custom ud pulley

[sam c.]

I've been looking at UD pulleys for the 2.5l base and had a quick question for all you guys with more knowledge on the subject...

It seems all the UD pulleys currently on the market were cut to the 4" spec dictated by race rules which were put into place to protect overheating PS components.

The UD pulleys were not cut to a size that squeezes the most power out (or more accurately, minimizes the most parasitic drag) while leaving the stock functions intact (in particular the cooling and charging functions).

So my question is has anyone done the necessary testing to see how small a pulley can be cut without impacting those important functions for a strictly street car that has no concern for race rules? If 4" pulleys are fine, would a 3.5" or 3" pulley be ok?

I've talked to some CNC owning shops and can get some custom pulleys prototyped for approximately $250/each but was curious if anyone else has already been down this road and what were the results of their efforts? As it sits, I plan on installing a 4" off the shelf pulley until my 3.5" and 3" prototypes are cut and then begin testing.

[Jake Raby]

The smaller the pulley the more under driven it is.. The 4" are the smallest commercially available. I have tried 5" and the gains weren't enough to justify the install and I have 4" on some engines that are daily driven and have been installed for 5 years now with no issues.

[sam c.]

Thanks for the reply, Jake. The smaller the diameter, the more the components are under-driven and the less parasitic losses. That was really my question... has any research been done to see how small the pulleys can be made without having an adverse affect on cooling and charging? (This would be for a strictly street driven car interested in the performance gains that would not be held to any rule mandated pulley size).

Thanks!

[Homeboy981]

Well @sam c., looks like YOU are OUR research and development department.

Let us know IF you decide to do it and how it 'turns' out.

Way to think outside the Box.

Good Luck!

[sam c.]

Well alright then... off we go. I'll be sure to post what I discover.

[Topless]

It's a simple math problem really. Calculate the change in wp rpm at different pulley sizes and make your best guess. A flow rate gauge on a coolant line would be useful also. Ideal flow rates for rads is pretty well understood.

I have never seen a smaller one than 4" so I assume someone did the math already.

[The Radium King]

100%. especially since the first UPDs were developed to slow the PS pump to reduce overheating at the track, not necessarily to to increase power. they may have got to 4" and said "hey, no more PS overheating" and stopped there.

however, you may want to figure out what the alternator needs to work; i have heard that some big subs can start to draw down the battery with big bass and a UDP.

[Jake Raby]

I have found that cooling is better at higher speeds with the under driven pulleys. I've only been testing different diameters of them for over 6 years now.

I have driven a car 28,000 miles in one year with a 4" UDP on the street and track in winter and summer and had no adverse effects, even with every accessory on in traffic on a July day with a 95F ambient temp.

Results always vary.

[sam c.]

Originally Posted by Jake Raby

I've only been testing different diameters of them for over 6 years now.

I was hoping you'd weigh in. Everyone seems to conclude that the 4" UD pulleys demonstrate no negative effects on anything while at the same time, do result in very real gains in efficiency, power, and driving dynamics. That and our PS fluid doesn't burn through the reservoir and dump all over the track which is always a plus.

Have you ever cut and tested a crank pulley with a diameter less than 4"? If so, what diameters and what were the quantifiable results?

I figured someone most likely looked at this which is why I asked... Just to avoid wasting time/energy on a path thats already been exhaustively explored.

[sam c.]

Originally Posted by Jake Raby

I have driven a car 28,000 miles in one year with a 4" UDP on the street and track in winter and summer and had no adverse effects, even with every accessory on in traffic on a July day with a 95F ambient temp.

I think this statement pretty much sums up the starting point and foundation for this conversation. Everyone agrees that a 4" pulley is good to go. Lets see what happens with a 3" pulley.