What is the "underspeed pulley" mod?
 

I have seen a few members reference this. Can someone explain what this is and how it helps? Thanks

It is a smaller crankshaft drive pulley that drives all of the belt drive accessories at a slower speed. This takes less horse power to do, so there is that much more for driving the rear wheels.
FLAT 6 INNOVATIONS :: Engine Parts :: Under Drive Pulleys/ Harmonic Dampers

The underdrive pulley was developed for motorsport applications to preserve the power steering pump and slow the accessories on a motor that spends a lot of it's time at high revs (4-6K). A side benefit is that it reduces parasitic drag so you get a bit more torque/hp to the rear wheels at all rpm.

If you spend long hours stuck idling in traffic with headlights on and the stereo cranked you probably don't want it. If your car spends most of it's time between 2-3k rpm, you don't want it. If your car is a weekend joyride/track toy and you use all of your rpm range, this is right up your alley.

Topless - I fitted my Raby underspeed pulley 3 years ago, and most of my normal driving is between 2-3,5k rpm - no problems as yet.
When I installed the pulley, I temporarily connected my multi meter into the car to get some idea of the voltage output of the alternator. Above 1250 rpm, the voltage was back to standard @ 14.3v, so as long as your alternator is in good condition & you're not continually in heavy traffic with a high electrical load, you should be OK....

Good to know, thanks for the meter read. Now go find a nice tight canyon road and wind her up! :D

No canyons around here - eastern coast Australia is "canyonless", but does have some good driving roads - the police mobile radar guns being the speed limiter.........

Is the Flat 6 part a Porsche part? Can any of you guys really "feel" at 5 HP gain? Is this a "while your in there mod" when doing the Serpt. Belt? Thanks.

No, it's a custom 4" pulley. I did the pulley as a reliability mod to prep my car for track use. My original P/S pump is still intact at 100k miles and 70+ track days... Yay! I cannot say I feel a difference but my car has some weight removed also. Others have ridden in my car and thought it was faster than their 2.5L but it's all subjective until you measure it.

If your car is mostly a street car and you love your stereo, don't bother with this.

No and yes.

No, you can't really (truthfully) feel the slight increase in power.

But yes, it does make a difference because my car does not have this mod and when I am chasing someone on the track who does have the mod, I can see that they have a bit more acceleration out of turns. Not a lot, just a little bit more. It might only be a few tenths of a second per lap, but in 5 laps they can be a full second ahead of me and then I have to drive the dang wheels off the car to keep up.

Of course, there are many, many variables involved, but when someone with an identical car on identical tires can pull away just a litle bit out of every corner, my man-pride forces me to conclude that something else HAS to going on!

The only other conclusion is that my driving sucks so I really prefer to believe that the underpulley helps. :o

I just did this mod yesterday and it was 103* outside. You can definitely feel the extra HP. Went out with the wife and she said "your car seems faster" which was great being out in insane heat.

I was concerned about getting the 24mm bolt out that everyone talks about. If you use the proper tools (an extra long 24mm wrench) the bolt will come out easily. I reused the bolt but others say to buy a new one.

I couldn't find the Flat6 pulley on their site so I purchased the RSS kit that comes with the new pulley, serpt belt, and special wrench. The hardest/longest part was cutting the boss down IMO. I just took a hacksaw blade off the saw and sawed through with the blade in my hand. Rest of the install was a breeze.