What will one more inch do for me

[Race Grandpa I]

OK get your minds out of the gutter. :hammer:

The question is what difference would one see on the dyno by having a larger tire. Say moving from a 14" tire to a 15" tire?

[Tom @ Fast Forward]

More speed, same HP.

[socal pat]

Power is power. You will see no difference on a dyno. However, you should see a bit quicker acceleration on the low end with a smaller wheel/tire, but the larger one should give you a bit taller gearing. I went from 14 to 15 for a HUGE increase in available tires and tire sizes. In 14's you have 195/55 and 195/60 and that's it. In 15's you have....
195/50
205/50
225/50
225/45 and they all fit!

[Race Grandpa I]

I was told it would make a difference on the dyno - by the Spec-Miata shop owner that had no idea how to tell the tuner how they should tune the engine.

Just goes to show you that even the well known and highly respected people in the business can have no clue about some basic things. :hammer:

[Gman7007]

Technically since you do see quicker acceleration on the low end with smaller tires that would equate to a steeper front end of the torque curve. But not more torque or HP overall. Negligible, hence why we say no difference.

[Steve in VC]

If I understand you question, I think you are asking about the flywheel effect in wheel / tire combinations.

Most dyno's are a flywheel where the rate of acceleration with HP is known.

If your old 14" wheel / tire combination, and a new 15" wheel / tire combination of the same diameter, same weight, you may see lower HP on the dyno because the weight in the 15" is usually further out, therefore has more flywheel effect. If the 15" is heavier, you will see lower HP.

If you see a 2HP reduction on the dyno, you will experience 4HP on the road because the dyno didn't "spool up" the front tires.

If you are on a dyno that can load the drive train to maintain RPM & HP, you will see no difference once the tires are up to speed.

[Tom @ Fast Forward]

It's like making a run in a different gear, WHP is still the same. Basically, it is a matter of speed vs. time. If you increase the diameter, you will make more MPH vs. RPM but, if you plot against time or speed in both cases, the slope should be the same.

Once upon a time there was a guy on mnet whose name ( 'cough' J_Man) shall remain anonymous, tried to convince people the WHP and crank HP are the same. That's not true either. ;)

[Steve in VC]

Race Grandpa I is going from a 14" wheel to a 15" wheel, don't know if the tire diameter is larger. It will act like a higher gear ratio only if it is taller.

A 225/45/15 is a lot more tire, but the stock diameter.

Anything that rotates in the drive train is effectively a flywheel (or anything that rotates in the engine for that matter). With a given power curve, a heavier flywheel will take longer to spool up than a small one.

Crank HP eliminates all the rotating stuff in the drive line, behind the flywheel. I believe it also removes most of the bolt on power parasites. Apples and oranges.

[Tom @ Fast Forward]

Yes, anytime you add/subtract mass/weight at the wheel, it will take more/less HP to spin it and that should show up as a loss/gain of RWHP. Typically, when you move from less mass 14" rim to more mass 15" rim, it is mostly offset by a lighter tire. They mostly cancel on the weight. If the OD stays the same then the top speed stays the same as well and the HP change is still nill.

Steve, how much WHP is lost if the rim/tire goes from 25# to 30#? Assume LSD so we are spinning both wheels

[Steve in VC]

Yes, anytime you add/subtract mass/weight at the wheel, it will take more/less HP to spin it and that should show up as a loss/gain of RWHP. Typically, when you move from less mass 14" rim to more mass 15" rim, it is mostly offset by a lighter tire. They mostly cancel on the weight. If the OD stays the same then the top speed stays the same as well and the HP change is still nill.

Steve, how much WHP is lost if the rim/tire goes from 25# to 30#? Assume LSD so we are spinning both wheelsUsing some crude calculations, each 25Lb tire requires about 3.5HP to accelerate from 35MPH to 100MPH (2500RPM to 7200RPM in 4th), for a total of 7HP on a dyno, 14HP 0n the road. The 30lb tire takes about 4.2HP, or about 1.4HP more on the dyno. The problem is determining the distribution of weight, so I took a SWAG between all the weight on the outer edge and a cylinder.

Does anyone have actual numbers?