7AG Compression ratios

[TauhuTauhu]

Hi,

Would anyone in here know:

1. thickness of a stock 4ag head gasket.

2. Compression ratio of a:
- bigport 7AG with 8.9 gze pistons, stock head gasket.
- smallport 7AG with 8.9 gze pistons, stock head gasket.
- silvertop 20v 7AG with 8.9 gze pistons, stock head gasket.
- Blacktop 20v 7AG with 8.9 gze pistons, stock head gasket.

Thanks!

[TauhuTauhu]

rough figure anyone?

Does the CR increase or decrease with these GE heads?

[oldeskewltoy]

1.2mm for the OEM head gasket

[TauhuTauhu]

oh ok. thanks for the info. all the while i thought it was 1.1
im just deciding on the thickness of the head gasket i should get to get the CR im looking at.

i know 7AGTE + 8.9 pistons + 20v head / 16head are quite common out there. has anyone actually measured them to find out the comp ratio?

[The Real Roadrunner]

its not hard to figure out if you know the Head CCs and dish CC's + HG thickness but it will only give you a rough estimate as you need to have the block machined and rods/pistons fitted to know how far above or below deck you are. Then you can work out exactly how thick you need your HG to be.

Cheers
Linden

[Leeswf]

There are a number of viables here that make it difficult to give a definite answer. I know the head CCs for 16v are as follows.

Bigport 39cc
Smallport 36cc
20V I don't know

Putting 4ag pistons in a 7A block will result in the squish area of the pistons being (from memory) 0.030 below deck which is not good from a performance view. Normally you would deck the block so pistons are within a couple of thou below. Also to optimise squish you would use a metal gasket, say 0.8mm.

I did some math and assuming that a standard 4AGZE runs 8:1 CR, going to a 85.5mm (7AF) stroke, using the same pistons/decking the block and using a 0.8mm metal gasket you would end up with a final CR of 8.8:1.