Dyno Results And Wheel Sizes

[EVLKNEVL]

I think I can clear this up.

I'm no expert so feel free to correct me if need be, but think of it this way:

PASHEN, your 19s have a larger overall diameter than your old 17s (both with tyres on), so for every rotation of your axles, your car actually travels further. The total circumference of your tyre is the distance you travel for one rotation. The extra power required to move your car the extra distance is the few kilowatts you lose on the dyno. That's why larger wheels have the effect of a taller diff ratio. The pay-off is that on the highway, your car mustn't rev as hard to keep you at a given speed (just like a taller diff). turbo6man, a dyno will measure tractive effort, but this is always effected by gearing and drivetrain power loss. It is acually a good thing because it gives you a real world indication of how well your engine puts down the ponies, relative to your car and its set-up.

As I said I'm not an expert, so disregard this post if you want, but at least think about it. Makes sense

Matt

[Turbo6man]

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.

Any figure taken from the dyno has to be manipulated by the software/operator in order to make figures the customer can relate to and is happy with.

People who use dynos to extrapolate absolute power figures are kidding themselves. It is a tuning tool ONLY.

I don't know how many times this has to be explained before it will be accepted as THE TRUTH.

I suppose however, that without dyno figures to talk/argue about, this forum would be a pretty boring place so ... ALL POWER TO THE DYNO!!!

<{POST_SNAPBACK}>

so why do they use 1to1 gear ratio......traction effort is the torque reading in newtons on dyno chart which is not nm of torque and gear ratio will effect rwkws ...changing wheel size or inflation will change finale gear ratio.

<{POST_SNAPBACK}>

Where do you get that idea from? And how do you do a dyno run in an auto and lock it in 1:1 gear?

[Turbo6man]

I think I can clear this up.

I'm no expert so feel free to correct me if need be, but think of it this way:

PASHEN, your 19s have a larger overall diameter than your old 17s (both with tyres on), so for every rotation of your axles, your car actually travels further. The total circumference of your tyre is the distance you travel for one rotation. The extra power required to move your car the extra distance is the few kilowatts you lose on the dyno. That's why larger wheels have the effect of a taller diff ratio.

Not necessarily so - the tyre profile is lower which compensates for the larger diameter rim. The overall wheel circumference may alter by about 2%. The difference in the readings we have been given is much more than 2% I think. In fact it was about 7% to 8%.

turbo6man, a dyno will measure tractive effort, but this is always effected by gearing and drivetrain power loss. It is acually a good thing because it gives you a real world indication of how well your engine puts down the ponies, relative to your car and its set-up.

As I said I'm not an expert, so disregard this post if you want, but at least think about it. Makes sense

Matt

<{POST_SNAPBACK}>

Well why aren't all dyno runs done in first gear? Surely with your argument this is going to give the highest possible reading???

[Turbo6man]

OK, for all those people who have done dyno runs or witnessed a car being done ...

Did the operator ask you what your diff ratio was? Did he measure the rolling circumference of your wheels? Did he run in a gear through the gearbox that gave 1:1 ratio? Did he weigh your vehicle?

I'm guessing (no I'm not, I know) that he didn't do any of these things eh?

So how does he come up with this magical power figure for your engine?

[slapnut]

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

[slapnut]

So according to the calculator

235/45 R 17 circumference = 79.6in

255/35 r 19 circumference = 81.8in

the difference is plus 3%.

So if you have 360 RWKW running on 19's (just like my car NOT)

235/45 R17 will get 3% more - 371 RWKW - wooo hooo

Anyone want to swap my 19" for stockies??

[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???

[Blown BA]

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}>

why don't dyno operators need to know these things in order to calculate their results???

<{POST_SNAPBACK}>

Cause they don't give a sh*t.......so long as you pay them :lol:

[slapnut]

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.

Edited 22/03/05 09:24 AM by slapnut

[ktford]

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

why don't dyno operators need to know these things in order to calculate their results???

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