Compression vs. PSI

[JRaptor2000]

Say I have 10:1 compression and am running a 5.5 psi and getting 350 horsepower. Now if I buy low compression pistons and have 8:1 compression and run at 14 psi will I have more horsepower?

 

[xcelr8n]

what engine is this??

 

[psychlow]

Depending on the design of the engine and the size and efficiency of the turbocharger and your intercooling, you most likely would get more power out of the latter setup. Assuming you're still in the efficiency range of your turbo and aren't heatsoaking, you'll be able to tune the car for power more easily than with the higher compression engine.

 

[n0va91TSI]

A low compression high boost turbo setup can be much faster than a high compression low boost setup.

This is why drag-racers run +30psi turbo setups!

Dont think about horsepower, think about how much air/fuel you can burn in the engine in a given amount of time. Hi-flow is power and speed. Thats why ppl buy exhaust and intake, it aids in flowing a lot of air which gives big power gains.

 

[JRaptor2000]

It isn't a DSM. It's the Nissan 350z engine. He wants to get the twin turbo setup and apparently it only runs at 5.5 psi unless you buy new low compression pistons. My friend was arguing that by lowering the compression and raising the boost, you'll just lose horsepower. I decided to ask around to see who was right.

 

[Irrational]

n0va91TSI "think about how much air/fuel you can burn in the engine in a given amount of time."

 

[VOTBLINDUB]

isnt compression high in n/a and no boost, and low compression and boost in turbo?

 

[JRaptor2000]

usually compression is around 10:1 in N/A cars while most people want 8:1 in their turbo cars. He now argues that you can't ever put low compression pistons because of the composition of the block.

 

[eyetrauma]

I don't know if this is right at all but:

Atmospheric pressure is 14.7 so every pound over that is boost.
Assuming ideal conditions:

10:1 compression pistons at 5.5lbs of boost will have:
(14.7 + 5.5) x 10 = 202lbs of pressure in the chamber at combustion.

8:1 compression pistons at 14.7lbs of boost will have:
(14.7 + 14) x 8 = 229.6lbs of pressure in the chamber at combustion.

So it seems the higher boost, lower compression combo has denser air in the combustion chamber. At the same RPM the pressure in the intake manifold will be higher so it must be flowing more air. Seems like its a win win situation.

Anyone want to confirm or refute this?

 

[JRaptor2000]

sounds right to me.

 

[crankbender]

I didn't check the calculations because i am sooooo dog tired today. But you seem to be referring to effective compression ratio.

Do a search on google and you should get like 8769856986 pages.

Turbo cars have been known on occasion to get an effective compression ratio of 30:1

 

[JRaptor2000]

according to this whole thread it seems like less compression with higher boost = more horsepower. now my friend just has to wait until sleeves come out for his engine so he can change the pistons so he can have more boost. apparently the 350z block handles up to 500 horses, anything above that you need to rebore the cylinders and insert sleeves.