I've been searching for info on the commonly accepted powertrain loss for EVOs. I would assume that this topic has already been hashed and rehashed. Please point me to the old threads, 'cause I'm not finding them.

Is powertrain loss a fixed HP number? I would presume so.

Or, is powertrain loss a percentage? I presume not.

Some people say that the only real powertrain loss is in the differential, with little to no powertrain loss in trannys. Most RWD or FWD cars will lose approx. 25 hp. However, since a EVO/STi/WRX has three differentials the powertrain loss will be approx 75hp. Does anyone here ascribe to this theory?

Thanks,
Jim

 

Powertrain loss is percentage,not value.There are a thread around Advance forum.

 

It is by no means a fixed HP. No, FWD and RWD cars do not randomly lose 25 whp...not sure where you got that. At best, the loss is a percentage, but it's not a static percentage. Basically, there is no answer due to all the different dyno types and differences between cars, and it's not important anyway, because all that matters is what you put to the ground. We don't even know what our crank HP is unless we use an engine dyno, and there's no reason to speculate otherwise.

 

I'm surprised by your comment, "there's no reason to speculate otherwise". That seems rather closed minded and not something I'd expect from someone with your obvious level of experience.

Look at it this way: Two identical looking VIIIs are traveling down the freeway side by side at 80mph. The only difference between the two cars is that one is stock - I'll call it 271chp - and the other is highly modified - I'll call it 542chp.

How could the 542chp Evo possibly by using more hp to maintain the 80mph than the stock Evo? It would seem clearly self-evident that both cars would be using the exact same amount of hp to maintain the exact same speed under the exact same conditions.

I'm not trying to be argumentative, but I have never found anyone who believes that drivetrain losses are a percentage who will actually explain their position.

Thanks for trying to help me understand,
Jim

 

Your reading into warr's post too much.

It works very much like fluid dynamics such as IC piping etc up to speeds.

It takes a certain amount of horsepower at some gear X to get the wheels turning. Then it takes a percentage loss as the loads increase. This percentage loss is NOT linear as a function of load. Generally as load/torque increases on the drivetrain components your going to see an Increase in the drivetrain loss.

Simply put your putting more pressure on the bearings to do their job and the efficiency factor starts to fall. Plus you have other factors like heat generated oil efficiency under load. The numbers are crazy.

In your example your saying at 80mph two different cars. Assuming they are IDENTICAL cars minus power output their loads should be identical thus loss identical from a drivetrain standpoint.

 

Why would not the same thing hold true with the same two cars on side by side identical dynos? Each dyno creates the very same load on each car. The car with twice as much hp is simply able to overcome the load more quickly than the stock car, therefore its whp will be much more. However, the load exerted on each car by the dyno is identical.

Jim

 

A dyno wasn't the question it was cruising at 80mph which simply means maintaining a current energy level required to keep the car in motion.

As a matter of fact Horsepower is literally defined as Work/Time meaning the one with double the horsepower should do the work twice as fast under the "same load".

By your definition though of overcoming the "load" more quickly in essence defines horsepower but completely disregards torque. Torque essentially is the force you need to move something via rotation to overcome the mass of the object. Since you must move this object more quickly you will need more torque to do so.

This increase in torque comes at a penalty of course called friction and was roughly explained in the last post i made Simply put more torque is going to mean more loss in the drivetrain.

 

Drivetrain loss should be a percentage but it is unlikely to be a constant percentage.

Losses come from a number of items, basically can be summarized as these:
1.) Friction from gears. In rough terms this is from the the amount of sliding contact between teeth and so on.
2.) Churning loss. Gears whip up lubricants and the viscous drag takes a force to overcome.
3.) Seal and bearing loss. The torque neede to overcome the drag cuased by seals and bearings.
4.) Transient 'loss'. Arguable not a loss but the energy stored in the moving parts. Think of a heavy flywheel.

As you can see, all these relate to speed of rotation and therefore increases with speed.

It cannot be a constant power figure as this would mean the torque required to keep the drivetrain going at a high speed is lower than that at low speeds as rotational speed x torque = power. Although this is perhaps true with losses in seals where stick-slip regions exist, other types of losses do not exhibit this characteristic. At least not in the region we're talking about. Obviously each type have different significance depending on the complexity of the gears and the speed at which you're going.

Finally, the car with double with horsepower can do the same work twice as fast. However, presented with the same load and same speed, say going up the same hill, same weight , same speed, both will produce the same amount of power. The whp value is the 'max' value at a given rpm. At a certiain rpm, it can produce any amount of power up to the max value.