which head gasket? (see engine combo inside)
Thread Starter
Racer
Joined: Sep 2003
Posts: 261
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From: Edwardsville, Illinois 88 stroked s10, 92 stock 6spd
which head gasket? (see engine combo inside)
388cu (3.75 stroke, 4.060 bore), 4340 6inch rods, 9000 series crank, kb flat tops, extensively ported l98 aluminum heads- around 58cc- among other things- the combustion chambers were cc'd and the 2.02/1.60 valves were unshrouded in the chambers, cc306 cam, and a converted lt1 intake. i want the most compression i can have and still be fine on 93 octane. which head gasket and what thickness should i buy?
Le Mans Master
Joined: Jul 1999
Posts: 8,683
Likes: 6
From: Phoenix AZ
Originally Posted by bumpnzx3
388cu (3.75 stroke, 4.060 bore), 4340 6inch rods, 9000 series crank, kb flat tops, extensively ported l98 aluminum heads- around 58cc- among other things- the combustion chambers were cc'd and the 2.02/1.60 valves were unshrouded in the chambers, cc306 cam, and a converted lt1 intake. i want the most compression i can have and still be fine on 93 octane. which head gasket and what thickness should i buy?
What compression ratio are you going for? You could push 11:1 on 93 octane pretty safely, slightly more depending on how your combustion chambers have been shaped. If you're looking for some of the strongest gaskets you can find, check out the Cometic metal gaskets...but they can get pricey (close to $100 each), but they will make them in almost any thickness you want to allow you to fine tune the compression ratio.
http://www.cometic.com/
Large Impressive Member
Joined: Jul 2003
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From: Good health is merely the slowest possible rate at which one can die
St. Jude Donor '04-'05-'06-'07
Do you know how many different flat tops KB makes? 
Would have to know distance from top of piston to the deck at TDC, piston cc, etc. Have you checked valve to piston clearance? This could be the final determining factor if you're on the edge.
Would have to know distance from top of piston to the deck at TDC, piston cc, etc. Have you checked valve to piston clearance? This could be the final determining factor if you're on the edge.
Pro
Joined: Dec 2002
Posts: 603
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From: 84 383 XFire Chandler, AZ
What is the deck height of the block. If it is 0 decked then a compressed gasket thickness of 0.039 to 0.041 would be inorder. Just my humble opinion... But you need the deck height number to figure out what the quench distance is.
Instructor
Joined: Jul 2003
Posts: 114
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From: Colorado Springs CO
Compression Ratio ??
If you have a stroker, and flat-top pistons, your compression ratio may be a whole lot higher than you think. My 383 needed a dished piston to get 10.8 compression with a .038" gasket, yours might be closer to 12:1 with flat-tops. Check with KB, or other piston manufacturers that have worksheets on their websites. J&E has a good one, check there.. You're probably gonna need a pretty thick gasket to get below 11:00 or so.
Le Mans Master
Joined: Jan 2003
Posts: 8,596
Likes: 19
From: Portland Oregon
Originally Posted by BillWelter
If you have a stroker, and flat-top pistons, your compression ratio may be a whole lot higher than you think. My 383 needed a dished piston to get 10.8 compression with a .038" gasket, yours might be closer to 12:1 with flat-tops. Check with KB, or other piston manufacturers that have worksheets on their websites. J&E has a good one, check there.. You're probably gonna need a pretty thick gasket to get below 11:00 or so.
You can get a cometic gasket that is around .042 compressed. But I don't recommend using a shim type gasket any thicker that that. The reason being is that going with a thick gasket will increase the quench or squish demension which is not a good idea. And ideal squish demension is < .055 and this is figured if the pistons are at zero deck. So if your pistons are below the deck then that has to be figured into the equation as well.
If you have flat tops with typical valve reliefs then I would suspect you have a piston with -5cc of valve releif volume, figuring that with 58cc chambers and at zero deck you are probably over 11:1 compression now. If the pistons are below the deck .005 to .015 then you can figure two to three tenths reduction in static compression.
Melting Slicks
Joined: Feb 2002
Posts: 2,699
Likes: 2
From: The Black Hole LA
I just decided to sell my stock heads, after much reading and researching I found that with flat top KB's (which I'll be using) and the same setup basically you'll be going with (383 ect) you'll be pushing 13.1 compression. You might could get it down to 12.1 but I doubt any lower. Like I said though thats what I have found from reading numerous posts/articles on the subject, I haven't actually done it yet, I'm going to talk with a machine shop tomorrow about the block work and my internals. I decided to go with after market heads with around a 64 cc chamber in order to safely be around 11.1 so I could use pump gas and not worry about blowing a piston up.
Thread Starter
Racer
Joined: Sep 2003
Posts: 261
Likes: 0
From: Edwardsville, Illinois 88 stroked s10, 92 stock 6spd
i am pretty sure that cam has a descent amount of overlap- so that should bleed off some of the compression. the block has not been zero decked- just some take off the top to square it up when the machine work was done. i will have to check to see what the deck height actually is though.
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