Chassis dyno vs Engine dyno

408 stroker, 326RWHP @ 5300 on a chassis dyno.
Using a C6 trans and 9" axle, the tester is claiming 25% drivetrain loss which doesn't seem out of line. However he calculates this to be 440 at the flywheel.

I can't imagine losing 114HP unless the parking brake is left on. I'm throwing up the BS flag. By my calculations, I get only 407.5 HP. Can someone check the math?

Maybe he is throwing in extra HP because your C6 and 9" are eating some power. Your chassis dyno number could be 5-10% higher with a C4 and and 8". Not as strong, however.
Besides, who cares what the flywheel horsepower is? RWHP is what you want. I have a buddy who is a former NASCAR engine builder. He says after a test on an engine dyno, he subtracts 25% to guess on a chassis dyno number and he is never disappointed. You are doing it the other way, but you get the idea...

<font size=-1>[ This Message was edited by: 67VertMustang on 3/28/07 7:13am ]</font>

Entirely possible I would say, the C6 alone will account for half of that.
I feel a chassis dyno is a ball park figure anyway...play with your tire pressures and you suddenly have an extra 20hp??
440 - 25% = 330 doesn't it?

<font size=-1>[ This Message was edited by: cmf60 on 3/28/07 8:30am ]</font>

"440 - 25% = 330 doesn't it"

I'm still looking for the math.

440 x .75 = 330 yes
though not the same as
440 divided by 1.25 which is 352

326 x 1.25 = 407.5
407.5 divided by 1.25 = 326

18% calculator.
http://www.fordmuscle.com/calculators/horsepower.shtml
4000lbs, 12.91 ET
326 RWHP x 1.18 = 385 FWHP
Same as 385 divided by 1.18 = 326

Not the same as
385 - 18% (385x.18 is 69.3) = 315.7

Yes the percentages, 18% vs 25%, are different but the online HP calculator does the math my way.

Doesn't work that way Brian as you are trying to add 25% of the lesser figure to get back to the original....which can't work.
An easy example is 400 dyno hp...that will loose 25% at the wheels -

400 - 25% (or times .75 if you like) = 300hp

To work from the other way round...

300 + 25% ( x 1.25) = 375hp ..... just lost 25hp

The second calculation is obviously wrong as you are using 25% of the CORRECTED figure to try to get back to the original....can't work.

Then you're saying the FM online calculator is wrong. Because if you use 4000 lbs, 12.75 ET you get an example 400FW & 339RW.

400 - 18% or (or times .82 if you like) = 328

To work from the other way round...

328 + 18% ( x 1.1[img]/forums/images/smiles/icon_cool.gif[/img] = 387hp ..... just lost 23hp

Percentages aside, I've read the C6 costs about 75HP. I can't see the 9" diff losing an additional 40.

If you are trying to calculate engine hp from a percentage of drive-train loss it doesn't seem right to me. If you continued to calculate back and forth several times for the same combo, the "error" would get greater and greater....because of using the corrected figure first??
From my understanding the C6 is worth about 60hp. Can't comment on the 9" but I would imagine around 30hp+. Also don't forget that you are probably driving accessories now that weren't on the dyno...power steering, fan, alternator etc etc .... add in enertia and drag of the driveshaft, uni joints, axles and bearings, rolling resistance of tires etc... It all adds up pretty quick !!

My current 410 made 330 or so on the chassis dyno.

Runs 110MPH in the 1/4

I've seen plenty of higher HP numbers run slower MPH and plenty of lower HP number run higher MPH in the 1/4.

Use the dyno as a tool to gauge gains from one combo to another. Consider this the baseline and look for ways to improve performance. Go to the same dyno next time and you can get a 'pretty' good read of the improvement net.
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On 2007-03-28 18:56, cmf60 wrote:
Can't comment on the 9" but I would imagine around 30hp+. Also don't forget that you are probably driving accessories now that weren't on the dyno...power steering, fan, alternator etc etc .... add in enertia and drag of the driveshaft, uni joints, axles and bearings, rolling resistance of tires etc... It all adds up pretty quick !!
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Do the math. If the 9" really is 30HP then look at the torque input at 6500 RPM

HP = TQ * RPM / 5252 so imagine you're in D or 4th (1:1 with the engine) and you're at 6500.

The TQ would have to be 24 lb-ft. Something would have to be dead-wrong in the rear. Mine reads (with drums installed) under 5 lb ft which would extrapolate down to ~4HP at 6500.
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I don't get what you mean allenman85?
Are you saying that it takes 5 ft/lb to rotate your 9" on axle stands?