As car enthusiasts and all-around gearheads, we are all part of the debate surrounding a car's power and how it "feels." We all assume that a car makes less power when accessories like headlamps, rear defogger, and air conditioning are on - at least that's what our "butt dyno" tells us. But how much do they lose? We figured we'd set out to see if, when, and how much, in today's modern cars, power drops off with additional accessories in use. We laid our hands on a 2008 Corvette Z06 (advertised at 505 hp) and a 2007 VW Rabbit (150 hp) - two ends of the normally aspirated engine spectrum - to put some numbers down and see what kind of losses to expect from each, if any. We also wanted to determine if the parasitic losses are more significant in a high-horsepower car vs. a low-horsepower car.
But first, a little light tech: When manufacturers test an engine to determine specific crank output, they put the engine itself on a stand in an engine dynamometer room. Manufacturers use the SAE J1349 standard to rate horsepower, which means all accessories - alternator, starter, water pump, power steering pump, A/C compressor, all idler and tensioner pulleys, etc. - must be installed but not necessarily running (like the air conditioning compressor). Hoses are hooked up to provide the necessary fluids to run the engine, the stock exhaust and intake system are installed and the engine is connected to a measurement device, and everyone involved in the test retreats to a control room behind thick glass to run the engine through its revs. The SAE standards ensure consistent testing techniques.
The reason for the drop in power when utilizing accessories is due to things like the alternator, oil pump, water pump, and air-conditioning compressor - among other mechanical devices - need to "borrow" engine power to produce electricity, pump fluids, or provide air conditioning. Each of these devices sap power like a parasite from the engine, causing what is referred to as parasitic power loss.
Since engine dynamometer facilities are few and far between, we do all our power-measurement testing on a chassis dynamometer (or dyno for short). This means that we measure power at the wheels or hubs and that power will be lower than the advertised because it has to travel from the engine through the transmission and driveline, losing some output as it tries to spin gears, driveshafts, and heavy reciprocating components. There are a few varieties of chassis dynos on the market, but the one we regularly use is a DynaPak dyno that takes the wheels and tires out of the equation and attaches directly to the hubs. On high-horsepower cars, this ensures that we won't have to worry about wheel slippage issues and removes any discussion about tire compounds and balance. It is important to remember that the numbers we publish are the numbers specific to our dyno (we use the same one each time) on a given day and testing environment. They can NOT be compared with other dyno numbers as there are always slight variations from one unit to another (this goes for all dyno numbers). So while the Corvette Z06 you see here may be putting 479.8 hp to the hubs, your Z06 on a different dyno will likely produce different numbers - this is normal, despite all the Internet debates and bickering.
This is also the reason you won't see us attempt to take our wheel/hub power measurements and interpolate what the crank horsepower is. The purpose of the dyno is to make a number of runs through 3rd or 4th gear to get a baseline power number -meaning the base horsepower you start with before tinkering. Tuners that need to dial in fuel delivery, boost pressure, and more use a chassis dyno to "tune" a car, carefully looking at things like power curves and air-to-fuel mixtures to determine whether the changes they are making are producing more power or causing more problems. So for A-to-B comparisons on the same dyno on the same day (like the test we're doing) the chassis dyno works well.
In our test, the first thing we did was get the Corvette up to operating temperature and complete a series of baseline runs on the dyno with all accessories turned off (no radio, climate control, headlamps, etc.). We removed the Corvette's rear wheels and attached the DynaPak base units directly to the rear hubs. Then we accelerated car leisurely through first gear and shifted to directly to fourth gear so the test run can be done. At around 1500-2000 rpm, we accelerated in fourth to redline (and an indicated 160mph!), completing the test run. Our Z06 and Rabbit both took about 6-8 runs to stabilize power output and get warmed up. Our Z06 put a stout 479.8 hp to the rear hubs with all the accessories turned off. Next we turned on the headlamps, air conditioning (set to a medium fan level), and rear window defogger, and did more runs on the dyno to see if we lost any power due to these parasitic devices. The outcome? The Z06 lost about 18 peak horsepower with all the accessories turned on. Looking at the torque curve, you can also see a significant loss of power throughout the rpm range.
We conducted the same test with our Rabbit test car and saw a 9- to 10 hp loss with the same accessories on. Base horsepower without the accessories fell to around 146 hp, but dropped to around 137 hp with accessories turned on.
So now we have some hard numbers to back up what our "butt dyno" has told us for years - running the air conditioning and other accessories saps engine power. What we didn't expect was a near 20 hp loss on the Corvette and the 10 hp loss on the Rabbit, which underscores the importance of turning off the air conditioning and other accessories if you want maximum performance. A number of variables determine the total amount of loss, from total number of accessories attached to the overall quality and design of the accessories that feed on engine power. So while no two cars will likely be show the same losses, it is clear that from the most basic Rabbit to the 505 hp ZO6 Corvette, running accessories will lower your engine output.
We're looking for more ideas along the lines of this test to put some basic car myths to rest. Let us know in the discussion forum topic following this article any additional tests you'd like to see us conduct and we'll do our best to put them together in the future.