Dear members,
I have measured the alternator regulation capability in order to determine if installing an Under Drive Pulley (UDP) will result in any undesirable effects. A UDP can be installed when one wishes to reduce HP losses or place less stress on vehicle components when a high RPM is maintained for some time. The disadvantage is the reduction of battery voltage regulation and lower RPM for other accessory items like the A/C compressor at idle speed.

The Pulley considered is the BBI model.
Supplier says it under drives by 16%.
Cayman Idle speed is 680 RPM. Therefore RPM reduction effect is equivalent to 571RPM. Battery nominal voltage =12.5V.
Car was basically in idle mode for 10-15 minutes while testing etc.

Idle RPM______Battery Voltage NB: All accessories turned off.
>1500________14.25
1500_________14.23
1000_________14.18
800__________14.10
700__________14.05
680 (Idle)_____14.02
600__________13.96 Engine loaded by riding clutch to force lower RPMs.

So 600 RPM is close to the RPM reduction effect of 571RPM and the regulation is clearly ok with minimal load (all accessories off).

Now we load fully at Idle.
High Beam Lights, Fog Lights, Demister, Cooling Fan max, Radio up etc, but with the A/C off however and at the same time I load the Engine down to 600 RPM. Which is rather hard to do without stalling the Engine or wasting too much of the clutch or driving into the wall. Note to self; handbrake hard on.
Battery now reads 12.7V. So its neither being replenished nor depleted from its original state. Acceptable IMO.

So now for the really interesting bit (if you call this interesting that is). I turned the A/C on flat out with High Beam Lights, Fog Lights, Demister, Cooling Fan max, Radio up etc. The Engine compensates for the extra load and the Idle is auto increased to 800RPM! By now all the fans are running including the Engine bay and Radiator fans. All good stuff.

Next tests should be done with the UDP installed.

Excellent post, thanks for testing this.

BTW, Mantis' pulley is rated at 20% underdrive. I wonder how much of a real-world difference there will be between a 16% UDP and a 20% UDP (other than 4%, you wiseguys.... )

4by4,

Great idea on this thread. So if I understand correctly, your test was done with the stock pulley, right?

It will be interesting to see the difference after pulley installation. There should be a measurable and noticable difference, but I doubt that difference will be problematic.

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4by4

I really appreciate your efforts and the thread you have started. We need to get you an award for this. It is really wonderful to have good hard data like this.



Don't be surprised if some member on the CC offers to send you the MantisSport UDB in order to test that 20% offset in real world conditions.

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Don't miss scraping my master's windshield, winter oil, winter tires, chains, pluggin' in the car over night--block water heater! and interior warmer. And having to put anti-freeze in my GAS!

Do miss slippin' & slidin'.

Thanks for the kind words.
Yes the test was done with the stock pulley and the loading of the engine to simulate the lower RPM if a UDP was installed.

I think you guys should read this first. Also, note who the author is. I think it's a credible source.

Welcome to ATI Performance Products.com

The difference is the BMW uses a harmonic dampener, ours don't. I've looked at the stock pulley we took off my car and it's no different that what we replaced it with (ok, wise guys, EXCEPT the size!), and I spun the new one on my lathe to be sure it wasn't grossly unbalanced before we put it on, and it was straight, true and in balance (Thanks, Mantis!)

23,000 miles since I installed it, and I have no issues. Obviously I can't extrapolate to 200,000 miles, but I don't think we're talking the same issue as in the referenced thread.

Greg

I can appreciate that. I don't do many mechanical mods to my cars, so I'm not an expert by any means, but figured it would be a good counterpoint to the conversation.

Interesting reading, but I have to go with Gemars on this one. The pulley on the Cayman is not a harmonic balancer. Harmonic balance is much more necessary on a BMW engine, by design.

The BMW engine has a very long crank shaft and all pistons are pushing in the same direction, down through the crankshaft.

The Porsche Boxer engine has a much shorter crankshaft (because it's more like a V-6, not like a straight 6), and each bank of pistons is pushing against the other, which naturally cancels vibration and stresses. This is one of the many fundemental advantages of a boxer type engine.

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I have the BBI Pulley installed now.
Remeasured the electrical.

Condition__________Before_____After
Idle 680RPM_______14.02V_____14.06V
1000RPM__________14.18______14.14V
Full electrical load__12.7V______12.57V
With A/C on Idle___800RPM_____800RPM Volts same.

Obviously no problem the the Alternator regulation.
A/C seems unchanged.

Thumbs up for this mod.

Great news! Thanks............