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.

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.

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.

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!)

I am not sure if any benefits would be found here. Having a mechanical WP that varies with engine speed is a good thing. Higher RPMs = higher heat = higher coolant velocity. Also electricity is not free. With an electric water pump, first you must convert mechanical energy to electrical energy with the alternator (maybe 20% loss) and then convert it back to mechanical energy at the pump (maybe another 20% loss). By this very rough calculation an electric pump would consume 40% more energy than a mechanical one for the same coolant velocity. The mechanical pump looks better to me. What am I missing?

There are significant advantages to an electrically driven water pump:

1. Reduced mechanical drag on the engine (frees up HP); the higher the engine revs, the more power is saved (drag goes up with engine speed).
2. Slows down the circulation of coolant to a constant speed (higher RPM’s actually reduce the ability of the coolant to exchange heat with the air at the radiators because it is moving too fast).
3. Compact; no pulley or belt is required, so it takes up less space.
4. Works if the belt fails.

By-the-by, electrically driven water pumps are commonly used on race engines for the above reasons……………

Te primary reason that more aftermarket manufacturers are not involved with making them for the M96 engine is the small population size of the application.

1) where can I get this U/D pulley?
2) how much does it cost?
3) how much will my mechanic likely want to install it? (i'm not even going to try to do it myself)

this sounds like a great bang fgor the buck mod!

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

Sorry JFP, maybe I am just a little dim. I still don't get it:
1. You still have mechanical drag, just at the alternator instead of the WP, plus inefficiencies of conversion.
2. A smart variable speed pump that measures coolant temp and maximizes heat transfer at all rpms would definitely be useful. A real plus if there is such a thing. I have just never seen one this sophisticated.
3. We already have a belt and pulleys on our cars, no space savings.
4. Works if the belt fails, not if the circuit fails. Seems like a wash.

That is quite obvious.

1. You still have mechanical drag, just at the alternator instead of the WP, plus inefficiencies of conversion.

Many electric pump applications (racing apps) do not use an alternator; but those that do definitely see a measurably reduced drag (seen on the dyno) compared to a mechanically driven unit. It is not a "wash"; it is a net gain...........hence their use on race cars.

2. A smart variable speed pump that measures coolant temp and maximizes heat transfer at all rpms would definitely be useful. A real plus if there is such a thing. I have just never seen one this sophisticated.

You have not for a reason, they currently do not exist. Attempts to do this, even with digital electronics, proved to be overly complicated, not dependable, and added weight. Running the pump at a fixed speed electrically offers simplicity, better cooling, lower weight and free power without all the complications.

3. We already have a belt and pulleys on our cars, no space savings.

Electrically driven pumps are definitely smaller and lighter than their belt driven counterparts.........

4. Works if the belt fails, not if the circuit fails. Seems like a wash.

And if the car gets hit by a meteorite, or is swamped by molten lava, it doesn't work eithereither................ :confused:

AC compressors have a clutch, thus when the AC is off - the electric clutch is disengaged and the compressor pulley is spining free (not much drag there). Removing AC all together will save you weight, but wont give you any power gain.
Installing an underdrive pulley will slow down the AC compressor as well thus it will have less drag when the AC is on (similar to Honda guys installing smaller compressors on Accords I guess)
Just my 2 cents

So in conclusion, if we have a specialty race car with no belts or pulleys, no alternator, and electrical circuits that are far more reliable than Porsche circuits, a constant speed elect. pump would be a good idea. Boxster not so much, go with the underdrive pulley instead. Ok I understand now. Thanks for your help. Diplomatic as always JFP. :rolleyes:

Not quite: If an electrically driven, constant speed water pump was available, which it currently not the case, the M96 in your street-driven Boxster would run cooler, as would the oil; would make more power and probably (all other things being equal) live longer. But only if you understand why………………

But have a nice day anyway........................:ah:

I have evaluated the 4" pulley with both A/C and without.. The gains are real, not to mention reduced coolant temps and dramatically reduced power steering temps while on the track.

Actually, there is at least one aftermarket electric water pump with an electronic controller that varies pump speed to control engine temperature, manufactured by Craig Davies.

I don't think any automotive manufacturer would lack the resources to design such a system. They manage to map fuel injection & ignition timing using dozens of sensors, over all possible operating conditions, while controlling emissions. Controlling a water pumps speed would be a piece of cake.

pm sent......

Thats me!! :cheers:

We purchased one of these last year.. Problem is there is no way to efficiently route the water to/from the engine due to the OE water pump location.. There also isn't any place worth a damn to mount the electric unit.

The engine's design is the variable...

The only problem is that a better system would cost the OEM five cents more than the one they gave you....................and there is no "sex appeal" in cooling systems.

Yes, the Boxster's tight packaging is problematic, which of course an OEM could solve, but as JFP mentioned, they have no real incentive to do so.