16" to 19" Wheels Equal Loss of HP

Myxalplyx · #1 07-12-2007, 09:45 PM

Got a free dyno run at Verocious Motorsports this past weekend. Got to dyno 2nd from a raffle drawing and I had the SVX there.

I had a heavy 19" wheel/tire combo on. This was the same dyno but moved to a different location. At any rate, I got a paper hand out with the two dyno runs on it (that I can not find right now). Hp was around 165hp and torque was around 155lb-ft. So my hp was 10hp less and 20ft-lbs torque less than my previous run with the 16" RX7 wheel/tire combo.

If you didn't know and you don't want to be slow, Lower rotational mass so you can be fast. Just a quick heads up for those that want to be blinging. The 19's will stay on but will come off only when dyno and track testing. Too lazy to look for the paper now but when I find it, I'll scan it and post up. L8Rs..

svxpert · #2 07-12-2007, 11:02 PM

I think it had more to do with the weight of the wheel/tire combo than the actual sizes. (even though they usually go hand in hand.)

SSSVX · #3 07-13-2007, 12:30 AM

What is your weight ratio to your old 16" and newer 19"?

Trevor · #4 07-13-2007, 03:21 AM

The weight of the wheels has no affect on a reading in respect of HP at the wheels. Acceleration is the only factor which will change.
Any variation in rolling diameter will have a profound affect relative to HP measured/delivered at a given engine RPM, due to the change in overall gear ratio.

All that said, wheel dyno measurements taken at different times/place, by different operators, etc, etc, can not be relied on.

Landshark · #5 07-13-2007, 06:54 AM

Quote:

Originally Posted by Trevor

The weight of the wheels has no affect on a reading in respect of HP at the wheels. Acceleration is the only factor which will change.
Any variation in rolling diameter will have a profound affect relative to HP measured/delivered at a given engine RPM, due to the change in overall gear ratio.

All that said, wheel dyno measurements taken at different times/place, by different operators, etc, etc, can not be relied on.

i disagree - i added lightweight 10" wheels and gained 50hp.

Myxalplyx · #6 07-13-2007, 03:10 PM

Quote:

Originally Posted by SSSVX

What is your weight ratio to your old 16" and newer 19"?

"Wheel tire combo weighed in at 50lbs on a bathroom scale (vs 37lbs for the 16" RX7 wheel/tire combo)." A 13lb difference.

I posted about it in this thread-->
http://www.subaru-svx.net/forum/showthread.php?t=31601

Trevor · #7 07-13-2007, 06:27 PM

Quote:

Originally Posted by Landshark

i disagree - i added lightweight 10" wheels and gained 50hp.

Where did you fit the "added" wheels.

LetItSnow · #8 07-14-2007, 11:33 AM

Measured static, a lighter, larger diameter wheel can be harder to accelerate than a heavier, smaller diameter wheel. It's all about MOMENT OF INERTIA.

Torque is simply changed via leverage - in our case, gear ratios. In our case, if the tires are identical in diameter, the dyno should not notice the difference. Consider: at the crank, we get X torque. After the tranny, we get Y. Get to the axle, it's Z. Factor in the wheel diameter, and that's your result.

Changes in gearing become cancelled by engine speed; this is why second gear and fourth gear (say) don't actually result in different torque readings.

So, your torque is the same with any tires with the same diameter. This defies your claim, so let's prove it. If a car is sitting on a pair of 2700 lb. drums, and is responsible for accelerating (among other things) a set of tires weighing 50 lbs or 37 lbs, you can do a little bit of math to see how irrelevant the tires' weight is.

INCORRECT MATH:
(2700x2) + (50x4) = 5600 lbs
(2700x2) + (37x4) = 5548 lbs
Doing it wrong, you're looking at less than 1% difference in weight. But this situation isn't static - it's rotational! Moment of inertia must be considered.

I have no idea where the moments of inertia are measured on either set of tires, and certainly not the pair of 2700 lb drums, but I can guarantee that the drums have a tremendous increase in their influence in our equations after the moments of inertia are factored in, making 54 lbs worth of wheels about as significant as a moth fart in a tsumani.

Now, in a car's linear acceleration (i.e. quarter mile), there will be a notable change... but your torque is still the same.

Landshark · #9 07-14-2007, 11:41 AM

<Mr. Obvious>
to keep it simple, power isn't the only thing to affect speed/acceleration. weight, aerodynamics, etc. come into play.

my lightweight pulley adds no power, but it gets the turbo spooled up quicker, and that gets you moving faster, but it adds no additional power. same with adding or reducing weight.
</Mr. O>