29 replies to this topic

[Ethel]

57hp - are we having a sweep stake?

[Elisha-The-Cooper]

right, i didnt pump my tyres up and i do have a problem with my steering rack i think, i just cant aford to get it fixed

so maybe after i get that fixed ill go again

x

[Sprocket]

Nothing wrong with the 28bhp losses. Dont forget that the dyno calculates these from the rate the rollers to slow down (drag) so the resulting flywheel figure is not that far off actual flywheel figures. Also remember that the losses are higher on a twin roller dyno because the tyre it trying to be sucked down between the rollers, this does not happen on the road

[mk3 Cooper S]

An accurate coast down should be performed prior to testing to take into account diff ratio wheel rolling radius etc.

If this was not done I would ask the dyno operator what coastdown curve he used?

Edited by mk3 Cooper S, 09 December 2008 - 03:25 PM.

[Sprocket]

Coast down is done at the end of the run, at the begining of the run the dyno is calibrated to the engine RPM therefore overall gearing ratio can be calculated, that takes the wheel size, final drive and gear selection into consideration

[mk3 Cooper S]

Accurate coastdown data performed either on the rollers or paralell straights as required to calculate the roller co-efficients or vehicle coastdown prior to the test.This then simulates the correct road load.
The dyno Calibration has nothing to do with the specific vehicle fitted to the rollers it just calibrates the rollers for the day, temp pressure bearing condition etc.
For max power runs you could post process possibly?

[Sprocket]

Why do you need to recreate road conditions when this is a full throttle max power test. All you need is a load applied to the wheels, then calculate the loss of the drive train. Modern dynos calculate this on the coast down. One thing that nearly all dyno operators do not do is run a coast down on the rollers alone. I know a chap who ran his own dyno, ran the rollers up to speed on a motorbike and jumped off the rollers and calculated nearly a 3bhp loss on the rollers alone. That was a Sun dyno and was in the presence of the Sun calibration technician.

If you do not calibrate the dyno to the engine rpm, you cannot work back to a reasonable flywheel figure. On modern chassis dynos which measures power at the wheels, the flywheel figures are representative, but never as acurate as an engine dyno, which measures power at the crank.

Why do you need to factor in anything else. Am I missing something?


I do recal speaking to a technician at Shell research and they used to do a coast down over a 1 mile road, measure the time and the difference in speed over the mile, then run the same road the oposite way, take the same readings and then average them. This can then be used as a factor in working back from the wheels to the engine, that was in the day of analogue dynos. However, this factor also has the drag of the rear axles and wheels as well as aerodynamic drag. Thats fine if you are doing testing of Efficiency and MPG so you can acurately recreate representable road conditions.

OR

Am I missing something. Please explain. Sounds like you know something

Bottom line is a modern chassis dyno reads power at the wheel and can reasonabley acurately calculate engine power, an engine dyno measures power at the flywheel. Does that all really matter anyway as the dyno's real purpose is to apply a constant load at varying engine speeds to facilitate tuning of the engine. Power measurement is secondary to the that

[mk3 Cooper S]

Agreed what I am describing is preparation for emissions drive cycles and leglative teste etc hence the need for accurate load loads etc

If the power run is done in absorbsion mode and calculating the delivered power can you explain the drivetrain loss calculation to me?
Are the losses in the transmission and drivetrain measured by powering the rollers or by running a rolling resistance or coastdown type test?

[Sprocket]

How deep do you want me to go, as when it comes to the maths I cant be arsed and probibly couldnt work it out anyway

Its all to do with the mass and inertia of the rollers. The diameter, and mass of the rollers is a known value, therefore the inertia can be calculated. The manufacturer of the dyno knows the inertial loss curve of their perticular system up to around 200mph and write this into their software. Sitting a car on the rollers changes this inertia, the difference in inertia is used to calculate the power absorption. This can then be assumed to be the loss of the drivetrain. The curve is inverted and added to the power at the wheels curve to give a representable flywheel figure. The system is by no means perfect hence the bloke I was talking about seeing a 3bhp loss with nothing on the rollers. Thats not the end of the story as the drive train is in over run rather than being driven, then there is any slight drag in the clutch if the operator keeps the clutch out, and if its put into neutral, that will sway the figures again, not to mention the tyres are not being pulled down into the rollers as they were under power.

From experience we are talking around 3-4bhp difference in flywheel figures on inertial systems, and perhaps as much as 8bhp on absorption systems. The latter being unable to calculate flywheel figures using coast down as there is no inertia in the system.

The closest you get to road conditions is the Superflow and DynoJet single drum dyno where there is only one tyre contact patch, removing the extra losses generated by a twin roller pinching the tyres.

Its all very subjective as you can already see.

[rozzer1275]

76hp at 4,700rpm sounds good thought the fartory was 63
85f/lbs of torque, but it drops off early, is this car moded.

[smorg]

tempreture of the engine can have a HUGE effect on the cars performance aswell as these A series engines really suffer at high tempretures

[zap]

standard mpi with K+N , lcb playmini twn dtm.



x

any thoughts or comparison advice is helpful as noone from my local club has replied

x

Christ thats really good power, i only got 64bhp and 74.5lb. from my stage one spi.

but it is 53bhp standered so, i am happy with that, the tester said that is really good for my mini

hope to get a lot more soon, will post

[Elisha-The-Cooper]

yeahhhhh!!! x

[Elisha-The-Cooper]

76hp at 4,700rpm sounds good thought the fartory was 63
85f/lbs of torque, but it drops off early, is this car moded.

no mods i dont think, just stage 1

x

[zap]

76hp at 4,700rpm sounds good thought the fartory was 63
85f/lbs of torque, but it drops off early, is this car moded.

no mods i dont think, just stage 1

x

well that is good

x