Grahamwrx

Question to all you techies

Is the Drivetrain power loss a constant figure for each particular car regardless of any increases in engine power from modified exhausts, ecu’s etc.
i.e. If you increase power of engine by 20bhp is the full 20bhp realised at the wheels ?
Or is it effected by a friction coefficient so that only a percentage of the increase is realised.
Does anybody have rolling road figures to illustrate this ?

Regards

Graham

brickboy

No dyno figs to support but the drivetrain loss is a % of power output, rather than a fixed bhp loss per car.

So if a mod gives an extra 10bhp at the crank, you won't see the full 10bhp at the wheels.

Now if we didn't have those pesky 4WD systems ...

Grahamwrx

Thanks for your response brickboy,
Yeah % losses does seem to make sense, but not the answer I wanted to here!!!

Losses appear to be about 75 bhp on std impreza turbo’s which comes out to about 35%, does that sound about right ??? How does this compare to 2 wheel drive cars, any rolling road guru’s out there ?

Graham

hypoluxa

You will generally find that a FWD car will lose approx 15%, RWD 18% and AWD somewhere in the region of 25%. Claims of scoobs losing 40% or more is rubbish - your diff oil would be boiling to say the least...

Also 'calculated' Flywheel figures can and will be way out depending on which gear is used - the only thing a chassis dyno actually measures is torque at the wheels.

Suresh

Agree with hypoluxa. Big reported drivetrain losses do seem unlikely.

Given that energy is never created or destroyed but merely transformed, the drivetrain losses would have to be released as heat, noise or some other form of energy. The equivalent 75 horses of heat would probably set your car on fire.

Flaming exhaust anyone?

323GTR

Rolling roads do measure torque at the wheels, and power is calculated from the RPM and the torque. Gearing does NOT affect this, providing that both RPM and torque are measured correctly. It MAY affect a calculated flywheel power figure, depending on whether say 3rd gear is more / less efficient than 4th gear (very likely to be, but not by any huge degree). RRs generally use 4th gear, as the speeds and torques generated are more in line with the design of the equipment, not because of the affects on the reading.

As far as losses go, it's very difficult to see why they vary so much, but there are lots of factors involved, and it's probably a combination of them all. Any decent RR operator will tell you that tyre pressures affect the readings for a start. There are now 1 or 2 'chassis pack' dynos, which bolt to the hubs to get a reading, so there is no wheelspin or contact patch effect, these should be more accurate - has anybody had their scooby on one?

flat4

just had mine RR'ed on sat. ('92 legacy) and it came out at 258bhp and 168bhp at the wheels, which is exactly a 35% loss

but an STI RA (i think thats what it was ) came out at the same bhp at the flywheel but had 195bhp at the wheels

must be down to clutch/gearbox/diffs


kev

flat4

this thread tries to exlpain ithttp://www.scoobynet.co.uk/bbs/threa...hreadid=138472

i'm none the wiser for it but it makes a good read


kev