I saw on Speed the other day that putting on a big brake kit will actually lower your hp and was recorded to do so on their dyno. They had installed an exhaust that was shown to give 5hp on a RSX and when they went the dyno it showed a loss of hp. They made some calls and it was contributed to the big brake kit. Although the kit was actually less weight than stock it created more inertia and thus lowered hp. Interesting yes? They said that they wont lose anything on the track because of the excellent stopping power they will be able to brake later entering a turn.

 

I watched that show as well.
They didn't really explain it well: here goes...
the dynojet basically calculates hp by measuring the effort it takes to accelerate the rolls between two speeds. Anything that increases the moment of inertia of the wheels (like larger dia brake rotors) will add to the effort and appear to be a hp reduction. In fact, it is only a reduction with respect to the way they measure the hp...there is no change in actual hp.
The effect on the acceleration of the car on the road would probably be similar to the dyno result, however, as you need to speed up a wheel with more rotational inertia.
What their results imply is that by going to lighter wheels, you would gain hp. The change in brake rotors produced 5hp less. If that is accurate, how much hp would many of us be showing by going to 20 lb per wheel lighter wheel/tire combos?

My BS meter is illuminated, although i find the topic interesting.

 

They are correct. It's not a new phenomena by any stretch of the imagination. You can experiment yourself if you'd like. Just take a small 2.5lb weight or whatever you have lying around and tie it to varying lengths of string. I can garuntee you that the longer the string, the more force is required to move it at the same rpm.

Essentially you are moving the same mass at different speeds due to rpm staying constant while varying diameter. In a brake setup, flywheel setup, or what-have-you it will really depend on where the majority of the mass is located in relation to the axis.

 

So this inertial effect should translate to the entire drivetrain.

I'm wondering how many hp I picked up with the lighter flywheel and clutch?

And I am wondering how many you (jlm) lost after putting the OEM one back on before the last dyno run?

 

I know how moment of inertia works, but i am not seeing huge dyno hp gains reported (or any,actually) by those who have changed way more rotating mass than a brake rotor. My lightweight flywheel was a 15 lb loss of rim weight; both drive wheels lost 15lbs each. Using the 5hp for brake rotor example, I should have gained 15-25 dyno hp at least. Uh, uh.

 

The moment-of-inertia only has an effect when the car is accelerating.

If you are travelling a constant speed, it makes no difference.

So, I agree that the dynamometer is extra sensitive to rotating mass.

In a "normal" acceleration, the horsepower is accelertating both the
rotating mass and the MUCH MUCH larger non rotating mass, so the
effect of the moment of inertia gets lost in the much larger effect of
accelerating the entire car.

I can do some math with examples if someone can post the dimentions
and weights of a pair of brake rotors. If you know the moment of inertia numbers,
I can post back immediately, otherwise, I'll have to draw and guess a bit
to get approximations of them.

 

The effect of changes in mass and mass distribution on inertia dyno results is seen most in 1st gear. As higher gears are used, the rate of acceleration decreases. So, a lightweight flywheel may show 5 whp in 1st gear, 3 whp in 2nd gear, 2 whp in 3rd gear, 1 whp in 4th gear. This is one of those cases where the butt dyno is more appreciative than the real dyno since we use 1st, 2nd, 3rd gear all the time.

Here's an interesting article showing the effect of light-vs-heavy wheels on dyno results:

http://www.nissanperformancemag.com/may01/dyno.shtml

 

trippy:
the dynojet works by measuring the effort toaccelerate the rolls, hence the suggested effect of changing the moment of inertia causing chages to the hp reading.

the question still remains about lightening the wheels, for example, and no one reporting different dyno numbers.

 

>>I know how moment of inertia works, but i am not seeing huge dyno hp gains reported (or any,actually) by those who have changed way more rotating mass than a brake rotor. My lightweight flywheel was a 15 lb loss of rim weight; both drive wheels lost 15lbs each. Using the 5hp for brake rotor example, I should have gained 15-25 dyno hp at least. Uh, uh.<<

Yea, that's where I was going with this. Maybe you had those gains included?

I wondered why Eric used 4th gear and now I know.

Some guy on Mini2 in England had his car dyno'd in 5th gear. I wondered affect the overdrive ratio would have.