Thread: 3.5 vs 3.9 heads

3.9 heads have "waisted" inlet valves.
The ports and so on are the same.

Later 4.0-4.6 heads have oil impregnated guides and are machined for "teflon seals", and the exhaust port is cleaner, although to me this looks like just that they cleaned up the die.

3.9 and 3.5 heads have umbrella seals which do nothing much.

3.9 early heads to 93? have the 14 bolt holes while later 3.9 heads have 10 and are shaved to give 28CC combustion chamber for a composite gasket vs earlier 3.9 and 3.5 with a 34CC combustion chamber for thinner tin gaskets.
Regards Philip A

Hey thank Phillip, you hit the nail on the head. I was very interested in the chamber capacity.

I am just building up my 4.6 motor which was running with 3.5 heads from an 87 Rangie. I had heard that using these heads on a 4.6 block causes a loss in compression due to the larger chamber.

I have the opportunity to get some 3.9 heads which I was told had a 28cc chamber but I had no idea how this compareed with the capacity on the 3.5 heads.

Someone also told me that later model heads had bigger exhaust valves but I take it that this is not correct.

I am mainly trying to work out if it is worthwhile buying the 3.9 heads or just keeping my existing ones. I have just installed high comp pistons into the 4.6 block so I don't really want to lose any compression but I guess I could just get the 3.5 heads machined a bit.

If you want to use the 3.5 heads , just shave them 35 thou if you are using Ehlin?(sp) OEM 4.0-4.6 composite gaskets.
I have shaved my 3.9 "tin" heads by 50 thou and it is all good.

I have taken a bit out of the combustion chambers to get back to 28CC, mainly around the inlet valve which is badly shrouded in a Rover V8.

I had to shim the rocker pedestals by 20 thou to get between 20-50 thou preload on the lifters.
Regards Philip A

Originally Posted by walker

Hey thank Phillip, you hit the nail on the head. I was very interested in the chamber capacity.

I am just building up my 4.6 motor which was running with 3.5 heads from an 87 Rangie. I had heard that using these heads on a 4.6 block causes a loss in compression due to the larger chamber.

I have the opportunity to get some 3.9 heads which I was told had a 28cc chamber but I had no idea how this compareed with the capacity on the 3.5 heads.

Someone also told me that later model heads had bigger exhaust valves but I take it that this is not correct.

I am mainly trying to work out if it is worthwhile buying the 3.9 heads or just keeping my existing ones. I have just installed high comp pistons into the 4.6 block so I don't really want to lose any compression but I guess I could just get the 3.5 heads machined a bit.

If you have bored the cylinders out, i.e. if your new high compression pistons are oversize, then this will raise compression ratio as well and if bored out, then this should be taken into consideration, Regards Frank.