@atspeed racing, just wondering - why do you quote torque at the wheels rather than force? The raw figures to come off the rolling road are force and road speed. In order to calculate the torque and rpm you need the size of the wheels, and I would imagine it would be difficult to measure the wheel radius any more accurately than about 0.3%, which will give your final results only two and a half significant figures. Whereas the rolling road should be giving you better than three figures.

Yeah I'm not completely happy.

I was hoping for 155-160 and was hoping for over 300 lbs ft of torque

From what you've said, the figures above are what I would kind of expect as a minimum as flywheel figures. You got 144 ATW, which equates to about 160 at the flywheel. 275lb of torque at the wheels is pretty strong too - well over 300lb at the flywheel.

I would re-dyno. Try to find somewhere with a dyno dynamics RR as they seem to be pretty accurate. Also, ensure the operator has dyno'd dervs before.

@atspeed racing, just wondering - why do you quote torque at the wheels rather than force? The raw figures to come off the rolling road are force and road speed. In order to calculate the torque and rpm you need the size of the wheels, and I would imagine it would be difficult to measure the wheel radius any more accurately than about 0.3%, which will give your final results only two and a half significant figures. Whereas the rolling road should be giving you better than three figures.

although your right in saying force is the raw figure, without calculating the torque the results are pretty meaningless. higher force does not mean higher power. if you take a car with dyno it, then dyno it again with much smaller wheels your going to get a larger force but the engine is still developing the same power.

also, the transducers on the dyno may be outputting data at a high definition, doesn't mean its accurate data though?

Can't understand the fuss with rr figures, either the car is faster than the one next to it or it isn't, no amount of printouts is going to alter who gets away from the lights first or the best standing quarter.

We need to convert the world to use wheel figures and not flywheel figures, i don't think its gonna happen

Smaller wheels mean lower mph for the same rpm.

Power is proportional to road speed x Force in the same way that it is proportional to rpm x torque.

You have: rpm N m
You want: hp
       * 0.0001404315
       / 7120.9092
You have: rpm ft lbf
You want: hp
       * 0.00019039955
       / 5252.1131
You have: mph N
You want: hp
       * 0.00059949051
       / 1668.0831
You have: mph lbf
You want: hp
       * 0.0026666667
       / 375

Our engine testbeds are calibrated to within 0.1Nm and 1 rpm. I imagine rolling roads will have about the same accuracy - probably better than 1rpm at the roll, but the same 0.1Nm.

to be honest, im only interested in rolling road figures if there's a before and after run.

Can't understand the fuss with rr figures, either the car is faster than the one next to it or it isn't, no amount of printouts is going to alter who gets away from the lights first or the best standing quarter.

@atspeed racing, just wondering - why do you quote torque at the wheels rather than force? The raw figures to come off the rolling road are force and road speed. In order to calculate the torque and rpm you need the size of the wheels, and I would imagine it would be difficult to measure the wheel radius any more accurately than about 0.3%, which will give your final results only two and a half significant figures. Whereas the rolling road should be giving you better than three figures.

you dont need to know the size of the wheels or tyres or the rolling circumference or anything:confused:

on an eddy current brake dyno all you need to know is the size of the rollers and how much current is needed to resist the turning of the rollers to a set road speed, torque is then calculated form this:confused:

No. Force is calculated from this.

Or more precisely, the torque on the roll axle is calculated from this and then divided by the known roll radius to give force at the interface between the roll and the wheel.

The torque at the wheel axle is unknown without knowing the effective wheel radius. It is not the same, nor anywhere near, the torque at the roll axle. If it was, then gear boxes would not work.

Of course, there's also the fact that theoretical and actual accuracy, and theoretical precision, are 3 different things.

If someone reads off dyno figures to 2 places of decimals, they're almost certainly quoting a theoretical precision, that is the smallest change the dyno can read.

When you calibrate a dyno using known driving loads, you calibrate the theoretical accuracy, which is dependant on how accurately you can apply the known load.

The actual accuracy with a real vehicle is dependant on other unknowns or variables, like the tyre type, size, pressure and wear on the day.

So I'd agree that the best measure of "power increase" is before and after runs, ideally done on the same day and with the cell at constant temperature.

ive seen it to many times on here people saying their car has been chipped/tuned etc and its now kicking out 100bhp more etc cos the garage said it will and then go on a rolling road somewhere else to find it doesnt and then to moan saying the roling road are wrong. of course some places will adjust it to make it look like the upgrade theyve done is kicking out loads more power to what it really wants as it wants your money.

Hiya. Have to say I got a little bit lost in all the calculations re tyre size..etc..etc.

Tyre size has no effect on the results from a rolling road. the BHP figure is calculated from the RPM taken from the engine and not road speed.

As we have said earlier as others have, take your car to a rolling road, get a power graph...modify your car, take it back and get a power graph.

Doesnt matter what the machine shows, if you start with 100 dots and finish with 110 dots, you now have 10 more dots than when you started.

Our dyno cell is weather station controlled so figures are not affected by the climate conditions on the day.

As we have said before, our roller set is accurate to the top engine builders engine dyno. When we ask is this the sort of figure you are looking for at the wheels, the answer is yes.

These people dont play games. These are the people that know.

Earlier in this thread it was mentioned to ask if the operator has dynoed a diesel before. It doesnt make any difference if the car is on petrol, gas, diesel or vegatable oil the dyno is a big calculator and doesnt care what the car, van, boat, jet ski, train or bike is. It will still only measure torque/RPM and calculate BHP from that.

Atspeed Racing

Edited 15 July, 200817 yr by atspeed racing

What I meant was that different tyre type, pressure and size will change unsprung weight and rolling resistance, and hence both power measurement and rundown rating. I'm not claiming that these effects are large relative to peak power figures, they're typically 1 or 2%, but I am claiming that they're large relative to dyno precision, so quoting figures on different tyres to 2 places of decimals is MBF since the tyres introduce an uncertainty that's orders of magnitude larger than your quoted precision ok.

I accept that you know more than me about using a dyno, but I calibrate and produce error reports on other electronic measuring equipment as part of my job, so I know about measuring precision and error sources.

What I meant was that different tyre type, pressure and size will change unsprung weight and rolling resistance, and hence both power measurement and rundown rating. I'm not claiming that these effects are large relative to peak power figures, they're typically 1 or 2%, but I am claiming that they're large relative to dyno precision, so quoting figures on different tyres to 2 places of decimals is MBF since the tyres introduce an uncertainty that's orders of magnitude larger than your quoted precision ok.

I accept that you know more than me about using a dyno, but I calibrate and produce error reports on other electronic measuring equipment as part of my job, so I know about measuring precision and error sources.

Tyre pressure as well as all the vehicle drive train, bearings, brakes, tracking etc etc etc has a effect on BHP.

If you are searching for an ABSOLUTE accurate figure then you need a engine dyno.

A rolling road is never 100% accurate. Too many variables. It is a good tool to calibrate air fuel and ignition timing, and to locate faults.

A vehicle is constantly changing, oil temperatures, water temperatures, gearbox oil temperature, tyre temperatures and how tyres get stickier when warmer, internal engine frictional losses. how can you measure something that constantly changes?

Tyre pressure as well as all the vehicle drive train, bearings, brakes, tracking etc etc etc has a effect on BHP.

If you are searching for an ABSOLUTE accurate figure then you need a engine dyno.

A rolling road is never 100% accurate. Too many variables. It is a good tool to calibrate air fuel and ignition timing, and to locate faults.

A vehicle is constantly changing, oil temperatures, water temperatures, gearbox oil temperature, tyre temperatures and how tyres get stickier when warmer, internal engine frictional losses. how can you measure something that constantly changes?

Agreed in full. In fact I'm not even sure we're arguing, at least not so much as discussing how to tell a good company from a shower of MBF merchants.

well done that man. no argument. considering the variables a rolling road does a very good job at maintaining consistant (ish) results.

you have to think of an engine dyno as a vernier and a rolling road as a tape measure. both gives the same result, just one is more accurate.

main reason people choose rolling roads over engine dynos is cost and ease of use. drive the car on, drive it off.

with an engine dyno the engine needs removing from the car, bolting to the dyno, usually with custom fabrication involved, cooling, exhaust etc. you can spend £££'s just getting the engine onto the dyno.

£500 for a power run on an engine dyno, vs £40 for a power run on a rolling road... i know what one id pick. (bit biased because its free for me :P )

My old cossie engine got mapped on a dyno, as apposed to rolling road etc..Much more expensive, but i feel it was worth it, and beleived the figures to be extremely accurate.

Although down the pub i often quoted the rolling road figures i got once it were back in the car

So

Power = Roadspeed * Force

Wheel Speed = Engine Speed * Total Gear Ratio

Wheel Torque = Power / Wheel Speed

Assuming you know the total gear ratio and have an accurate figure for engine speed, and you probably do, that would work.

I still think road speed and force would be better and give more real-world applicable numbers, but whatever the punter wants. The punter expects to see rpm and foot-pounds, no matter that it might be more use to him to see mph and pounds.

the top engine builders engine dyno...These people dont play games. These are the people that know.

I know them well. I write the software embedded in their engine dynos.

I'm disappointed with the figures but the main reason behind this thread is that i wasnt happy the vehicle was locking up on run down, I've been to many rolling roads and seen plenty in action, and i've never seen a rolling road end like that ever.

your figures are correct, just because they didnt make the advertised power doesnt make them wrong. i can tell you that you have 500bhp if you want me to, but you dont.

once the software flashes up TEST COMPLETE it means that.. the test is complete.. nothing which happens after is added into the calculation, there is no BS calculated driveline figure, no run down tests nothing, so even if a herd of wild horses came galloping through the dyno bay it wouldnt effect it.. braking, or making the tyres screech doesnt effect it either. As you are aware the dyno cell was just built when you came down, and the auto-brake facility was not in use, it now is, so the computer stops the car automatically after a run. the rollers weigh over 2 tonne.. thats a lot of inertia to try and stop.

we have explained how the system works, and why we dont do the "calculated" run down tests, perhaps you would like to read it again, or perhaps do what you should have done in the first place.. and ask us first.