Ask Away! with Jeff Smith: Exploring Parasitic Power Loss in an Autocross Car

I have a small-block, four-speed ’71 Camaro that I race in autocross. How much horsepower do the following items use on a small-block at 6,000 rpm? Even a guesstimate would be great. Thanks.

  1. Flex fan – 18-inch diameter
  2. Alternator
  3. Water pump
  4. Power steering pump
  5. Exhaust 2 1/2-inch exhaust tubing with Flowmasters
  6. TKO 600 transmission


I have testing results for some of these components, but the rest will have to be estimates. Several are unfortunately tempered with way too many variables, but it’s an interesting question so we’ll take it on.

If nothing else, it will stoke the fires of opinion.

We did some testing a long time ago on both the engine and chassis dyno that showed that engine-driven fans can eat a lot of horsepower.

In one case, we saw on a small-block Ford that a larger clutch fan when fully engaged ate a whopping 22 horsepower at the rear wheels. Flex fans are also notorious for eating power and certainly 12 to 18 hp wouldn’t be unusual. Remember too, these last two examples are specifically rear-wheel hp numbers.

This makes the addition of an electric fan combination very appealing if you could gain anywhere from 12 to perhaps 20 rear-wheel hp. That’s huge, and I think it would be very advantageous on the autocross track. Just remember, the larger the fan, the more air it pushes and the more hp it will require.

Many will claim that electric fans are not “free,” which is true of course. The power required to drive the alternator for a pair of 12-inch electric fans however, is less than 1 hp. I would trade a 1 hp draw on the alternator for a gain in 17 flywheel hp.

Even an alternator cranking out full output of something like 140 amps at 14 volts is equal to roughly 2,000 watts (1,960 actually, but we rounded up). A full 2,000 watts of continuous power is equal to 2.7 hp. Assuming losses from heat and everything else, it might be 3 hp. But the reality is that your alternator on the autocross track will put out maybe 70 amps, which is half of the full-output amperage, so now we’re talking less than 2 hp. Which isn’t much in the grand scheme of things.

Water pumps are a bit of tricky question. We tested a complete truck accessory drive on an LS engine and saw roughly 9 hp improvement when we substituted an electric water pump. That number may not be accurate but I can’t explain why—it just seems high. The alternator wasn’t charging , there was no power steering pump, but the water pump was fully functioning and the rpm was 6,500. Lower engine speeds, an underdrive pulley setup, or different design pump could easily reduce this number to less than 5 hp.

A power steering pump doesn’t really demand much power at all and when it does—under a turning load in the corner, you are only lightly accelerating, so power really isn’t a concern. Weight might actually play a larger role than parasitic losses on the autocross, especially because the pump and box are located so far forward on the car.

The exhaust system might be one place where the added weight of a 3-inch section from the headers to the muffler could be an advantage. We’ve seen multiple examples of systems where a larger pipe from the header collector acts like an amplifier to enhance the reflected wave returning up the exhaust system to work on the induction system. On the engine dyno, we’ve seen longer, 3-inch collectors add significant torque below peak torque. Of course, there’s no guarantee that will work in all applications, but it certainly won’t hurt power.

A transmission will use the most power when in any of the lower gears (not 1:1). Because of the way the power is transferred through the box, each time power changes direction from the input, down through the cluster, and then back up through the output shaft, this creates heat and friction, and that consumes power. How much power depends on multiple factors so it’s difficult to say with accuracy, but a minimum of 3 to 5 percent is considered typical, so about 12 to 15 hp.

An automatic transmission will eat more power mainly because of the slippage in the torque converter, but also because of the loss from driving the pump that creates the hydraulic pressure plus having to spin the mass of the clutch packs and planetary systems.

Overall, with an automatic transmission, you can expect to see a loss of around 100 hp, and slightly less with a manual transmission compared to power numbers measured at the flywheel.

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  1. Charles Brooks says:

    Loss of 100 hp in an automatic transmission!!! I don’t think so.

    • What about a lenco, 3sp with a reverse. We used a lenco instead of a ford top loader and did not seem to gain much in et or mph. This was back in the seventies, 331 sb running 9:10 9:20. Is there an advantage with the lenco or did we waste 4000 dollars? We seemed stuck right there. I seem to remember the mph went about 5:mph more but no better et. We 6:76 in the rear and went thru the trap about 145 mph. Pro stock, Sprit of America , Vega.

  2. Jeff Smith says:

    Actually 100 hp loss through the drivetrain is about normal. I can show you a flywheel dyno test of a supercharged 4.8L engine that made 630 hp on race gas. We then put the engine in my ’66 Chevelel with a relatively tight converter adn 2004R automatic transmission and a 12-bolt with 3.55 gears. On Westech’s chassis dyno the car made 505 hp at the rear wheels. Granted, we pulled a little timing out to account for using pump gas instead of race gas. But the reality is the rear wheel hp was an honest 100 to 110 hp less than the flywheel hp. The car runs a 3-inch mandrel bent exhaust system vs a similar setup on the dyno that day. So the drivetrain and automatic ate 100 hp. I’m open to a discussion about where the power went if it was not absorbed by the drivetrain, but this is the reality. I don’t make this stuff up and I don’t make statements in columns like this without data to back up the information.

  3. Parasitic transmission drag is measured in percentage not in absolutes. Somewhere around 20% old school. I’ve heard around 10 to 15 % with today’s transmission technology

    • Jeff+Smith says:

      I have to respectfully disagree with that statement. Let’s use some simple math to illustrate this – if we retain the exact same drivetrain but increase power from 400 hp normally aspirated to 800 hp with a blower – according to your statement, the drivetrain loss with a 20 percent absolute would jump from 80 hp (400 x 0.2 = 80) to 160 hp (800 x 0.2 = 160 hp). If that is true, then how do we explain the additional loss? The truth is that there will be a slight increase in drivetrain loss because the drivetrain will accelerate more quickly, which will require more power to accomplish that acceleration. The answer is that if we had a 100 hp loss at a given power level, that might increase to 110 or 115 hp with double the power input where the acceleration is now quicker. But the loss will not double. I’m open to a further discussion on this point.

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