flatso

Was watching Sports Car Revolution and after they put a catback exhaust on their RSX tuner car they put it on the dyno and found that they actually had a decrease(12 hp) and did not understand why. Came to find out that the larger brake rotors because the inertia is further from the center of the wheel caused this loss of power. Thought that was interesting because who would think your brake rotors could cause that kind of HP loss.

lafrad

I don't think I would give my stopping power up for anything though... my '8 stops pretty damn good!

epitrochoid

i knew you'd loose a little, but damn, 12hp?!

Lawerence

that show is pure crap IMO. Like most of the other shows for the 'tuner scene'

rev-2-9k

hmm

lafrad

Huh? the brake rotors are mounted to the wheel lugs, so I really hope they "rotate" with the tires......

RX-8 friend

It's not a HP loss. It's extra rotational inertia which slows down the rise in wheel speed on a Dyno Jet. On a "real" dyno, where you load down the drum until it stalls the rise in speed it wouldn't show. This is one of the main reasons many don't like the Dyno Jet or their kind of "dyno". It assumes too much.

Same idea as using a G-Tech. If you don't enter the correct weight of the car and contents you will get erroneous HP readings.

MrWigggles

Yes, brake rotors rotate.

It would have to be extremely heavy rotors for it to cause the car to lose horsepower on the order of 12HP.

I've worked the math before but it would come out to something NO MORE THAN .1 HP per pound. So for 12HP that would be 120 pounds heavier brake rotors than stock set-up or >30 lbs heavier rotor per wheel

Something is wrong with their set-up because with only 2 wheels turning that would mean the extra 120lbs would be on just the two wheels or >60lbs extra per rotor.

I don't plan to upgrade my rotors, but it won't be because of this dyno test.

-Mr. Wigggles

MrWigggles

But it can't be 12HP anyway. That is way too much.

As far as the G-tech is concerned, I think they just have their algorithm screwed up for HP calculation. However, the concept of "in-car dynos" is perfectly valid. You are directly measureing the the amount of power the car is putting to the road which is the most important thing. High reving cars like the RX-8 "lose" more energy inertial losses due to higher gear ratios.

You have to extrapolate the engine HP. With the G-timer which appears to have better software and calculations, I have been able to get fairly close to the 238HP number provided by Mazda.

-Mr. Wigggles

Superfan

Maybe the catback they added has something to do with the missing 12hp..

wakeech

uh, you're an idiot... at least watch a show before you bash it.

no, it's not a "tuner" show, it's a show actually all about sports cars of every type (GN's, Miatas, RSX's, Vipers, everything), and the guys that do it are really quite knowledgable, and know how to make interesting and informative television.

yeah, the brake rotors WERE taking 12 hp away from power-at-the-wheels on their dyno. it had to do both with the increased mass of the rotors (nearly double) and the increased size (up something like 4" in diameter at the front).

flatso

yes and weight was not a factor as the new rotors weighed less then stock ones

epitrochoid

greater the radius greater the rotational inertia

rodmeister

Hmmm. I presume a lighter disc material would lessen the HP loss. What would be the ideal material for a disc and would it be worthwhile to replace the stock discs?

speedsector

i'm guessing that this totally irrelevent to us 8 owners if we were to upgrade the front rotors since we have rwd. the rsx on the other hand will probably suffer a little bit because it's a fwd. it's not that the car is losing hp, it just takes more hp to spin the wheel. same goes for upgrading to a heavier and larger wheel. just my .02

wakeech

it reduce the increased amount of energy that's required to turn the larger disk.

ideal material for a brake disc varies on application, but usually a cast steel disc deals very well with the heat, is cheap, and gives great pedal feedback. i don't know how ceramic discs perform differently, but i do know that the carbon fibre discs used in F1 (which is kinda making a disc out of pad material) only work well up around 500* C, don't last long at all (just like any other aspect of a GP machine), and give very little pedal feedback (extremely difficult to modulate and utilise effectively).

it's true that on a dyno a larger and heavier brake disc, but that doesn't mean that in the real world (actual use of the car) that larger brake discs and heavier wheels on any corner of the car, whether being turned directly under power or not, are not going to be detrimental to the performance of the car. having a greater amount of unsprung (and rotational) mass on the vehicle undermines its perforamnce whether it's on the front wheels with rear wheel drive, or on the rear wheels with front wheel drive.

it isn't that the bigger brakes and heavier wheels reduce your horse power, but simply take a lot more energy to turn as fast as something lighter and smaller, just like having a heavier car: it all needs to get moved by the motor in the end.

Broke_Apex_Seal

sounds fishy

flatso

So why was the dyno showing a lower HP then with the larger rotors? Thats like saying the stock exhaust does not decrease HP it's just that the catback is more efficient in bringing out the HP that is there but however you word it HP is decreased with the stock exhaust system and increased with the catback. Basically what is happening is decreased efficiency and HP is being converted into heat instead of energy.

Does a heavier car do poorer on a dyno(I'm asking I don't know).

Floyd

A dyno will only be affected by weight in the drive train. So, anything attached to the crank(brake discs, rims, fly wheel, etc) will have an effect on the dyno #'s you put out. Any weight reductions outside that realm will not give you higher/lower dyno numbers but will effect your real street performance.

Thats my .02

RX-8 friend

For an example:
Take your car (with roll cage and proper harness for the occupants for a top speed test. Say you reached 145 MPH. That is limited by HP. HP is fighting wind resistance.

Now change the wheels to much heavier ones (same overall diameter) or put on much heavier brake disks. On the Dyno Jet (or any other dyno that uses accelleration rate to calculate HP) you will loose HP. If you then take that combination for a speed test you should get the same top speed, because you haven't lost any HP, you just can't accellerate as quickly.

A true dynomometer loads the car to a constant speed and measures torque. It calculates HP as torque times rotation rate. This ensures there are no other factors affecting the accuracy of the reading. This type of measurement is about 30-50% more expensive to do, so the Dynojet and other accelleration type dynos are becoming more popular.

The dynojet and other accelleration type dynos are affected by rotational inertia. They can also loose accuracy due to the change in tire friction vs tire speed and change in torque output which can overpower tire friction on the drum.

Lawerence

Ive seen the show my friend.

I like how the first thing you say is a personal insult.

Im an idiot because i dont like the show
Notice i said IMO. Thats because its my ******* opinion.

Your opinion is that its a good show, mine is that its not. Its not nearly technical enough and its dumbed down for the mainstream (which might be you) IMO. (oh gee is that this opinion thing again?)

Im sorry i get my info from ppl who dont dumb thier stuff down for a national TV audience.

MrWigggles

Okay,

For a rotating mass not experiencing translational movement, the kinetic energy of such object is:

Kr = 1/2 * Iw^2

where I is the moment of inertia and w is the angular velocity in radians per second. For a 15 inch (.381 meter) BIG rotor that has its mass distributed at outer rim of the rotor (excessive worstcase), it has an I of:

I = mr^2 = .036m

and has a angualar velocity of:

w = 2pi * v / (2pi * r) = 3.03v

where r here is the radius of the 26" diameter tire. Substituting back in to the earlier equation we have:

Kr= 1/2 *.036m * (3.03v)^2 = .331 mv^2

So if the dyno was say done at 0-100 KPH (0 - 27.8 m/s) and it takes 5 seconds and the rate of acceleration is fairly uniform (i.e. flat torque curve) and we are using rotors 10Kg heavier (i.e. 22lbs heavier!) than stock. Let's look at that final second of the dyno run.

At t = 4, v =22.4 so Kr = 1637 additional Joules per wheel

At t = 5, v=27.8 so Kr = 2588 additional Joules per wheel

So in that final second 950 joules (or 950 Watts = 950J/1s) are being stored in each wheel so that it would be about 2.5HP since only two wheels are spinning.

So in this ridiculous example where I am using big rotors that weigh 22lbs more than stock each with all the weight of the additional rotors is on the outer rim of the rotor, and the car is doing normal accleration on the dyno (i.e. the dyno is not to slowing it down to a near crawl), we get only a 2.5HP number.

Actual numbers I would guess to be less than .5HP for an actual performance rotor upgrade.

12HP is a joke - something else is wrong with their test.

-Mr. Wigggles

Broke_Apex_Seal

E=mc square

NAVILESRX8

iTrader: (1)

haha I am not a fan of any of those "tuner" shows....none of them....even Car and Driver TV is dumb. I love Car and Driver magazine, but somehow the TV show just ain't the same.....

wakeech

as Floyd was inferring it's because of the way that the dyno measures power: it measures the rate with which a known mass (the drum) is accelerated at what speeds and from this can calculate how much force was being applied to the drum at what speeds. then you calculate the speed of then engine and can calculate power. because of the increased energy needed to accelerate the wheels the drum would be accelerated at a slower rate the dyno registers less force acting on the drum and thus less horsepower. so it isn't that a heavier car will register lower on the dyno, but it will accelerate slower in the same manner as a car with larger and heavier wheels, brakes registers lower power on a dyno (just accelerates slower, even with the same amount of power).

Mr.Wigggles might be correct that there could be other discrepencies between the car's performance on the dyno which accounted for some of the registered power loss, but the brakes are making a difference.