Mahle brand flat top short skirt pistons which from what I've read on here were used on '90-92 engines. '89 and older had longer piston skirts. As far as I know '82-83 LU5 CFI, '83-86 L69 HO, '85-86 LG4, '85-86 LB9 TPI and '87-92 L98 TPI engines all had flat top pistons that yeilded 9.5:1 compression. '82-84 LG4 and '87-92 305 engines had a small dish to provide 8.6:1 and 9.3:1 compression, respectively.

 

there are a few types of pistons used for L98s

early years 87-89 as i understand have a -12cc dish piston, while the later 90-92 years have a -7cc flat top piston. It is possible some late 89s and 90s could have gotten the -7cc flat top as well. My 89 has dished pistons unfortunately

All corvettes i believe used the flat top -7cc piston. Their compression ratio should be up near 10 to 1 if not a tad over. They generally make near 15-20hp more than Fbody L98s

90-92 speed density fbody L98s would have more compression than 87-89's and that is one reason they generally considered to having more power than older L98s. BUT the 91-92s i do believe have a slightly smaller camshaft than the 87-89's. Even tho rated at 9.3 to 1, the 90-92's should be closer to 9.8 to 1

 

My 1989 had the dished pistons. They probably went to the flat top pistons with the change to speed density. They changed the computer and the program so they could have taken care of the slightly higher compression at the same time. Just a theory based on no facts.

 

According to the "factory" service manual I bought back in '91, (not from the GM parts dept., but they gave me the address to the publisher of the manual..) the L98 was listed at 9.75:1 compression, the LB9, 9.3:1 squeeze. Some people I mentioned it to thought it was an error, and the L98 had 9.3:1. Plot complication, or just fuel for discussion?
Bill

 

My 89 had dished pistons. Be careful when buying pistons cause "flat top" aren't all the same. The Federal Mogal set I used turned out to be +0.020" in the hole. Add that to the stock 0.025" piston to deck clearance ends up with the piston being 0.045 down. Even with a 0.015" shim gasket that gives a 0.060" quench. Not terrible but not ideal either. I "lost" about 1/4 point compression than I thought I would have. Still runs great tho...

 

i plugged in the specs for an L98 with a 7cc flat top and it is indeed near 9.8 to 1 or 9.75 to 1 like you say. They did have higher compression than the earlier L98s

 

This is very interesting. I thought all 350s and 305s after 85 had 9.3 to 1 compression. Has anyone here measured the cc of there stock pistons, measured the deck height and head gasket thickness to prove this? Im not doubting ya, I just would really like to know the Actual compression since I need to pick out a new head gasket soon and that would really help me out here. Also if the MAP L98s have 7cc pistons is that in the valve reliefs or the over all dish? Im wondering if there is less valve clearance or not.

Kevin