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.

yeah, you def loose leverage as you go down in size. The key is to find that sweet spot, as Jake pointed out. And who knows, that sweet spot may end up being 4" and all this was nothing more than a validation of what's already being done.

My new pulley showed up in the mail yesterday so I went ahead an put it on the car. Pretty nice looking piece... paid $200 and that included the necessarily shorter 8 ribbed belt.

http://i1282.photobucket.com/albums/...ps78d05e87.jpg

Removing the pulleys made things a lot more accessible IMO. Also, I didn't monkey around with trying to remove the old belt.. I was just throwing it away anyway so I just cut it off and pulled it out (after removing it from the tensioner so it didn't snap back and put out my eye!)

http://i1282.photobucket.com/albums/...psa925d92b.jpg

I think I'm going to hold off on pursuing an even smaller pulley until I get the electric PS pump and electric water pump installed. That's quite a bit of added electrical loading and I don't want to over tax the charging system. If everything is good to go, I might throw a custom cut 3 or 3.5 inch pulley on there just to see what happens. I plan on sticking with the 4" pulley for now though to determine the impact of the other modified systems.

Looks like your engine struck GOLD! Looks cool! :cool:

You could also use a larger pulley to selectively underdrive one particular accessory, without changing the others.

That is true. If u want my honest opinion, the value of these pulleys are that they under drive the water pump, the alt, and the PS pump. The AC is being under driven as well, but ur controlling that load with the panel on the dash in accordance with ur own preference. So if u drive it slower with an UD pulley but ur still too hot (in a convertible.. :rolleyes:), u'll just turn it up anyways so the AC is a none issue in my opinion. That just leaves the other 3 loads.

Its been proven that the other 3 loads are being over driven from the factory which is why ud pulleys work.

In my case, because I'm removing two of those loads and effectively giving their work to the alt, I may just find that it needs to be driven closer to the factory speed. I seriously doubt it but maybe.

The other potential benefit of the ud pulley is that they could be made lighter than the factory pulley and therefore give less rotational mass/inertia for the engine to have to overcome to rev. However, from what I've seen, all these UD pulleys are actually slightly heavier then stock. If I were to cut a custom pulley, that's one improvement I would def pursue - weight - even if it meant going to a different material such as titanium.

Btw, replaced my idler pulleys yesterday... used just off the shelf 70mm pulleys that are used on dozens of Audi and VW cars. Price=$20/pulley. Found out too that the bearing on these pulleys are just an off the shelf bearing and super common. If you still have the stock metal pulleys, you can press out the old bearings and press in a new one for less then $4.

http://i1282.photobucket.com/albums/...ps861c8c25.jpg

Replacing just the bearings wasn't an option for me since the lower pulley was bent/warped as it it had been dropped and the upper was effectively MIA. Haha

http://i1282.photobucket.com/albums/...psd339b5eb.jpg

I think when we talk about under drive pulley diameter limits we are considering what belt speed reduction the driven accessories can safely tolerate throughout the rpm range. You probably dont want to start running your ignition off your battery. It is also important to consider belt drive dynamics with a reduced diameter. Will the arrangement cause undue belt stress or slippage? Crank pulley size controls the belt speed for the whole system and the torque load of every accessory is reduced with a smaller crank pulley. The torque load on the engine is directly related to crank pulley radius.

A lower belt speed turns each accessory slower. One can control the speed (reduce) of each accessory individually by increasing its pulley dia. The torque load for that accessory will be reduced as well, but you will not realize the power savings that a smaller crank pulley gives unless one makes all of them bigger.

Recall that, originally the aim was to offset accessory speeds downward for constant elevated engine rpm at the track, particularly the power steering pump.

I have noted my ps pump groaning under full lock at parking speeds with the UDP....so here is another design limit cue

My guess is that that crank pulley mass is not significant when compared the massive flywheel but a quick calculation will tell you whether fancy spokes are worth the machining cost.

Look into the failure rates of BBI pulleys before doing machining on the crank pulley.

Yeah, the fancy spokes aren't worth the potential for sheering off said fancy spokes. It ur PS pump is already struggling with a 4" pulley then I'm would tend to agree that might be the limit on size. With PS being a huge load on engines (more then AC or alt), its probably the limiting factor here.

The funny thing about mass is the stock pulley is remarkably light. The under drive pulleys I looked at all were about the same weight despite being substantially smaller in diameter. That's what I was referring to when I said the diameter might already be optimized. Where the next little bit of gain might be realized is with a lighter pulley (titanium for example).

Well, until the electric PS pump gets installed anyway...

My MR2 electric PS with integrated reservoir.
http://i1282.photobucket.com/albums/...ps36dc02c5.jpg