Bud2

Had the car dynoed the other day. Was thinking about it last night and I really don't understand something. They put the car on the rollers, run her up through the gears and when in fourth jump on the loud pedal till it hits the rev-limiter at 6000. Out comes a printout from the computer with the details. Now as I understand it, horsepower is shown 'at the wheels'. My question is what happens when I turn up next time with a different set of gears in the diff? As I see it it will show up a different horsepower curve when in actual fact the power is the same but the leverage, via gears, is different.

But that doesn't make sense.

What am I missing????

Bud.

AZDoug

The gear ratio is part of the calculation for HP and Tq. Change gears and they will figure it out, or just tell them and they will input it.

Doug

Faster Rat

All dynos measure torque. Period. The horsepower is calculated from that measurement. Torque and horsepower always equal each other at 5252 rpm.

If you can keep the same amount of torque, then the more rpm you can turn, the more horsepower you get!

zwede

Nope. Intertia dynos such as the DynoJet measure hp. They have a heavy drum that the car spins up, overcoming the drum's inertia. You need energy for this which translates to hp. They hook a sensor to a plug wire to get engine rpm and from this they can translate hp to torque.

If you don't hook the sensor to the plug wire you will get a printout with hp vs road speed.

Sigforty

Often with gear changes you will see a change in hp from the dyno. It really depends on what gears you have and go to. If you move to a lower gear ratio (numerically higher) you tend to see a drop in horsepower. On my Z06 moving from 3.42 stock gears to 3.90s showed about a 5 rwhp drop.

MotorHead

Hate to say it but Chasiss dynos actually measure HP and calculate the torque from the HP. This is not how or why I know this but if you have ever watched the screen on a chassis dyno then you will see it plotting a HP curve and the operator presses a button to get the torque. Don't want to argue about, there's plenty on the net to read about it

Your different gears might have a slight effect on the dyno prinout but very minimal as it uses time it take in intervals over the RPM of the motor to rotate the drum up to 5-6000 or higher RPM so the HP and Tq will remain near constant.

gkull

What your asking is why higher stall TC's give erronious RWHP readings until past their flash stall speed. So you just start the dyno runs at high starting rpms. gearing is just another factor and why you use top gear dyno runs. It gives the machine more time for calculations

A given motor only has the ability to do so much work. That is the calculated "HP" rating. You just need to read up on TQ and HP

MotorHead

That sounds correct, more heat and friction lost in the 3.90's than the 3.42's

Faster Rat

You are absolutely right and I stand corrected. Inertia chassis dynos measure hp and calculate torque. Eddy current chassis dynos (such as the Mustang) as well as most automotive engine dynos, measure torque and calculate hp.

Sigforty

Yep, they also impacted mpg as well. Fortunately, I am back down to the 3.42s now, but that is because the car traps too high in the quarter for my tire size and 3.90s.

Bud2

The dyno uses time along with force to give you a true reflection of HP at the wheels. So if you went in on Monday with a 4.11 and had a pull then returned on Tuesday with a 3.73, the different time taken to get back up to 6000rpm is taken in account (reflects on the different gear ratio) by the computer and will give a HP readout that would be similar to Mondays.

Or, as mentioned in a previous post, I could tell the operator of the gearing.

Is that correct or am I still off base?

Bud.

MotorHead

You don't need to tell the operator anything other than how high you want to go in the RPM department. Rear gears don't matter as well as I have had dyno's done in third gear.

gkull is correct, the inertia dyno takes a certain amount of measurements per second so if you have 4.11's or do it in third gear you will get less measurements, because the motor will rev to redline in a shorter period of time than if you had a 3.08 and done the dyno in 1:1 or 4 th gear. This will give you a slightly more accurate dyno printout as it had more time to make more measurements.

zwede

You always do your runs in direct (1:1) gear as that's where the losses are the least. I've done a run in 3rd once to see what happened. The number were a few percent lower than in 4th (1:1).

MotorHead

I get a little worried about running 110mph rated tires at 160mph in 4th gear at 6000rpm and the difference would probably be in allowable margin of error for the machine, the slight difference in reading due to the difference in friction between 4th and 3rd isn't going to be much

Now the 427ci with the ZR rated tires I don't worry about

Manuel Azevedo

This kind of discussion can get crazy, but your first comment is the one that is right. It doesn't matter what the dyno program shows you first, HP or a torque number. HP is still just a calculation of torque times rpm equals HP, end of game. You can have torque and no HP, but you can not have HP and no torque!! Think about it!

zwede

Not wanting to get crazy, but no, an inertia dyno (dynojet) measures horsepower and calculates torque. An absorption dyno (eddy current, water brake, etc) measures torque and calculates horsepower.

Manuel Azevedo

NP here but think about what HP is. HP can not be measured without first knowing torque. I have been through this kind of discussion many times, but this is something I deal with at work everyday and we have 22 dyno's, yes that is really 22, so I really am not talking from hear say or advertisements here. Have a great day!!

DRIVESHAFT

22 Dynos? Where in the heck do you work?

Faster Rat

Found this explanation in Wikipedia. Even websites that advertise Dynojet chassis dynos still admit they are measuring acceleration of the inertia drum and using computers and software to calculate horsepower. I retract my earlier confession and am sticking with Manuel. Sorry Zwede.

Inertia sweep: An inertia dyno system provides a fixed inertial mass flywheel and computes the power required to accelerate the flywheel (load) from the starting to the ending RPM. The actual rotational mass of the engine or engine and vehicle in the case of a chassis dyno is not known and the variability of even tire mass will skew power results. The inertia value of the flywheel is "fixed," so low power engines are under load for a much longer time and internal engine temperatures are usually too high by the end of the test, skewing optimal "dyno" tuning settings away from the outside world's optimal tuning settings. Conversely, high powered engines, commonly complete a common "4th gear sweep" test in less than 10 seconds, which is not a reliable load condition as compared to operation in the outside world. By not providing enough time under load, internal combustion chamber temps are unrealistically low and power readings, especially past the power peak, are skewed low.

toddalin

Not necessarily true. A jet or rocket produces pounds of thrust and this is converted to a horsepower equivelent.

Torque is a rotational force. So where is the rotation coming out of a rocket engine?

Horsepower is simply perfoming a given amount of work in a given time. Torque need not enter the equation.