Big Rotors = less horsepower at the wheels Options

[anthony]

I've been watching Sports Car Revolution on Speed lately. Of course most of the cars they feature are Japanese but I really like that they put everything through a dyno. The balance bling bling and performance mods pretty well.

Anyway, they are doing an Acura RSX project right now. They fitted the car with a big brake kit - you know, big rotors and big calipers. Their times around the track didn't really improve even though they were faster around the corners because they could brake later.

After dyno testing they came to the conclusion that even though the big brake kit was lighter than the stock brakes it resulted in a 7 hp loss at the wheels because of the increased rotating weight of the big rotors.

I thought it was a pretty interesting result.

[phantom914]

I don't think that a dyno, which measures horsepower at a fixed speed would show a change in horsepower with a change in wheel, rotor or flywheel weight. These things should only show up under acceleration, like on a track, in which case it could be said that the extra rotating weight is equivalent to losing x amount of horsepower. I think something else was wrong on their dyno run or other mods went awry, and that the difference in power measured on the dyno had nothing to do with the rotors.

Andrew

[machina]

I am not so sure. The rotors were much larger even thought they were a bit lighter overall. The extra diameter has a huge effect on rotating mass.

I never understand why so many cars have brakes that are clearly oversized for the weight/power of the car. Big rotors and calipers are heavy.

Best thing to do is learn how to slow down the car and how to carry the most speed out of a corner. Most people with big fast cars slow the car way to fast and way to soon at the end of the straight and then wonder why the brakes are fading.

dr

[Mueller]

hese things should only show up under acceleration,

you answered the question with that statement (IMG:style_emoticons/default/smile.gif)

accelerating a known weight.....

most of the cost-effective dynos are based on turning a large drum of a known weight and diameter.

less weight means faster acceleration...you must have a baseline first, either with more weight or less weight to compare the before and after

[phantom914]

Mueller,

Are you saying that these dyno's are taking measurements under acceleration? Or what was the question that you said I answered with that statement?

Andrew

[Mueller]

yes.....in fact with these type of dyno's. one could "cheat" by running lets say a 26" diameter tire and then by changing nothing else, run a 25" tire...you'll "gain" hp (IMG:style_emoticons/default/smile.gif)

[anthony]

Like I said, they put all their mods through a dyno on this show. They definitely measured the -7hp at the wheels. They were even stumped by the result at first because they knew the new brake package overall was lighter. Another expert they consulted suggested that it was the bigger rotors.

And then they did a tuned intake and cat-back exhaust and got their 7hp back.

[phantom914]

yes.....in fact with these type of dyno's. one could "cheat" by running lets say a 26" diameter tire and then by changing nothing else, run a 25" tire...you'll "gain" hp (IMG:style_emoticons/default/smile.gif)

Ahh, I see. With this type of dyno then, you generally wouldn't be able to convert wheel horsepower to crank horsepower in a straightforward manner.

Andrew

[phantom914]

OK. I guess this also explains how some manufacturers claim horsepower gains from lighter flywheels and pulleys (other than the "underdrive effect"). They really mean at the wheels under acceleration, not at the crank at static engine speeds. Pretty clever way to lie I guess.

Andrew

[anthony]

It doesn't seem like a lie though.

Aren't you still gaining usable horsepower if you can decrease the hp loss between the engine and the wheels?

[SirAndy]

yes.....in fact with these type of dyno's. one could "cheat" by running lets say a 26" diameter tire and then by changing nothing else, run a 25" tire...you'll "gain" hp (IMG:style_emoticons/default/smile.gif)

i don't consider that a cheat. you'll effectivley increase the RWHP if you go to a smaller diameter wheel/tires.

nothing "cheat" about that ...
Andy

[tat2dphreak]

IMHO: the "at the wheels dyno" is best used when you know the real hp/tourque of the engine, then you measure the hp at the wheel, to measure how efficient you are getting that hp to the wheels...

if you measure 100 hp at the flywheel, and then you measure the same engine in a car and it gets 90 hp at the wheels, that's a 90% efficiency...


of course if you look at it 1 way, the only hp measurement that counts(if any of them count for more than bragging rights) would be what you are putting through the wheels...

just to piss people off, I like posting this link... Horsepower-v-torque.htm

[phantom914]

It doesn't seem like a lie though.

Aren't you still gaining usable horsepower if you can decrease the hp loss between the engine and the wheels?

OK. Lie may be strong. Maybe I felt misled because I was thinking in terms of engine horsepower, rather than "road horsepower". The claims made are just not directly comparable to engine horesepower. Maybe they could be more clear and say their product "leaves more horsepower available at the wheel". Or am I just being too picky like an engineer ? (IMG:style_emoticons/default/smile.gif) It would be like measuring better acceleration times after installing lightweight chassis components. You wouldn't say you gained horsepower.

Sorry for being ridiculous. (IMG:style_emoticons/default/smile.gif)

I hadn't heard of this type of dyno before. Apparently, this type of dyno isn't meant for absolute horsepower measurement, but for comparing mods to a baseline.


Andrew

[Dave_Darling]

Actually, this type of dyno is meant to cost one heck of a lot less to buy and operate than the "fixed load" type. And it's much better at doing before/after type measurements (before and after a modification) than giving absolute numbers. But then again, with all the correction factors that have to be added to most dyno runs, you very rarely get comparable numbers to other dyno runs on other machines on other dates anyway...

--DD