No power gain from Underdrive Pulley?...

[heliguy]

Originally Posted by tommy583

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.

Good point.

[Gilles]

Originally Posted by Quadcammer

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.

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

[Porsche9]

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.

[Topless]

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.

[particlewave]

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.
They are for saving your accessories on the track.

[The Radium King]

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.

[particlewave]

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

[Gilles]

Originally Posted by Gilles

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 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

[Anker]

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!

[Boxtaboy]

Hey, it proved the mod gained 0.1 hp!

[edc]

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

[Boxtaboy]

Originally Posted by edc

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!

[edc]

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.

[edc]

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.

[heliguy]

So what would an ECU flash give?

[JFP in PA]

Originally Posted by heliguy

So what would an ECU flash give?

Not a lot.

[Anker]

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.

[j.fro]

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.

[Steve Tinker]

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.

[edc]

Originally Posted by heliguy

So what would an ECU flash give?

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