GP-1

Actually, there will be less torque to the rear wheels due to the parasitic loss of the transmission.. and the torque made by the motor (horsepower is just a mathmatical equasion) is all there is.. changing your gearing won't change it..

RMC

That's true, but the question relates to rwhp dyno #'s.

I'm curious how exactly does a dyno extract HP and TQ numbers from what it sees at the rear wheels. What basic calculations does it do to get a baseline value? Then from there what correction calculations does it use?

Does anyone know the actual math behind a dyno?

From that point it should then lead into discussing how the rear gearing will alter the output and how it is accounted for by the dyno or dyno operator.

GP-1

I'm not an expert, but here's what I know...

Dyno's measure torque.. Horsepower is an equasion computed from torque (HP=TQxRPM/5250.. this is why the torque and horsepower curves always cross at 5250).. Transmission is just like the gears on your 10-speed.. just because you're in a lower gear doesn't mean your legs are making more torque..

Highlander

AS i said before.. rear gearing WILL NOT affect the dyno output rwhp... the math behind it is the time it takes to accelerate from x to y of whatever the weight of the dyno is.

your legs might not be making more torque but torque is being multiplied by the gearing to the rear wheels.. thus the dyno has to account for that... if not.. I put on a 4.11 from a 342 and gain 30% more torque and HP?

In reality and the street you do gain more toruqe but the real output on the dyno will be the same... The dyno is there to show you the power of your engine not the multiplied torque to move your car.

You need more torque to get faster to the finish line.. Hence why people change rear diffs to earn those few extra tenths and more mph in the 1/4

RMC

He's talking about rw dyno's right? What I'm not sure about is this..

The dyno is giving rear wheel torque numbers right? Not engine torque numbers. So why should it account for changes in anything between the engine and the rear wheels. It's not an engine dyno.

If you change the rear gears you are affecting the acceleration which the dyno determines it's values from.

The dyno only knows the speed of the drum and the acceleration of it and the time at which the values occured. I think it would also be programmed in with the mass or weight of the drum.

With those you can find Torque.

Torque = moment of inertia times angular acceleration.
That would be 1/2 mass of the drum(if it's uniform solid) times the radius squared times it's angular acceleration

Take two identical cars but change the rear gear..one will accelerate laterally faster from one point to another over a limited length of time where the one car is geared to utilize engine output more efficiently.

Would this translate into the dyno wheel accelerating slower or quicker than the car without the efficient gearing over that relatively short period of time. Without correction would one be different?

So what's the real deal? Anyone actually know for certain what calculations a RW dyno uses and whether or not it corrects for gearing?

This is a very interesting thread!!

edit: I'm not certain exactly why a car geared differently accelerates faster through the 1/4 when it requires more turns of the driveshaft per wheel rotation.

brain

The way the dyno works depends on the brand of dyno. Dynojet is an inertia dyno. The other type is load based, I believe they are called brake dynos, with eddy current and water brakes as different types. They can actually apply a load to the engine and it will measure torque and calculate horsepower, similar to an engine dyno. The dynojet gets horsepower from its measurements of thrust force, since it times the acceleration of a known mass, i.e. the drums. It calculates horsepower from work, and then calculates torque (only if it has a tach reading) from horsepower. There is some Dynojet company magic in there to compensate for rear gears, but apparently it is not entirely accurate. Work is Mass times acceleration times distance. Distance is the circumfrence of the drum, mass is the weight of the drum, and acceleration is increasing wheel speed. So with shorter gears, acceleration is faster. There is 550 lb ft of work done in 1 second, so HP is Work divided by 550 times the time it takes. So if it takes less time to accelerate the car to the rev limit and it accelerates faster, I would figure HP would go UP, not down. It appears to be a general consensus that HP seen on a dynojet goes down on the dyno by using shorter gears. Anyone that knows any more, or need to correct anything, please do! Its been a long time since I've done any calculating and my math could be all wrong, but thats how I remember it.

Highlander

The point is that most of the time you are connected with an inductive pick up to a coil wire or something and thus have the correction there...

imagine you do it in first... the car revs too fast too quickly...

NewbieWar

well i did a graph on my computer to show engine power points on a stock lt1...
with an average stock lt1 dyno graph
i multiplied the horsepower & torque at each gear ratio at coresponding mph

so like here is first gear per rpm (everything is somewhat approximate and assuming a 60" tire (usually about 65 or so but im not sure)
Rpms_torque_hp _3.06_Final 1rs gear power 2.73
1250_285_ 67.83_ 207.56_566.64
1500_290_ 82.83_ 253.46_691.95
1750_290_ 96.63_ 295.69_807.23
2000_300_114.24_349.57_954.33
2250_310_132.81_406.4 _ 1109.47
2500_310_147.56_451.53_1232.68
2750_310_162.32_496.7 _ 1355.99
3000_310_177.08_541.86_1479.28
3250_310_191.83_587 _ _ 1602.51
3500_310_206.59_632.17_1725.82
3750_310_221.34_677.3 _ 1849.03
4000_310_236.1_722.47 _ 1972.34
4250_310_250.86_767.63_2095.63
4500_310_265.61_812.77_2218.86
4750_300_271.33_830.27_2266.64
5000_280_266.57_815.7 _ 2226.86
5250_270_269.9_825.89 _ 2254.68
5500_260_272.28_833.18_2274.58
5750_240_262.76_804.05_2195.06

now that last number does not account for drive tran loss...
but you the higher the rear gear will give you a higher total power. but probably might add or subtract from ur drive train loss, and will definatly lower your top speed.

RMC

Ok so now that I'm at home and have a little time I'm reading up on these.

I was right and one does use a big drum and measures acceleration of the drum, speed and the time it takes to get to that speed, it even calculates the mass similiar to it's moment of inertia.. However, it doesn't solve for torque it uses work like stated above to find HP and then calculates torque from that.

But for the hyrdraulic brake dynos they increase the load and measure torque from that then find HP from the torque.

Pretty interesting stuff if you're into physics and math.

I'm still reading up though.

Does anyone know mathematically for certain what gearing changes do in terms of HP and TQ and why it gives a faster 1/4.

Ahah I found what I was looking for.

"Generally, with rear wheel numbers, axle ratio is not considered in the torque computation. For comparison purposes, this makes more sense. ---->The computer factors out the axle ratio by using engine speed data in the torque derivation."

So it calculates HP which the gearing has no effect on and then factors out the gearing by calculating torque from engine speed. So I suppose torque could be higher with better gearing which would explain faster 1/4 but it gets factored out.. it's just not completely accurate and sometimes factors out too much TQ

brain

I did some reading too, and found that the hp is decreased on a dynojet by shorter gears because the gears themselves are using some of the power it had before to accelerate the car quicker, therefore leaving less for the dyno to report. Not sure if that is 100% true, but it sounds correct.

As far as the inductive pickup correcting for gear, I don't think so. It is there to pull a tach signal, which it needs to calculate torque. You can put a car on a dyno without connecting anything, but you will only get horsepower across speed or time, not engine rpm.

Highlander

You cannot measure HP.. you measure TORQUE!

NewbieWar

like in my post earlier

first gear at
4750 rpms...
u have aproximatly 300 torque (at fly wheel)
271 horsepower (at the fly wheel)
830 horsepower in the transmission (not accounting drivetran loss assume 20% less)
2266 horsepower to the wheels (not accounting drivetran loss assume 20% less)

now thats with the 3.06 1rs gear and 2.73's in the rear

same thing w/ the 4.10's

4750 rpms...
u have aproximatly 300 torque (at fly wheel)
271 horsepower (at the fly wheel)
830 horsepower in the transmission (not accounting drivetran loss assume 20% less)
3403 horsepower to the wheels (not accounting drivetran loss assume 20% less)

altho because it takes more rotations of the drive shaft to make a full rotation of the axel, now ur looking at 1rst gear topping out at
25-30 instead of 37-40 and so on

OldSStroker

Yep, that's how inertia-only dynos work. Brake dynos do measure torque directly. It's easy to get confused.

One reason more gear can give less rwhp is because the driveline is accelerating faster and rotating inertia losses in the vehicle driveline are higher. That's why you usually run in the highest trans gear (or 1:1): it gives higher rw numbers. Yeah, there may be more friction in a 4.10 as opposed to a 2.73, but I'll bet the setup of the gears is a bigger factor. Hypoid gears can really generate heat under big power loads.

Many engine dyno tests are run at a fixed 300 rpm/sec rate. OEM numbers are steady-state (no acceleration) step tests. Running a chassis dyno test at about 300 rpm per second would make sense to me. Some chassis dynos have eddy current brakes in series with the rolls and can do this accurately. They probably give the best correlation with engine dyno tests.

As to why gearing effects 1/4 ET, that's another subject. If the gearing is such to keep the engine from overspeeding in the lights, gearing changes don't have much effect on trap speed, especially for cars under about 130 mph trap speeds. Why's that?

Highlander

I beg to defer...

In 1997 we had a 94 camaro with forged pistons S/C 9psi and 150 N2O we ran here in PR A4 11.2 consistently @ 124... went to 4.10s and 10.98@128mph... we gained 4mph...

Its got to be related to the RPM range where the engine is running, The car will accelerate faster also and that is obvious.

OldSStroker

Sorry, this isn't correct. Horsepower isn't multiplied by gears, but torque is. HP is torque x rpm divided by 5252. The same gears that multiply the torque divide the rpm so hp remains the same (except for friction losses which decrease everything).