Increased whp by having lighter wheels

[mechaghost]

So I installed some 16.4 lbs wheels on the BRZ (see thread: http://www.ft86club.com/forums/showthread.php?t=10244) and dynoed it during virtual_bob's dyno meet in Auburn, WA.

Also note that I had a drop in k&n filter (which I doubt contributed to anything based on other threads about air filters)

This graph is not SAE corrected, but bob showed me a SAE corrected chart during the run and saw 170whp instead of the 167 we see here.

I don't have any baseline runs on my brz without the mods so I don't know exactly how much it gained, but Tainen's BRZ baselined at 159 whp, so I'd say you can gain ~2.5whp per lbs lost on the wheel.

[Mitch]

How much did total diameter change of the wheel+tire combo change with the change of wheels? That factors into the overall effective gear ratio.
If there's a way for you to get the torque curve vs road speed, you can see if the curves are offset, and if so by how much.
How many runs on each configuration? How much time to cool down between runs? Lots of factors here.

[mechaghost]

The new tire+wheel combo should have not changed, maybe a little since I'm using 215 on a 7.5 rim? I don't have access to the other information other than this graph. There were only 3 runs on this configuration, like I said there were no baselines done so my baseline values are based off other BRZs/FRS.

It was a free dyno day, so yeah

[Dimman]

It's pretty much accepted that lighter wheels reduce drivetrain losses. Rotating mass and inertia thing.

[rice_classic]

Lighter rotational mass=higher dyno numbers. This is common knowledge.

[ImAwesome]

^^these. And if you want to really get into detail Think overall rotating diameter and mass. If a fly wheel is 10 inches across it has less rotating inertia then a wheel that is 26 inches tall.

What I'm trying to say is if you take 2 pounds off a flywheel due to ratting mass its like removing 10 pounds. If you get tires that are 2lbs lighter, due to the larger rotating area and power needed to turn them its like removing 26 lbs for every 2 lbs.

I didn't actually do any math here. This all fact but my numbers are theoretical

also from I've seen, the fact that you have an AT and dyno'd that high is impressive

[fatoni]

Originally Posted by ImAwesome

^^these. And if you want to really get into detail Think overall rotating diameter and mass. If a fly wheel is 10 inches across it has less rotating inertia then a wheel that is 26 inches tall.

What I'm trying to say is if you take 2 pounds off a flywheel due to ratting mass its like removing 10 pounds. If you get tires that are 2lbs lighter, due to the larger rotating area and power needed to turn them its like removing 26 lbs for every 2 lbs.

I didn't actually do any math here. This all fact but my numbers are theoretical

also from I've seen, the fact that you have an AT and dyno'd that high is impressive

just to be fair, you probably should consider the fact that between the flywheel and the wheels there are a bunch of gears changing the relations between the two. in first, second, and third the flywheel is going to be way more important than the wheel weight and that is probably more important to most enthusiasts

[xwd]

Different days, different cars, etc. That difference is probably within standard deviation, so it might as well be zero.

If you had swapped wheels around during your dyno run, the results would be usable.

[mechaghost]

Yeah I wish I had the opportunity to do so but it was a free dyno run and I'm just going with data that we have

[Captain Snooze]

Originally Posted by rice_classic

Lighter rotational mass=higher dyno numbers. This is common knowledge.

"Common knowledge" is not so common. It relative to cultural and specific interest groups.

Dynamometers measure torque and calculate power. I don't see how lighter wheels will increase power. Lighter wheels will decrease inertia giving higher acceleration but not more power.

[Dimman]

Originally Posted by Captain Snooze

"Common knowledge" is not so common. It relative to cultural and specific interest groups.

Dynamometers measure torque and calculate power. I don't see how lighter wheels will increase power. Lighter wheels will decrease inertia giving higher acceleration but not more power.

They decrease losses to the the dyno. You do know that a Dynojet makes its calculation by how quickly the car accelerates a known mass, right?

[civicdrivr]

While I do agree that lighter rotating mass can contribute to higher whp numbers, Im not going to agree that the number is 10whp. Without a baseline of your car, theres just no way to say what the gain truly is.

Either way, the numbers are pretty good. How many miles do you have?

[rice_classic]

Originally Posted by Captain Snooze

"Common knowledge" is not so common. It relative to cultural and specific interest groups.

Dynamometers measure torque and calculate power. I don't see how lighter wheels will increase power. Lighter wheels will decrease inertia giving higher acceleration but not more power.

Correct and you also proved why I selected the wording that I did. Notice how I did not say lighter wheels equal more power?

The words we choose are important.

[Captain Snooze]

Originally Posted by Dimman

You do know that a Dynojet makes its calculation by how quickly the car accelerates a known mass, right?

Ahhhh....nope.
But I still don't understand why this should result in an increase in power.
F/m=a. Decreasing the mass (or inertia) increases the acceleration. Understood but you are not increasing the power. The engne output has not been changed.