[jadcock]

Drivetrain losses aren't a percentage of the engine or anything -- it's a particular number (that is usually never specified by the OEM). For instance, it may take 20 horsepower to turn the transmission and propeller shaft and axle half shafts on a given model (say the Corvette for instance). 20 horsepower to turn all those components.

Now...you put in a 100hp 4-cylinder engine. It takes 20hp to turn the drivetrain, so you have 80hp left to push the car with.

Next...you swap out the 4 banger and put in a 350hp V8. It still takes 20hp to turn the drivetrain, so you have 330hp left to push the car with.

The drivetrain losses are a static number, and are not related to the engine installed. Obviously, all of the above is in very general terms.

 

[jadcock]

The torque tends to be greater in lower gears, when the car is trying to speed up. Once at cruising speed, all the engine needs to do is keep the car moving.

The torque of what? The engine or at the rear wheels?

The torque at the rear wheels is NECESSARILY greater in the lower gears, because a lower gear offers a speed reduction, and a torque multiplication. A 3.71:1 final drive multiplies the input torque by 3.71 and that is the output torque number. The speed is also reduced 3.71 times. It's an inverse relationship.

 

[jadcock]

Yes, that's certainly true. I think we're talking about the drivetrain losses, though, and on any given vehicle, the engine makes a certain amount of power/torque, regardless of the gear the transmission is in. True, the actual power produced and power lost will be slightly different between two given vehicles.

 

[jadcock]

I'm not quite sure how this thread got into parasitic drivetrain losses.

Probably because he was asking about rear wheel power, and you should always include (or at least mention) the parasitic losses when you're talking about power at the wheels. I understand that the original question was the relativity between one car and another in terms of torque numbers, but other related discussion is always good to have as background information.

 

[jadcock]

Great.