3.5 Compression ratios

[Speedball1]

Evening all. Is changing the compression ratio of a standard 3.5 as simple as just swapping for a different type of head or is it to do with stroke etc as well?

Phil

[katanaman]

Neither its the pistons that determine the compression. You could use 4.6 heads and tin gaskets right enough or just skim the heads you have but that depends on what cam you are running. If you have a high lift cam you run a good chance of hitting the pistons with the valves.

[ChrisJC]

Err neither.
The compression ratios are set by the pistons - the lower the compression ratio, the bigger the dish in the pistons.

Of course you can skim the head, but that mucks up the hydraulic tappet preload, as does mixing late heads with the early thin tin head gaskets....

There's plenty of standard comp. ratios to choose from.
8.13
8.25
9.35
9.75
10.5:1 to name a few....

Chris.

[ppyvabw]

What others have said, except its not entirely the pistons that determine the compression. Having the head/block skimmed, thick or thin head gaskets, or having the chambers polished and opened out can change the compression but mainly to change compression ratio one changes the pistons on the rover.

For example i think a composite head gasket can lower the compression by as much as 0.6. Just skimming the head will mean adjusting tappet preload, not hard in itself, and making sure you have valve/piston clearance.

Also, this will only determine static compression. The dynamic compression is determined by the cam you use.