Flow vs psi?

[Ruan]

Engine air consumption in CFM is a formula.

CFM = ( L * RPM * VE * PR) / 5660

Cubic Feet per Minute = ( Engine size in litres * Engine speed * Volumetric Efficiency * Pressure Ratio) / 5660 (constant to correct to CFM).

This is how you calculate the increase in airflow since you're multiplying by the pressure ratio So say on your 1.9 litre engine you want to run 2.0bar boost - plug that into the equation...

1.9 * 4200 * 85 * 3.0 = 2034900 / 5660 = 359.522 CFM = (roughly) 25.1 lb/min

Say you want to calculate that for 1.0bar boost -

1.9 * 4200 * 85 * 2.0 = 239.68 CFM = (roughly) 16.7lb/min

Yes - more boost equals more air... However look at that graph - if you ran a GT15 at 2.0bar, it'd be moving 25.1 lb/min @ 2.0bar -



So that's a fail - I also know for a fact that that top shaft speed line corresponds to about 180,500rpm - so you can imagine the sorts of speeds you'd be talking about making a GT15 try and run at that.

However it will turn your turbo into a TOTAL hairdryer and be completely pointless and provide absolutely no more power - only succeeding in damaging components.

I understand most of that, but (please tell me where I am going wrong here) when I think about it as long as your turbo is in its efficiency range and the turbo flows enough for the engine. The higher the psi the better (within the island)?

Yep - absolutely.

The other thing that confuses me is on the graph for the gt 15.. how does it flow the same at 2.2 and 1.8? surely it should flow more at 2.2? If not why would you run it at 2.2, when you can save some of the turbo life?

Airflow increases with increases of pressure with the same airflow demand. All things being equal - you can't move the same amount of air with a lower pressure unless you increased your volumetric efficiency - and once you're to 100% VE, you're moving as much air as the engine can move, unless you make the engine bigger! Your only way forwards is more pressure. You'll notice that the shaft speed would be lower for the lower pressure at the same airflow rate, that's what happens when you have a smaller engine

And is the flow a positive correlation between that and the psi? as when you fulfil what the engine requires (e.g the cylinder) and there is a restriction, only then will the psi go up. Obviously the more you can flow the higher the psi can get to (Given its the same engine)? or am I talking out of my arse?

Absolutely - regardless of the turbocharger, if you keep applying more pressure (at the same temperature/density, i.e. not from a turbo running at 1000000*C) to the inlet, your engine moves more air which equals more power.

So basically to all those who say "There's no replacement for displacement" - well judging by the CFM calculator, there's quite a nice replacement, positive inlet pressure