Dyno Results And Wheel Sizes

[SPIKO]

Dynoed mine on HPF's dyno with my factory 17's on, 305rwkw.

Dynoed on HPF's Dyno with my 19's on, 305rwkw

<{POST_SNAPBACK}>

I failed to mention that you can adjust the dyno to compensate!

Provided that they know how.

<{POST_SNAPBACK}>

Dyno settings were not touched.

<{POST_SNAPBACK}>

that's impossible!

What dyno make and model?

<{POST_SNAPBACK}>

Failed to read the prvious post humble apologies.

Measure 19,s and 17's circumference eg. around the tread and see if and how much the difference is in length.

[Geeseman]

Thanks JB, I thought as much.

When I get around to it I'll measure the 17 and 19inch wheels to see just how close they are.

[cobrav8]

You can of course get an even lower profile 19 inch tyre, and a lighter weight wheel, and therefore be better off than the 17 inch wheel if your rolling circumference and weight are both better.

Light weight 19 inch wheels, and very low profile 19 inch tyres will both cost a lot of money. Also - the very low profile tyres should handle better - but will provide a harsher ride.

[cobrav8]

Also - dont overlook the difference in how many rpms are needed to achieve the same speed.

A small difference in tyre circumference can make a big difference in road speed. Potentially the 19inch wheels with a larger rolling circumference will be going a few km faster - and your speedo will be inaccurate. Look out for the cops!

[Cobra]

haven't we agreed that the rolling diameter of the various wheels was going to be considered the same for the purposes of this thread ???

and that some guys were reporting power variations due to the rim sizes only ....

go the 5's

[cobrav8]

If that was the case - you should compare different brand mags with the same rim size, as the weight is the only factor left.

[EVLKNEVL]

Fact is, cobrav8, that tyre circumferences are not the same, even if only by small margins.

Now for those who say the negligible difference in circumference does not translate to amount of power loss, try multiplying that difference by the rpm of the wheel. Don't forget that 1mm difference in diameter equals 3.14 extra mm in circumference (old pi equation).

Yes, weight of the wheels is also a contributing factor, possibly more so than circumference. It is an inertia-related issue and must also be considered.

Turbo6man, why would your dyno operator need gearing information? what would he use it for? even if he knew these variables, they shouldn't be used to change power readings, and if they were, I'd be finding myself a new tuner. Tyre size ABSOLUTELY MUST alter dyno figures. If you don't believe it, actually explain to me how this is not so.

My original point is that if you want to know your engine's actual power, put it on an engine dyno. If you want to know how the engine copes with all other loads, friction, inertia and gearing-related, and the actual amount of force your tyres will translate to the tarmac, use a chassis dyno. That's what they're for.

[cobrav8]

Or if you want to be entirely accurate - use a drag strip - that takes into account weight, power, gearing, wind resistance, wheelspin, etc, etc.

[Cobra]

ofcourse wheel sizes alter the readings on a dyno ... different gearing=different results ..... but we were talking about the actual tyres and the rims they are on ... not gearing ... I think

(why is it all dyno threads go weird ??? )

as for a change in rolling diameter of 1mm, the distortion of a tyre at high speeds would have more impact .... you'd have to be calculating the perimeter of an ellipse or some other weird a$$ shape except a circle

also the differential wear on any pair of rear tyres could easily be more than a few millimetres .... what then ??

oh no

quoting the thread starter

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.

the magnitude of the difference (nearly 10%) and its repeated result I find interesting ... and yet to be explained (to this dumb a$$ anyway)

[Turbo6man]

Turbo6man, why would your dyno operator need gearing information? what would he use it for? even if he knew these variables, they shouldn't be used to change power readings, and if they were, I'd be finding myself a new tuner. Tyre size ABSOLUTELY MUST alter dyno figures. If you don't believe it, actually explain to me how this is not so.

<{POST_SNAPBACK}>

I think you must have totally misunderstood anything I have said. Please go back and read my posts at #10,#12 and #25 for some clarification of my position on this.

PS your own argument has holes big enough for my bus to drive through --- you state that even if a tuner knew the gear ratios he shouldn't use them to alter power readings BUT you maintain changing the rear wheel circumference is OK to produce different readings. They are one and the same thing ... you can't argue it both ways.

Like I said before if you want to see the highest readings possibly use first gear ... or a 12" mini wheel ... or hey, go for both and get yourself a starshot dyno figure to brag about eh.

Gee thanks Geeseman for bumping this thread!