Lextreme said:
Blue,
You should stay with stock compression pistons because if you change compression ratio, you will change pressure level. If you are running 9 psi at 10.5:1 right now, you might only get about 3 or 4 psi with 8.5:1 compression ratio.
Yes its correct. However, since the speed/size will be constant. Therefore decrease compression ratio will increase chambar volume and pressure will drop. Since the 3uzfe crank pulley is about 5.00" its will be very hard to increase boost.
Boost x VE (%) give us how much air that actually get´s inside the cylinders. (and intake temperature, but lets not complicate the matter for now)
Compression ratio says how much that air get compressed on the compression stroke and nothing about what we se on a boost gauge.
Boost itself isn´t anything we want because pressure creates resistance. Flow in weight is what we want. a good enginedesign needs less boost for a specific flow of air.
Test:
VE 80%, comp 10.5:1 @ 0,5 bar boost gives wich compression?
VE 110% comp 8.5:1 @ 1,0 bar boost gives wich compression?
If we change the compression ratio with low comp pistons, the combustion chamber and gets an increase in volume, but the stroke and bore is the same, so the "work volume" of the engine is the same, but the engine can take more boost without knock, pre iginotion, detonation and so on.
An engine should have the highest possible compression ratio to atomize the fuel better and increase pressure in the early phase of combustion = more power.
If we step back to the supercharged engine in this thread - the extremly small drop that he may se on his boost gauge isn´t of any interest, the flow trough the runners/head is the same, so the combustionchambers will get the same amount of air as before or slightly more on the (intake stroke), than the intakevalve closes and the compression stroke begins and compression ratio becomes important.