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DYNO Numbers? wheels make a difference? 

 

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 Post subject: DYNO Numbers? wheels make a difference?
Posted: Wed Oct 10, 2007 11:31 am 
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Does the size and weight of your wheels make a difference on the dyno?

I was told by a very reputable tuner that my 20 inch wheels, size and weight of them cost me upto 20rwkw on the dyno? He said hes seen it time and time again, people with big wheels get lower Dyno numbers with the same car. He said if i popped on a set of 17s there is 20rwkw there?

Is this a porky or does it have some merit?? Maybe it will effect the gearing? Maybe the weight of the wheels but 20 rwkw??

 

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Posted: Wed Oct 10, 2007 2:55 pm 
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20rwkw is a big difference.
if you went to a lighter wheel then it would make more power as their is less resistance on the axles ,but that much gain?

I was also reading in some jap crap magazine that grippy tyres will rob you of horsepower as it relates back to rolling resistance.

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Posted: Wed Oct 10, 2007 4:36 pm 
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I think 20rwkw may be a bit over the top but yes, bigger, heavier rims most certainly rob you of "dyno" power.

 

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Posted: Thu Oct 11, 2007 2:03 pm 
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thats s**t!. wheel size has nothing to do with power.....but with torque.

 

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Posted: Thu Oct 11, 2007 2:13 pm 
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larger diameter wheels slow the car slightly when taking off at low speeds because of the extra intertia, but when the car gets up to speed its fine
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Posted: Thu Oct 11, 2007 7:16 pm 
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according to my physics teacher, max kw will be increased, but the torque will be reduced...

 

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Posted: Thu Oct 11, 2007 7:43 pm 
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The dyno will cancel out the larger rolling diameter. Extra weight and friction of wider tires will change the reading somewhat, but 20rwkw is a bit of a joke. Off the top of my head I would say no more than a couple of HP. I googled the question and found quite a few dyno tests and I noticed a max difference of 4hp between extremes of wheel size.

In one of the first hits on google they go from a 13" rim/tire that weighed 13kg and went to a 17" rim/tire weighing 17.5kg and they lost 3.8hp.

The results I found also suggest that the more powerful your car is the smaller the % change.

 

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Posted: Thu Oct 11, 2007 8:20 pm 
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Larger diameter is effectively changing your final drive ratio. Slower acceleration yet higher top speed.

 

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Posted: Thu Oct 11, 2007 8:27 pm 
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i heard that tyre pressures make a (large) difference... most dynos run tyres at or over 40psi

 

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Posted: Thu Oct 11, 2007 8:52 pm 
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The relationship between power & torque is

Power = Torque x RPM

So anything that robs your torque at a given RPM, like bigger rolling diameters, robs power.

And extra weight adds to rotational inertia - harder to spin up.

So bigger heavier wheels will rob power - but how much is the question isn't it.
But they do look good.

 

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Posted: Thu Oct 11, 2007 9:05 pm 
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smithy_1 wrote:
Power = Torque x RPM

So anything that robs your torque at a given RPM, like bigger rolling diameters, robs power.


Not quite. The (incomplete) power:torque:rpm relationship you have posted applies to power at the flywheel - before the wheels. Dynos measure power at the drive wheels and and use the same basic principle but there is a twist. Bigger wheels do lower torque, but they increase speed proportionally. So for the same engine speed changing to a larger tire means you have lowered the torque but increased the dyno roller speed and the end result is the same power.

 

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Posted: Thu Oct 11, 2007 9:18 pm 
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thanks stockstandard
The plot thickens!

Sounds like someone needs to go to a dyno and do some tests with 20 inchers and 15inchers and report.

I just love fordmods.

 

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