No power gain from Underdrive Pulley?...

[Nine8Six]

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

[Boxtaboy]

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

[edc]

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.

[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.

[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.

[Timco]

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

[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

[jcslocum]

Originally Posted by particlewave

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

A lighter flywheel will add no HP to the car at all. It will allow the engine to spin up quicker, essentially "waking" the engine up a bit. The lighter flywheel will also add to the engine braking because the less momentum it has.

I wish I had added a lighter flywheel to the Box when we did the clutch and IMS. The car feels a slow to rev...to me.

Just my .0002

[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.

[edc]

Originally Posted by heliguy

So what would an ECU flash give?

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

[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.