The Underdog

hi. im just wondering about how a rolling road calculates flywheel bhp from the wheel bhp.

what im wondering is for my car now with a lightened flywheel and lightend pulleys and no air con it has gained more wheel bhp but obviously not flywheel. i understand that bit.

but what im confused about is how a rolling road would interpret this. if it sees more wheel bhp is it going to over calculate the flywheel bhp? does that make sense. so for instance if my car standard was a genuine 170bhp (not likely) and stock for argument sake it had 130 at wheels and my modified one had 140 at wheels would the computer inflate over the 170 when it estimates flywheel figures?

am i making sense? hope so but thanks for input

RichieST

iTrader: (2)

When it hits the Rev limiter the operator puts clutch in and allows wheels to stop turning by themselves. Time taken for wheels to stop can be calculated into how much resistance is in the transmission, thus how much power is being lost, if you get me!

The Underdog

but would it still give a correct flywheel figure or would it be over estimated because of higher wheel figure or am i seeing it all wrong?

Fullflush

Put it on a hub dyno

RichieST

iTrader: (2)

What you have done is made more power at the wheels by making less resistance through the drivetrain / ancillaries.

The RR would see the higher power at the wheels, but also less resistance because the wheels are taking more time to run down to a stop, so it should then calculate the same flywheel HP.

RichieST

iTrader: (2)

The only thing a hub dyno takes out of the equation is an incorrect power figure from wheelspin and instability on the rollers etc, it would still need to calculate losses through the drivetrain to calculate a flywheel figure. The most accurate way would be to remove the engine and measure power directly from the flywheel.

The Underdog

thanks i understand it now.

RichieST

iTrader: (2)

Don't forget also your lightened flywheel will make the car accelerate quicker as there is less weight to get turning, however when on the run down, it will stop quicker, as there is less weight to keep it rotating. These two figures should balance themselves out though to not affect the final reading.

Think of it as a roundabout in a park. When it's empty it will be easy to get spinning, but let go and it will stop quickly, but put a load of fat kids on it and it will take a lot of effort to get going to the same speed, but let go and it will take much more time to come to a stop!

The Underdog

what if a few of the fat kids fall off? lol

RichieST

iTrader: (2)

Then it'll just be funny lol

Fullflush

I don't get why anybody cares about flywheel power. You can only use the power at the wheels.

BigPeBe

I think the reason flywheel power is still preferred most of the time is that car manufacturers have always had a habit of announcing flywheel power on their specs for their cars. So it's easier to compare power outputs and everyone has kinda "calibrated" their brains for the flywheel output so it's easier to understand what is lots and what's not.

I agree that it would be much more logical to talk about power on the driven wheels, but I still prefer using flywheel power because it just seems to be the way around.

scoooby slayer

iTrader: (4)

it will be hugely dependant on which dyno you use as to what correction needs to be applied to get a close estimate at the flywheel power imo.

there will always be people saying flywheel figures are rubbish I just ignore them and do as I please

surrey rolling road is a dyno dynamics dyno and known for being fairly accurate, certainly not generous anyway.


so here is the flywheel plot from my gtr at 1.4 - 1.6 bar of boost





now assumeing that 2wd correction was applied (which is the minimum it can add) then the wheel power for that run was about 710 WHP with 18% added for rwd losses that the dyno applies.


so we have a dyno dynamics dyno measureing 710 WHP from my car at 1.4 - 1.6 bar.
now heres a dynapack plot from my gtr on the exact same 1.4 - 1.6 bar tune



so dyno dynamics made 710 whp, dynapack made 812 whp.

so the dyno dynamics needs 130 hp adding to show correct flywheel power, dynapack needed 28 hp adding to show the correct 840 hp it has at the flywheel.

there we have the same tune run on dynos operated correctly with no fudgeing involved and the wheel/hub figures varied by 102 hp !


the tiny gains your talking about with the flywheel being lightened can easily disappear and more through dyno variation, if you didn't get it dynoed before the mods I don't think you will be able to make a good comparison now imo.

and your aircon will make no difference, yes you've shaved a little weight but when the aircon isn't active the clutch isn't engaged so the pulley spins freely, an air con pulley with no air con engaged wouldn't even sap 1 hp imo.

The Underdog

thats just mental scooby.