Quick question
I've been using
HP X BSFC = Lb/hr required for fuel

but when I use

for CFM
Cubic inches X RPM / 3456 = CFM @ 100% VE

and convert it to Lb/min

either my ideal gas law translation is off, or my math is off, because going backwards from:

Cubic inches(RPM/3456)(VE) X (Bar of boost) X (compressor efficiency %)
I usually wind up closer to the actual horsepower on the dyno using:
(CFM) X (bar of boost) X (.069 "constant") to get horsepower, without even pulling compressor efficiency or actual VE into the equation, even with poor tuning.

The only thing I can see happening is the constant could be way off, but thats what most of the popular compressor maps use...

 

Last week we dyno'd a 240sx w/ an RB25DET.
The particular turbo, according to the compressor map, should have been in the 75~ Eff range near 1.4Bar

SO the simple math would say
(154 X 7400 / 3456) = 329CFM@100%VE
now the drop in VE by 7400RPM 329 X .90 = 296CFM
This isnt volume, just air moved, so temp (density) doesnt play a roll yet, and no turbo.

Now the boost (additional VE) to keep it simple lets pull the efficiency out of the boost to keep our engine volumetrically stable
1.4Bar X .75(%) =1.05bar @ 100% effiiency
That takes into account for the efficiency of the compressor, now lets take care of the efficiency of the engine's current VE with no boost:
296CFM @ .069(constant) = 20.4Lbs/min
20.4 X 2.05(bar) = 41.8Lbs/min
41.8Lbs/min or roughly 420Horsepower worth of airflow

This motor made 470 to the wheels.
I must be missing something, because when I just do
(154X7400/3456) X 2.05 X .069
I get my 49Lbs/min (490horsepower) which minus drivetrain losses is exactly what the car made.

 

This is all just theory, dont be afraid to input your opinions

 

I don't know much but it seems like you are making the mistake of adding compressor efficiency into the equation (1.4Bar X .75(%) = 1.05bar @ 100% efficiency). The engine doesn't know how efficient you're turbo is, all it knows is that it's getting 1.4Bar worth of boost in addition to ambient. So instead of 2.05 you'd want to plug 2.4 into the equation. Also, when you did this calculation:
(154X7400/3456) X 2.05 X .069 - you said you got 49 but the math is wrong. I got 46.6. I believe this is the correct equation: (154X7400/3456) = 329 and then 90% VE = 296 X 2.4 X .069 = 49.0176. I'm pretty sure that's your answer.

 

So your just adding in the boost without taking into account the drop in VE (due to density losses at the compressor)? The only way that would work is if the constant .069 took this into account, but I dont see that since its to correct for ambient temp as far as I know.
Just the process of adding pressure reduces density (effieicny) due to heat from the compression... this not accounted for at all unless you subtract it directly from the effieicny of the turbo using the map and plotting your point,

otherwise I could just say my T-25 @ 3Bar would do (CFM X VE%) X 2 X .069 = horsepower

and we all know that a T-25 cant even get close to 3bar of boost. So how do you account for effiency of the compressor, let alone the turbine...

Its all a bit befuddled. Thats why Im throwing out there, to see what others think.
And your right, my math was off, I did that part in my head but didnt show my work.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Kingtal0n &raquo;</TD></TR><TR><TD CLASS="quote">So your just adding in the boost without taking into account the drop in VE (due to density losses at the compressor)? The only way that would work is if the constant .069 took this into account, but I dont see that since its to correct for ambient temp as far as I know.
Just the process of adding pressure reduces density (effieicny) due to heat from the compression... this not accounted for at all unless you subtract it directly from the effieicny of the turbo using the map and plotting your point,

otherwise I could just say my T-25 @ 3Bar would do (CFM X VE%) X 2 X .069 = horsepower

and we all know that a T-25 cant even get close to 3bar of boost. So how do you account for effiency of the compressor, let alone the turbine...

Its all a bit befuddled. Thats why Im throwing out there, to see what others think.
And your right, my math was off, I did that part in my head but didnt show my work.
</TD></TR></TABLE>

I don't see how a turbo can lower the VE of the engine significantly or measurably and that's why I don't think you would account for it. Without a turbo the exhaust gases are just wasted so if it's making using of them how could it have a negative impact?

 

Ok I guess you (or I) am confused about how I am negotiating the VE based on compressor effiiciency.

I think of it this way;
a 355 cubic inch motor @ 6000RPM with 100% VE would(should) pull 616CFM
616 CFM * .069 (from volume to mass) = 42.5Lbs/min or roughly 400 horsepower worth of air.

You have to think of this, then
same motor (355 cubes) @ 6000RPM with 200% VE would (should) pull double the CFM. right? Thats 1232 CFM or 85lbs/min or roughly 800 Horsepower worth of air.

But how did it go from 100%VE to 200%VE? You cant do that naturally aspirated. It needs to be forced in (via forced induction).

SO how much boost would it take to go from 100% VE to 200% VE? Double the current atmospheric pressure. Thats 14.5PSI or 1 Bar of boost.
That means to hit 200% VE we now have 14.5PSI of pressure. But that 200% is not without its thermal and pumping losses. That motor wont make 200%VE worth of horsepower... Its losing some power due to compressor efficiency and turbine efficiency (blow down anyone?) So there has to be a way to correct for that lack of efficiency. I subtract it directly from the VE, because thats really whats happening. If a compressor is only 70% efficient then out of that 14.5PSI of boost, only 70% of it is actually helping to make power (your MASS FLOW Doesnt follow linear with boost and CFM. It drops as boost and VE and CFM go up- because the air is getting hotter and is less dense)

So if our compressor is 70% efficient then only 70% of the additional 100% VE is doing any good.

So I take it out. 14.5 * .70 = 10PSI, of 100% efficient boost from the turbo. it may be making 14.5PSI, but that comes out to only 10PSI of actual MASS of AIR compared to the atmosphere.

The other thing I think you are missing is that turbos do take power to move. Thats why turbine sizing plays a roll in how much power you make and where you make it- the smaller the turbine housing the easier it is to get the turbo going, but the quicker it starts to restrict the motor.

And thanks for your responses they got me thinking.