Rwhp

[Trappy]

For some time iv'e read people talk about RWHP and the amount of power that is lost through the drivetrain. How do you calculate this loss figure? Are all cars totally unique? And do front wheel drive and four wheel drive cars transfer the power better, or worse, than front engined rear wheel driven cars? I would imagine that rear/mid engined cars lose the same amount of power as front wheel driven cars.

[g11ary]

Im not expert on this but all cars seem to lose bhp but I think on an fd it is between 17 and 20%. How effect they are is also meaured in tourque (not 100% torque reliant though).
Now someone shoot me down and please put me straight.

[richrx]

funny i should read this now but i was talking about that the other day and i got the formula for it. But first you need the power at the wheels.

BHP + 10 * 1.11 = flywheel power

i dont know why you add 10, i wanted to know why.
Im not really sure if 4wd is the same formula and im not sure of the rest of your questions.

[Trappy]

Surely if you divide flywheel power by 1.11 then -10 you get wheel horse power? (255/1.11)-10=220 How does that sound for standard RWHP?

[bladerider]

I must say I chuckled quite alot at this "formula"!!

Its somebodys own personal way to calculate their own transmission losses I presume.

If you want to know what your transmission losses are then you will need to visit a rolling road to have them measured. I know that rolling roads are not the most accurate of devices, and a lot of bad feeling can arise from different outputs from different rollers, but generally they measure the same percentages.

So if one set of rollers say you have 250bhp at the flywheel and 200rwhp at the wheels, then youve lost 20%. You should find that another rolling road will give you similar results, its just that the specific outputs might change to say 275bhp and 220rwhp, still about a 20% transmission loss.

I hope this helps.

James.......:cool:

PS You will probably find that some people on here with tuned cars may already know what their transmission losses are and this should give you a fairly accurate figure for most rx-7's of that type, obviously different clutches and so on will make a little bit of a difference, but it should still give a rough guide.

[Robertio]

As per James reply: it is very much car specific (change your wheel size, or tyre pressure and you can affect it) and the only way you can really find out is by visiting a rolling road that measures drivetrain losses.

Front what I've seen FWD is generally the best closely followed by RWD with AWD throwing away power all over the place.

My FD is booked in for 13th April, so assuming I get the rebuilt engine in the car and run in by then I'll be able to post what I get :)

[richrx]

where will you be taking it to be rolling roaded?

[Robertio]

Star Performance in Glenrothes, I've been up there in previous cars and can't recommend them highly enough. There is a bunch of us going up (Skylines, Scoobs, Gti-rs, etc) for the day, if you've got your car by then, then let me know and I'll check with the organiser if they can squeeze your car on as well :) If you just want a passenger run up let me know, as long as I'm in Glasgow that weekend you are welcome to come along.

[richrx]

ye they've got a good rep. when are you going up? unfortunetly i'll be getting my 7 in a few MONTHS:reallymad

[Glenn Butcher]

Hi guys,

Just picked up on this thread.

I put this together on my site:

Formula for calculating Flywheel Horsepower:

Drivetrain loss on an FD RX-7 is about 18%. Therefore take 18 from 100, and you get 82. Then take the rear wheel horsepower figure and divide it by 0.82.

e.g. 217rwhp / 0.82 = 265fwhp (rounded)

Drivetrain loss on the JC Cosmo is around 24%

e.g. 222rwhp / 0.76 = 292fwhp


Also have some Dyno video's there: www.mymazdarotary.com (not sure if you have seen them).

[bill shurvinton]

To calculate FW HP you need to do a 'coast down' test on the rolling road. This will measure the transmission losses and calculate your flywheel HP.

If you really must know what your HP is, this is the only way. If the RR operator gives you a blank loss and says (20%) go look elsewhere for power figures.

Of course HP figures are more about bragging rights than anything else. ;) You can tune a car to the nth without knowing what the actual HP is.

Bill