Does the size of your tires (meaning the overall diameter) have any effect on your hp or tq readings?

I know it will affect gearing, but it shouldn't matter on the dyno, right?

 

I would assume a smaller circumference will give you a (slightly) higher dyno reading...do to the ability for it to spin up the dyno quicker perhaps?

 

No.

Assuming you are talking the small differences in diameter in different "street type" tires (24-30"), it wil have no measurable effect.

One might think that a smaller tire will allow for more torque, the rpm/mph calculation done by the dyno software will compensate for that.

However, the weight and distribution of the tire/weight combo would have an effect, but still, these differences would still be within the range of error for any dyno (+/- 2%)

 

Incorrect... Well, disregarding any weight changes, rolling resistance, etc., if the overall diameter changes the WHP will also change. This is not unlike gearing changes will produce different WHP.

AHP = HP at the Axle
WHP = HP at the Pavement
R = Wheel/Tire Radius

12 / R * AHP = WHP

So a stock 215/50/17 will have a radius of 12.732 Inches, lets assume a AHP value of 200:

12 / 12.732 * 200 = 188 WHP

Now lets take a small change to a popular size of 225/45/17 and use the same AHP value of 200:

12 / 12.486 * 200 = 192 WHP

So a smaller diameter will put more power down at any given rev. But there are too many other variable that can also be affected with may or may not influence overall gains. Such as shift points being affected, the afore mentioned rolling resistance and the potential weight difference.

Oh yea, and traction might be more limited in the smaller tire/wheel combo.

 

How would different tires sizes affect 1/4 miles times???

If I switch from stocks to 235/45/17 how will that impact my track performance...?

Obviously traction will be increased (that's the reason for the upgrade). But what other affects will take place??

What would the difference be assume traction stayed Equal between the stocks and the 235/45's??

THANKS

 

Those are so close in over all diameter the difference in WHP would be less than 1. So it is not significant enough to make a difference.

Yes, traction might be increased and it might not. You may not have enough flex in the side wall with the 235's and may experience wheel hop. But let's say it does help your short time a little, there is increased rolling resistance and maybe a little more weight possibly negating any potential traction advantage.

But in the end I doubt you would see anything significant, plus or minus.

 

Well... with all this information... what size tires do you guys think would provide the best track times??



-- Wouldn't lower tire pressure help to eliminate wheel hop??

 

Without drastic differences you won't see all too much in gains or losses with just tire changes since there is a limited range.

If you want all out track performance then just go with a 3500 - 4000 stall converter with slicks up front and skinnies in the rear and block the rear springs. That could net you 1.8 short times for sure. I just don't know how long the transaxle will last with this.

But seriously, with the stock 17" rims there is a limitation in range limitating potential gains in just swapping tires. Working on your start is the best way to get the best times.

 

 

Everyone told me my 19"s would slow me down, I think it feels the same

 

So am I incorrect or not? You seem to be calling my answer out at first, but then you backpedal and say there are other variables, etc. I stated that there are no "measurable:" differences on the dyno and I am right.

 

Pull_T, when's the ETA on SC installation completion?

 

There are measureable difference based simply on the diameter change. This has been tested before by keeping weight similar and just varying the diameter through rim and tire changes.

Your statements seemed to indicate that a change in diameter would not make a difference in WHP readings. This is where you were incorrect and if I misread your statement I apoligize. However, a change in wheel/tire radius changes the leverage effect and will alter the torque value at the pavement (or rollers).

Backpedaling, hrmm?? I look at it more as clarification as the statements were not well defined.

 

Here's a little more info I found on this subject. It deals with changing gear ratios, not tires, but should still apply.

These questions have been long standing debates among car enthusiast, so I recently emailed DynoJet and asked these exact questions. Below are the responses I received from the DynoJet representative.
QUESTION:
A car having 3:27 rear end gears is placed on a DynoJet chassis dyno and has a dyno performed. Then this same car is taken off the DynoJet chassis dyno and has the rear end gears changed to 4:10. The car is then placed back on the DynoJet chassis dyno and another dyno is performed. Will the second dyno show a loss of horse power caused by the 3:27 to 4:10 gear change?

ANSWER:
Yes

EXPLANATION:

"The 4:10 gear will show less horsepower than the 3:27. The reason is due to rate of acceleration changes. The rate of acceleration is quicker with the 4:10 because of torque multiplication being increased to the rear wheel. The horsepower will show less because the targeted RPM is met before the horsepower has a chance to overcome the rotational mass (dyno, drive line, etc.) or moment of inertia in speed. Because the speed is decreased and the RPM is met faster, the horsepower never has a chance to catch up with itself, so to speak. The overall ratio of 1:1 will always produce the most horsepower on the chassis dyno. Having said this, a similar problem can occur with horsepower loss when the rear gear is too high. The horsepower is being absorbed in just trying to keep the rotational mass spinning. Please keep in mind that your engine's horsepower never changes but what gets to the dyno or drive surface does. If you have any further questions please don't hesitate to ask. Thank you."

 

If the height in tire is more than stock yes, it will affect it, or smaller. The weight as well will affect it, but width does not.

 

Tom, will you email me at gavriil1@yahoo.com with the answer to this question please?

"What did you mean when you said that the auto mode in the SMG II is not as people expected it".

I thihk that is what you said at lease. Clarify please.

 

Since the example was a Mustang, I'll chime in. With my stock 3.27 gears (36 & 11 tooth) compared to my 4.30s (43 & 10 tooth), I can expect to dyno 3-4 horsepower less. I was under the impression that it was due to additional frictional losses in the differential (more "tooth on tooth" action) and not due to the change in the overall ratio in and of itself.