Torque through driveline losses

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James Clews

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Hi my 410 small block was dynoed on a chassis dyno a few years ago and produced 355 rwhp and 530ft lbs of torque. I know we loose about 25 pecent of hp through the driveline but does anyone know how much torgue we loose if any? It runs 11.4 @ 119mph in a 3000lbs car with 3.91 diff ratio with 4 speed manual!
 
Every dyno is different - but the losses are more like 18% for automatics, and 15% for manuals. Your results may vary too if the convertor is loose.
 
Torque is locked to hp. Google the factor 5252. They are "one in the same." In fact a dyno does NOT measure HP, it measure torque and calculates the HP via math
 
119mph/3000 = 415fwhp - 15% drivetrain = 352rwhp..
 
Ooh Ooh, AJ: Do a 65 "S" 16.1 ET at 2900 lbs with a 235 HP/4sp and 3.55 SG compared to a 15.9 ET of a 2974 lb 271hp/4sp 3.89 rear K-Code Mustang. Those formulas hit pretty close.
 
what about other parasitic losses ....air filter,alternator,water pump,fuel pump,fan?
At 5000 rpm another 40hp loss?
 
It's not that precise. The calculation from MPH and weight will always be "right". Any dyno will always be some form of "close". I've seen shops use numbers all the way up to 28% stating its the accessories or whatever. My experience is the most solid conversions are 18% or 15% assuming the calibrations and testing were done properly.
 
No one has answered the question I asked yet! If my motor puts out 530ft lbs at the back wheels what would it likely put out at the flywheel?
 
James Clews said:
No one has answered the question I asked yet! If my motor puts out 530ft lbs at the back wheels what would it likely put out at the flywheel?
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As said, net torque is going to track HP losses. So the 15-18-20% numbers should apply.

530 ft-lbs at each axle? If that was the case, then you have to know the transmission gear used for measurement , the rear gear, and the approximate efficiency loss. OR, if the dyno 'translated' the torque to top gear (4th gear) numbers.

Assuming it translated the results to top gear: If you had 530 ft lbs at each axle (both tires biting well so the torque out of the rear gear splits equally) for a total rear axle torque of 1060 ft lbs , a 3.91 rear gear, and were in top gear (1:1 gearing in the trans) with a 15% efficiency loss, I do believe that would work out to around 319 ft lbs at the flywheel.... which seems low. With a 25% total efficiency loss, then the flywheel numbers rises to 361 ft lbs.

But, I do not know in what format that dyno gives you a wheel torque number. Does it account for tire size, and account for gearing? Is it torque per axle or total? Is the torque on the rolling drum or the vehicle axle? (If that 530 number was the total for the rear axle, then the number at the flywheel would drop in half of what I show, and that does not seem right. So it seems correct to assume their dyno number is per wheel.)

Do you have dyno sheet that might show the input info? (Like, trans type and tire size?) Did you make 'runs' on the dyno, or measure at steady state RPM's?
 
James Clews said:
Hi my 410 small block was dynoed on a chassis dyno a few years ago and produced 355 rwhp and 530ft lbs of torque.
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Those numbers don't make sense at all, sorry.
If you apply the simple math and count whatever percentage on top of your numbers, say 18% for drivetrain loss, you'll arrive at a 410 making roughly 420 horses and 625 ft lbs of torque. No way.
 
stixx said:
Those numbers don't make sense at all, sorry.
If you apply the simple math and count whatever percentage on top of your numbers, say 18% for drivetrain loss, you'll arrive at a 410 making roughly 420 horses and 625 ft lbs of torque. No way.
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This need to be computed with the rear gearing and trans gearing in the equation, and know if it is torque per axle, to get back to the flywheel.

If the dyno gives torque per axle (i.e., per wheel..the answer to which I do not know) then:
  • Total torque to the rear axle is 2x that, or 1060 ft lbs.
  • Divide that by the rear gear ratio (3.91 in this case) to get to the driveshaft torque (with no losses).
  • Then if the trans is in top gear, then you ignore the trans gear ratio and the driveshaft torque is the flywheel torque (with no losses).
  • Divide by [1-(loss%/100%)], which is .82 for an 18% total driveline loss.
  • Comes out to 330 ft-lbs net torque at the flywheel for 18% driveline loss, with all the losses in the engine like water pumps, fans, air cleaner, hot air under hood, etc.
 
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stixx said:
Those numbers don't make sense at all, sorry.
If you apply the simple math and count whatever percentage on top of your numbers, say 18% for drivetrain loss, you'll arrive at a 410 making roughly 420 horses and 625 ft lbs of torque. No way.
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Why is it so impossible to believe? Those numbers are similar to the numbers my 408 Magnum recently spun out on our local chassis dyno. 382 hp & 525 ft lbs of torque at the wheels.

A well tuned injected 408 with ported big valve aftermarket heads, a well-matched roller cam, 10.2 to 1 compression and full TTI 3" exhaust makes an efficient & surprisingly potent street motor. Especially when coupled to 3.90 gears, an automatic with overdrive and a properly set-up rear suspension with drag radials..

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Don't know a thing about chassis dynos, but I espected (kind of) that it would give numbers that are in "tune" with the numbers taken on a engine dyno.
While I follow your equation the result doesn't make sense either, since 330ft lbs is mighty low for a 410 stroker.

The numbers given by James and jbc do not correlate at all... horsepower is a mathematical function of torque (as has been said many times), and 525ft lbs
doesn't make ANY sense. When max torque is at, say, 4500rpm, this would [mathematically] equate to 449 RWHP. ??

Scratching my head... and not trying to compete with AJ.
AJ, you're a good man!
 
the percentage conversion is wrong. for example - manual trans, say 20%, so 530 at tires becomes 662, so the drivetrain sucks 132. but a say 300 chassis +20% becomes 375, drivetrain only sucks 75? of course the higher the rpm the more the drive sucks, but for a manual trans, and depending on the rear etc. , I say a straight number of 90-110 depending on variables
 
jbc426 said:
Why is it so impossible to believe? Those numbers are similar to the numbers my 408 Magnum recently spun out on our local chassis dyno. 382 hp & 525 ft lbs of torque at the wheels.

A well tuned injected 408 with ported big valve aftermarket heads, a well-matched roller cam, 10.2 to 1 compression and full TTI 3" exhaust makes an efficient & surprisingly potent street motor. Especially when coupled to 3.90 gears, an automatic with overdrive and a properly set-up rear suspension with drag radials..

.
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Do you have a time slip for your 408?

525 ft lbs at the wheels is stroked big block territory.
 
Again = too many variables to make it gospel. The calculation off MPH and weight come close to matching, so I'd say it's as close as it's going to be. If you want to get a torque number, call it 530x1.15 or 610. Being that the HP figure is as low as it is, I can accept a higher torque figure. It's undercammed.
 
why are we doing a torque per axle when a SG will pretty much lock the rear end to a 1:1 per side? what would happen if you ran a spool? would the dyno give you different results?
 
stixx said:
Don't know a thing about chassis dynos, but I expected (kind of) that it would give numbers that are in "tune" with the numbers taken on a engine dyno.
While I follow your equation the result doesn't make sense either, since 330ft lbs is mighty low for a 410 stroker.

The numbers given by James and jbc do not correlate at all... horsepower is a mathematical function of torque (as has been said many times), and 525ft lbs
doesn't make ANY sense. When max torque is at, say, 4500rpm, this would [mathematically] equate to 449 RWHP. ??
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Mistakes are being made by not making torque and RPM conversions for the gearing in the drivetrain. And yes, stixx, 330 ft lbs from the engine is indeed too low for the ET's per pure peak HP numbers but I'll explain below.

If you take the OP's 355 RWHP, and use a 18% loss factor, that is 433 HP at the crank. Plug that into the Wallace calculators with 3150 lbs (car + driver), and it spits out the ET of 11.29 and speed of 119 mph. So the wheel HP number looks plausible.

Where the OP's numbers 'go off the rails' is for the torque that the OP reports. Converting from RWHP to WHEEL torque is done at the WHEEL RPM's, not the engine RPM's. If the peak wheel HP is at an engine RPM of say, 5500 RPM then the wheel RPM's are 5500 divided by 3.91 = 1407 RPM. (Assuming the trannie ratio is in 1:1 gear.) Take that to any online HP to torque calculator, and you get 1325 ft-lbs of torque for the whole rear axle, or 662 ft lbs per each wheel. (This is not peak wheel torque but wheel torque at peak wheel HP.)

Since peak torque at the wheel in a fixed gear run-up is at a lower RPM, then the peak wheel torque can be expected to be something like 5-10% higher, and would be around 700 ft-lbs per wheel or 1400 ft lbs for the whole rear axle.

So, the OP's torque number does not look like a wheel torque number..... or is just a flat out mistake.
 
pishta said:
why are we doing a torque per axle when a SG will pretty much lock the rear end to a 1:1 per side? what would happen if you ran a spool? would the dyno give you different results?
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My question was: What does this torque number that the OP posted mean and/or how is it presented? Is it reduced to a per wheel number or is it a whole axle number? Just trying the make sense of his numbers, and, as above, the torque number does not compute as a number at the axle.

If traction & load & any slippage is equal on both sides, the torque will split 50-50 between the 2 wheels.... sure-grip, open, or spool..... For a chassis dyno with a drum turned by both wheels, one would assume the car chained/strapped down so that both wheels are turning the drum with littler or no slippage. If one is doing a single wheel dyno (hub dyno), with something like a spool or locker to keep the torque going to the loaded wheel, then the whole torque goes to the one wheel with the dyno.
 
Fast340cuda said:
Do you have a time slip for your 408?

525 ft lbs at the wheels is stroked big block territory.
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My stroked big block is actually up in the 680 ft lbs area. That was on an engine dyno.

I think the 408 is artificially high too. The graph they gave me shows three lines with one indicating the torque at 495 ft lbs at just under 3900 rpm. The horse power peak is up around 4600 rpm. The line that indicates the 525 ft lbs sweeps in from the left side of the chart. They didn't indicate what color represents what. I was there to verify my A/F ratios, not get numbers.

I tried to run it at the closest track, which is about 3 hours away. I had just broken the second gear apply band mounting post, so I couldn't get a decent time.

I've since had the transmission repaired and freshened, and will make another attempt in the spring. I expect the car should be somewhere in the 11's.
 
A sorted out stroked small block in a sorted out a body will run 11’s
 
Doing a ruff calculation torque at peak hp is around 92% of peak torque. 530 x .92 = 488

So say you were making peak hp at 5500 rpm. So 488 x 5500 / 5252 = 511 hp at crank. Either your losing a **** ton of power through your drivetrain or you peak way lower like 4900 rpm. Which would give you a 455 hp at crank and 80% of that is about 355 hp.
Something seems wrong.

Whats the gear ratio trans during test, tire size, torque at peak hp and rpm ?
 
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