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.

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.

Underdrive pulley answers
 

1.- No
2.- No
Happy Boxstering,
Pedro

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.

That's what it was, the PS pump efficiency

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.

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.

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.

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.

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.....

What would this mod cost?

Thanks, Simon

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

or you can order it with the type of belt you need, here:
RSS Under Drive Pulley for Porsche Boxster, Cayman, Carrera, Targa

This means that the effort (read hp) to turn the pump increases as you turn it faster

are you sure about that?

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.

Be sure to use the harmonically dampened version if you have an engine swap/LWFW.

RSS Underdrive Pulleys - RSS

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.

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.

My Service manager just left and gave me a Mantis Racing (new style) underdrive pulley for free.

I have to give a big thumbs up to the guys at GSR. I am in Australia and asked them some questions about the right sized belt including shipping from the U.S. - They were really happy to help out and insisted that I buy it locally to save money on freight. They gave me the model number and what vehicles the shorter belt is used for to help cross-reference other brands - excellent customer service.

Here is the link to the belt sizes:

HELPFUL HINT: What size serpentine belt to run when using an under drive pulley? - GSR Autosport Community

Just had a thought...damned engineering education makes you worry about everything.

Aluminum Pulley mounted on a steel? flange.

I am wondering if there is any dissimilar metal contact issues we need to be aware of and whether the two metals need to be separated by a barrier of some kind....plastic, paint etc.

Aluminum strength suffers quickly when this is an issue and with the rubber belt whirring around I imagine there might be some significant electrical potential being developed.

Any reports of issues with this?

The factory pulley is anodized aluminum as well.

Both the aluminum and the alloy used on the shaft are relatively anodic. Also one must consider that is little opportunity for an electrolyte to get to that part of the engine compartment. After all, the block is made of aluminum as well.

I received the UDP and belt today. Will install this weekend.

Let us know how it goes and if you don't mind, who you ordered from?

I installed the UDP and new belt over the weekend. Took about 2 hours (I wasn't in any hurry). Hard to tell if there is more power by the seat of the pants but I'll be at the track again this weekend and will be better able to gauge any improvement there.

UD Pulley alert.

Underdrive Pulley Alert - Rennlist Discussion Forums

The RSS underdrive pulley that I installed is mostly a solid disk (with a lot of solid material between the holes) as compared to the drilled out spoke design of the pulley that cracked. Not saying that the RSS pulley couldn't crack, just saying that it seems less likely.

http://i1114.photobucket.com/albums/...isc/rssudp.jpg

The RSS pulley is currently on sale at GSR Autosport.

https://store.gospecracing.com/underdrivepulleyrssboxster986.aspx

*** Follow up ****

After having the UDP for a couple of months, here are my impressions;

Engine and accessories run and operate exactly like stock with the UDP. I can't say that I notice any increase in engine power but the car IS faster on the track. I picked up about half of the difference in acceleration between my BSX and the 500 lb lighter BSR Spec Boxster race cars. If I could find one more equivalent upgrade, I could give the BSR guys a run for the money. Overall, a great upgrade for the price.

That upgrade is headers, on my 986 they provided at least as much change as the UDP, and maybe more.

hi ghost.
was wondering which headers you use on your 986, and do you experiences any loss in gain at low mid RPM?

thanks.
Meir

I had the Chinese set, I thought they were pretty sweet for the price. The biggest issue for a 986 is not snapping the hardware when you remove and replace. I felt that the headers improved the performance without any sacrifice other than a drone with the top up. I also had a Dansk sausage SS muffler as well. All the small tweaks on a 986 will add up to better performance, what's nice about the headers is they don't truly scream unless you are on it, which is nice for around town motoring.

Good info for rowing a 7 speed Ghosty! I had the same ???s as @Meir.

Did you flash (ECU) when you put in headers? I may have asked this in the past but am closer to doing the mod now….my little 986 is starting to come alive! If you flashed the ECU, where did you get it done, and did it effect gas mileage or drive-ability? Since you are taking away those readings from the ECU. Was the drone as bad as these ???s

The spyder is all stock this was on my 986, I didn't get the ecu flashed as the car died before I got the itch for that mod. I do believe that getting a flash after you have done all your mods by a company that takes into consideration said mods would eek out a bit more. Then again, I have read numerous times how the ECU flash is snake oil it's all personal choices.

gonna embarrass myself (again).
what dose "snake oil" means?
and did you feel any loss of torque at the low/mid RPM?

I didn't feel any power loss at all, snake oil refers to expensive mods and the wild claims of gains they make. You can even see differences between the chip makers that are "tough to believe".

thanks for the explanation.

Meir
Just to put your mind at rest, I fitted a set of Chinese sourced headers (from Delubois Parts) onto my 2001 S. No other mods mechanically. The fitting went well, bolted strait on to the heads & secondary cats with no drama's. There is definately no loss in mid range, and the balanced length header pipes really come into their own at 4,000 rpm. In fact, as far as I am concerned there are no downsides apart from not being able to fit the rear plastic undertray.
One difference for you though is that here in Australia we don't have cats in the header - this may or may not have an impact on US cars.
The headers worked so well that I bought the cat delete test pipes from Delubois, and bolted them onto my stock rear muffler. This combo gives a really good sound above 4,000 rpm without the dreaded drone.