OLDSKOOLRYAN

hello everyone, you may have seen my last thread about my full body restore and now the bottom ends let go, anyway can any one tell me EXACTLY whats involved in low comping my engine block? what parts are going to be needed(pistons etc), can i use my standard pistons and what boost can you safely run.
thanks in advance Ryan.

rs_cabby

you can drop the compression just by buying different pistons
but i have found the best way to go is to buy forged pistons,
keeping the standard compression.
you will be able to run more boost with lower compression
but you dont always gain from this i.e.
standard compression with 10 psi =150 bhp
lower compression with 10 psi =130 bhp
these figures are not true but just an example.
forged items will be stronger, allowing you to run
more boost anyway

JesseT

IMO the standard comp achieves it's "sweet spot" around the 1.5 bar (22psi) boost range. After that it becomes increasingly difficult to tune and to make more power with the increased boost. Anything less than 1.5 bar of boost and you're just wasting power by going low comp. The sweet spot of a N/A 9.5comp cvh seems to be around 1.0bar of boost.

However, a lower comp will relieve rods from a little extra stress. It also will make the car sluggish before the boost builds, i.e. 1000-3000rpm.

To acchieve a lower comp, you need to leave more volume for the combustion chamber and there's a number of ways to do this.
-fitting lower comp ( pref. dished) pistons
-dishing the original pistons, there's quite much material on the crown really. Around 7mm.
-fitting a thicker head gasket
-fitting a separate de-comp plate
-machining the combustion chamber