DYNO question...

[twboy1999]

hi

let say u have 2 car.. both have same hp... let say 400hp
one is around 1500kg, and one let say around 1300kg

dyno on the same day, same time,,, would they produce the same amount of power? or the lighter car would produce a better result?

[bmwmpower]

im pretty much sure result will be same, its all about question of engine not weight

[RC45]

The heavier car may generate better traction and be less prone to wheel spin on the roller and as a result you may have a SMALL increase in measured torque - but not that it will matter.

[gottacatchup]

If the dyno operaters knew what they were doing and the decreased weight didnt cause the tires to slip on the rollers than they would produce the same result.

[loliea]

On a dyno the car is not moving thus it's weight doesn't play any role.
You have different way to measure the HP of a car on a dyno: flywheel horsepower and wheel horsepower. For the first you plug the dyno "directly" on the engine and for the second... well you measure the output at the driving wheels.
Then if the 400HP has been measured using a flywheel dyno, you could have a slight (even thin small or non existent) difference (between the two cars) when you repeat the same test on a wheel dyno (the diff is coming from the transmission components and eventually the wheels breaks... and other rotating components that the engine has to put in motion).

[RC45]

^^ well - the accepted number is 15% drive train loss for flywheel vs wheel measurements.

So a 400hp car will put about 350hp down at the wheels.

The weight might play a small role with regard to traction on the roller, but strapping the car down will normally offset this issue (if it's a chassis dyno measuring wheel torque) - remember, torque is measured and HP is calculated.

[loliea]

Originally Posted by RC45 ^^ well - the accepted number is 15% drive train loss for flywheel vs wheel measurements

I guess the difference depends on the dyno brand... I had 22% for difference between the two measuring system...
Anyway, doesn't matter, flywheel HP > wheels HP.

Link about measuring HP on dyno

[RC45]

Yes - I misread your piece there - at first it appeared that you indicated very little difference between flywheel hp and wheal hp - but you were indicating that fly wheel for flywheel the numbers should be very close.

[TransAm]

Originally Posted by RC45 ^^ well - the accepted number is 15% drive train loss for flywheel vs wheel measurements.

Is this just for manual transmission cars?

What about the 'loss' due to the fluid in the automatic tranny torque converter?

Or is my train of thought completely wrong on this?

The reason I ask is that I ran 202.5hp at the wheels and 249.5lbft of torque when I ran the Saab on the dyno after the SW/filter/catback upgrade. I know that MT Saabs run a slightly higher torque figure as the auto tranny self-limits output to 250lbft...

Regards,

Confused of Fort Worth

[RC45]

Dear Confused of Fort Worth,

While we all would like to get along with our family members, you are really not obligated to sit on Uncle Jeremy's lap when he is wearing his Union Jack speedo.

Kind regards,

Abby of Houston.

[RC45]

^^^ oh buggar... I didn't mean to respond to that PM on the public board... ops: ops:

[TransAm]

:roll: Ask a stupid question...

*TA slaps RC with leather driving gloves, SS style*

Vee haf vays of making you talk!

[RC45]

As far as I know, yes - auto cars have a higher drive train loss. I think the number tossed about for LS1 based drive trains is 20%.

[TransAm]

In which case I don't need to adjust the numbers in my sig

I take it there is no similar calculation (i.e. drivetrain loss percentage when measuring 'at the wheels' on a dyno) for torque, as it is a different kind of force than power?

Trans "I never claimed to be an engineer" Am2001

[RC45]

Actually the dyno calculates the torque required to either turn a known weighted drum to a certain speed or rotate the drum against a given brake force (in my layman's vernacular) and this torque figure is then used to calculate the hp figure.