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Transmission Losses

Found this article about how to calulate transmission loss on any car, seems to work pretty well too.

The 1.8 Vauxhall Astra GTE (new shape from 1985 on) has a frontal area of 20.5 sq feet and a Cd of 0.31. With two people and some test equipment on board (which is how most reputable magazines do their tests) the car weighs about 2460 lbs. The engine is rated at 115PS (about 113 bhp) and the tested top speed is about 123 mph. lets see how much net power is required to achieve that speed.
Rolling resistance power is 0.013 x 2460 x 123 / 375 = 10.5 bhp
Air resistance power = 20.5 x 0.31 x 0.00256 x 123 cubed / 375 = 80.7 bhp
Total bhp at the wheels must be about 91.2 bhp to achieve that speed. If we apply my formula for FWD cars to the quoted flywheel power we get (113 x 0.9) - 10 = 91.7 bhp at the wheels. Hmmm - so you gonna step outside and fight me about 0.5 bhp or is this starting to make some sort of sense?
Lets try a more powerful car.
The 2WD Sierra Cosworth was rated at 205 PS (about 202 bhp). Top speed was in the 145 mpg region according to most magazines. Test weight with 2 people and 50 lbs of equipment on board is around 3060 lbs. Frontal area is 21 sq feet and Cd is 0.35.
Rolling resistance power is 0.013 x 3060 x 145 / 375 = 15.4 bhp
Air resistance power is 21 x 0.35 x 0.00256 x 145 cubed / 375 = 153 bhp
Total net power required is 168.4 bhp. Apply the RWD formula to 202 bhp and we get (202 x 0.88) - 10 = 167.8 bhp.
The conclusion here is pretty obvious. If transmission losses were as high as 30% then there just wouldnt be enough power left at the wheels to achieve the top speeds that the cars actually show. The Cosworth would only have 141 bhp at the wheels if this were the case and its top speed would therefore only be around 136 mph. You can work that out for yourself by applying the formulae above. The Vauxhall would only have 79 bhp at the wheels and be capable of around 117 mph.

So does anybody have the figures for the 172????

difficulty here is the calculation of weight and the 0.013 as a constant.

It dont take into account tyre pressures or contact are either - or, actual transmission / drive losses, a lot of mags also test top speed from speedo, without using decent test gear.

But, its a good guide.

What do you mean it doesnt take account of actual transmission/drive loss? Good point about taking top speed from speedo though :)

Another point I was thinking about, if transmission loss really is as high as 28% than with a 172 producing approx. 130kW, that would mean the transmission is soaking up something like 36kW minus the energy used to turn it of course, surely it cant be that high?

You are correctt re the 172, there is something wrong!.

What I mean by taking into account the transmission loss etc is the use of the 0.0013, and in fact the 375 as constants. this, I presume, is in lieu of actual figures.

Also, looking closer, the rwd conversion also uses a constant that must be applied as a fixed loss. Thus, the formula is actually applying fixed losses, not attempting to caclulate them.

hope that makes sense..

oh.........and what with the spread sheet bit?

and joe, i think Dave Bakers *ill say it again lol* ROUGH guide to working out basic power loss is fine for everyday, quick approximations......

and a hell of a time easier than what u posted.......LOL

But hell, somebody will always be correcting somebody else till you get it right, which is the point of such forums i guess.....

actually no, its JD

John De Armond, WD4OQC, Marietta, GA
Performance Engineering Magazine

i meant the first topic post calculations..........

OH, do you read race car engineering BTW? pretty good, if a little americanised......lots of good contacts at teh abck for bespoke parts.

ok ok..........

but there is no way i would walk about with a pda!!! with exel sheets already made! LOL

i would need the matching top pocket pen protectors!!!

and i dont have any masking tape for me glasses....LOL
  BMW 320d Sport

Im not even gonna get involved in a row about trans loss. Suffice to say I dont believe any car built in the last ten or twenty years would have a trans loss of over 20% even if its 4wd. To lose more than a fifth of the engine power through the transmission is just silly.

Still, the myths and facts about trans loss still abound; not helped by the fact that manufacturers who surely must know exactly how much power a car makes at the wheels, will never release that information in case it makes their car seem slower/less efficient than someone elses.

Yes the extract is from an article on Puma Racings website. The two examples he gives seem to work well and the argument over the transmission soaking up 36+kW cant be ignored.

Does anyone have the figures for the 172, then at least yo could see if his formula (with its questionable constants) gives a figure anywhere near Renaults claimed amount?

nope, we talked about this before, but to no avail.....looks like nobody knows such fugures and cant be bothered measuring up the FA.