I was watching a tuner episode today on speed channel where they were modding a RSX - after adding different rims (same weight as stock) , bigger brakes BBK (lighter than stock), and upgraded exhaust (ruled out by returning back to stock exhaust -12hp loss) - they lost power at the wheel on a dyno.
After their analysis - they stated that the car lost WHP due to the larger diameter brake rotors even though they were lighter.

So if you lose the BBKs - you should get larger numbers?

 

yes if your dealing with a Dyno.

The problem is Rotating mass. Which also hold true for rim and tire selection. For Dyno runs IF your tring comparitive measurements of engine mods and not Chassis tuning. then Things like flywheel, clutch , spindles, axles, brakes and wheels should all be the same each time you run (also light wieght crank pulley if the same size). By changing the size or wieght of any of these will improve or Lessen the WHP on a Dyno but no loss at the crank. This happens by increasing to decreasing the effenceny on which the drive train xfers the power to the ground

 

you could easily estimate if this is true. the brakes are probably close enough in topology to just compare them using the uniform disk equation for rotational inertia: 1/2 X mass X (radius)^2. do you have the weights and diameters?

 

Okay, thanks I remember seeing this show a while back and thinking WTF are these guys talking about? Brakes don't rotate! It has just now dawned on me that, as when I think brakes, I think calipers, what we're really talking about is the difference in rotor mass. 12HP seems like a heck of a lot of loss for just two rotors! Are the Acura rotors that flimsy? Hadn't thought much about this before, but this would seem to be a significant inherent disadvantage of FWD, as with RWD, if you upgrade the fronts, you would never see this effect.

 

I watched that show and was unconvinced that all of their losses could be attributed to the BBK. They made several mods, got results worse than they had hoped, and they chalked it up to the BBK. It would have been simple enough to swap the stock brakes back on to see if their theory held merit.

I have no doubt that a BBK that has greater mass than stock and has that mass concentrated further from the center of the hub than stock, will accelerate worse than stock. However, that effect would be most noticeable in lower gears and I doubt would show a 12 whp difference.

 

hi andy,

Yeah - I agree there are too many variables in the mix to pin it down. Though I thought it would be interesting if someone could do a comparison test on a car - prior to a BBK upgrade on a dyno to bust this myth.

 

The guy on the show said about 5 wheel HP loss attributed to the new wheel & brake combo.

 

Couple things going on...

1) The wheels were the "same weight", but they were a larger diameter. Even when overall diameter stays the same, plus-sizing a wheel tends to move the weight further to the outside of the diameter.

2) Yes, increasing rotor size will hurt acceleration. Again, the overall "kit" may have weighed less, but I'll guarantee the rotors weighed more. That's part of what makes them more effective (for heat dispersion).

3) Highly unscientific results. Luckily for them, their laptimes improved regardless. Else, I bet they'd have been re-filming to keep from pissing off the advertisers even more.

 

I have gone back and forth with wheels on the dyno...just to see if an 18 vs a 16 with smaller tires would make a difference in WHP...

what it came down to was that sure the smaller wheels and tires changed the gearing...which helped to accelerate faster..but the WHP was not really different...we are talking 1 or 2 WHP...and anyone who has spent time on a dyno knows you can get that every run with out changing anything...due to heat and other variables...so...uhhh yeah...I raise the BS flag on the HP diff...only an acceleration variable...so make more HP!!! then it won't matter