AWD lost HP

[eps]

People say that AWD loses horsepower and I was wondering why this is. If power is going to all the wheels, and all the wheels are on the ground, why does it lose HP?

Also, just a quick question, is 4WD the same as AWD?

[Mediocrity]

All cars lose power to the ground because the power has to travel to the wheels - resulting in the loss.

in AWD cars - the power has to be split between all 4 wheels... thus a greater power loss.

That's the extent of my knowledge... someone else can come in here and give the real scientific version of it for you.

[RACER D12]

Originally Posted by Mediocrity

All cars lose power to the ground because the power has to travel to the wheels - resulting in the loss.

in AWD cars - the power has to be split between all 4 wheels... thus a greater power loss.

That's the extent of my knowledge... someone else can come in here and give the real scientific version of it for you.

Thats about as much as I understand to.

To answer your second question no, 4WD and AWD are not the same. I believe 4WD when turned on sends power to all 4 wheels and the amount of power sent to each wheel doesn’t change. (You see a lot of trucks with 4WD) AWD on the other hand in some cars isn’t always on. Like in our old Audi it would come on when it detected wheel slippage. Also AWD systems don’t always send the same amount of power to each wheel. The power sent to each wheel can very depending on traction. So that’s what I think the difference is but I may be wrong.

[eps]

The way I understand 4WD/AWD is that the transmission is connected to the rear wheel driveshaft. The rear wheel driveshaft is connected to the front wheel driveshaft via the drive chain. So when the RW driveshaft is spinning, the FW driveshaft is also spinning. I can see some power being lost in this transfer, but it wouldn’t it be minimal?

As for AWD, if the AWD system is only active when a loss of traction is detected, wouldn’t it be just like a RWD car after the launch? So on the launch when the rear tires start to lose traction, the AWD kicks in and puts power to the front tires as well. After the launch and the tires have traction, the AWD system is turned off and the car is running off its rear wheels which would mean no power is lost.

This is all theory. I don’t even own a car, heh.

[flylwsi]

wrong.
there aren't any vehicles on the road that use a chain system, especially not anymore.
they use transfer cases.

awd sends power to a center differential (on cars) or a transfer case (on rwd vehicles, truck based).
the diff or transfer case splits the power front/rear.

awd and 4wd aren't the same.
awd is constant, 4wd is switchable, like in a truck.

awd systems are not all the same, and are not all traction related, as you mentioned.
different systems in different cars do different things, and act in different ways.

there's a "tech" forum here, under "cars in general" which may be a much better place to get your question answered.

lost hp, as you're referring to it, is driveline loss.
all drivelines lose power, which is used to spin the driveline.
manufacturer hp claims are at the crank, before you hit the trans/driveline.
therefore, whp is lower than crank hp, b/c power is used to spin the driveline and wheels.
makes sense that spinning 4 wheels will cause more "lost hp" than a 2wd car, huh?

[eps]

Thanks for taking the time to explain it to me.

[flylwsi]

not a problem...
the tech forum is your friend...

also, try www.howstuffworks.com

awesome amounts of info there

[StupidBrodie]

is a viscous coupling considered a center diff???Thats what i have on my car.. i think DSM's are the same...

[syr74]

When push comes to shove the real problem is that there exists no real set way to say "this is awd and this is 4wd", the line gets blurry sometimes. While the previous statement that awd is full time and 4wd is not has some merit, it is not always true.

Case in point. My 1979 Jeep J10 pickup had full time Quadra Trac 4wd. It was constant like a typical awd setup, but it also had locking differentials that can be set by the driver with the turn of a knob. Many of these trucks (J10's, Grand Wagoneers, etc.) came with dual-range transfer cases as well.

There are a lot of very knowledgable people (and I mean off-road gurus and legends) who will tell you that if it has a dual range transfer case or locking differentials it is a 4wd. Put simply if you have a hi-lo transfer case AND locking differentials like the J10 does it is likely best called a 4wd. But, the full time nature will throw you off too so it can be difficult to pin things down.

[Skavoovie]

Typical drivetrain loss numbers I see are:

FWD or similar layouts (MR, RR) = 10-15%
RWD = 15-20%
AWD = 25%+

Keep in mind these are rough estimates, some drivetrains are more efficient than others. DSMs are notorious for being pretty inefficient (around 30% loss, from what I've read). Also, layouts like the Corvette with it's transmission on the rear axle can help boost efficiency.

The more moving parts you have, the more loss you're going to have. Power is lost in the form of heat from friction. Differential lube can easily get up to 180 degrees C and probably much more (the hard numbers I have come from the lube in a GM 10.5" rear end which operates at around 160 degrees C with peaks up to 180).

Typically, the current definition of 4WD vs. AWD is that 4WD uses a transfer case for selectable low and high range gears. AWD is lacking this. 4WD drive systems may or may not have a center differential. This is mainly due to the purpose of each system. AWD is designed for slippery on-road conditions while 4WD is the system of choice for off-road. Most of the new full time 4WD employ a center diff. because a transfer case will not allow the front and rear tires to spin at different speeds which is less comfortable and promotes greater tire wear in non-slippery surfaces.

Speaking of differentials, what we traditionally call 2WD and 4WD is really 1WD and 2WD. A normal open diff. will send 100% of the torque to the wheel with the least amount of traction. So with 4WD vehicles you have one drive wheel on the front axle and one on the rear. Limited Slip Diffs. help solve this problem and are commonly used on AWD vehicles.

LSDs (sometimes called by the GM brand name PosiTraction or Posi) come in three basic flavors: Clutch-type, Torque Biasing, and Viscous Coupling. Clutch-type using springs and clutches to lock the two sides of the diff. together. Problem with them is clutches wear out and it isn't the most elegant solution. Torque biasing (sometimes called by the brand name Tosen) use additional gears to, as the name implies, bias the torque between wheels. It's an elegant solution, but expensive. Viscous coupling diffs are commonly used in center diffs. They contain a bunch of plates and a liquid that becomes viscous under friction. So when your rear wheels start to spin, the liquid becomes more solid, causing the plates to start making the front wheels turn, transferring torque to those front wheels.

Locking differentials are also used by hardcore off-roaders. All LSDs will never provide a perfect 50/50 torque split. The only way to do this is by locking both sides of the diff. together. This is done several different ways. Mechanical systems can lock the diff. under wheel spin like an LSD, selectable systems can be mechanically, electrically, or air actuated from a button in the cab, or you can have a full time locker. Full time lockers, called a spool, can simply be a Lincoln locker aka the spider gears welded to the carrier, or full out racing spools or mini-spools. The rule with spools are unless you KNOW you need a spool, you don't need a spool.

Edit: Fixed spelling