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Old 10-02-2012, 08:28 AM   #1
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Underdrive Pulley Questions

I track my car extensively and an seriously looking at installing an underdrive pulley but I have two concerns and would like some feedback. Both questions are a result of the 100F+ temps that we've been having here in SoCal recently;

1. Engine Cooling - Does the UD pulley have any noticeable effect on engine temps (since the water pump isn't turning as fast as normal)?

2. Air Conditioning - Does the UD pulley have any noticeable effect on A/C cooling (since the A/C compressor isn't turning as fast as normal)?

Thanks to all.
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Old 10-02-2012, 08:34 AM   #2
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Some claim the underpulley actually helps cooling, if I recall it was related to the water pump efficiency at a slower spin speed. My car never overheated as a result of the pulley, no issues whatsoever. As for the AC, it still worked well for NY now how would it be in Miami in August? I think you might notice a loss but you wouldn't be dying for AC. You will feel the underpulley difference immediately and the installation is pretty easy.
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Old 10-02-2012, 08:37 AM   #3
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Underdrive pulley answers

1.- No
2.- No
Happy Boxstering,
Pedro
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Old 10-02-2012, 09:46 AM   #4
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I tested our 4" UD pulley for 2.5 years, even with a 28 channel data logger it was impossible to tell which plots came from the factory pulley or the UDP and that included different operating environments from mid teens to over 100F, both track and street. The biggest difference you'll see on the track is the temp of the power steering pump and fluid. We originally applied these pulleys in an effort to extend PS pump and fluid life on track.

We see a solid 5RWHP gain with every UDP that we install.
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Old 10-02-2012, 09:56 AM   #5
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That's what it was, the PS pump efficiency
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Old 10-02-2012, 12:31 PM   #6
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Would you notice much increased effort on the steering wheel required at lower speeds due to the fewer RPMs on the PS pump, say while parking?

I'm considering it as well.

I spend most of my drive time above 2500 rpm and frequently peak it up in the 6k-7k region with spirited driving.

1st gear is a pain but Im starting to get the hang of it and not redlining anymore.
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Old 10-02-2012, 12:52 PM   #7
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I have seen no difference in coolant temp, A/C, or steering efficiency with the UD pulley. My car now has 102K miles, 70+ track days, and the original 14 yr old PS pump. I inspect it every oil change and no sign of failure yet.
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Old 10-03-2012, 05:57 AM   #8
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Great information guys! I will put the UD pulley on my "to-do" list!

I too was worried about "street-ability" of the mod. That is the only reason I have not pursued it.

Glad the question was posed to our resident experts. I wanted to ask, @thstone thanks for posing the ???. Happy to glean information where I can about our street-legal racecars!

Is the HP increase throughout the RPM range?

So that 5HP increase will drop our F-150 passing time by about a 1/10 of a sec or so. Yippie!
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Old 10-03-2012, 07:02 AM   #9
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Great information guys! I will put the UD pulley on my "to-do" list!

I too was worried about "street-ability" of the mod. That is the only reason I have not pursued it.

Glad the question was posed to our resident experts. I wanted to ask, @thstone thanks for posing the ???. Happy to glean information where I can about our street-legal racecars!

Is the HP increase throughout the RPM range?

So that 5HP increase will drop our F-150 passing time by about a 1/10 of a sec or so. Yippie!

Disclaimer: I don't have an UD pulley...yet

But let's think about this critically. We are not talking about gains in engine horsepower, we are simply removing parasitic drag from the drive line so that more of the existing horsepower is transmitted to the rear wheels. So the gain at the wheels is equal to the drag removed from the engine minus whatever the drive train losses are. The drag being removed in this case is dynamic in nature as opposed to static. This means that the effort (read hp) to turn the pump increases as you turn it faster. So this leads me to believe that the HP gain at the rear wheels follows the same curve as the HP required to turn the pump does, across the rev range.
If this is in-fact the case, then no you would not see 5hp gain across the range. You would see little to no gain at idle with a progressive increase in gain as RPMs increase peaking at Jake's 5HP number at the top of the rev range.

Still, 5HP is 2% in my 02 S and in 2.5% in a base. While not huge, it is likely just enough to make the car feel like it has been awoken a bit. Kind of like when you go for a drive on a crisp 40df fall morning compared to a warm summer morning...the car just feels like it's doing everything a little bit better in the cool air...not a huge difference, just healthier.
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Old 10-03-2012, 07:27 AM   #10
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As a reliability mod I think this is worthwhile. Don't get too excited about hp gains though. My butt dyno is kinda old and worn out so I didn't feel a thing. Others get in my car and say "wow, fast 2.5L" but I don't notice much difference. The placebo effect can be a powerful drug.
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Old 10-03-2012, 07:28 AM   #11
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My 100% non-scientific view is:-


No ill effects whatsoever and a bit more pep.

Go for it, as you see above no reason not to.....
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Old 10-03-2012, 07:42 AM   #12
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What would this mod cost?

Thanks, Simon
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Old 10-03-2012, 08:18 AM   #13
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UP can be felt as soon as you accelerate, I wouldn't put gain numbers on it but you can feel the difference easily, that usually isn't the case with other 986 mods where you aren't sure if it just sounds faster.

As for cost, here's the one I had, might as well replace the roller bearings and you need a new belt so add about 40 extra.

RSS Underdrive Pulleys - RSS

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Old 10-03-2012, 11:00 AM   #14
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or you can order it with the type of belt you need, here:
RSS Under Drive Pulley for Porsche Boxster, Cayman, Carrera, Targa
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Old 10-03-2012, 11:04 AM   #15
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This means that the effort (read hp) to turn the pump increases as you turn it faster

are you sure about that?
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Old 10-03-2012, 11:43 AM   #16
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Awesome, awesome! Thanks for all of the feedback and I just ordered the RSS UDP with the corrrect size belt from the guys at GSR.
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Old 10-03-2012, 11:43 AM   #17
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Be sure to use the harmonically dampened version if you have an engine swap/LWFW.

RSS Underdrive Pulleys - RSS
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Old 10-03-2012, 12:16 PM   #18
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Awesome, awesome! Thanks for all of the feedback and I just ordered the RSS UDP with the corrrect size belt from the guys at GSR.

You have to saw a "nub" of Aluminum maybe half inch for accommodation for the new pulley. That nub has a threaded hole right under it (I think it's under) anyway the point is DO NOT ruin the threaded receiver during nub removal. Apparently it's important for locking cams? I don't remember I just remember Jake saying do not ruin it which is simple if you are aware of it. The Aluminum cuts like butter too so no need for a grinder or aggressive saws all, I hand sawed mine.
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Old 10-03-2012, 07:52 PM   #19
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You have to saw a "nub" of Aluminum maybe half inch for accommodation for the new pulley. That nub has a threaded hole right under it (I think it's under) anyway the point is DO NOT ruin the threaded receiver during nub removal. Apparently it's important for locking cams? I don't remember I just remember Jake saying do not ruin it which is simple if you are aware of it. The Aluminum cuts like butter too so no need for a grinder or aggressive saws all, I hand sawed mine.
Ghost - thanks for the installation tip! Sometimes knowing what NOT to do is just as important as knowing what TO do.
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Old 10-03-2012, 10:27 PM   #20
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Originally Posted by Ghostrider 310 View Post
You have to saw a "nub" of Aluminum maybe half inch for accommodation for the new pulley. That nub has a threaded hole right under it (I think it's under) anyway the point is DO NOT ruin the threaded receiver during nub removal. Apparently it's important for locking cams? I don't remember I just remember Jake saying do not ruin it which is simple if you are aware of it. The Aluminum cuts like butter too so no need for a grinder or aggressive saws all, I hand sawed mine.
The hole is for the pin that locks the crankshaft at TDC (or 180 degrees from it), and is not threaded.
The point is not to cut the boss off completely; cut enough the clear the smaller UDP, but leave some to accomodate the locking pin.
However, keep in mind that the locking pin cannot be used once the UDP is on there. So, if you need to do anything that requires the crankshaft to be locked at TDC (such as replacing the IMS bearing), do it before you install the UDP. Otherwise, you must devise an scheme to lock the crankshaft from the flywheel end.
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