I have been trying to answer this question for a while now, but I cant seem to figure it out. Say an engine makes 500 hp, 470 ft lbs torque max at the flywheel. If that engine was installed in a car, why wouldnt there be 500 hp, 470 ft lbs at the road, providing that you have perfect traction. Where does the power go? How does the power get lost in the transmission? Does some of the power also get lost in the drive shaft and the rear-end? Thanks for the insight.

Simple answer? It takes power to operate an automatic transmission,or manual for that matter,turning the gears,power to turn the driveshaft,power to turn the differential which transmits the power to the wheels and tires. Thus a 20-25% loss to those tires and wheels.

Overcoming the natural friction of the rotating and meshing parts in the trans/rear etc comsumes power.

So only the transmission and the rear end would decrease the power?

the drag of the brakes also, among other things, just think about it, anything rotating takes power to rotate it, so it uses horsepower.

Dont make the assumption that you are making more power by using lighter weight parts, you just accelerate them faster, making for a faster accerelating car.

Anything that causes drag on the driveline will take up power though.

Everything behind the engine takes power to move it,whether its the trans,driveshaft,differential,axles,wheels and tires...that stuff don't move on its own,the engine makes it all go.

You can tell where the lost energy goes pretty easily, it goes to whatever gets warm while driving. Automatics are probably the biggest horsepower drain, especially the ones without a lockup torque converter. Even with a torque converter, you'd have the pump to turn. Overdrive transmissions will drain a little more power than a direct drive transmission. Manual transmissions aren't as bad, but they still do have some drag from the meshing gears and from the oil. There's a formula to calculate the power lost in gears, but it's been a while since I used it, so I can't remember it offhand. I can look it up if you'd like. The differential is similar to the manual transmission, there's power lost because of the gear rotation, bearing resistance, etc. After a long drive on the highway, you can feel the heat from the tires because of the rolling resistance of the tires as well. An underinflated tire will give more resistance than a properly inflated tire, that's why an underinflated tire will get warmer, and give worse mileage. Thinner oils will help get back some of the power losses, but at the expense of not being able to lubricate as much of a load.

don't forget the accessories on the front of the engine too. The alternator,power steering pump and water pump all take powre to turn also. A mechanical fan can use up 20 hp easily.

And what's the power in % that we'll lost on a high performance engine? (everything in good condition,and no AC!!). :confused:
15%?....20%?........,If you have a engine that produce 500 hp on the flywheel...you keeps 375/400hp on your wheels?

EDDY merlin.

20 is not all that bad a number to start with.

Anybody else??

pdq67

Originally posted by pdq67:
20 is not all that bad a number to start with.

Anybody else??

pdq67 Sounds reasonable to me Paul. Personally I've only witnessed a handful of back to back engine to chassis dyno comparisons. The lowest loss was about 17% and the highest was 28%. Everything else in-between. These were with the engine dyno done as it would be in the car; all accessories, exhaust system the car used, etc.

Jody

Wow, I didnt know that all of that stuff drained power. I was only thinking inside the box pretty much.

Johnson, if it wouldnt be too much of a hassle, would you mind looking for that information on how to determine how much power you are loosing? Thanks a lot, I would appreciate it, and same to everyone who explained their answers.

Power loss is not exact, every trans, rear, tire/wheel combo, etc is a little different..

Also the more HP you make, it will actually lower the % you lose. How? Well if it takes, say 100hp (to use nice round numbers) to turn all the "stuff" in a combo a 300 hp engine will lose ~33%. Same stuff on a 500 hp motor still only 'drains' about 100hp, so its now ~20% of total output. Of course these numbers are thoretical, but it makes the point.

Heavier duty components usually take a little more oomph to move, like 9" rears, th400 trans, and big slick tires. But these are required on big power cars to make use of the power produced without breaking.

Take a skinny front runner and spin it with your hand, dont take much to make it turn huh? Can do it with a finger. Do the same with a huge MT Drag slick from a Pro-street car, takes more effort to make it rotate doesnt it? On a car its the motor doing that effort, which bleeds hp.

Originally posted by BusDriver:
Power loss is not exact, every trans, rear, tire/wheel combo, etc is a little different..

Also the more HP you make, it will actually lower the % you lose. How? Well if it takes, say 100hp (to use nice round numbers) to turn all the "stuff" in a combo a 300 hp engine will lose ~33%. Same stuff on a 500 hp motor still only 'drains' about 100hp, so its now ~20% of total output. Of course these numbers are thoretical, but it makes the point.

Heavier duty components usually take a little more oomph to move, like 9" rears, th400 trans, and big slick tires. But these are required on big power cars to make use of the power produced without breaking.

Take a skinny front runner and spin it with your hand, dont take much to make it turn huh? Can do it with a finger. Do the same with a huge MT Drag slick from a Pro-street car, takes more effort to make it rotate doesnt it? On a car its the motor doing that effort, which bleeds hp. The % of loss will vary on all combos. However, if theres a 20% loss from engine to the tires on a 300 HP setup, say you lose 60 HP, a higher HP setup will not lose the same 60 HP. It will lose more because the extra HP/torque will accelerate everything faster which increases the load/friction, etc. on the parts.

At a static load like cruising down the freeway at 65 mph things may be pretty close as far as drivetrain loss; there would be a certain amount of HP lost to maintain that speed, and swapping to a higher output engine should see about the same loss (although that can be different also). But in an acceleration test like a chassis dyno a higher HP engine will eat up more power than a lower HP one while accelerating the trans/driveshaft/rear end, etc.

Jody