Dyno science – why do some cars make less power after a tune?

In the latest episode (see it HERE) of Mighty Car Mods we see MOOG's VW RS3 run on the dyno and some people will be wondering if a stock RS3 is rated by Audi at 394hp (294kW), why did MOOG's car make LESS than that after all those mad Revo mods?  Actually, kids, it didn't.

*SPOILER ALERT* DON'T READ ANY FURTHER ON IF YOU HAVEN'T WATCHED THE VIDEO AND WANT TO KNOW HOW MUCH POWER MOOG MADE

When manufacturers display how much power a car produces, they are claiming this as measured at the crankshaft, not at the wheels or hubs like most aftermarket tuners do. The other fact which can be confusing is the difference in measuring power, between kilowatts and horsepower, as one measurement is metric and one is imperial. I'll get to those differences later, but let's look at why cars make less power "at the wheels" compared to their rated power at the engine...

A car's drivetrain (transmission, differentials, etc) all chew up little bits of power through what is known as parasitic loss because it takes little bites of energy to get all the parts of a drivetrain (transmission, tailshaft, driveshaft, wheels and tyres) spinning to put the engine's power to the ground. Back in the old days it used to be thought that as much as 25% of a car's power would be "lost" by the time it got to the wheels, though modern cars seem to be much more efficient.

This relationship is not always linear, as a 1000hp car isn't necessarily going to lose 250hp through it's drivetrain, just like a 100hp car may not lose 25hp. But, you get the idea: gearboxes and diffs other rotating parts all change up the numbers

So, a brand new RS3 will have 294kW (394hp) as measured at the engine but most will only produce around 230kW (308hp) at the wheels, indicating a parasitic loss of 64kW (86hp) through the stock drivetrain. MOOG's RS3 made 290kW (388hp), approximately 60kW (80hp) more than what a stock RS3 should make at the wheels. If we estimate his car also lost 64kW (86hp) through the drivetrain then his engine should produce 353kW (474hp).

Why would a different dyno read different power amounts? Some of it comes back to the type of dyno used, whether it is a chassis dyno or a hub dyno.

A chassis, or roller, dyno is what most people think of when they hear power figures. These dynos use a car's wheels to turn drums, and the dyno computer reads the RPM the drums spin at and the force they're being moved at to calculate horsepower, torque, and about 4000 other things.  

It is far easier to tune cars and record power figures using a chassis dyno as you can literally roll the car on, strap it down, and go. Measuring power through a hub dyno, or on an engine dyno, takes much, much more work to set-up, which takes time some businesses don't have spare.

Hub dynos involve bolting the dyno's measuring drive flange directly to the hub of the car, thereby removing any potential power lost through spinning wheels. Wheel spin on the dyno can be an issue for very powerful cars, or cars whose power ramps in very quickly, though the readings on a hub dyno are normally higher as you don't need to waste power turning the wheels and tyres. 

Here you can see Choptimus Prime's had its wheels removed and the dyno attached directly to the hub, hence "hub dyno".  The Barra-powered monster went on to make over 1000hp at the hubs minutes after Haltech took this photo.

There are other factors that can affect power read-outs, too. Differences in the weather and atmosphere affect power as it all comes down to how easily an engine bay can breathe - cold, dense air makes more power than thin, hot air. So a car on the dyno in the tropics will generally make less power than a car in cool, temperate weather.

So what is the difference between horsepower and kilowatts? 

"Horsepower" came from James Watt, the father of the steam engine, who worked out a mathematical equation for how much work a horse could do and therefore how much power that horse had. Old Mate Wattsy reckoned one horsepower was required to lift 33,000 pounds of mass, one foot vertically, in one minute. 

Of course, this was in the Back In The Day Times of imperial measurements, and most countries use the metric system today. Today Watt's equation works out to be around 15,000kg lifted around 300mm in a minute, which is the equivalent of 0.7457 kilowatts (geddit? Kilo... watts... ho ho!)

Further complicating power figures is the old way power was advertised by car brands way back when bumpers were shiny and chrome. American brands typically published "SAE" power figures from the Society of Automotive Engineers. These were the amount of power an engine made with open exhausts and no power-robbing ancillaries like power steering pumps, water pumps, and the like, thereby falsely inflating the true figure of what the engine could make when installed in a car.  

Car manufacturers today have to stick to an international standard for measuring the power their engines produce. However, you may see mention of "PS" (pferdestärke), "CV" (Cavallo Vapor) and "ch" (cheval vapeur) from Germany, Italy and France, respectively, as these figures are metric measurements of "horsepower" but use their local language to describe it.

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