L98 max compression ratio
Thread Starter
Racer
Joined: Apr 2004
Posts: 306
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From: Cotati CA
L98 max compression ratio
Since I've done a couple of searches to find this info and had no luck, what is the max CR I can aim for w/my '89's engine? I'm planning to do a rebuild w/a 383 stroker and don't know which style of pistons to use.
Any help/tips appreciated!
Any help/tips appreciated!
Team Owner
Joined: Aug 2000
Posts: 24,125
Likes: 3
St. Jude Donor '14
If your gonna keep the stock heads, the pistons are gonna be dished between 28 and 30cc yielding around 9.0:1 comp. Perfect for boosting 
12cc dish will be closer to 10.75:1
12cc dish will be closer to 10.75:1
Last edited by 8388; May 21, 2005 at 11:36 PM.
Race Director
Joined: Feb 2004
Posts: 12,518
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Originally Posted by V's Vette
Since I've done a couple of searches to find this info and had no luck, what is the max CR I can aim for w/my '89's engine? I'm planning to do a rebuild w/a 383 stroker and don't know which style of pistons to use.
Any help/tips appreciated!
Any help/tips appreciated!
Team Owner
Joined: Aug 2000
Posts: 24,125
Likes: 3
St. Jude Donor '14
Originally Posted by Pete K
On pump gas, fuel injected,aluminum head cars I shoot for 10.5-10.75 to 1. This makes me happy. I am also carful to keep the quench at a minimal amount. This makes a more efficient package that is less octane sensitive and more powerful.
With the quality of gas I'm seeing at the pump these days, I would have to think twice.
Thread Starter
Racer
Joined: Apr 2004
Posts: 306
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From: Cotati CA
Good point about the quality of gas...I forgot to mention that here in Nor. Calif. the top octane at the pumps is 91, and I'll be using my stock aluminum heads. So, given these additional factors, what is the top CR I should aim for?
Team Owner
Joined: Aug 2000
Posts: 24,125
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St. Jude Donor '14
Originally Posted by V's Vette
Good point about the quality of gas...I forgot to mention that here in Nor. Calif. the top octane at the pumps is 91, and I'll be using my stock aluminum heads. So, given these additional factors, what is the top CR I should aim for?
Back in 91 I built a 383 with only 9.0:1 and this engine surprised every body in how well it ran. I used an old trick (special cam) the cam made the engine think it had more compression than it actually had. The 3500 lbs + car had a best run of 121 mph in the 1/4. It would pull factory vacuum and had all the manners of a stock factory car. Idled like a stocker too.
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Team Owner
Joined: Aug 2000
Posts: 24,125
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St. Jude Donor '14
The cam is made by Wolverine Cams. Wolverine is or was a sister company of Crane cams. The specs : Billet roller 224/232 @.50 lift was .490/.510 lobe sep. 115. Cam did real well between 2000-6500 rpms, on the 383.
Racer
Joined: Jul 2001
Posts: 322
Likes: 1
From: Los Gatos, CA
I'm running a real 11/1 with .040 quench and JE flat tops. Combustion chamber design and cam specs will have a huge impact on how much compression you can handle with pump gas. In my application it works. I just use 100 octane unleaded at the road race track for insurance. It's only a couple of times a year so I can afford it. On the street I haven't had any issues.
Good luck
Good luck
Cruising
Joined: Nov 2004
Posts: 11
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the cam has a large affect on how high of a CR you can run. ive run a iron head 350 with 11:1 on 91 octane gas and had no problems. but i would rather keep it around 9-10 and not risk having problems. i cant remember what cam it was but i do know they quit making it a few years back. the quality of pump gas these days seem to have gone down like 8388 said so thats another factor to look at. personally i would try to stay down below 10:1
Burning Brakes
Joined: Aug 2000
Posts: 1,146
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From: Albuquerque NM
Originally Posted by V's Vette
Since I've done a couple of searches to find this info and had no luck, what is the max CR I can aim for w/my '89's engine? I'm planning to do a rebuild w/a 383 stroker and don't know which style of pistons to use.
Any help/tips appreciated!
Any help/tips appreciated!
I was running 11.1 compression ration with a lot of ping...I used a fat head gasket 0.051 to lower CR.
I am now running 11.6 compression ratio with no ping using a .039 gasket with 0.005 piston out of head clearance...mind you with upwards of 39 of advance (still tuning!)
quench hieght is definitely important...I say let whatever reasonable CR using a forged piston (hyper are nice, but tend to melt/crack/explode under more extreme circumstances like over heating)
p.s. I am using cryo treated and coated pistons and rods
