Underdrive pulley recommendations? - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Boxster44 · 12-04-2009, 10:17 PM

Ques - when you say Underdrive Pulley and HP gains, are you assuming w/o A/C?... or still have A/C connected?

Would there be more HP gain w/o driving A/C?

thanks,

kj

Lobo1186 · 12-04-2009, 10:25 PM

yes no A/C would "gain" you more HP, if you were to remove the whole system also some weight would be lost giving you even more...

however, its more along the lines that you will just have more HP that the car makes already available to you.

correct me if im wrong.

amondc · 12-05-2009, 06:11 AM

Correct. They pulley does not create horsepower it only reduces the factors that rob hp from the motor. Another way to think of it is that the motor is just an air pump. It sucks, bangs and blows. If you don't change the amount of air it sucks or how big the bang is or how it blows you do no "create" hp.

That said all we really care about is how much hp we have at the wheels not how we get it, and the pulley does work.

Boxster44 · 12-05-2009, 06:34 AM

... sorry, I asked the question the wrong way. I know it doesn't create HP, I was just surprised at the estimate of 10 extra available to the drive train... that's pretty darn good for just an external pulley change.

I've been thinking about the pulley and instead of full A/C delete, just go with the shorter belt and eliminate the A/C drag from the equation. This pushes me a little farther along.

You're right about the weight, that compressor feels like it weighs 20 lbs. alone... the whole system ought to net 40ish?... no?

thanks,

kj

amondc · 12-05-2009, 06:49 AM

no worries boxster44. I think the reason there is such a large gain is because you are slowing down all the accessories but Jake can give us a much better answer.

and yes if you remove all the a/c stuff you will pull about 35-40lbs out of the car.

blue2000s · 12-05-2009, 07:21 AM

Spinning the water pump, alternator, and AC compressor takes power. In the case of the pump and compressor, that power increases by the third order with speed. In other words, it takes 8 times more power to drive a pump twice as fast.

You can imagine that if you spin the pumps more slowly, more of the engine's power can go to twisting the wheels. And because of the third order relationship the faster you spin the engine, the more power is saved compared to a larger pulley.

The drawback is that at engine lower speeds, the pumps may not flow sufficient water to keep the engine cool in an extreme environment, or the alternator may not provide enough current to a car with a very high draw stereo with the lights on.

As Mr. Raby has found, everythimg works fine in his region.

stephen wilson · 12-05-2009, 08:16 AM

I surprised that no manufacturer has gone to an electric water pump, with it's speed electronically controlled. There's lot's of power and efficiency to be gained, though I'm not sure it would ever be as reliable as a belt driven pump. (though Boxster water pumps haven't proved that reliable!)

Lobo1186 · 12-05-2009, 08:09 AM

Quote:

Originally Posted by Boxster44

... sorry, I asked the question the wrong way. I know it doesn't create HP, I was just surprised at the estimate of 10 extra available to the drive train... that's pretty darn good for just an external pulley change.

I've been thinking about the pulley and instead of full A/C delete, just go with the shorter belt and eliminate the A/C drag from the equation. This pushes me a little farther along.

You're right about the weight, that compressor feels like it weighs 20 lbs. alone... the whole system ought to net 40ish?... no?

thanks,

kj

im sure that is what you meant anyways but its good to clarify for people who come on here and may not know. but yes im not sure how much the whole AC weighs but it is substantial.

if money is no object. (but usually it is!)

you could do AC delete and replace all panels on the body with Fiberglass or carbonfiber and that would net you quite a bit of weight. I have also heard that the sound proofing removal is quite good too.

Boxster44 · 12-05-2009, 11:17 AM

Lobo,

Yes..... I think amondC (Chris, right?) can really go into detail there as I believe he's with the Spec Boxster group in Texas and they do lots of things with their cars.

kj

amondc · 12-05-2009, 07:34 PM

Quote:

Originally Posted by Boxster44

Lobo,

Yes..... I think amondC (Chris, right?) can really go into detail there as I believe he's with the Spec Boxster group in Texas and they do lots of things with their cars.

kj

Thats me!!