Torque calculations

[Bob K]

Do the same principles apply with torque as they do with horsepower with regard to calculating flywheel versus rear wheel? When Dodge promotes the 2003 as having 500 hp and 500 lb-ft of torque, is the torque at the flywheel or the rear wheels? My GTS has 475 lb-ft of torque at the wheels. What does that translate to at the flywheel? Inquiring minds want to know.

 

[Venom Lover]

Yes, the same conversion factors apply for torque as for power, at least based on the numbers in the VCA FAQ.

www.viperclub.org/faq/powerfaq.html

Numbers cited for the '03 are flywheel.

For your '96, take your 475 ft-lbs and divide by .87 to get ~545 ft-lbs.

 

[Serious Eric]

Driveline losses affect both torque and hp. Same scaling factor applies to both since HP is derived from torque via the old familiar equation HP = Torque(ft.lb) * rpm / 5252. The reduction factor is much debated and probably only known accurately within team viper ... I've heard anywhere from 13% (a more traditionally accepted value) all the way up to a claimed 17% for viper specifically. The 17% claimed for viper was allegedly due to the greater reciprocating mass of IRS/halfshaft setup. Dunno if I buy that though.

The 500/500 numbers quoted by DC are flywheel torque/hp. Manufacturers always quote the higher number.

If you're making 475 ft.lb at the rear wheels then your flywheel torque would be somewhere around 475 * 1.13 or about 537 ft.lb. Pretty strong.

 

[jcaspar1]

Dividing by .87 is the correct method. It is a 13% loss from flywheel, not a 13% gain over RWHP. FWHP-13%=RWHP or FWHP-.13(FWHP)= RWHP or FWHP(1-.13)= RWHP. So.... FWHP=RWHP/.87
We should have this in the FAQ as so many people get it wrong.

 

[Janni]

Always thought this was the easy way to remember:

RWHP .... (1- .13)
_____ = __________
X ......... 100
(FWHP)

RWHP is known from your dyno results

Assume 13% drivetrain loss

FWHP = X

Solve for X

As you can see it ends up being RWHP x 100 / .87, which is definitely NOT the same thing as X * 1.13.....

Math - not my strong point... but fractions I understood...

Hope this helps.

 

[Bob K]

OK, one person divides by .87 and the other multiplies by 1.13 giving different results. Which is the correct way to do it?

 

[Jack B]

I have had my gen 1 on a Dynajet dyno several times. The shop I use is also the manufacturer of the Dynajet unit. He has done over 100 vipers and has worked for one of the dodge factory racing efforts.

When I told him about the 13%, he claimed that that was low. When he compares his chasis dyno to a documented flywheel dyno, a car with half-shafts is typically 15%. He further stated that the factory racing team claimed a new viper was at 15% and as the car put on miles the losses would actually increase to 17%.

At my last dyno , he had an open wheel track car that had 12% losses with direct drive and fixed axles. Another point of interest, they are getting ready to deliver a four wheel drive dyno (a first I believe) to speedvision so that they can dyno the Audi's