bob r

iTrader: (46)

with a vast amount of chit chat on here concerning "my car is quicker than yours" which is sometimes annoying but interesting i wonder if some of the so called facts are correct.
i have read posts stating a cars weight or bhp/ton and transmission loss all play a factor but i wonder. a recent thread stated that a subaru with 220 bhp less 25% transmission loss made it considerably less something like 160 bhp at the wheels which sounds unlikely. these figures are usually given to undermine the scooby owners but surely how power is delivered also plays a major factor in the acceleration. ie gearing, powerbands, torque and even aerodynamics the point of this thread is for me to get more of an understanding of comparisons from one car too another and if there is any truth in my thoughts as i have read so much about bugeyes being slow and 2 wheel drive cars being faster (not from personal experience, quite the opposite) any thoughts anybody?

rmtypeR

When I had my car dyno'd it lost 76bhp from the engine to the wheels = 28% loss. Someone else I know had his MR2 turbo tested on the same rollers and lost 68bhp (22% loss). This is slightly annoying for me, as it means I would need to produce 340bhp at the engine to equal his power at the wheels. Of course 4WD has advantages in other areas but if were talking pure straightline speed, I'd be at a disadvantge.

As regards power to weight, the power delivery would also make a difference. Example being you could get 2 cars of identical weight and identical peak power, but if one of those cars only makes it's power briefly at the top end of the revs, and the other has a flatter curve, then the flat curve will be faster. This was the case with my old Golf TDi - peak torque was 228lb/ft which sounds impressive, but it only made that around 2000 rpm, the dropped off back to about 150-ish for the rest of the time, hence it had a similar 0-60 time to the petrol engined 150lb/ft Golf GTi.

Out of interest, here's an overlay I did of my mates MR2 and my Impreza dyno graphs...

StiX

iTrader: (4)

I had mine dynod and lost 102bhp to drag.

272bhp, 172bhp at the wheels. This was at Powerstation just over a week ago.

Now thats 37% loss - hmmm?

rmtypeR

I have read in other posts that Powerstation tend to produce above average figures for flywheel power.

StiX

iTrader: (4)

I disagree. If you look at their dyno results compared to others its the other way around? They seem conservative compared to Power Engineering etc.?

rmtypeR

As I understand it, the dyno measures the power at the wheels. The flywheel figure is then calculated from that.

StiX

iTrader: (4)

OK, so lets say that Powerstation are 100% accurate for at the wheel power.

You are saying that perhaps their dyno is not converting that to an accurate flywheel power, as in too high?

It just seems comparing the different dynos that Powerstation are lower than anywhere else - for flywheel I mean?

Confused!

rmtypeR

I'm definately not accusing anyone of being inaccurate! It's just that a 37% transmission loss seems quite high.

The engine power is worked out after the power run - i.e. when the wheels begin to slow again. The dyno then starts to turn the cars wheels as opposed to at the start when the car spins the rollers. I think the dyno then measures how hard it has to work to keep the car wheels turning, which is then added to the power at the wheels measurement, to give the final flywheel figure.

I do stand to be corrected on this though - having only had my car dyno'd once. Hopefully someone else will be able to explain this fully?