Do Undersized Pulleys Work


I’ve heard changing out the belt pulleys and installing undersized ones results in more power, as much as 8-9 h.p. Does anyone have any experience with this? My engine is a GMC 4.3L six.


Yes, it will help but do not expect a noticeable gain. It will not be something you will feel when stomping the pedal.
The 8-9 HP may be possible but there’s too many variables to know for sure.

Underdrive pulleys allow the engine to work a little easier and also slow down the rotation of the alternator, water pump, air compressor, and power steering pump, but the latter is not normally a problem.




if you want 8-15 more horses, install a cold air intake, or even a k&n air filter


if you want 8-15 more horses, install a cold air intake, or even a k&n air filter

I would avoid that K&N filter unless it was an older car. Unless the owner is very careful about oiling them they can damage some modern car sensors. There is also some question about how well the actually filter, although I really don’t see that as a big problem and in at least some gasoline (as opposed to diesel) they can offer some power increase.


Caddyman said it perfectly: “Save-Your-Money”

One experiment you could try would be to briefly remove the belt entirely (from a cold engine). Then drive the car around the block to see what the upper bound is on power gain.

While that would satisfy your question on whether this approach has any noticeable power gain, I still wouldn’t recommend pursuing it.


Yes, they do work. Many enthusiasts install UDPs (undersized drive pulleys) and claim an increase in horsepower. Maybe not 8-9 horses, but there is a gain.


If the gain in horsepower is at 5,000 rpm you must drive in second gear at near wide open throttle to realize the gain. What is the advertised power gain at 2,500 rpm? Most late model trucks operate at under 3,000rpm at highway speeds and the power gain is negligible there. Headers, cold air intakes, undersize pulleys, and re-programmed ECUs are not cost effective for any but the most demanding drivers who are operating at the upper limits of the vehicles ratings… Or as was already said so succinctly, Save-your-money.