Do you know how you estimate bhp from desibred boost pressure ?Turbo-Brown wrote:Right, lets say that we're talking about the M45 for an M30 (that's confusing!)
The M30 displaces 3.5 litres?
Let's also take 6000rpm as being our crank speed.
So, at 6000rpm, the M30 consumes this amount of air (based on a few assumptions):
((3.5x6000)/2)*.85) which is the ((displacementxcrank speed)/2)*VE)
= 8925 litres/min
The divided by 2 is there as each cylinder only inhales every other crank revolution, and VE is the volumetric efficiency, which I've made up as being 85% for the sake of arguement)
From the graph above, we can see that the M45 can shift about 320 cubic feet of air / min at 14000rpm.
Now one cubic foot of air = 28.316846592 litres.
So at 14000rpm, it cam move 9061.39090944 litres of air / min.
Sounds good so far!
However, if we want to calculate the boosted air flow required, we need to select a boost pressure so lets take the 9psi suggested above.
So, to work this out, we first need to calculate the pressure ratio which is:
(Absolute pressure/atmos pressure)=PR
Absolute pressure is atmos+boost pressure so we have:
((14.7+9)/14.7)=1.61
Which essentially means that our engine will be able to consume about 61% more air at 9psi than it would if it were running normally aspirated.
If we compare the figures again, we see that the engine under 9psi will now want this much air:
8925*1.61=14369.25 litres / min
whereas the most the M45 can supply is 9061 litres / min
Quite a defeceit not quite so great
I fancy playing with a few numbers this afternoon.
u should still have my PM....
then to atmos!