ATW figures explain

[Ali]

Why do people use this as a productive measure?

Surely at the fly/crank are far more productive measures of BHp/Torque.

We tested mine and it produced different ATW figures for each gear?!

Useless??? Explain?

 

[Saner]

A rolling road measures whats at the wheels, it then, 'guesses' the output at the fly. So the only accurate measured figure is at the wheels.

Thats how i see it.

 

[Ali]

^Not sure thats accurate...Because there's a common denominator to calculate the loss @ the transmission etc to get the "ATW" figure. ?? /

 

[MarkM]

You cant measure the fly figure unless the engine is out on a dyno, so best you can do easily is measure the wheel figure.

Dont they factor in the wheel speed when calculating the power figure?

lol, I dont know the answer either.

 

[Saner]

but there isn't, people use a calcutaed guess for the losses. In truth there is no way of cosst efectivly telling how much each individual car is loosing through its gearbox and other mechanical parts.

 

[stan*]

yuo, a rolling road measures power at the wheels...thats it. even that figure isnt 100% accurate due to many variables in the exercise. flywheel power figure can be guestimated by reducing the figure from the wheels (usually) by 15-20%. this again isnt accurate as transmissions vary, as do the losses. also transmission losses rise as a factor of power being produced eg a 1000bhp engine will prduce more transmision loss than a 200bhp engine...even on the same transmission.

so, in short, the only REAL way of measuring your engine power is on an engine dyno...anything else, including a rolling road, isnt as accurate

 

[Ali]

rite....lol...ok.

 

[drsmith]

the newer rolling roads are equiped with new software and technology which allows them to more accurately work out the % transmission loss rather than using a standard equation and 'guessing' using general figures, i.e. 15-20% as quoted

 

[Guy]

they use a gear that is as close a ratio as 1 or something.

depends on cars

on the saxo vts boxes i know doing it in 3rd gave an over inflated gear so best to do it in 4th..

think punto turbos they do in 3rd?




most rolling roads use a coast down method to ''calculate'' the transmission losses then add these ontop of the measured wheel figure to give the fly wheel figure

manufacturers quote fly figures to sell cars.



but as you say.. rolling roads are pretty much useless for comparing figures.. they vary so much from time to time.. get a bit of damp on your tyre and your losing power from lack of traction etc.. warmer gearboxes mean thinner oil so less resistance for the transmission.
but then if the engines hot it will have a hotter air going in so produce less power too.


theres so many factors. just take it with a pinch of salt.

 

[Ali]

Thing is the first time i did my power run he used 3rd and the last time he used 4th??!! Inconsistancy surely?

 

[Guy]

did the 3rd have the higher figure by any chance?

maybehe did one in 3rd, one in fourth and recorded the average between them?

 

[Ali]

no no diff day.

Higher BHp & torque in 3rd

 

[Martin]

no no diff day.

Higher BHp & torque in 3rd

if it was a different day there is no way to know what caused the different reading, there are hundreds of variables.

 

[Paul J]

Different roads use different gears, as said a lot use 4th as its close to 1:1 on most cars but some roads are designed for lower speeds and 3rd gear.

Engine dyno's give a nice indication of power from engine but to compare with other cars is also a near useless figure. Different components will reduce power eg water pump, exhaust and the car may make no where near that on the road.

Chassis dyno's and ATW figures are better for indicating which car will be quickest on the road if used correctly in the same conditions

 

[Guy]

no no diff day.

Higher BHp & torque in 3rd

because of the gearing..

as said above most places use 4th as its close to 1:1

 

[stan*]

coast-down method is as inaccurate as multiplying by 0.85-0.8.

 

[Guy]

theyre all innacurate. some are mor einacurate than others

bets thing to do is really not care about figures and prove stuff on the track.

 

[Justin]

i had a rolling road session a while back on my ibiza, and it came out at 165bhp.. so is this at the wheels? surely not, its supposed to be 150 standard!?

 

[Djw John]

Cant you also change theatw figures dependin tyre pressures as well?! Only good for finding flatspots I guess

 

[thedoc]

IIRC you can gain a few bhp on coast down by dabbing the brakes..

 

[stan*]

yep, or icreasing tyre pressure, or fudging ambient correction, etc etc

 

[Paul J]

^ which will have no effect on the uncorrected ATW figures;

Two cars run one after the other on a chassis dyno and the ucATW result will give a very good idication of how they will compare on the road (taking into account power curve aswell as peak power).

 

[Fred@BTM]

4th gear is generally nigh on 1:1 ratio

so its in theory direct drive from the engine

in 3rd, or 5th, you would be over/under driving the wheels and giving a false reading

thats the most simple way i can explain it

 

[stan*]

^ which will have no effect on the uncorrected ATW figures;

Two cars run one after the other on a chassis dyno and the ucATW result will give a very good idication of how they will compare on the road (taking into account power curve aswell as peak power).

of course it wont, but who quotes un-corrected figures? people always want the highest power figure whetehr its realistic or not....

 

[stan*]

4th gear is generally nigh on 1:1 ratio

so its in theory direct drive from the engine

in 3rd, or 5th, you would be over/under driving the wheels and giving a false reading

thats the most simple way i can explain it

That only applies to torque (gear ratio's), and eddie current dyno's measure power direct, so power will be un-affected