Underdrive Pulleys – Our Testing

by James Walker, Jr. of scR motorsports (2000)

A few comments from our dyno experiences at team scR.  Since we are allowed to change accessory drive components, we have looked into issue this on more than one occasion.  With all of the wild horsepower gains advertised, we figured that it was at least worth a solid investigation.  Here’s what we found.

Alternate ‘Crank Pulleys’ 

At first glance, the most obvious change to make to your accessory drive system would be to change the biggest part in the system – the crankshaft pulley.  If you go to the Saturn factory parts book, however, you will see that the crankshaft pulley is not referred to as a pulley – it is known as the crankshaft harmonic balancer assembly.  Here’s why:

During the hectic life of a crankshaft, it is subjected to numerous loading conditions and consequently will ‘flex’ or ‘bend’ at very high frequencies.  If this ‘flexing’ is severe enough, the crankshaft will fatigue and break into lots of small pieces.  In general, this is not viewed as a performance enhancing modification.

Now, in order to reduce some of the more severe vibrations, the Saturn Powertrain designers elected to hang a big old hunk of metal on the end of the crankshaft to damp out (absorb) those frequencies which occur at the natural frequency (or resonant frequency) of the crankshaft assembly.  By reducing these frequencies, the life of the crankshaft would be extended, and during its life, the noise and vibration levels from the engine would be reduced.

To make sure that the big old hunk of metal damped the right frequency range, calculations were performed to determine the exact weight required to attack the most severe frequencies.  Math, I know, but don’t run away just yet.

From this, you now realize that the weight (or mass, as the engineer would say) of the balancer is CRITICAL to the happiness of the crankshaft.  Vary from this weight, and best case you will get more vibration in the passenger compartment.  Worst case results in spewed parts under your car…and money spewing from your wallet.

Case in point: last season our engine builder was experimenting with alternate crank pulleys on BMW engines.  Like the other BMW teams (six or so) in the series, they all elected to run these pulleys on a given race weekend.  Anyway, four of the six cars went home that weekend on the trailer with broken cranks.  Tell you something?

This does not mean that alternate Saturn crankshaft pulleys will cause your crank to snap in two instantly, but that pulley is big and heavy for a reason.  A very good reason.  If nothing else, it’s something to think about.

Other Pulleys

In general, the other pulleys in the system are not sensitive to weight like the crankshaft pulley, so harmonic imbalance is not a concern here.  If you do decide to underdrive, though, be aware that while there may be a slight horsepower gain (read more on this below), your components will not be operating in the ranges determined by the factory.  If you are on a race track at constant high RPM, this may actually prolong the life of these components, but on the street, it could go either way – a water pump or power steering pump which is designed to operate at a given speed may not be as efficient as necessary when underdriven.

Another perspective is that auto manufacturers such as Saturn are always trying to squeeze the last 1/10 of a MPG from their vehicle’s fuel economy estimates.  Big dollars and big engineering budgets are dedicated to this task, and you can bet that if any component could be underdriven further (a potential plus for fuel economy) and still meet its performance and durability bogeys, it would have been already.

The Horsepower Story

With various aftermarket manufacturers claiming up to 15-horsepower gains with their underdrive pulleys, we set out to the dyno to find out how much power WE could expect on our 1999 ITA-prepped Saturn twin-cam powerplant.  The results might surprise you, but here they are.

Because there were no pulleys available at the time, we went to the extreme and actually REMOVED the accessory drive belt (only for one run, of course) to see what the impact would be to ZERO drag due to the accessory drive – sort of pulleys with infinitely small diameters.  We figured that if we saw a big gain without a belt, that the pulley gains would be somewhere in between, right?

We fired up the dyno and low and behold, removing the belt netted a whopping three HP (and we’re rounding UP).  That’s it.  The number was repeatable, and even correlated to data run earlier on the doomed BMW motors – they saw similar results in their application.

So, in light of the fact that three HP was completely without a belt, we estimated that the best we could do was to underdrive a few components by 20% – or a theoretical 0.6 HP gain (3 HP x 0.2). After that, it hardly seemed worth it to try to custom fabricate pulleys for less than a one HP gain.

I know, I know – stock pulleys, like stock suspension bushings, aren’t sexy, and you can’t brag about them in the pits…but in racing horsepower talks and marketing BS walks.  We chalked up this exercise as yet another learning experience in the quest for REAL horsepower.  Maybe someday we’ll find some.