Dyno Results And Wheel Sizes

[Damo]

The only dyno comparo one should worry about is one against your own ride. Like the 1/4 mile it is all about trying to improve your PB. So make your mods, go back to your own dyno & see what difference it made !!!

Scotty

Damn straight. I thought that was the point of them...!

[Mustang D]

Do wheel sizes affect the dyno result?  I mentioned that they do to Scotty last year and he pissed himself laughing.  Would like to hear from people who:

1.  ARE dyno experts

2.  HAVE had their cars dyno'd, especially if on 2 different wheel sizes

My car was tuned to 333rwkw on 17 in tyres.  Fitting the 19's showed the power figure to be 310 rwkw.

I had the power de-tuned back to 310rwkw on 17's.  A recent dyno with the same power level on my 19's showed the power to be 285rwkw.

Have others found the same thing?  If so, for brag value, obviously everyone will be quoting power figures from dyno runs with smaller wheels?

Would appreciate a discussion on this.

<{POST_SNAPBACK}>

Different Tyre/Rim combinations definately effect RWKW readings, the reason is INERTIA. A 19" tyre/rim combination has more weight toward the outer circumference of the wheel, so the 19" tyre/wheel will consume more Torque to accelerate than a 17" tyre/wheel. The true effect of different Tyre/Wheel combinations can be measured when a Ramp Up, then Ramp Down, is performed on a Chassis Dyno, and half the difference between the 2 runs is the Drivetrain Inertia. Not one of you would of ever had a Ramp down performed on your cars, as it is not great for Turbo Charged cars, and any owner would grimace in pain watching their car being ramped Down.

People in the racing industry are normally the most interested in measuring Drivetrain Inertia.

The differences you are seeing are quite normal. I recall a BMW M3 that I had dyno'd many times at 170RWKW with the standard wheels, the owner then fitted some 20" wheels and the Power fell to 150RWKW. I've also seen a WRX pick up 8AWKW from just changing to a lighter set of wheels.

[PASHEN]

Thanks all....now clear as mud.

So what about this scenario: My automatic gearbox is warrantied to handle over 360rwkw. If dyno's are just a tuning device, why state something like thatand how do you measure your power levels (wheel sizes etc)?

[Turbo6man]

Tyre circumference MUST influence that RWKW.

The question is .. by how much?

It depends on the difference in the overall circumference of the tyre.

Here is a link to a tyre size calculator that may help.

http://www.miata.net/garage/tirecalc.html

<{POST_SNAPBACK}>

well, using that argumenat so does diff ratio, gearbox ratio, etc etc so come on - answer the Qs - why don't dyno operators need to know these things in order to calculate their results???

<{POST_SNAPBACK}>

Hey Turbo6man

I am not a dyno expert, just a hack, so back off man.

But everything behind your flywheel MUST affect the RWKW.

auto versus manual, - diff ratio - what grease and sh*t is in there - if your handbrake is on or not...

I don't believe that dyno dudes will ever give you an actual figure for fly wheel KW.

If they do give you a figure for FWKW, it is an estimate based on experience and factory claims for FWKW.

The only accurate way to measure FWKW is at the flywheel not at the wheels.

<{POST_SNAPBACK}>

You've obviously missed the point - when was the discussion about FWKW? We've only been talking RWKW so don't make it any more complicated than that eh? So far no one can show even how the RWKW figures are determined or why they vary with different wheel sizes so don't get into FWKW at this point.

If you go back over what I've said - it is that the figures presented mean J@CKSH!T.

If anyone truely believes that running 17" wheels instead of 19s is going to suddenly give their car and extra 7 or 8% increase in power then how about we all go out and get 12" mini wheels?

[iTec]

You've obviously missed the point - when was the discussion about FWKW? We've only been talking RWKW so don't make it any more complicated than that eh? So far no one can show even how the RWKW figures are determined or why they vary with different wheel sizes so don't get into FWKW at this point.

If you go back over what I've said - it is that the figures presented mean J@CKSH!T.

If anyone truely believes that running 17" wheels instead of 19s is going to suddenly give their car and extra 7 or 8% increase in power then how about we all go out and get 12" mini wheels?

RWKW = FWKW - DRIVETRAIN LOSS

A dynometer is an accelerometer, the inertia of the steel drum is a known fact and this means that calculating the power from this spinning drum is pretty easy to do. Wheels, driveshafts flywheels clutches and pretty much anything else that makes up your drivetrain* will effect the DRIVETRAIN LOSS this in effect increases your RWKW it does not increase the FWKW (as this is measured without a drivetrain)

if you want examples here are some

Light weight flywheel = more power

Light weight wheels = more power

carbon fibre driveshafts = more power

weight requires inertia to move, reduce the weight of your drivetrain and you reduce the inertia it takes to spin your drivetrain meaning more power ends up on the rollers

If you really want to see this taken to the extreme take a look at the Jap JTC race cars, they may be rice but they have extreme amounts of power being made reliably out of little engines and they incorporate all the lightweight parts I listed as examples above.

p.s pashen I would be interested to see the weight difference between your 17's and your 19's

[aiboart]

...and whilst the dyno can give you and indication of your power gain using lightweight (rotating) components, it will under read this gain, in wheels and light weight brakes in particular. (for 2WD cars)

The dyno will only register a variation on the driven wheels. (2WD for XRT) But all four wheels and brakes are spun into rotation when the car is on the road. The driveline has to spin up all four wheels when the car is on the road because the road couples the driveline to all four wheels. (we hope) The big high rotational intertia (power sapping) brakes are also on the front wheels don't forget. A (4WD dyno on a 4WD car will however pick up the variation.) Note - friction associated with all these systems is another but related matter.

There is hoever one thing that a dyno cannot measure and that is pure translational inertia, or mass. Imagine you car on ice, with nothing spinning and imagine how much inertia is added with every extra kilo. That includes big heavy brakes, wheels (and the spare), big sound systems, us!

When you introduce lightweight rotating components you gain power on two fronts, a reduction of rotational interia, and a reduction of translational inertia. You also gain performance in the bouncy bits, the wheels, they can track the ground better.

Regards,

aa

[Ezy10z]

Do wheel sizes affect the dyno result?  I mentioned that they do to Scotty last year and he pissed himself laughing.  Would like to hear from people who:

1.  ARE dyno experts

2.  HAVE had their cars dyno'd, especially if on 2 different wheel sizes

My car was tuned to 333rwkw on 17 in tyres.  Fitting the 19's showed the power figure to be 310 rwkw.

I had the power de-tuned back to 310rwkw on 17's.  A recent dyno with the same power level on my 19's showed the power to be 285rwkw.

Have others found the same thing?  If so, for brag value, obviously everyone will be quoting power figures from dyno runs with smaller wheels?

Would appreciate a discussion on this.

<{POST_SNAPBACK}>

Different Tyre/Rim combinations definately effect RWKW readings, the reason is INERTIA. A 19" tyre/rim combination has more weight toward the outer circumference of the wheel, so the 19" tyre/wheel will consume more Torque to accelerate than a 17" tyre/wheel. The true effect of different Tyre/Wheel combinations can be measured when a Ramp Up, then Ramp Down, is performed on a Chassis Dyno, and half the difference between the 2 runs is the Drivetrain Inertia. Not one of you would of ever had a Ramp down performed on your cars, as it is not great for Turbo Charged cars, and any owner would grimace in pain watching their car being ramped Down.

People in the racing industry are normally the most interested in measuring Drivetrain Inertia.

The differences you are seeing are quite normal. I recall a BMW M3 that I had dyno'd many times at 170RWKW with the standard wheels, the owner then fitted some 20" wheels and the Power fell to 150RWKW. I've also seen a WRX pick up 8AWKW from just changing to a lighter set of wheels.

<{POST_SNAPBACK}>

I witnessed a chrysler charger being dynoed and for the first run it was using 28.5" McCreary drag tyres and measured 260hp ATW but because of the soft compound the tread was being ripped to shreads so fitted everyday 225/60 fitted on 15" rim and the horseposer dropped to 220hp. Judging from earlier posts everyone will say I have this mixed up but its not the case. No dyno settings were changed maybe this was the reason. And I did witness this in person !!!

Robbie.

[PASHEN]

p.s pashen I would be interested to see the weight difference between your 17's and  your 19's

<{POST_SNAPBACK}>

Thanks iTec

I dont know the weight difference between the wheels mate, but I'd say it's substantial. The 19's are 9.5 inches wide, wearing 275m rubber, 30mm profile.

[PASHEN]

If anyone truely believes that running 17" wheels instead of 19s is going to suddenly give their car and extra 7 or 8% increase in power then how about we all go out and get 12" mini wheels?

<{POST_SNAPBACK}>

Turbo6man, I dont think anyone believes that at all - that's crazy. But what I am questioning is the difference in power levels shown when dyno'd, with the exact same power levels on a car, with different wheel sizes.

[Guest xrtwpn]

The dyno measures tractive effort at the rollers. It doesn't matter what sort of car it is, what size wheels or what diff ratio or gear ratio. It doesn't even care who the driver is.

The dyno doesn't measure driveline losses, wind resistance, tyre resistance or weight.

<{POST_SNAPBACK}>

Turboman, correct me if I am wrong but the diff rratio, and rolling diameter will affect the torque reading won't it?

<{POST_SNAPBACK}>

yes you are right it makes the dyno read more

But at the end of the day who cares what your car makes on a dyno we arent racing dyno's it all comes down to what you can do at the track