Hi

You always here people quoting their BHP figures and all that crap but I love the impreza due to the way it drives not the "my car will beat your car" but it got me thinking how much power does the 4 WD system cost impreza drivers?

I know the benefits to me far out weigh the loss of power from the flywheel

 

hi mate im pretty sure from the flywheel to the wheels there is roughly a 22% loss

someone please correct me if im wrong

steve

 

Mine loses around 16%, a fwd car is around 12%
Diff will also play a role in how much losses you have (as does tyre width/size)

Tony

 

+1

In September, I was at SC waiting for my car to be remapped by JGM.

Whilst waiting, I was working out the other cars' (before me) power losses - and it was consistently c.20-22% (my own car worked out to be a 21.79% loss).

But of course, that's based on an equation for just one brand of rolling road - thus different RRs may have different wheel to flywheel conversion factors and, therefore, differing figures (like for like).

 

16% to the hubs used on my car

 

i worked it out at SC aswell but as you say diff RR may give diff loss figures

steve

 

Correction. My car was 'only' 20.79% loss (not 21.79).

 

Cheers for help guys was only for an idea but all answers much appreciated thank you

 

Basically 16-22% lol

Steve

 

Doesn't a rolling road measure the actual wheel horsepower, then use a guess as to the loss to estimate the flywheel horsepower? For example on the output from a MAHA dyno you get lines for estimated engine power, measured wheel power, and assumed drag power, where engine power = wheel power + drag power. If that's the case, then you can't use a rolling road to work out the loss, only to find out what loss the operator has entered as a parameter.

Having said that, the fact that RR operators consistently assume a roughly 22% loss does suggest that is a typical figure for the actual loss. Of course it's more complicated than that, since the assumed loss on an RR accounts not only for drivetrain loss, but also slip on the rollers; the power calculation takes into account a range of environmental factors; and operating a RR is all in all a black art.

 

you can reduce the bhp diffrence by reducing paracitic weight though i think. i.e lightwieght alloys, lighter flywheel, driveshafts etc.

 

Some dynos will use a lookup table for the transmission losses and don't measure it, for example Dyno Dynamics, a more accurate way for your individual car is to measure during the rundown phase how much drag is in the system, this is added to the wheel torque and then they calculate the BHP at the flywheel using "torque x rpm / 5252" (the reason why all torque/bhp lines should cross at 5252 rpm assuming both use the same scale, some mix metric kg/m and imperial BHP and dont crossover)

The other figure they take into account is the drag of the rolling road itself which is not always accurate and varies hugely from RR to RR. An operator can increase the notional value of the RR drag just to show you a higher after figure, or decrease it to lower your before figure.

22ish percent losses are typical for an Impreza and Lancer 4wd systems, in a nutshell, the higher the losses the higher the flywheel calculated figure for a given whp, but each individual loss is a measure figure.

Given all these variables in dynos, it is not unusual for a 300bhp car to show between 270 and 330 bhp on different dynos, never be disheartened or over excited by some of those numbers as they are all pub talk figures, how the car drives and the way it delivers it's power are far more important as most of the experts here will tell you.

 

Thanks Galifrey, that's really helpful