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Written by Devin Durham
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Friday, 18 July 2008 |
Underdrive pulleys are marketed as an easy 10 horsepower bolt on that benefits any car that any shade tree mechanic can install in an afternoon. But do they work? And how do they work? And what are the disadvantages, if any? This is a very common performance part that is commonly seen in the drag racing pits, but is it just a bragging rights part or does it actually produce?
Theory March's UD Pulley set
There is also the consideration of weight. Most UD pulleys are crafted at a lighter weight than factory pulleys, and have a corresponding decrease in rotating mass. This may also provide a performance increase.
HistoryUD pulleys got their start on racing motors but not as a way to increase power. Back in the day, racing engines were almost always stock motors that had been severely modified and were running at significantly higher RPMs than the factory ever intended, as a result the accessories were being spun at an excessive rate. This can cause problems with the alternator as they tend to melt themselves if spun too fast and with the water pump which can enter a state called cavitation, where the water is only being churned up instead of actually being pumped anywhere.
UD pulleys were the remedy for that. By reducing the ratios that the crank pulley and accessory pullies were operating at, the accessories slowed down greatly, allowing them to run at a regular speed even though the engine itself was revving very high.
Sometime after that the idea came into fashion that the same mechanism could be applied to regular engines to increase power output to the wheels.
Potential Problems New water pump pulley versus old. Shiny!
It is important to point out that the majority of these issues are most likely to rear up when the car is at idle, and most of the UD pulley manufacturers recommend that the car's idle be increased by 100 rpm or so to alleviate them. This can be problematic on any car with electronic engine computers, as most of them use some sort of electronic idle control that isn't easily changed, so that needs to be taken into account.
Installation Lincoln works on the Lincoln
This particular set piggybacks onto the existing harmonic balancer, a concession to the need of a balancer that in many ways eliminates any rotating mass gains that would come from the overall lighter pulley design. Steeda in particular makes a 4.6L pulley set that comes with its own harmonic balancer and it is possible that it offers better performance because of that, although that can be debated (and evidence of that can be found with a simple Google search for "Underdrive Pulleys.")
After removing the radiator cowling and cooling fan, an impact wrench was used to remove the existing nuts holding on the accessory and crankshaft pulleys. The accessory nuts came right off, but the crankshaft bolt was stubborn, so a breakover bar was brought to bear on the problem. That ended up just cranking the motor... since the Mark VIII has an automatic transmission, the motor freewheels in park. Experimentation showed that the motor turns regardless of engine setting (the torque converter was allowing it to spin) so how on earth do you crank on/off that bolt?
 Harmonic Balancer removal tool
The harmonic balancer has to be removed using a pulley puller, or in this case a special harmonic balancer removal tool. Once removed it was bolted onto the new pulley and the combined assembly was bolted back onto the engine, the rest of the install flew by. The entire installation took about an hour and a half. If these pulleys were installed on a front wheel drive car, you could expect the install times to increase greatly.
ResultsIt is important to start this section with a warning that will foreshadow what is to come: any product that promises a 10 horsepower increase is mostly likely just a marketing gimmick. 10 horsepower is extremely difficult to track unless you have immediate before and after access to a well tuned Dyno, and it is something that is very easy to talk yourself into feeling. The buttometer can not be trusted!
That being said, the negatives of the new UD pulleys came into play almost immediately. At idle the car certainly was not charging the battery as well as it once had (and in the long term this caused a few problems where the car almost did not restart after a long session of idling since the battery was nearly depleted). The coolant temperatures at idle and on the road were higher than before and the air conditioner clearly blew colder air if the engine was above idle.
For reliable street duty, the idle will need to be increased by 100 or 200 rpm, but that was addressed by the manufacturer so it can't really be classed as a failure.
On the plus side, these cues did prove that the UD pulleys were doing one of the things that was advertised: the accessories were turning more slowly. But does that translate into wheel power?
After hundreds of pre-install 1/4 mile runs and dozens and dozens of post install 1/4 mile runs it can clearly be stated that the underdrive pulleys made NO noticeable difference in the performance of the Mark VIII. The car has some shiny new parts, and "Underdrive Pulleys" can be added to the bolt on brag list, but when the rubber meets the track the value just isn't there.
Why is this? At the moment the leading theory around here is the fact that the accessory that offers the biggest drain on horsepower is the alternator, and the alternator is a regulated system. The computer knows how much power it needs to charge the battery, and it will demand that of the alternator no matter how quickly it is turning. All slowing it down does is make it work harder, but the net end result is the same: the alternator draws off as much power as it needs, period. The why is not as important as the results though: the UD pulleys made no difference in any 1/4 mile metric... the 60', 330', 1/8, 1000' and 1/4 mile times were all unaffected.
Interestingly there was one positive outcome that might make the whole system worth it. The Mark VIII has an onboard computer that keeps track of, among other things, constant and long term fuel economy. After the pulley install the in city economy went up by about 1 MPG and highway economy went up by about 2 MPG. This would seem to argue that the UD pulleys are in fact in some way translating power to the ground, so maybe it is picking up those 10 horses after all? In today's world of high pump prices, it's nice to have found a performance modification that added economy instead of lowered it, even if it didn't deliver at the track.
In the end the UD pulleys get a +1 for economy, but a -1 for failing at the drag strip and a -1 for the negative effects at idle. All tallied together this is not a performance addition that we can recommend.
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Last Updated ( Friday, 18 July 2008 )
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