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Originally Posted by yieldsign2
Very interesting on the HP! I would assume you can't double up gaskets on the 3.9 and get more power  |
No Devon. It works the other way.
All other things being equal, the size of the combustion chamber influences engine compression. If you reduce the size of the combustion chamber, the more it will compress the same air/fuel mixture and the bigger a bang that will result. If you increase the size of the combustion chamber, you will reduce the force of the combustion.
The size of the combustion chambers are determined by their shape in the head and part of their height is the thickness of the head gasket. Racers often skim the heads to lower the combustion chamber height and increase horsepower.
LR head gaskets for the Buick 215s, 3.5s, 3.9s, 4.2s are thin metal. The thick composite gaskets were made for the 4.0 and 4.6s and are/were an attempt to cure the constant issue of easily blown head gaskets with the earlier models. However, by switching to thicker gaskets and nothing else, LR would have increased the height/size of the combustion chambers and that would have lowered the compression and bhp. So they compensated by skimming the heads (lower) to match the added height difference between their new composite gasket and the old metal ones.
Moving a 3.9 to composite head gaskets makes your engine far less likely to have a head gasket issue, lowers its compression (which makes it easier on the bottom end) and lowers its horsepower. If you want to have the same compression and a composite gasket, either skim your heads or switch to 4.0/4.6 heads (though I would be concerned with the lower row of bolt holes...4.0/4.6s have 10 head bolts each and the other engines have 14).
For the 4.0/4.6 people, the switch is hassleless. Switch in an older metal gasket for the composite. As the heads are already skimmed, that increases the compression to 1960s levels and bhp increases accordingly.
There are so many ways to increase horsepower...especially with these engines. I would take a wholly different route with your model. However, whatever the model, it must be remembered that a vehicle is designed as a cohesive assemblage. Change one thing, and you begin a domino effect.
James
