'90 compression ratio
Thread Starter
Racer
Joined: Dec 2003
Posts: 345
Likes: 0
From: Rochester NH
'90 compression ratio
I am getting a few different numbers for the compression ratio for stock aluminum heads on a 1990. I hear 9.5:1 and I hear 10.25:1 (due to different pistons). Does anyone know the correct ratio? I am considering having the heads milled down a bit while they are out being ported and polished. Has anyone done this?
The car is a 90, 6 speed. With normal mods. I am planning on putting headers on when I bolt it back together.
The car is a 90, 6 speed. With normal mods. I am planning on putting headers on when I bolt it back together.
Thread Starter
Racer
Joined: Dec 2003
Posts: 345
Likes: 0
From: Rochester NH
Now you guys understand what I am talking about. The compression ratio for '90 and '91 are all over the place.
Haven't decided about the intake. Maybe a SR, but need to check the funds.
Haven't decided about the intake. Maybe a SR, but need to check the funds.
Pro
Joined: Jan 2002
Posts: 718
Likes: 0
From: W.V.C. Utah
Originally Posted by Swirk
I am getting a few different numbers for the compression ratio for stock aluminum heads on a 1990. I hear 9.5:1 and I hear 10.25:1 (due to different pistons). Does anyone know the correct ratio? I am considering having the heads milled down a bit while they are out being ported and polished. Has anyone done this?
The car is a 90, 6 speed. With normal mods. I am planning on putting headers on when I bolt it back together.
The car is a 90, 6 speed. With normal mods. I am planning on putting headers on when I bolt it back together.
Drifting
Joined: Jan 2002
Posts: 1,261
Likes: 4
From: Middle River MD
Originally Posted by 88421VETTE
are you sure you want to mill the heads the combustions chamber is already pretty small I know I wouldn't put a bigger cam in if they were milled
Yeah, my 113's have been milled and I think I have a compression problem now...too high.
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Thread Starter
Racer
Joined: Dec 2003
Posts: 345
Likes: 0
From: Rochester NH
Originally Posted by AS84
I took .010 off my D ports and really wish I had gone more. I really wanted to take another .005 off the block but I didn't want to pull the motor. Compression makes power
First of all, nice car. I really like the steel blue cars.
Have you had any detonation problems? I only run 93 octane or better.
What head gasket did you use? Stock thickness?
I am getting the porting/polishing work for cheap (friend of my father) and he asked if I wanted to mill them down a little while they are out. I was just checking my facts before I let him cut them down.
Drifting
Joined: Apr 2003
Posts: 1,379
Likes: 4
From: rome ny
I will clear this up for everyone. 90 and 91 had a piston change the gm part # is 10159437.. with a standard .039 head gasket and stock deck height which is .020 or .025 the comp ratio with 58 cc heads will be 10.1/1 give or take .1.... the pistons are around a -7 inverted dome.
the piston change in 90 did raise the comp ratio from 9.5/1 to 10/1. and they are a performance hypo piston.
This piston is for the 5.7L high output engine assembly; second design with ZZ4 and LT1 (high silicon) aluminum. This 350 standard or high-limit, pressed pin, high performance piston has a 10:1 or 9.6:1 compression ratio with 58cc or 62cc head chamber volume
this piston in the 90 and 91 are the same as the 92-96 lt1's.
the piston change in 90 did raise the comp ratio from 9.5/1 to 10/1. and they are a performance hypo piston.
This piston is for the 5.7L high output engine assembly; second design with ZZ4 and LT1 (high silicon) aluminum. This 350 standard or high-limit, pressed pin, high performance piston has a 10:1 or 9.6:1 compression ratio with 58cc or 62cc head chamber volume
this piston in the 90 and 91 are the same as the 92-96 lt1's.
Drifting
Joined: Apr 2003
Posts: 1,379
Likes: 4
From: rome ny
Originally Posted by Trog
Cool info. If that is true then the performance head gaskets I used should bump my compression to 10.5:1. A nice number for a little more power but not so high that I can't find fuel easily.
what size gasket did you use. i will calulate you new compression
i need the gasket bore size and compressed thickness.
Last edited by mos90; Sep 10, 2004 at 02:07 PM.
Team Owner
Joined: Sep 2001
Posts: 62,119
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From: Athens AL
C7 of the Year - Unmodified Finalist 2021
C4 of Year Finalist (performance mods) 2019
That engine DOES use different pistons from earlier L98s, the CR was raised as a result to about 10.25.
Drifting
Joined: Apr 2003
Posts: 1,379
Likes: 4
From: rome ny
multiple site and many performance shops confirm a compression ratio increase from 89 to 90. so i dont know who is right. all i know is i did a lot of research and every one says increase from 9.5/1 to 10/1.. 
Melting Slicks
Joined: Jan 2002
Posts: 2,238
Likes: 4
From: Milan
Originally Posted by qtcoupe
Have 1990 vert. Plate on ashtray cover says
L98
350 cu in.
245 HP
345 torque
10:1 comp.Ratio
L98
350 cu in.
245 HP
345 torque
10:1 comp.Ratio
L98
350 cu in.
245 HP
345 torque
9.5:1 comp.Ratio
I don't know how GM says the right CR...
When I rebuit the engine I made calculation with compressed Head gasket, bore of the gaskets and deck height, 4valve reliefs piston and cc comb. chambers and the result was about 10.1:1 (piston was flat w/4 valve relief.
The same calculation has been made on my actual engine with KB pistons (with 2 valve reliefs) and slightly thinner head gaskets and resurfaced block and heads.
If I remember correct The final CR was 10.7:1
-Beppe-
Burning Brakes
Joined: Apr 2000
Posts: 980
Likes: 2
From: Chandler AZ
Not to hijack, how can you tell with a gauge?
Just pretend for a moment that I know little about compression ratios. If I, an average guy have the same problem and wanted to know what my C/R truly is, how can I tell. I know that at sea level the atmospheric pressure is around 14.7 psi. but when I test my car and I have 180 lbs showing on the gauge, I know it doesn't have 12.25:1 compression. What is the true formula? This may help answer the which C/R do I have question.
Thanks in advance.
Thanks in advance.