It's sort of both. Power loss does grow with power input, but it's not really constant. There are a few factors you're dealing with.

There's dynamic friction--losses due to materials moving against each other which acts counter to the force applied. Yeah, sure there are bearings and all that but there is steel on steel contact, stuff is sliding at some point even if it is just the bearings against their housing. Friction is quantified using a friction coefficient, which you can multiply by the applied force to calculate the friction loss. It is usually a fairly linear relationship and is pretty constant with temperature for metals. In a transmission you could probably calculate an overall friction coefficient, it would depend heavily on the construction of the particular transmission though. But basically, this particular part can be thought of as a percentage.

Another factor is inertial losses. The more you're trying to move, the harder it gets, the higher the losses. With a bulky transmission, huge flywheel, heavy clutch, and big-ass 22" chromies you're wasting a lot of energy turning all that weight instead of making the car move. And the inertial losses are not constant--they increase as you try to make things spin faster.

There are other reasons that I can't think of right now because laundry is not condusive to high brain activity. tongue.gif

 

I must be totally stupid because it still it's making sense in my head why a 100 HP engine would lose 15HP through the trany but then a 200HP engine would lose 30.

Ok someone really explain this. Random or REd would be nice right about now.

Thanks for everyone elses posts so far.

 

Well I was hoping Red or Random would reply again and really go into detail why this happens. Can some one please go into detail about this because I'd like to fully understand it.

So far I'm just not getting it.

 

I thought Ego did a good job of explaining it.

There are three camps. One.. hp loss is a % of flywheel hp. Two... hp loss is a set number for each tranny/diff/wheel setup. Three... it is a combo of both % and a set number.

Although most people just use a percentage as power loss through the drivetrain, that is probably not totally accurate. More likely it is a set amount of horsepower for the inertial losses... + a % of horsepower dependant on frictionally losses. This becomes a difficult subject. Even Dynojet doesn't have a firm stand on this.

What it really comes down to is... how much power is being delivered to the wheels and how you use it (1/4 mile times). Everything else is BS. Some people use outrageous dyno correction numbers, automotive companies artifically inflate or deflate their hp numbers.

 

One more thing to think about the situation of a transmission losing a set amount of horsepower, not a percentage or a combination of the both.

If I made a very inefficient driveline that lost a set amount of horsepower, say, 75 hp, and bolted it to a 150 hp motor, I would put down 75 hp to the ground. No arguments, right?

Now, if I bolted that same driveline to a weaker version of the same motor, that made 70 hp, would I make negative horsepower?? No. Would it turn the driveline at all? Most likely. It wouldn't put EXACTLY 35 hp to the ground (50% loss like before), maybe 25 hp to 30 hp. That would account for the set hp losses and the percentage hp losses. Now I know that these numbers are not even close to real world examples, but they seem to get the message across. Hopefully that helps a little.

 

Cheuk and Ego did a good job of explaining it.

There are no TRUE rules or written in stone guidlines on drivetrain losses.

In general 15% to 25% for manuals, and 25%-35% for automatics. In general FWD have less loss than RWD, but there are examples of really efficent RWD drivetrains, and examples of really poor FWD drivetrains...

There is no set "power" to move a drivetrain. In the example by Tib in CB is perfect.

In reality, the power sapped by the drivetrain does not stay proportional with engine output. a 100HP engine that puts 85 HP to the wheels (15%) that does not mean that a 1000 HP engine would only put down 850 HP to the wheels (15%). In reality, the 1000 HP might put down 925 WHP. Who knows. There is a point of diminishing returns, where you have to beef up the transmission/gearset/workings to handle the high HP so that it actually RAISES the drivetrain losses in ultra high HP cars with weak factory components, but some cars which can use the basic stock factory drivetrain to insane HP levels might only have a 10 or 15% loss even for a RWD car.

You would think that a transmission that saps 15 HP from a 100 HP engine would only sap that same 15 HP from a 1000 HP engine? Think about the physics of it man. There is 10 times the load on the transmission of the 1000 hp ENGINE. That does not directly translate into 10% more drag/ware/lost HP, but it does somewhat work against the HP gained in the engine, and what, in general keeps the drivetrain losses fairly consistent across HP levels using the same tranny.

Lastly...there's no way to prove ANY of this unless you are willing to dyno with 2 seperate engines, and then dyno each engine on a engine dyno and compare the 2 results to determine drivetrain losses. tongue.gif