Help.....problem with lowering my compression.....

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Mar 19, 2015 | 09:32 AM

LS6427

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Help.....problem with lowering my compression.....

So here's my deal: I really do not want to change my pistons.....

I had my AES 390ci built with Flat top pistons. I want to use a single PT88 turbo.

My engine info and planned parts:

Iron LQ9

4.030" bore
3.825" crank
6.125" rod ((1.180 pin height))
LS9 .051 gaskets
74cc chambered heads
Flat Top pistons
9.225 deck height

The compression calculator at http://www.eaglerod.com/....IF I DID IT CORRECTLY.....says 10.28:1 compression.

I'm trying to get it to 9.8:1.......

How thick can I go on the head gaskets ((Brand and part number)) and still have the gasket be very strong and reliable.......? That the only place I see any possible wiggle room to lower cr any more.

I know 10.28:1 cr is fine if I use full time E85....but Id rather not if I don't have to.

Thanks...

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Mar 19, 2015 | 10:19 AM

Forcefed86

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If you have good deck and head surfaces cometic can make you a custom head gasket to hit most any compression you want, there is no limit really. Very common for the import crowd to run 3.5+mm thick HG's (.138" thick). I believe they sell a cheaper .070 shelf LS gasket as well.

Mar 19, 2015 | 10:28 AM

stevieturbo

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How thick are the piston crowns ? Could they be machined ?

Likewise can chambers be opened out more ?

Cant say I've ever seen anyone using a gasket more than 2mm thick, and even that's rare.

Mar 19, 2015 | 10:54 AM

Forcefed86

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Wouldn't need that thick of a gasket to drop a half point of compression.

3.5mm is common in the BMW turbo crowd…

http://www.jegs.com/i/Cometic+Gasket...FZSKaQodNWoAwg

I ran a 3mm thick gasket on my Sc300 2JZGE setup as well.

Mar 19, 2015 | 06:10 PM

LS6427

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Quote:

Originally Posted by Forcefed86

Ok....I did the compression calculator again using .070 gasket......it dropped me to 9.87:1......

That should be good......

Again...if I am figuri it all out correctly with calculator. I used the one here:
http://www.eaglerod.com/

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Mar 19, 2015 | 06:11 PM

LS6427

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Quote:

Originally Posted by stevieturbo

How thick are the piston crowns ? Could they be machined ?

Likewise can chambers be opened out more ?

Cant say I've ever seen anyone using a gasket more than 2mm thick, and even that's rare.

I was also wondering that......because when I enter 78cc chamber volume and .051 LS9 gasket........and everything else I listed the same......that also drops me to 9.86:1

So can a 74cc chamber head like the AFR 245 be ported 4cc more in each chamber.......?

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Mar 20, 2015 | 07:26 AM

LS6427

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Would this be the proper gasket to use for 4.030" bore....?

http://www.netgaskets.com/COMETIC_He...%20c5751-b.htm

Is .070 thickness perfectly fine for 18-20 psi.......???

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