Friday, May 24, 2013


I ran across a neat modification that wasn't too expensive: Underdrive pulleys. When I first saw the name it looked uninspiring. "Under" sounds like less. However just like with weight, when it comes to engine drag, less is more.

When your engine runs it turns the wheels to move the car. That much everyone knows. It's easy to forget that the engine turns a bunch of other things, the alternator, the water pump, the AC among them. Each of those things takes a certain amount of energy that would otherwise be available for turning the wheels and making you go fast. These accessories are turned by a pulley on the serpentine belt, and the size of the pulley for the alternator determines how fast the alternator spins. (of course for a race car, one simply removes the AC all together...)

Spinning these accessories faster of course takes more energy, and slower takes less. This is how an "underdrive" pulley helps. It spins the accessory somewhat slower leaving more energy for the wheels. This also reduces the wear on the accessory somewhat. It also makes the car slightly easier to start, and reduces engine drag on lift.

So why didn't it come like that from the factory? Because factory cars are optimized for street driving at lower RPMs, whereas race cars spend most of their time at high RPM. Since accessories such as alternators and water-pumps are needed to cool the engine there is a limit to how slow you can go. Additionally to spin slower you need to increase the size of the pulley. At some point pulleys would start touching (and that would be BAD), so that's another limit.

I got my new pulleys from Monkey Wrench Racing, and they claim that it's worth about 3whp on a celica that makes 158 peak wheel horse power, which is just a tad less than most Elise's make with the same engine. A stock Elise makes around 162whp. My engine tune from charlie-x adds 8bhp which is around 6-7whp, and this pulley should give me another 3whp... so that should put me in an (unverified) range of about 170-172whp. My muffler and air in take might possibly add a little more, but most folks say the CAI doesn't do anything for power, and muffler changes don't have much effect until you improve the catalytic converter, which is the limiting factor for the system.

What's 3whp worth? Not a whole lot, but the pulleys were only 70 bucks each. I'll probably gain around 0.010 to 0.020 seconds on a 60 second course... but I've lost places by as little as 0.001, so I'll take it. Every little bit helps. Many days 0.02 is enough to move up one place in the pax standings.

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