Thread: Head and manifold machining for higher compression on V8

I dont think its a practical/economical way to gain compression, if you mill the heads too much you will also need to mill the manifold and fit spacers under the rocker shafts and you may only gain a few .?? of compression - if its a 3.5 it would be easier just to find a hicomp motor to start with.

Sorry. Will clarify:

This is a 4.6 crate engine with 9.35:1 on the block.

Will have heads from a 8.35:1 ratio 3.9ltr put on it. 3.9 has deeper dished pistons than the 4.6.

Get some 4.6 heads, they are already machined down a bit to compensate for the composite head gasket.

I know that you want high compression for gas, but I did a bit of searching to see if anyone makes hicomp pistons.
The best avenue I know is "Real Steel" in UK but they do net seem to make them. Google them and drool.
You can probably mill about 50 thou from a 3.9 head to get about 1.0 more compression, but the ports will be out a little.
BTW 3.9 heads and 4.6 heads are different only in that the 4.6 heads are about 34thou milled to compensate for the composite gasket. 3.9 heads have 34CC combustion chambers and 4.6 have 28CC. Both will give the same compression on either a 3.9 or 4.6 if the matching gaskets are used.

The only CR difference between locomp and hicomp v8s is in the piston bowl volume.
The only other suggestion I have is to look up the guy in "Australian Classic Cars" who makes custom pistons for vintage cars. But would be exxy. Another thought I just had is that I understand Bruce Davis uses Holden 253 pistons in his 5 litre conversions. Maybe there is a piston/ rod combination that can work.
regards Philip A

My 3.9 has different pistons and the compression has been lifted to about 9.8:1

Bore it out and fit oversize pistons, that will raise compression ratio, or get in touch with some speed shops like Sainty, Rocket, Ross pistons in USA, etc. someone like TRW will have flat-top pistons, Regards Frank.

Measure up a 240 Volvo piston.They fit the 4.4 P76 engine so that could be another way to go.You get floating pins as well. Pat

AFAIK the 4.4 has the 3.5 bore. For 3.5 there are lots of options as they originally came as 10.5:1 in P6B AFAIR.
But I have not yet found any 94MM hicomp pistons.
I do not think its feasible to "bore out" a 3.9 any more than 20thou, as the sleeves become thinner than usual , and this would only exacerbate the slipped liner problems.
Regard s Philip A