[tholyoak]

Joe,

I'm not 100% sure but at least within the same design years I would guess that the journal diameters are the same. You are correct though, the top end would need modification to deal with the larger bottom end. This is why I think it would be cheaper and easier to just start with the bigger motor rather than try and build up a smaller one. Custom engine work isn't cheap and all things considered, crate engines from Porsche are.

Although if you built up a 2.5 or 3.4 into a 3.6/3.8/4.0, you wouldn't have to deal with the electronics issues that can complicate some engine conversions. Problem is you then open up a new can of worms as far as needing custom ECU programming that wouldn't be cheap to do correctly.

Also, a large amount of the difference between the 3.4 swap that was in my car and the subsequent 3.6 engines I have had in the same car comes not from the displacement increase, but the significant torque improvements from the increased cam timing changes that came with the variocam plus system over the basic variocam setup (40 degrees vs 25 degrees if I remember correctly). You wouldn't get this benefit if you modified an earlier motor.

Just my $0.02.

Todd