Rear wheel HP calculator
09-23-07, 02:09 AM
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Rear wheel HP calculator
I've been trying to wrap my head around turbos selection for the last few days 
, and ran across the post by Zero R about calculating RWHP & flow rates.
To cut a long story short, I got bored today and whipped up a little java script that quickly calculate RWHP based on the maths in Zero R post & a little guess work of my own.
Anyway have a go and let me how close I did or didn't get to your real world figures.
http://homepages.xnet.co.nz/~taz/rotary/rwhpcalc.html
, and ran across the post by Zero R about calculating RWHP & flow rates.To cut a long story short, I got bored today and whipped up a little java script that quickly calculate RWHP based on the maths in Zero R post & a little guess work of my own.
Anyway have a go and let me how close I did or didn't get to your real world figures.
http://homepages.xnet.co.nz/~taz/rotary/rwhpcalc.html
09-23-07, 03:50 AM
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Comments:
What are the units for the engine displacement?
What are the units for boost pressure?
Pressure Ratio should go to at least one decimal place.
There is usually a 1-2 psi difference between the compressor discharge and the intake manifold due to losses from the intercooler and intake plumbing. This requires the turbo to operate at a higher PR in order to produce a given level of boost as read on the boost gauge.
It looks like you used about 0.07 lb/cu ft for the weight of air, which is on the light side. It may be better to use a more conservative number because I think it would be preferable to slightly over-size a turbo rather than under-size it. Most engineers use ISA (International Standard Atmosphere) numbers for estimates, which would put the weight of air at about 0.0765 lb/cu ft.
Strangely enough, the HP calculation seems to be a good estimate for something that has a LOT of variables.
What are the units for the engine displacement?
What are the units for boost pressure?
Pressure Ratio should go to at least one decimal place.
There is usually a 1-2 psi difference between the compressor discharge and the intake manifold due to losses from the intercooler and intake plumbing. This requires the turbo to operate at a higher PR in order to produce a given level of boost as read on the boost gauge.
It looks like you used about 0.07 lb/cu ft for the weight of air, which is on the light side. It may be better to use a more conservative number because I think it would be preferable to slightly over-size a turbo rather than under-size it. Most engineers use ISA (International Standard Atmosphere) numbers for estimates, which would put the weight of air at about 0.0765 lb/cu ft.
Strangely enough, the HP calculation seems to be a good estimate for something that has a LOT of variables.
09-23-07, 05:59 AM
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Pressure Ratio now calculated to two Decimal placed.
There is usually a 1-2 psi difference between the compressor discharge and the intake manifold due to losses from the intercooler and intake plumbing. This requires the turbo to operate at a higher PR in order to produce a given level of boost as read on the boost gauge.
It looks like you used about 0.07 lb/cu ft for the weight of air, which is on the light side. It may be better to use a more conservative number because I think it would be preferable to slightly over-size a turbo rather than under-size it. Most engineers use ISA (International Standard Atmosphere) numbers for estimates, which would put the weight of air at about 0.0765 lb/cu ft.
It looks like you used about 0.07 lb/cu ft for the weight of air, which is on the light side. It may be better to use a more conservative number because I think it would be preferable to slightly over-size a turbo rather than under-size it. Most engineers use ISA (International Standard Atmosphere) numbers for estimates, which would put the weight of air at about 0.0765 lb/cu ft.
That bumps the flow rate required noticeably, but better to be on the safe side as you said.
(This is a bit of a quick & ugly fix at the moment, I'll probably see if I can find a better way of working this into the calculations)
Thanks for the feed back.
Last edited by TAZ-NZ; 09-23-07 at 06:05 AM.
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