No power gain from Underdrive Pulley?...
 

This guy posts up some interesting videos about the Boxster. This one shows no gain after installation of an underdrive pulley...

https://youtu.be/CvFdH11y4EI

When I installed a UDP, it wasn't because I thought it would give me more hp, but rather because it drives the accessories at a lower speed, especially when you are on the track! Power steering pumps, and the like, don't like to run at constant, near redline speeds and can cause problems or fail when pushed too hard.

I don't doubt that statement. Years ago, I actually had a water pump failure right after I had redlined my engine during some spirited driving. :) What I find amusing is the sellers of these UDPs claiming instant 5-10hp gains.

VJ should have included a before dyno run in the same environmental conditions as he tested the UDP afterward. I think you loose 10~15HP between seasonal air densities. It can be difficult to track small power gains sometimes.

My Boxster gains about this +10hp... when the wind blows from behind loll

Perhaps he forgot to install the shorter serpentine belt required to make the kit working?

I think he says in the video that he put in a "longer" belt! :D

oh did he. missed it

Correct. Pumps cavitate (the fluid stops flowing and creates voids Article on wikipedia) when turning too quickly. Water pump, power steering pump, oil pump. Their pulleys are all designed to run at low(1000) to mid RPMS(4000). That's why when at a stop some cars forces the engine to rev a little higher when it senses that its not providing enough cooling or power steering help. Now I personally wouldn't replace it unless you are really having spiking temperatures when on the track. Most of us will get stuck in traffic more often then run 7K RPM constantly at the track. Then again its an easily replace part.

Example:
My Sr20det revs up to 8k RPM and I was chasing some power-steering and cooling gremlins. Past 7k for too long and I was running into some cooling problems. I under-drove the power steering and water pump and it mostly helped the cooling side. But its more of a band-aid. Finally installed some power steering cooling lines with a filter and that was able to keep the viscosity in check and help deaerate the oil.

Back to the power. The pulley doesn't have enough significant mass to provide any substantial effect on the inertial of the system. Actually smaller pulleys would make it harder for the engine at lower RPMs to turn accessories by the loss of mechanical adavantage.(lever arm)

* Torque the bolt to 50 NM (37 ft-lb) and then tighten an additional 90 degrees.

I don't wanna believe this. :(

+1 ^^^^

The dyno baseline test was on 10/22/16 whereas the UDP test was on 11/26/16. The temp and humidity correction (or poor correction) could easily result in a margin of error equal to the magnitude of the difference under test (3-5hp).

He needed to do a baseline and the UDP test on two consecutive days at the same time of day when the weather was generally constant to be able to accurately measure the difference.

Me neither...

Maybe psychological but I could feel it when I did mine, mostly in first and second, but I primarily did the mod for power steering longevity.

Considering the difficulty of the change. This could be planned on the same day. Then again we are talking inertia change. Just like a lightweight flywheel but at a microscopic scale. Engine will rev faster without load. That's if the parasitic drag of the equipment doesn't increase from the pulley size change. With load, nothing will change.

Hey if you are looking for most horsepower output, try running a lighter oil both in the engine and transmission. You will have way less parasitic loss and more income loss.

To be honest, I feel my boxster would benefit from a beefier flywheel looses revs too fast for my taste which makes up shifting a rev match fest(6 speed).

Agreed. I thought I could feel mine too. But I did the secondary cat delete pipes at the same time. So easy to break the tires loose in 1st and 2nd now.

Any truth to some comments in the video that the stock pulley acts as a harmonic dampener, and that replacement risks engine damage? One comment even sites an article claiming so.

Can't personally confirm this scientifically (quickly anyway), but yes, natural freq will certainly have different critical speed as "per design" requirements originally set by Porsche Eng. You'll introduce a whole new set(s) of vibrations to the engine (in engineering terms)

Read Rotor Dynamics

Hmmmm...interesting. Anyone get over 100K miles on an UDP yet? :D

not on the 986. Its a simple aluminum pulley. It has not harmonic dampening whatsoever aside from its weight, which is not much different than the aftermarket ones.

On 987 cars, they have a true harmonic dampner which is why you have to buy a much more expensive underdrive.

Whats the word on this boxster making 273 wheel hp? Or is this one of those stupid dynos that converts to engine hp?

It's not wheel horsepower. It's estimated at crank.

I use this same dyno. You get print out of both. The owner normally talks the crank figure because he knows most customers just want to hear the big number and these are the figures peopke can relate to when they read the manufacturer spec on the websites.

Wouldn't you want to compare the wheel HP before and after? The pulley wouldn't actually create more HP, just more of what the engine does make would get to the wheels. At least that was always my understanding of how it would work. It is supposed to FREE Up HP not create it.

I have absolutely no doubt it gave me felt low end torque / HP.

Good point.

On my 987 the OEM pulley did Not have a balancer.., but I replace it with an UDP to reduce the speed of the accessories after frying three steering pumps on the track (thankfully replaced under warranty) and it's been installed on the engine for at least 20k without any issues with the water pump on the SoCal summer with heavy traffic..

I believe that the only 987's that came with the dampened pulley are the engines on tye GT3 and perhaps the 987 Turbo 911's

My UDP has been on my car for 20,000 plus miles with no ill effects. Only a little more power to the wheels and no issues at the track.

Wow, 273.8 RWHP sounds like a lot for any Boxster. That is about what my brother's 3.8L sleeper Cayman puts out.

Bottom line: Changing the underdrive pulley to preserve your PS pump might make sense for a track rat. Chasing a hp increase might not.

Placebo effect. ;)

I see no way an UDP has any significant gains, especially not anything you can feel. If you can, there are some scientists that want to examine your super-human abilities. :p
They are for saving your accessories on the track.

less rotating mass (lighter pulley, less belt) = better responsiveness of engine
less friction/resistance to rotation (smaller diameter pulley, shorter belt - less surface contact for friction) = more horsepower

unknown - how accessories respond to different rpm - resistance to rotation may increase with rpm, or may decrease. ultimately, however, if the accessories output less when turning slower, all while still meeting the needs of the automobile, less unnecessary work is getting done. less work over time means less parasitic horsepower loss.

Inertia. The difference in mass between the stock pulley and the smaller one is not much. Once that mass is spinning, inertia helps a lot, considering it is spinning at around 800 RPM at idle.
I'd be shocked if the gains from the change in mass were more than 1/10th of 1hp.
Once a mass is spinning, as in a gyroscope or stabilizing flywheel, it takes very little energy to maintain the spin. Spin up (accelerating that mass from a stop) is where more energy is needed, but since the pulley never stops spinning at idle (inertia), the gains would be very small.

The flywheel has a lot of mass. I wonder what the supposed gains from a lightweight flywheel are? If the UDP beliefs are true (5-10hp), then the gains from a lightweight flywheel must be 5x more (since the flywheel has probably at least 5x the mass as the acc pulley).
I doubt a lightweight flywheel adds 25-50hp, lol :D

I am sorry but forgot to mention that along with the UDP I also installed an oil cooler on the steering rack return line, by copying a design posted by Eric323 (Hergesheimer), but this story belongs on a "how to save your steering pump" …LOL

Getting 10HP from an underdrive pulley is a wet dream. Driving the accessories at full tilt requires less than 10HP and the UDP just reduces the load by a fraction no larger than the ratio of the regular to UDP pulley's diameters.

In actual fact, once the water pump starts cavitating it requires less power to run. The generator drag is close to proportional to the number of amps it delivers, and they don't go up by more than a small amount with speed. The AC is turned off on the track, I am sure, and the power steering pump also draws the amount of power required to turn the wheels.

So happy dreams!

Hey, it proved the mod gained 0.1 hp! :D

I've done this mod twice now on both my Boxsters and barely notice if any difference in feel or power.

So you're saying it's worth it! :D

I've never thought it would make any significant power so have never bothered to dyno the mod. That's saying something as my 2 Boxsters have probably been on the dyno about 8 times between them measuring the mod difference.

The mod is cheap if you know where to buy. I always say it is almost free if you are due a new belt anyway.

So what would an ECU flash give?

Not a lot.

The underdrive pulley reduces wear and tear on the accessory bearings and reduces the possibility of cavitation in the water pump. It is worth it if you plan to race the car and drive at high RPMs for extended periods.

Are we talking 4" or 5" UDP's?? Both are out there.
Also, I think I remember Raby commenting that the UDP was the one mod he found to produce hp... But only 5hp.
FWIW, I've had a 5" on my car for 43,000 miles & 5 autocross seasons.

I think there is some misunderstanding regarding the underdrive pulley.

There possibly is a 5hp gain with the reduced parasitic loss, but the gain is @ low to mid range rpm, not max rpm. A good dynamometer chart would show only a slight bump in HP @ 3,000 - 4,500rpm not @ 7,000rpm.
And dyno charts with compensation numbers "built in" for gearbox / transmission losses are pure nonsensical - who wants to prove that a gearbox soaks up 12% or 15% or 20% of the rated engine power output, let alone adding in different daytime barometric pressure, temperature and rolling road calibration changes.

I dyno'd about 6 bhp from a remap after my mods.