Rolling road "drag losses"

[phillpot]

A little while ago my 2.9V6 S3 went on the rollers at Austec and recorded a healthy corrected power figure of 184.9

At the time I was happy with that and even happier with the way it drove after a few "tweaks" by Paul so didn't take a lot of notice of "all the other figures" .

So, getting to the point "drag power" was 56 leaving me 129.5 at the wheels. I've looked around a few other forums and 35 - 40 ish seems a bi more like it for a front engine/rear wheel drive car?
Others mention 20 - 25% but don't see how that works, my transmission drag would surely be the same whether I had a 100 bhp four pot up front or 300 horses of V8 ?

Just out of interest, Torque was 177.4

At the time the car had drum brakes on the rear, could it be as simple as they were a bit "tight" ?

[LongBaz383BHP]

That's 43% loss through the drive train, seems high?
Most would go 16-20%
If the brakes were 'tight' shirley that would have affected the RWHP

Edited by LongBaz383BHP on Wednesday 26th June 22:40

Edited by LongBaz383BHP on Wednesday 26th June 22:40

[Graham]

the transmission losses will erm depend on the transmission. the same trans on 1 car should be roughly the same as an other identical car so your right a 4ltr chim should have the same losses as a 5ltr chim so that shows how using a % figure is a pure guess.. a t5 box Btr diff, a couple of cv's and some wheel bearings will take the same power to spin them at a particular rpm regardless of what is making them turn.


the thing that is really mad is that a rolling road measures the power at the wheels, it has to as thats the only thing thats turning it. they then estimate the power at the flywheel based on the losses they think your getting, and then give you a wheel power by guessing the losses again in reverse...

ergo flywheel figures are fantasy unless the motor is out of the car and hooked up to a dyno. calculated wheel figures from a flywheel figure double fantasy... imho the only figure that means anything from a rolling road is the actual power measures at the wheels..

[Vee8ight]

Pump your rear tires upto 60 psi and watch the extra horses rear their heads

[Sardonicus]

phillpot said:

A little while ago my 2.9V6 S3 went on the rollers at Austec and recorded a healthy corrected power figure of 184.9

At the time I was happy with that and even happier with the way it drove after a few "tweaks" by Paul so didn't take a lot of notice of "all the other figures" .

So, getting to the point "drag power" was 56 leaving me 129.5 at the wheels. I've looked around a few other forums and 35 - 40 ish seems a bi more like it for a front engine/rear wheel drive car?
Others mention 20 - 25% but don't see how that works, my transmission drag would surely be the same whether I had a 100 bhp four pot up front or 300 horses of V8 ?

Just out of interest, Torque was 177.4

At the time the car had drum brakes on the rear, could it be as simple as they were a bit "tight" ?

Is the engine stock? cant see a stock Cologne 2.9 making that fly figure I recon corrected would be about 170ish probably nearer 165 IMO

[Julesprivate]

If I read the posts correctly it made ~130 at the wheels and the operator corrected it by ~40% to give a flywheel rating. That correction seems a bit excessive, I'd have expected ~25-35%.

But that's a figure chosen by the operator...well actually to be fair it might not have been (it depends on the dyno) but it is a "calculated" figure based on other similar typs of vehicle so not really apllicable to a SPECIFIC car being tested.

Oh I guess he could have just dipped the clutch and then it would calculate the coast down drag...but that's not a very precise measure either..


130 at the wheels would be "about right"

Flywheel figures are pointless unless the engine is on a bench. RWHP is the figure to focus on..

Edited by Julesprivate on Thursday 27th June 13:49

[phillpot]

Sardonicus said:

is the engine stock?

little bit of "cleaning up" on the inlet ports and a mild Piper cam.

TVR claimed 170 for the 2.9 cologne and Piper reckon cam can add upto 15 so thats why I was pleased with the 184.9

[Sardonicus]

phillpot said:

little bit of "cleaning up" on the inlet ports and a mild Piper cam.

TVR claimed 170 for the 2.9 cologne and Piper reckon cam can add upto 15 so thats why I was pleased with the 184.9

A good one stock made 145 (from Ford) so I reckon with those TVR tickles/mods about 165 all in it would certainly explain the trans losses if it goes well who cares?

[TA14]

Julesprivate said:

That correction seems a bit excessive, I'd have expected ~25-35%.

That would make the transmittion losses for an S about twice that for any other TVR. Hmmm...

[Julesprivate]

TA14 said:

That would make the transmittion losses for an S about twice that for any other TVR. Hmmm...

Really? Well I guess I could be (and probably am) wrong.. I suspect that's why the "corrected HP" figure is always such an emotive subject. TBH even a coast down check really doesn't give a good correction figure.

That's why I only ever look at the RWHP figure saves all that confusion. I guess realistically it doesn't matter what the flywheel HP is, you're not really measuring that on a RR, you're measuring the speed at which the WHEELS accelerate the drum, assuming your other base settings are corrected for variences in air density, temp etc then that's the only really "reliable" figure..and even that can vary from dyno to dyno..

[Byker28i]

Which probably explains all the claimed bhp figures that never match up when we have rolling road days?

[TA14]

Julesprivate said:

Really? Well I guess I could be (and probably am) wrong.. I suspect that's why the "corrected HP" figure is always such an emotive subject. TBH even a coast down check really doesn't give a good correction figure.

That's why I only ever look at the RWHP figure saves all that confusion. I guess realistically it doesn't matter what the flywheel HP is, you're not really measuring that on a RR, you're measuring the speed at which the WHEELS accelerate the drum, assuming your other base settings are corrected for variences in air density, temp etc then that's the only really "reliable" figure..and even that can vary from dyno to dyno..

Quite so. RRs are best to monitor changes - leads, cams, etc and study their effects relative to an initial figure.

[300bhp/ton]

phillpot said:

A little while ago my 2.9V6 S3 went on the rollers at Austec and recorded a healthy corrected power figure of 184.9

At the time I was happy with that and even happier with the way it drove after a few "tweaks" by Paul so didn't take a lot of notice of "all the other figures" .

So, getting to the point "drag power" was 56 leaving me 129.5 at the wheels. I've looked around a few other forums and 35 - 40 ish seems a bi more like it for a front engine/rear wheel drive car?
Others mention 20 - 25% but don't see how that works, my transmission drag would surely be the same whether I had a 100 bhp four pot up front or 300 horses of V8 ?

Just out of interest, Torque was 177.4

At the time the car had drum brakes on the rear, could it be as simple as they were a bit "tight" ?

I don't quite know why in the UK there is such a fascination with fly wheel figures. Personally I'd always want to see what it's making at the wheels.

The flywheel guesstimate will take the wheel figure anyhow and estimate from that. So starting with the wheel figure is a more accurate way than trying to back track to it.


Anyhow. There is no surefire way of knowing exact loses by using a generic calculation. There are many different ways to guess it. Some say for a RWD manual car that the 15% rule works well, or 12%+10hp.

[Brummmie]

I work on roughly 15%, but different dyno's also give different readings, especially different manufacture of dyno, at TVR Power on their dyno jest it gave 538RWHP, on the Flymo Dynamics it gave 586bhp at the fly.
So 635bhp minus 15% equals 539.75rwhp, so take your pick for which bar you are leaning on!

[wedgeinald]

I would say that (as a few above have) that the BHP@Wheels is the one to go on, and if your transmission is not as lossy as they say it is (very likely) then the engine is "down a bit" on power but this is normal for an emgine that is not in its first flush of youth.

I'm sure some RR guys COULD (not saying it is the case with this one by any means) ask what BHP you "think" it should be and then tweak the "losses" to make the flywheel figure close to what you were expecting.

I was going to get my car RR'd as it felt down on power compared to the same engine (actually the same one not a similar one) in the previous car (which I did take on a RR once), I got my MOT guy to check the mixture (as I suspected it was lean) and now the car pulls nicely again. Having just changed the Tyres I'm sure the Toyos were causing a little (not much but enough to notice) bit of "drag", might just be my imagination of course.

Either way my wegde is now running much nicer than it was even a few months ago. I'll know for sure when I take it down the runway at Dunsfold as part of the motoring demonstration for wings and wheels. (the only place I can get an approximation of its true performance)

[phillpot]

Thanks for all your comments and views guys.

Guess it can't be too bad, seems to be up to pace with other V6 S's and has managed to "hang onto" the occasional V8 version