Driveline and Torque

I was discussing with some guys at work today about torque and driveline.

One guy said that the torque at the crank gets multiplied to the wheels by the rear end ratio. Ie: 500 ftlbs torque at the crank is approximately 1500 ftlbs torque at the wheel with a 3:08 rear end.

He said that is why the stock rear ends are more susceptible to breakage than the driveshaft on high torque vettes (C3). We were discussing drag racing and slicks.

True...or at least in line with the truth?

Well maybe not so much the pumpkin/gear set itself, inherently but certainly the drive shafts as a vette is IRS and has U joints in there....

and yes obviously your math is very close.....

the saving grace is the tires break loose....

but even that is not the whole story in that with a stick shift you can have a 2.5-1 something first gear....so add in that factor....and if it's a automatic, that initial surge/launch off the line has a typical additonal 2.5 to one on top of the trannies 2.5-1 for most autos in the mechanical calculations...

:smash:

Wow, I remember hearing that from school or somewhere long ago----So does that mean if I dyno 320 lb/ft torque at the rear wheels with a 3.36 rear end I only have 95 lb/ft at the flywheel and so a 300lb/ft rated T5 trans would be way more than adequate?? Oh man, that would be great! (See 'parts for sale' for my fantasy-du-jour)
Of course that means my motor is really a piss-ant after all.....I don't pay much attention to flywheel dyno numbers because the rubber-to-pavement connection is the only place it really counts.....

(PS Redvetracer--you still have that pedal assembly for sale??)

Well lets not get too pessimystic there, 300 ft lbs at rear wheels would be something like 140 ft lbs at the engine....something like a 20-25% loss of power in the tranny/drivetrain/diffy....total......varies a bit, but that's typical so I understand....seems awful high to me though as lets say my engine actually makes a average of 100 hp for a nice easy drive at 80 mph.....

being a automatic that means I loose some 20% of the power there....

so that would be another say 5% or the diffy's....now I know the typical diffy gets hottern' hell on a typical decent run....

ok the tranny is getting 4x the power losses in there somehow....

I look at that as 20 hp minimum being dissipated at heat....at 750 watts/hp that is 15000 watts, more than your stove puts out at full tilt....ovens also...

which is why I sorta question those calculations....I can't see the air blast on a tranny or diffy being enough to dissipate the stated standard power losses, without frying all the internals after a month....

:gurney:

JPhil said:

Wow, I remember hearing that from school or somewhere long ago----So does that mean if I dyno 320 lb/ft torque at the rear wheels with a 3.36 rear end I only have 95 lb/ft at the flywheel and so a 300lb/ft rated T5 trans would be way more than adequate?? Oh man, that would be great! (See 'parts for sale' for my fantasy-du-jour)
Of course that means my motor is really a piss-ant after all.....I don't pay much attention to flywheel dyno numbers because the rubber-to-pavement connection is the only place it really counts.....

(PS Redvetracer--you still have that pedal assembly for sale??)

Click to expand...

See..now that's why I get confused....maybe it's the 1500 lbs torque is the stress that the rear end takes, but not neccesarily what you would see at the wheel?

He said the mehanics are very similar to a torque multiplier wrench.

I just got back from talking to an engine builder of good repute here. He's been in business for probably 20 or more years and builds, among others, racing engines for all types of competition. He has both an engine and chassis dyno.

He said the torque multiplication factor we were speaking of does not come into play in the readings of the chassis dyno. In numerous cases where he has tested the same engine on the engine dyno and then installed it and run it on the chassis dyno, he finds a similar drop in torque as the horsepower shows. That is, if you have a 20% loss in horsepower, there will be a similar 20% drop in the torque reading from engine to chassis dyno.

So if I show 320lb/ft at the rear wheels through an old TH400, add 20% or so to that number and I am putting out in the neighborhood of 390lb/ft at the flywheel. Therefore I would need a tranny rated for that number.

He also mentioned that driveline alignment is often overlooked as a potentially great power robber, that he can often see up to a 20 HP gain to the wheels just from setting that up properly.

JPhil said:

I just got back from talking to an engine builder of good repute here. He's been in business for probably 20 or more years and builds, among others, racing engines for all types of competition. He has both an engine and chassis dyno.

He said the torque multiplication factor we were speaking of does not come into play in the readings of the chassis dyno. In numerous cases where he has tested the same engine on the engine dyno and then installed it and run it on the chassis dyno, he finds a similar drop in torque as the horsepower shows. That is, if you have a 20% loss in horsepower, there will be a similar 20% drop in the torque reading from engine to chassis dyno.

So if I show 320lb/ft at the rear wheels through an old TH400, add 20% or so to that number and I am putting out in the neighborhood of 390lb/ft at the flywheel. Therefore I would need a tranny rated for that number.

He also mentioned that driveline alignment is often overlooked as a potentially great power robber, that he can often see up to a 20 HP gain to the wheels just from setting that up properly.

Click to expand...

Ok. So the torque multiplier exists when it comes to rear end stress, but has nothing to do with rear wheel numbers on a chassis dyno. I always thought that the torque numbers at the rear tires via chasis dyno or calculated was the same stress on the rear which made it hard to understand parts breakage.