Horsepower/Torque-Flywheel vs Rear Wheel?

[RDABIKE]

I have heard the standard loss of 25-30% for an automatic to the rear wheels. On this line of thinking is it a percentage loss or is it "X" amount of horsepower loss. Example: Stock 5.9 rated at 250 hp -30% loss equals 175 hp at the rear wheels, a loss of 75 hp. Now we build the engine to 600 hp. Do we have the same 30% loss which now equals 180 hp, or do we just have the origional loss of 75 hp?. Does anyone have any facts on this? Now does anyone know what the loss should be for my 4 wd, it has an automatic and the power also runs through the transfer case.

[dewme5]

it's all goign to matter on your exact setup. not only auto / manual, but which auto you have. a 904 robs less and an equally built 727. Axels, in your case 2 of them, add in a transfer case you are looking at a decent amount off loss there.

I've read alot of 20% loss average on a car, but then I've seen alot of people dispute that claim saying it's closer to 5-10% and that 20% would create enough heat in a rear or tranny to melt it. 30% considering the extra axle, and the transfer case. might be a little to much, but still close

The higher the number, the bigger the ego inflation. which would explain the "i can run mid 14's but I have 500hp at the crank." *must be some high drive train losses* lol

basically no matter the numbers, if you do a tune up, and gain 10hp, no matter what, you gained 10 hp

using 10% or 20% numbers are a way of saying 'this will get you in the ball park, but nothing is accurate"

[9dodge]

I've done both tests on race motors, before and after.

A 318 with 265 at the crank was only 175 at the rear wheels.Another example, a 360 with 240 on the motor was only 155 at the crank.

Rear gear ratio has alot to do with it too. A 4:56 gear will put more horse to the ground than a 3:23 gear on the same motor.

A chassis dyno run with the tranny in second and you would be surprised at the power loss.

20% is a good rough number, just remember it could be more or it could be less.

Michael

[jelsr]

Most chassis dyno outfits are saying 80 to 85% of crank output is delivered to rear wheels. Like dewme5 says, there are variations. Kind of like the "one size fits all" sales pitch. 4WD vs 2WD, Std vs A/T, Syn vs petroleum, all will have an effect on output.

[Maxwedge]

even length of the Driveshaft and stiffness of the chassis also affect that percentage
Dramatically. Other variables include weight of drive train components. Bigger tires dont
necessarily mean more power. Bigger tires and their bigger rims weigh ALOT and absorb alot
of power, especially when you are at the top of your run..
MW

[Jims451]

Many years ago Hot Rod or Car Craft listed the power different automatic transmissions used.
I think the listed the 727 as using about 75HP, and the 904 at about 50 HP? It really did not say if this was at a constant RPM (frictional losses) or acceleration losses (Losses due to inertia or the weight of the moving parts in the transmission.)

The thing that really kills my engines power is Altitude! I think the HP lost to Altitude HP at 6,000 ft is over 15%?

[jelsr]

Another consideration is the fact that when tested in a dyno cell the engine is without the power robbing accessories like alt, fan, p/s, etc. All the majors lost a bunch of adv. HP when the SAE standard was changed to "As installed in the vehicle" vs a temp and humidity corrected test cell without accessories.

[dewme5]

I know it doesn't matter now.. but back in the musclecar era.. insurance rates were rising fast as horsepower numbers were getting bigger and bigger. would have been nice to dump the hype, and publish RWHP numbers. Sounds alot better to the insurance, and lets you get a much better idea of what your car would do anyway.