Under drive pulley - How much improvement?

[99ParkAvenue]

Looked for previous discussions and did not see one that answered my question, and that is, how much gain in mpg from using an underdrive pulley? I know it make a little more horsepower, but I was wondering how much gain is there from the reduced accessory drag? 2%?

[t vago]

Not a whole lot, if my experiences were any indication. I ran a 20% UDP on my truck last year, and did not see any beneficial increase in fuel economy.

[SevenT6Vet]

underdrive pulleys do two things well: prolong accessory life and increase peak hp. anything else is pretty trivial in MOST cases.

[redpoint5]

The gain probably wouldn't be more than 1%, which is undetectable with my instrumentation. I'm running a 20% under-drive on the TSX and didn't notice anything.

There is the risk of not having enough alternator output to keep up with electrical demands with the under-driven system. Here is a chart showing the measured voltage of a TSX with a 20% under-driven pulley.

Original post

[justme1969]

effect is dependant on Idle run time, city/ hiway, day or night,
dont expect a miracle though. all things add up over time.

[UltArc]

Originally Posted by justme1969

effect is dependant on Idle run time, city/ hiway, day or night,
dont expect a miracle though. all things add up over time.

100% agree. All things add up, over time.

I don't know this for fact, or if it applies to this situation, but I have read a lot about higher performance vehicles who switch to these for increased performance (as mentioned).

I would think that similar to a lighter drive shaft or wheels, no performance is "gained" but there is less parasitic loss. YMMV, and how it works in your vehicle may not be very valuable, but I couldn't even speculate on that.

It seems as engines become more advanced, upgrading this component is less and less effective.

[redpoint5]

Just as a radiator must be sized for worst case scenarios, the water pump, alternator, AC, and other components are over-sized for typical use. They are sized to deliver adequate performance under severe conditions that are not often encountered.

Just as grill blocking may be appropriate for some applications, under-driving various accessory and cooling systems may be appropriate. The gains from under-driving are miniscule to be sure. The overall load of the alternator, for example, still has to meet the demanded draw. While it may not output as much power when under-driven, it will make up the power difference by running longer. The same can be said of the AC.

The fuel savings really just come down to slightly less frictional loss due to spinning all accessories at a slower rate. In an efficient system with good bearings, this represents a very small gain. There are also tiny gains in fuel efficiency by having lighter weight pulleys that spin up to slower maximum speeds; less energy is wasted in accelerating the rotational mass.