how to lower compression for turbo (shims?)

stooges701 · Sep 5, 2006

I need to drop my 306's compression to around 8.8 or lower. Right now, I have 10:1 with AFR 165 heads (58cc), .032" head gasket, and the piston comes up .004" out of the bore.

I do not want to change the short block. With that said, is there some way of dropping the compression through either thicker head gaskets or head shims to obtain a 8.8:1 ratio?

Mr. Rustypwnz · Sep 5, 2006

I think yer gonna have to change the heads or the pistons, either one.

68torinogt · Sep 5, 2006

if your shimming your heads to run more boost they will just blow out

R2D2 · Sep 5, 2006

shims? Pull the heads, figure out the cc size of the head you need, send em to a competent head shop and see if they can get the chamber large enough. Your not gonna run a thicker head gasket without serious quench area problems. Your stuck!

85_SS_302_Coupe · Sep 5, 2006

Port/bowl blend the hell out out of your combustion chambers on the heads and run the thickest head gaskets you can find.

90lxcoupe · Sep 5, 2006

you can order cometic head gaskets as large as .125''

Saw an article in Hot Rod with a 454 that was N/A making like 11.5:1 compression and they got it all the way down to sometinhg like 8.5:1 if i remember right.
Just make sure your tune is good if you use cometic cause if it detonates the gaskets gonna hold but the pistons might not

Hickfied · Sep 5, 2006

Have the combustion chambers worked as much as possible, and don't go over a .054 think head gasket in this case.

R2D2 · Sep 5, 2006

dont go and run a thick head gasket, you need to maintain quench. Is the block 0 deck?

vristang · Sep 5, 2006

Another vote on keeping the head gasket height around .050"

Here is a thread in the Corral...
http://www.corral.net/forums/showthread.php?t=829880

I am just learning about issues with excessive Quench Height, so I am not sure I can explain it well.

jason

tvanlant · Sep 6, 2006

The heads are AFR 165s, so I don't know how much I will be able to pull out of the chamber. The piston comes out of the hole by .004". Only a .050" head gasket? No shims? Is this going to be possible?

What compression do I need to obtain to run around 12-14 psi on 93 octane?

Hickfied · Sep 6, 2006

With the piston comming out of the hole .004, you can run up to a .054 head gasket if you have too.

vristang · Sep 6, 2006

tvanlant said:
The heads are AFR 165s, so I don't know how much I will be able to pull out of the chamber. The piston comes out of the hole by .004". Only a .050" head gasket? No shims? Is this going to be possible?

What compression do I need to obtain to run around 12-14 psi on 93 octane?

I would think 9 to 9.5 would be ok, if the tune was right.

This is just a guess on my part though, I would like to hear what others have to say.

BUMP!

jason

Xpipe · Sep 7, 2006

can he enlarge the valve notches a litl ?? would it work

GroverDill · Sep 7, 2006

get a good tune you will be ok.

good tune + compression + boost = happy motor.

how to lower compression for turbo (shims?)

stooges701

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