Ask Away! with Jeff Smith: Drivetrain Loss & Comparing Engine Horsepower vs. HP at the Wheels

I hear numbers thrown all over the place about the horsepower loss from flywheel power to rear wheel power. I’ve heard 10 percent up to 25 percent—which is a huge difference in power.

Can you throw some light on this for us guys trying to figure this all out? I know that most folks are disappointed with how much power they make at the rear wheels and I think that’s where that 25 percent number comes from. But 25 percent seems a large difference to me.

T.M.

This question and answer will definitely generate some opinions. Let’s look into this and see what we can come up with.

Understanding Engine & Chassis Dynos

Flywheel horsepower is the number generated by the engine at the flywheel and most often determined on an engine dyno. Wheel horsepower is the power generated at the rear wheels (assuming a rear wheel drive car) and is measured with a chassis dyno.

Nearly all engine dynos employ a water brake to act as an absorber. Think of the absorber as a torque converter where the engine spins the input side while a calibrated lever arm is attached to the opposing turbine wheel that measures the deflection in pounds of force over a one foot lever arm. This creates the torque which is then used to calculate the horsepower using the classic formula:

HP = (Torque x RPM) / 5252

So for example, 400 lb.-ft. of torque at 6,000 rpm = 456.9 horsepower.

There are some engine dynos that operate electrically where they measure the amount of electrical power in watts that is then converted to horsepower and torque. These are generally very expensive dynos mainly used in development work.

Chassis dynos operate a little bit differently. The most popular chassis dynos are acceleration dynos. These operate with the vehicle’s rear wheels driving a large diameter drum. The dyno company knows the weight and diameter of the drum and measures how quickly the drum is accelerated. This elapsed time is then used to calculate the torque and horsepower available at the rear wheels. There are several companies building these dynos.

The second version is often called an eddy current dyno. In this application, a large acceleration drum is also used but mainly to provide a large footprint for the driving tires to prevent slippage under load. With the eddy current dyno, the power used to accelerate the drum is again measured in watts of energy created much like if you were driving a large electrical generator. These dynos are often thought to be slightly conservative in terms of output especially when compared to acceleration style wheel dynos.

Comparing Flywheel & Wheel Horsepower

It would take a small book to list all the different variables involved with comparing flywheel horsepower to wheel horsepower. There are so many that a simple percentage just does not work but yet persists within the performance industry. The most common answer to this question we’ve heard is the “10 percent loss” claim.

The problem with any given percentage is that it does not take into account the power level of the engine. It should seem logical that if we have a vehicle with a given drivetrain that an engine with 400 flywheel horsepower that it may lose as much as 100 horsepower through an automatic trans and rear axle.

That’s where this 25 percent comes in.

But if we add a 200 hp nitrous system that now makes 600 hp, will we still only lose 100 hp through the drivetrain? Probably not—it will probably lose more than 100 hp Here’s why.

Let’s say we’re driving a big heavy C6 transmission behind a 351W small block Ford engine making 400 flywheel horsepower. Now if we bump that to 600 hp, it will accelerate the chassis dyno wheel more quickly but it also must accelerate the big direct drum in the C6 automatic at a much faster rate. This will require more power to accomplish this, so now the amount of power required to drive the rear wheels has increased. How much? It’s difficult to say without specific testing.

To take this idea even further, with 1,200 horsepower that same drivetrain will demand even more power to spin it because with that much flywheel horsepower, it won’t take long to accelerate that big chassis dyno drum. So there are way too many variables to assign a straight percentage number to rear wheel horsepower versus flywheel horsepower.

Let’s give you examples of two completely different power numbers from flywheel to rear wheel on two different vehicles to see how the power changes. The first vehicle was a small block Ford tested by our good friend Kevin McClelland many years ago when he was at Flowmaster. He tested a small block Ford that made 360 flywheel horsepower and then tested the engine again in the car on a chassis dyno using the same headers but with a complete accessory drive hooked to an automatic transmission. The rear wheel power dropped to 260 hp. That’s a loss of 90 hp or a 25 percent loss.

The next car was a 455 Buick that belonged to his dad, Dave McClelland. It dyno’d with 330 flywheel horsepower at 4,500 rpm because it was near stock. With the engine in the car and spinning a Muncie four-speed and 12-bolt, the rear wheel horsepower dropped to 280 hp which is a difference of 50 hp. This calculates out at a 15 percent loss through the drivetrain. This reduced percentage loss is mainly due to the minimal loss through the manual trans and the 12-bolt rear axle which requires less power to spin than a Ford 9-inch.

You can see that there’s a big difference between 25 percent and 15 percent and neither were even close to 10 percent. The point here is that a simplistic percentage number isn’t going to be valid across a wide variety of vehicles and power. There are literally hundreds of variables that can affect rear wheel power. For example, our engine testing has shown that just removing the truck accessory drive from an LS engine can be worth roughly 10 to 12 flywheel horsepower. This is important since most engine dyno tests use an electric water pump instead of the normal accessory drive for convenience and also to make the flywheel horsepower numbers better.

So the bottom line is there is no simple way to estimate the power loss through the drivetrain to come up with a flywheel horsepower from rear wheel numbers and it should be obvious that there is no simple percentage number that can be employed across a wide variety of applications. There are just way too many variables. Certainly we can estimate and be close but that’s about as good as it gets.