Lose HP with bigger brakes???

[98RedGs]

Thread cleaned of the useless kiddy arguments.

 

[95blackGsTurbo]

Originally posted by igs
You are completely wrong. Stop the madness. If you don't know what you are talking about DON'T POST. Just because you heard it on some ricer TV show does NOT mean it's true. Even if you put on heavier wheels you will not lose that kind of hp. End of discussion. Until you do the calculation yourself stop posting BS.

Dude STFU, I watched them dyno it. They didnt just say they lost it they showed it with and without the rotors, and the rotors actually wiegh less, its just more rotating mass, so obviously you need to stop the madness.

 

[Doug99RS]

I think there's a quick and easy way to reproduce this to prove that less weight with a larger rotating diameter WILL cause a loss in horsepower. Take a fat drill bit (heavy but small rotating diameter) and put it in a drill. Spin it up and just watch the rate in which it rotates. Then take a cutting wheel (lighter but larger rotating diameter) and spin it in the drill. Though it's lighter, it will spin fewer RPMs.

This is a really good thread from a technical aspect. It would be really nice if it could remain civil and informative. If you can't bring up valid points to prove your case and do so in a mature and civil manor... don't respond. The warning in the cream colored area above your post before you submit it DOES say not to post unless you're sure about the answer but it also says not to start or add to a flamewar.

This is good discussion and should stay that way.
Doug

 

[pneumo]

Don't some dynos use the rate of accelleration against a load to calculate HP? What if the dyno load was set so the engine accellerated through the gear very quickly? Wouldn't that skew the results and favor a car with lighter rotating mass?

I agree with the engineers that say loosing 12 HP is too much for the weight difference, so maybe the dyno isn't getting a realistic HP reading. Maybe the rotors are two piece and use lightweight aluminum hats with extra heavy fricion surfaces. Maybe the upgraded wheels have more of their mass located farther from the axle, like typical large diameter rims with low profile tires.

 

[Todd TCE]

That's the reason for asking about TQ readings. HP is only the final result of the equation and not always the most important.

 

[Zenja]

There's a chance that the brakes might be rubbing or something.

I understand how this works in respect to the rotating mass change. I've actually thought about this a few days ago in a different context. The radius makes a big difference. However, considering the likely size difference between the rotors, I can't fully believe that it could result in a 12hp change. On a car with lots of hp this would be more like 30+hp loss. 12hp is actually a huge amount of power if you think about it. The change of characteristics of the rotating mass here just doesn't seem to translate into a loss of 12hp.

 

[Hercules]

I dyno'd on a dynojet with 16" then heavier 17" wheels. The dyno showed basically no loss of hp or torque but....the run took a full 2 seconds longer to complete. Note: both wheel and tire sizes were of the same diameter.

 

[Doug99RS]

I talked about this with a few other people at work and they mentioned the brake drag as well. Really couldn't explain the loss any other way. I dunno, I'm lacking in a degree in engineering.

 

[PaulPDX]

Same car with same mods running on the same dyno can produce different results even if the runs were done one after the other. Dyno figures are not absolute and should only be taken as a point of reference. Larger brakes can certain account for some, if not all of the power loss...

Considering a very simplified scenario... say you have a 10lb weight tied to a 10" long string at one end and you are twirling the whole thing by the other end of the string, the moment of inertia is 1000 lb-in^2.

Increase the string length to 13" and the moment of inertial increases to 1690 lb-in^2.

That's a 69% increase with just 3" of increase in the string length.

In that light, I don't think a 12 hp loss (+ or - 4 hp variance in dyno readings) is completely inconcievable.

For those who think it's impossible and have flat out proclaimed the believers to be wrong... you better provide some evidance to back up your opinion... because anyone can say "you are wrong" and it's far too easy, convenient, and often incorrect to assume opinion to be fact.

 

[ckozlgod]

I don't think it's "impossible"...just not likely

 

[igs]

Originally posted by PaulPDX

Considering a very simplified scenario... say you have a 10lb weight tied to a 10" long string at one end and you are twirling the whole thing by the other end of the string, the moment of inertia is 1000 lb-in^2.

Increase the string length to 13" and the moment of inertial increases to 1690 lb-in^2.

That's a 69% increase with just 3" of increase in the string length.

In that light, I don't think a 12 hp loss (+ or - 4 hp variance in dyno readings) is completely inconcievable.

That is if 100% of the mass is located at the very end of the string. Rotors are not like that.

 

[PaulPDX]

Originally posted by igs
That is if 100% of the mass is located at the very end of the string. Rotors are not like that.

No shit. I did say it's a very simplified example did I not? or was that statement too complicated for you to understand?

To do the actual calculation you'd have to use calculus and it's been more than 10 years since I took college level calculus physics courses and I am not about to go digging around for information just for your benefit. One look at the simple I=mr^2 shows that it is possible, taking dyno result variance into account.

Plus, in order to make the calculations you'd have to know the exact dimension of the cross section of the rotor and the density of the exact alloy used to make the rotor.

If you are so convinced everyone else is wrong... why don't you offer some concrete evidence to back up your mere opinion? or can you?

Dyno figures can be off as much as 4 HP per run. So the 12 HP loss could potentially be 8 HP in reality. That's 4 HP per side... not entirely impossible nor improbable.