The V8 Owners Forum -

Hi , i am rebuilding a 3.9 engine with a mild cam upgrade and was thinking which heads would give me some slight performance improvements,I have some slightly ported 3.5 heads some 4.6 standard heads and some standard 3.9 heads .Which do you think I should fit , regards David.

Go for 4.6 heads. Exhaust port is bigger near valve guide vs old style heads.. Also you get more compression ratio.. You can also port those 4.6 heads.

kokkolanpoika wrote: ↑Wed Jul 15, 2020 5:39 pm Go for 4.6 heads. Exhaust port is bigger near valve guide vs old style heads.. Also you get more compression ratio.. You can also port those 4.6 heads.

With standard heads, compression ratio comes from the pistons used, not what heads are used.

Heads designed for a steel gasket are higher than those for composite. If you use heads for steel with composite, you'll end up with a lower CR than the pistons say.

To clarify here, there are two thicknesses of head gasket. Thin 'tin' ones and thick 'composite' ones. The heads are machined to suit a particular gasket type, so a head for a tin gasket is about 1mm taller than for a composite gasket. The idea is that overall, the combustion chamber volume is the same, and the compression is set by the piston.

You can mix & match gaskets and heads, but it does mean that the hydraulic tappet preload will be incorrect (as it depends on the distance between the camshaft and the rocker shaft), so you have to compensate for it by shimming the rocker shaft pedestals!

Best bet is to keep head & gasket type matched, and choose pistons to suit your end-goal.

Chris.

And use waisted stem valves