We all know that some rolling roads calculate power at the fly, others at the wheels and some can do both.

What I'm wondering is say you had a car with 200bhp @ wheels and 250 @ fly (due to transmission losses etc) and then tuned it a bit so it produced 250 @ wheels, would it then show 300 @ fly. Conversely would 350 @ fly mean 300 @ wheels.

I'm not meaning in relation to coupes here, but in general terms. ie is there a liner increase in @ wheel power along with @ fly power?

Or does the greater the power, the greater the losses?

Depends how the rolling road measures the wheel power ferg.

Maha dynometers measure the drag, to convert from wheel to fly figures through a complicated calculation. Whereas AVA Turbo measure the bhp at the wheels only, through neasuring torque.

Ross

The greater the power you get proportionately smaller losses as you can only loose so much through the transmission e.g. 50bhp @ 350bhp, 100bhp @ 800bhp. The higher the gearing and greater rpms will also loose you more.

What I'm driving at guys,

is to say increase the wheel figure by 100bhp, would that mean the fly figure would be increased by the same amount or would it be more?

I'm thinking in real terms as well as on RRs.

Usually, wheel HP is almost 85% of the fly HP. This is approximate figure for a car with an engine, transmission,... running in a good condition

Rear wheel drive cars (with drive shafts) have more lossed than front wheel drive ones

Of course there are calculations for this but it depends on each car's setup.

Hani,

Ferg, if you increase your wheel bhp 100bhp, then who cares!!

Your gonna actually feel the difference more!!

Ross

It is similar to what hani says the transmission losses are a percentage of total power and this varies from car to car even of the same model due to tyre pressure transmission wear tranny oil (it dresses up in a corset and sussies) etc.

The way it works it out is totally hit and miss and not really that accurate. Another forum i am on has agreed to quote only wheel horsepower as it is the bit that matters really.


Perhaps a forum wide gentlemans agreement could be put inot place here to agree on WHP only.

Wheel power is open to many factors too and therefore cannot be relied upon for accurate comparissons.

But technically its the only measurable factor. You can only measure what the wheels are doing, bar getting the engine out and measuring the flywheel power.

I know what Flea is saying tho. I went to one rollers one weekend, another the next and the "presumed" flywheel figure was near enough the same (315 v 319) but the wheel figure was TOTALLY different. First one was 278 and the other was 245.

So again, Barbz and Dan have the plate when they say, "put it on the strip/track then say how fast it is!"

Ross

If EVERYONE only quoted wheel bhp, then the playing field would be far more even.

Its the conversion to fly bhp (whether by measuring coast-down drag or by calculating it by some method) that skews the results.

For example, Flea's car ran 420bhp at the fly at Perfect Touch (about 390 at the wheels ), then VERY soon afterwards, with no further mods, ran 480 at the fly at PowerStation, but with a similar 390-ish at the wheels

I'm fairly sure that with the 405 I ran at Perfect Touch, a trip to PowerStation would show me at about 460bhp

Either way, I have about 375 at the wheels, which is "adequate"

The wheel power is not a constant unfortunately as Ross has shown. I got 398bhp @ wheels and 420bhp @ fly during my mapping at PT in November. I followed this up with a cold run at Powerstation a week later where I got 358bhp @ wheels and 480bhp @ fly. A cold run is good for 10-20bhp @ fly as I and DrFrag have shown.

So that is a discrepancy of 40bhp @ the wheels which goes to show there can be a significant difference between wheel power between rollers. I know that Owen Developments also show massive fly losses, both Barbz and Nyssa lost 100bhp which suggests that only one is correct, either the wheel power or fly power. On the road, it was proven against Barbz that the fly power was indeed correct.

Nigel, when you last ran at Powerstation you got 302bhp @ fly which was to be expected for your 28R setup but the wheel figure was very low at only 191bhp. Now we know you didn't have 191bhp @ wheels but 302bhp @ fly was very consistent with your mods at the time.

Different rollers use very different loads but as long as you can find one that is consistent on each visit you will know with a high certainty whether your mods have had the desired effect or not.

A rolling road actually measures torque at the wheels when running under load, then on de-acceleration, calculates the wheel bhp. Then its a guess for fly bhp. Some cars lose as little as 9-10%, others as much as 30%!!! Problem with so many rolling roads, they are very inaccurate. Some vendors actually set their rollers to show an artificially high power/torque figure. Ive seen the same car make 48 bhp more and 40lbs more of torque at another RR. I know Northampton Motorsport RR is deadly accurate. They always adjust for different ambient temps, and calculations can be made with set acceleration times through the gears to attain the power and torque. Each time they match. As i said, this same car made much more on another well known rollers. I wont say who they are, but as they say, the BS stops when the flag drops!!

Andy.

Well a 1/4 mile time is only ever as good as the driver off the line My previous setup of 370bhp showed absolutely nothing in it on the road against Markus's 378bhp (same rollers) but he has the special ability to consistently do cracking 1/4 mile times which were a good 0.3 seconds quicker than me!

Now on the track...

Having spoke to Mark at Owen Developments, higher revs = increased losses

Last week's map showed 587 fly, 504 wheels at around 8000 rpm. Lost a bit less than last map (562/455), put that down to sending it out on road tyres this time rather than slicks which the rollers destroy

Don't use percentages, thats just a load of rubbish.
My car produces peak power at stock rpm and it loses not much more than a stock car either.

Thats why Trev. some RR are moving away from rollers and using the wheel dynometer or whatever its called.
How is the Kappa? Did you find the misfire problem?

Andy.

wheel dynometers are a load of c*** IMHO. They neither measure the real power ATW or ATF as they remove the tyre and wheel from the equation which are a large cause of ATW losses, but do not take off the transmission. They are only good for engine mapping.

Actual face value figures are meaningless. Any value you get is just the result of a calculation. "brake" power by definition is a calculation not a measurement. Brake power = power x efficiency.

i.e. Electric motor produces 90kw of usefull power by work. Electric motor also emits 10KW of heat. Brake power of the motor is 100KW. For an electric motor its easy to work out as you can measure the power the motor is using. For a petrol engine that is not so easy.

Wheel loss measurement on "run down" is just a joke. A gearbox will burn up a whole lot more power under load than on run down. The dyno is not measuring the loss, its just another piece of data they use to add to the loss calculation.

I prefer wheel power measurements, power, not brake power. Power and torque at the wheels makes a car move. Its directly compareable to any car, FWD, RWD or 6WD etc. Fly HP is totally irrelevent. We all know its just going to be a bit higher.

At the end of the day it does not matter how accurate it is as everything is relative. As long as its consistant you can compare the output to other cars. And your car with changes. This is the point.

This can only be done with figures from the same dyno. Dont try compare figures from different dynos.

I read an article in PPC that said ATW hp was inaccurate and meaningless and it was only flywheel hp that meant something.

I am now totally confused about all of this!

All sorted Andy - see the kappa power thread. Was runing too rich as a result of the new alternator pumping out 14.1v rather than the 12v or so the map had been done with - more voltage = more fuel delivered from the pump

Now runs as sweet as she ever has, if it wasn't for the clutch I could go shopping in her

Horse Power is also meaningless really. What accelerates you is torque.

Andy.

Ever thought of running without an alternator? Using a race gel battery will easy last a race and no alternator will add power.

Andy.

Horsepower is far from meaningless given it has an absolute direct relationship to torque! What you are referring to is the spread of the torque band instead of peak torque which translates to your acceleration curve all other variables being equal.

Been down the no alternator route in the past with the Y10, never again. Motecs get a bit stressed if they don't have a consistent power feed

If the dyno is designed to give flywheel power then yes, the face value wheel power may be inaccurate, or totally wrong. In these cases wheel power is just a number they use to achieve their flywheel HP figures (which they want to measure accuratly)

Other dynos may wish to have accurate wheel power figures. It depends on how its all setup.

Yes, but not for that reason. Watts is a far better unit than Horse for this day and age.

Andy, find out what power actually is.