Rolling Roads and the fly power...?

[freebone16v]

i ran a rolling road yesturday an managed to get 117 bhp atw's (its a valver with a decat, full stainless system and a panel filter). There were 2 standard valvers there also which ran 105 atw's.

now i have been told 2 different things about how to work out the losses and want to see what is correct..

the first is that it is generally known as a 20% loss...which gives me a fly figure of 140.84bhp

the second is this
Guys, just a quick lesson on losses. It's not a percentage. Quite tricky to explain but if car X had 400 BHP and it made 200 BHP at the wheels the loss is 200 BHP. If this car was tuned to 1000 BHP then the loss would still be 200 BHP ie. 800BHP at the wheels.

So you need to find the result from the most stock 172/182 respectivly (I'm guessing around 135/145 but I'm no expert) then you can work out drivetrain losses.

The CTR standard will do 160 BHP at the wheels, wheras the Fly figure is 197 BHP. Thats a 37 BHP loss. Today I made 179 BHP so it's running about 216 BHP at the fly.

which working mine out the same way gives me
so if the standard factory fly figure for a valver is 137bhp and then tomS's (totally standard) ran 105 that would be a loss of 32bhp....

so i then add that to my atw's figure to 32 + 117 to make 149bhp at the fly...


can someone shed some light please..i am confused..lol

[FATBOY]

u aint the only one confused mate! lol since last weeks rr day at hillpower im still none the wiser! (ok i admit i was shite at maths at skool!!! too many numbers together confuse me)

[freebone16v]

well i can work the equations out but don't know which one it is...lol

i am pritty sure its the first equation as i think 140 bhp is more likely than 149 bhp from the mods i have got

[OliS]

I found this equation in another forum on the net last night, but it's just another variation of the 20% rule:

117bhp divided by 80% = 1.4625bhp per one percent
1.4625bhp/% multiplied by 100% = 146.25bhp at fly

The problem with this equation is that it just assumes that transmission lost is 20%, I don't where this figure comes from, or why it is used but it seems to be the given percentage for fwd cars. However, surely transmission losses can vary for for each and would depend on the car's condition?

[stew]

I wouldnt bother too much with AT THE FLYWHEEL FIGURES (bhp)

Its more an indication of how powerfull your car is. The AT THE WHEELS FIGURE (hp) is a true indication of power. This is where the power is measured so there is no guessing at transmission losses!

The observed power is the power your car made at the wheels on the day.

A corrected figure is at the wheels, and nullifys air temp, air pressure and humidity.

There is no way of getting a true AT THE FLYWHEEL FIGURE unless the engine is taken out of the car, the gearbox removed and tested on an engine dyno!

Cars power is always quoted at the flywheel (bhp) but in reality this means nothing. A 280bhp scooby only has 200 hp at the wheels!

Most people use a conversion factor (for fwd cars) of either 20% losses,

bhp = 120% atw hp

or they use,

bhp = 110% awt hp + 10 hp

[OliS]

Agree with you totally, it's the atw figures that count, but as manufacturers quote flywheel figures, these are the ones that people normally want to compare against, it's just annoying that you can never with 100% accuracy work out the flywheel figures!

[matty]

so if the standard factory fly figure for a valver is 137bhp and then tomS's (totally standard) ran 105 that would be a loss of 32bhp....

This is assuming that engine is still 137bhp and that it was 137bhp when it came out factory or was ran in and loosened. All based on an assumption of a figure that cannot be proven (the 137bhp). Seeing as most 172/182 owners fin their power to be up or down depending on many things like inlet manifold etc.

Only ever true indication of mods is to get before and after runs on the same day/same RR, same conditions when doing something to car. Even if you do something else 6 months down teh road and use same RR the RR calibration could have changed as would all other conditions.

[BenR]

power lost through the running gear increases with bhp for the same system, but it is not linear. A running gear of a vehicle will not only consume 200bhp (as per your example) and no more or no less.

[kj16v]

Read this:
http://www.dynamometer.fsnet.co.uk/p...ywheel-BHP.htm

[stew]

^^that is a very interesting website. Loks of good info there.

Its basically saying that losses arent a percentage, but more of a constant due to rpm and road speed.

IMO lets just speak about SAE corrected ATW hp! Much easier....