What is the formula to convert engine bhp to wheel bhp and vice versa?

 

There isn't one. It depends on the losses in the particular transmission system (e.g. autos and AWD are less efficient than 2WD etc). On a dyno you can do a coast down to try to measure transmission loss - but most will not give you a very accurate figure.

 

you loose roughly 26% from fly to wheels

 

Eh??

Where do you get that figure from?

 

I have read on here about 22% is the figure but would be surprised if anyone has actually done it properly and taken an engine out to check.

 

dyno dynamics, not an exact thing but a rough guide

 

I've been told somewhere betweeen 18-20%

 

dunno, all i can go on is what DD told me when i rang em, if you wanna argue the point give em a bell.

would have thought they'd know though pmsl

 

bob rawle allows 22% using the same stretch of road, well he did with me. So tidgy do you subtract 22% (in my case) from your flywheel bhp to get atw

 

To be honest, as an engineer, I think most of this is nonsense.

First off, remember that energy cannot be created or destroyed - it has to go somewhere. In a transmission most energy loss is through friction or hydrodynamic losses in the transmission oil - it ends up as heat.

Lets take an example of a car giving 200bhp at the flywheel. Now knock off your 22% transmission loss - that's 44bhp lost in the transmisison. So at the road wheels we get 156bhp.

Now let's mod that car, remap it etc., so its now giving 300bhp at the flywheel. Now put it back on the dyno. Knock off your 22% transmission loss - that's now gone up to 66bhp. So at the road wheels you have 234bhp.

BUT - you have not touched the transmission - however the transmission loss has gone from 44bhp to 66bhp !!

22% is far too high anyway. On a standard WRX that would be about 50bhp. 50bhp is about 38kW. Imagine all that as heat going into your transmission - that's about a dozen three-bar electric fires heating up the transmission components! The transmission oil would be boiling after a few miles.

 

what about drag efficiency of the car, rolling friction of the tyres, bearing losses at the wheels... friction from brake pads, inertia of wheel masses, there is far more than straight gearbox losses....
IMH engineering opinion....

 

There are both static and proportional losses within the drivetrain, and can't be accurately calculated for a single vehicle as there are far to many variables. Lets of tuners and operators give an approximate figure for calculating the flywheel bhp.
You have to remember that just dropping your tyre pressure by 5 psi will increase drag and reduce your ATW reading. Turn those wheels even faster and the drag will increase further.

 

By using a percentage to measure transmission loss means the more power you have at the fly the more you loose at the wheels, how can increasing engine power affect transmission loss? the power is still going through the same components to get to the wheels.

 

its
- 10bhp from wheels then x by 0.84

 

Which would then = 16% Loss

 

ish yeah

 

currently i have 605ish at the flywheel
work it out at the wheels ill give you a gold star lol

 

Yes, there will be power losses additional to the transmission. Chiefly in the tyres (rolling resistance and energy dissipation in sidewall deformation). Losses in the brakes and wheel bearings during normal running should be small compared to other losses.
Inertia of wheels, tyres etc is not responsible for power loss - its energy stored during acceleration that you get back during coast down.

I am not saying that there are no power losses, just that any powertrain engineer worth his salt would say that extrapolating drivetrain power loss by using a fixed percentage is deeply flawed. My own experience (from working in powertrain development) is that it is best approximated to a fixed amount, say around 30bhp, whether the engine is developing 150bhp or 350bhp.
I suspect some tuning companies like the use of a high fixed percentage because it gives more favourable 'at the flywheel' figures and makes the power gains seem higher.

Anyway, an interesting discussion.

 

This is something I've wondered, and its vaguely related to this topic...

If a car is on a Rolling Road and it was running 30psi in its tyres, would you get a higher result than if it was running 36psi in the tyres... as when the car is coasting down from the 'actual' run the lower tyre pressure would give a higher resistance than the higher tyre pressure, meaning the machine would over compensate, or is that complete cr@p??




Cheers,
Grant

 

iv got 284bhp at the hubs ,,had it done at aet turbos ,,and its true to 0.001bhp or some thing like that

think we should all talk bhp at the wheels, as flywheel bhp is pub talk

stu

 

Dan I bet yours is around the 260 WHP mark as you ran 302BHP at the RR and Zaks Road Dyno WHP is usually bang on for a 16% conversion to BHP on Engine Advantages RR.

 

This might help

Dyno Power Run - Calculators and convertors

 

Keep up the good work. At last, someone who knows that it's ridiculous to try and attempt to use a simple percentage to work out transmission loss.

The straightforward answer to this whole problem is of course, to simply ignore 'flywheel' figures and just go on wheel horse power. Much harder to fudge that, and I notice a lot of US tuners talk about whp a lot.

 

500 ish