custom ud pulley
 

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.

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.

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!

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!

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

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.

Maybe I'm just cynical about human nature, but my couple of decades in mechanical and nuclear engineering suggest to me quite the opposite. It may not be once a week, but it's prob not too far fetched to say once a month, I hear the general equivalent to this sentiment: "if that was a good idea and/or worked better than what we're doing, someone would have already done it." :rolleyes:

The shorter, more common version is: "...this is how it's done and it's always been done this way."

While I have tons of respect for any experimentation/trials/prototyping, even if the results were simply to prove what doesn't work, I honestly have zero respect for any reason for why something isn't done that sounds even remotely like "this is the way we've always done it and there's nothing better."

Just my $0.02

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.

The other thing I'd be interested in is how much cavitation is occurring in the stock pulley driven WP.

Right now, it's known conclusively that a stock 6" pulley can be swapped with a 4" pulley with no negative effect on cooling despite the WP turning a third or so slower. There's a number of variables in the cooling system including (but not limited to) the radiators heat xfer coefficient, the fluid friction through the system and engine coolant channels, the heat xfer of the block, the thermostat, ambient cooling, oil cooling, etc. All together though, the one thing we know for certain is that the WP is currently being turned a lot faster than necessary.

I'm with you RK... I'd guess the alt. is really the bigger question here.

Math and experimentation time then! Let us know what you find.

I grew up around scientists and engineers so I understand your burning curiosity very well. One of my friends used to be a model rocket hobbyist and always wanted to build bigger and better. He was building liquid fueled rockets in his garage until a man with money and vision offered him a partnership. The company is now called Space-X Technologies.

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.

Wow! Maybe I'm a bit of a dreamer, but I believe SpaceX is one of a small handful of companies that are currently in the process of changing the course of human history right before our eyes. Very nice!

I absolutely agree. Man on Mars in our lifetime I think. These guys are very focused and determined and currently have a $2Bil satellite launch schedule on the calendar... And Tom owns a Boxster. :)

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.

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.

...and who says capitalism and free enterprise don't work? ;)

I have not tried a 3" pulley. My hypothesis is it would be great on the track, but would suffer greatly in highway scenarios. I say this because 4" seems to be on the ragged edge.
Have fun.

Send it to me and I'll dyno it back to back and test some other dynamics of it and how they impact the engine, too.

Just like bigger is seldom better with an engine displacement and "just right" is the hardest sweet spot to find, there becomes a point where smaller is not better with something like these pulleys.

Awesome! Will do.. yeah, that sweet spot is what I'm curious about. I plan on cutting a 3" to start with and monitor the real world impact on cooling, charging, and drivability. Getting some dyno numbers would be great data. If the 3" turns out to be crap, I'll step back and do a 3.5". If the 3" is good to go, I'll have a 2.5" cut just to see what happens.

i'd also look at the size of other pulleys in the system. get too small and you are actually increasing the amount of work the engine has to do, as you lose the benefits of mechanical advantage.