fwhp calculation on rolling road with supercharger
Discussion
Supposing two cars run on a rolling road; one is n/a, the other supercharged...
I know whp is easily measured, but if the machine then does its best to calculate fwhp from the transmission losses (through running it down?), how can it compensate for or factor in the parasitic losses of the supercharger in this calculation, which may be something north of 100 bhp... (?)
Let's say the two Monaros, or whatever, both put out 500whp - people quote around 18-20% losses for drivetrain, but that's for n/a. So it's making maybe 600+ fwhp. The FI car will surely have to be making a lot more than that to drive the supercharger as well, so how can that amount be measured on a RR - or can't it? Will you just get the same fwhp figure?
I know whp is easily measured, but if the machine then does its best to calculate fwhp from the transmission losses (through running it down?), how can it compensate for or factor in the parasitic losses of the supercharger in this calculation, which may be something north of 100 bhp... (?)
Let's say the two Monaros, or whatever, both put out 500whp - people quote around 18-20% losses for drivetrain, but that's for n/a. So it's making maybe 600+ fwhp. The FI car will surely have to be making a lot more than that to drive the supercharger as well, so how can that amount be measured on a RR - or can't it? Will you just get the same fwhp figure?
In theory it shouldn't make a difference! The supercharger is driven directly off the crank, which has the flywheel bolted to the other end. So any loss induced by the charger will be felt through the flywheel, exactly the same way the extra power it adds is felt through the flywheel. Eg if your charger is giving 300BHP but takes 100BHP to drive you will only ever see the 200BHP benefit. The other 100BHP is cancelled out by the charger (that's where the efficiency of the charger makes a difference). That force will always be felt, unlike the transmission that can be disengaged to give flywheel and wheel power figures. Easier to explain than to write down! But 500 at the flywheel is the same for charged or N/A. And equally you will have exactly the same transmission losses so WHP will be the same for both cars too. Does that help or is it as clear as mud?!
Think of it as 4 parts
a is the wheels
b is the transmission
c is the flywheel
d is the engine
You can measure the power at a.
You can guess the losses because of b.
You can combine a and b to guess c.
What d is made of makes no difference to a, b or c, the fact is, d turns c (the engine turns the flywheel) and that's all you're measuring. It doesn't matter if the engine is a hamster or a big block, all that system does as a whole is turn the flywheel.
a is the wheels
b is the transmission
c is the flywheel
d is the engine
You can measure the power at a.
You can guess the losses because of b.
You can combine a and b to guess c.
What d is made of makes no difference to a, b or c, the fact is, d turns c (the engine turns the flywheel) and that's all you're measuring. It doesn't matter if the engine is a hamster or a big block, all that system does as a whole is turn the flywheel.
A 500hp engine is a 500hp engine. Supercharger parasitic losses and efficiency ratings are measured differently. So if you have a 300hp supercharged engine and a 300hp NA engine on the same exact car, theoretically the drivetrain losses will be the same. Now you will have a different power delivery but that's another matter.
hi,
whilst understanding the points being made about the measured flywheel HP being the same, I think the question is more about power being produced or lost rather than the flywheel measurement. I seem to recall that some or most manufacturers quote engine HP by excluding water pumps, alternators, AC, belts and even oil pumps so in reality it's not possible to "have" that high a HP from their engine.
In reality for us, the potential impact could be that STD pistons are okay at 650 for NA or 500 for FI.
Hope this helps
Paul
whilst understanding the points being made about the measured flywheel HP being the same, I think the question is more about power being produced or lost rather than the flywheel measurement. I seem to recall that some or most manufacturers quote engine HP by excluding water pumps, alternators, AC, belts and even oil pumps so in reality it's not possible to "have" that high a HP from their engine.
In reality for us, the potential impact could be that STD pistons are okay at 650 for NA or 500 for FI.
Hope this helps
Paul
I see where you're going with the 560 BHP thing. But, You're right and wrong! Yes, but only half of the engine! The front half of the crank will be feeling 560, but NO, the back half (the bit that is measured and does the driving) will feel exactly the same 500 regardless of how it is produced. You're digging deeper now, and diving into a huge can of worms really! Yes forced induction puts stress on an engine, but how much depends on fueling, cooling, mapping, type of induction etc etc. But to answer your original question. No, WHP and FWHP have the same relationship regardless of induction type. Bloody good question though :-)
I see where you're going with the 560 BHP thing. But, You're right and wrong! Yes, but only half of the engine! The front half of the crank will be feeling 560, but NO, the back half (the bit that is measured and does the driving) will feel exactly the same 500 regardless of how it is produced. You're digging deeper now, and diving into a huge can of worms really! Yes forced induction puts stress on an engine, but how much depends on fueling, cooling, mapping, type of induction etc etc. But to answer your original question. No, WHP and FWHP have the same relationship regardless of induction type. Bloody good question though :-)
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