2012s4

Drivetrain Loss: Horsepower Losses Through FWD, RWD and AWD Drivetrains

Knowing your horsepower at the wheels is important in calculating your power-to-weight ratio, which is a good indicator of how your car will perform against other cars with known power-to-weight ratio. But do you ever wonder what is the flywheel, engine horsepower on your 400 whp Subaru STI? Or what is the approximate wheel horsepower of the stock 510 hp Viper? Let’s set the record straight on the drivetrain losses in All-Wheel-Drive, Rear-Wheel-Drive and Front-Wheel-Drive cars.

Drivetrain horsepower loss is the loss of horsepower between the engine (flywheel) and the wheels due to (mostly) friction. It takes some power to turn transmission gears, transfer case, driveshaft, axles, etc. – before the power gets to the ground (wheels). Generally, car manufacturers advertise horsepower figures as measured on an engine dyno, at the flywheel. They do not say how much horsepower does the car make at the wheels.

Wheel horsepower is a more precise measure of car’s performance than flywheel horsepower, since drivetrain losses vary between cars. If you know your car’s wheel horsepower and weight, you will be able to roughly gauge how your car will perform against other cars in acceleration. Yes, you can figure in gear ratios, traction limits and other factors, but for a rough estimate of how will your Subaru WRX / STI perform in acceleration versus say, a stock C5 Corvette, you only need to know the power-to-weight ratio of both cars.

For example, a stock 2006 STI makes advertised 300 horsepower at the flywheel, and about 235-240 hp at the wheels. A stock 1998 C5 Corvette has 345 hp at the flywheel and about 299 whp. Of course different dynos would read differently, we take these sample whp figures to illustrate an example. Corvette weighs 3246 lbs, and the STI weighs 3351 lbs, both without drivers. Now you can calculate power-to-weight ratio of both cars. STI: 3351/240 = ~14 lb/whp. Corvette: 3246/299 = ~11 lb/whp.

Based on the calculations above and taking into account traction issues that the RWD Corvette will have on the street, you can safely assume that the STI will beat the Corvette off the line, but the Corvette will fairly quickly catch up and leave the STI in the dust. (Usually that happens right past the 1/4 mile mark but that’s not the point here). Racing from a roll will end in the STI being left behind, due to more weight per one wheel horsepower.

So does it matter what the drivetrain loss is? Since the manufacturers are unwilling to advertise their cars’ wheel horsepower, yes it does. Knowing the flywheel (crank) horsepower is good not only for bragging, but for doing rough reverse estimates of car’s wheel horsepower, if you know the estimated percentage loss for the drivetrain type (FWD, RWD, AWD).

The drivetrain loss is commonly measured in percentage, but it is never an exact number, only a rough estimate. The drivetrain loss is not linear and will vary based on RPM, drivetrain load, and other factors. So you must know that when we say ‘drivetrain loss’, we are really talking about a rough estimate which is not a scientific ratio or exact percentage.

However, it makes sense to refer to the drivetrain loss as a horsepower loss percentage range and not exact horsepower loss. Let’s say a stock 2006 STI has 300 flywheel horsepower and dynos at 240 wheel horsepower. That is a 20% drivetrain loss, or 60 horsepower. If you were to add another 200 hp to the STI through engine/turbo mods, the drivetrain hp loss will not remain at 60 horsepower. It will increase due to the additional stress on the drivetrain components and added friction/heat from putting more power through the drivetrain. So it would be safe to estimate the drivetrain loss as a percentage range, say 17-25% for AWD cars.

There is no way to calculate the exact amount of horsepower loss through the drivetrain unless the engine is dynoed separately from the car.

You will find tons and tons of conflicting information on the Internet about what the loss percentages are for different drivetrain types. Below are the correct drivetrain loss percentages for each drivetrain type. Keep in mind that the drivetrain loss is about 2-5% higher in cars with automatic transmissions.
■FWD: 10-15% loss;
■RWD: 10-18% loss;
■AWD: 17-25% loss.

http://rusubaru.com/drivetrain-loss/

sakimano

this is funny

actually, isn't that just more 'conflicting information on the internet'


There is no 'standard' drivetrain loss range for any driven car for 2 reasons

1. there is not one standard dyno configuration...I can calibrate a Mustang/Dynojet/Dynapack/DynoDynamics dyno to tell you that your B8 S4 made 200 whp or 400 whp. To that end, the 'range' for AWD would need to include those two readings, which would indicate -39% to +23%.

2. drivetrain 'loss' is a measurement of dyno power vs. factory claimed engine hp. Since none of us have an engine dyno, and tuners don't like pulling engines to dyno cars, the only engine hp levels we have to use are factory claims. When companies (routinely) lie about the horsepower their cars make talking about 'loss' to compare back to factory claimed levels is absolutely useless unless you're a math major and are doing some light division for fun.

The Nissan GTR routinely exhibits '10%' drivetrain loss. The Audi S4 exhibits the same. Why? I thought 17-25% was 'correct' for AWD cars? It's simple...both companies underrate the power at the crank in marketing material...thus that information is useless (as is worrying about drivetrain loss.

NWS4Guy

Quite. DTL is not a fixed number or percentage. It changes based on speed.