I just bought an underdrive set from lmperformance the ASP brand for about one fifty. I hear all the bad things but im still gonna try it out. Ive heard that you are supposed to get a slightly shorter belt but there were not reccomendations when i bought my set does anyone know where to buy a certain belt like that.

 

Most of the guys who like an underdrive have modified cars, the raised idle for a bigger cam takes care of the low speed charging issue.

That said, most of us find about .1 going belt LESS on the track, stands to reason a 34% UD would only give you 34% of .1 so .034 at the track. IMO those who "feel" the gain are just wishfull thinking.

As covered, the gains can be more substantial on belt driven WP cars.

 

So, going back to my original question, for those of you with OD pulleys, are you running a bigger alternator (than stock) to compensate?

 

Its not just a matter of a "larger" (higher max amps) alternator. You have to find out if the alternator will supply the amps your system needs to hold 13.5V at your engine's reduced alternator RPM at idle.

 

Like I said in post 3. Stock alternator pulley.

 

Well i heavily agree with the larger cam theory due to the higher idle i appreciate the advice. Also for the guy asking for advice on the alternator the best one to buy is on partstrain.com this one is the stock 140 amp output but has a 300% more charging compacity which will help in stop and go traffic sitting at idle. Its called the mean green alternator which is what i got but it runs four hundred bucks which is about the highest price out there and is a direct installation made specifically for the vehicle.
its part #MG8172

 

Hmmmm.... buy a $400 alternator, so you can use a $150 ASP U/D pulley set, that produces very little in the way of performance gains.

And the one "review" of the Mean Green alternator I found online (for an International Harvester Scout) indicates it achieves "300% more charging power" by using an undersize pulley.

 

Heh - good point, Injuneer.