Dubai944

How sensitive are the Bosch alternators to high rpm?

I had two alternators die on me this race weekend in short order. Both worked fine for a short time then nada.

I am assuming it is because I ran a large custom 7 inch crank pulley for the first time and with 7500rpm and the alternator pulley still at stock diameter, it is obviously spinning the alternator a hell of a lot faster than usual. I didn't expect such sudden failures though.

Before I go to the bother of installing alternator number three and making an underdriven pulley, can anyone confirm this is a typical problem? It may have been something else unrelated, but I can't think of anything obvious.

V2Rocket

what is the purpose of the larger crank pulley on your car? the 7" is a full third larger around than the standard pulley.

JohnKoaWood

Yes, they are sensitive... I do quite a bit of driving, mostly highway, and have killed 2 in less than 4 months...

No problems in 2000 miles with my Nissan alternator though!

Dubai944

Its an 8 rib custom setup that allows bigger drive pulleys on the supercharger to reduce belt slip.

V2Rocket

What SC are you using? An SFR kit? Or Carl's kit on a S2/968 motor?

Dubai944

I started with an SFR kit a couple of race seasons ago. It worked ok originally on a fairly stock 968 engine at the class level I was running, but belt slip was an issue from the beginning and it never made the type of power SFR advertised except on a wildly optimistic dynojet.

As I have developed the car and moved up classes, I have kept the C2 procharger, but the motor was rebuilt for higher boost, the belt system completely changed to run big pulleys, more belt wrap and gear the supercharger to maximum design speed but control boost with one of Carl's limting valve on the intake side. Boost comes in a lot earlier now. Currently running to a maximum of 10lbs of boost reached at about 6300rpm, but that seems to still be limited by belt slip rather than the BLV. Still needs further tuning and still not completely solving belt slip issues even with 8 rib belts and big pulleys.

Car is reasonably quick now, but not quite yet where it needs to be with the level of competition in my current class, GT4 Astons and Maseratis, Ginettas ,Cup Cars etc. Hope to push it a bit further if possible over the next few months.

Here's some video of the car in action.

http://vimeo.com/20399968

Arominus

Nice vid, does your class allow hybrids? if you cant get enough umph out of the SC'd 968 motor maybe you should try to steal that corvettes engine and swap

Anyhow back on topic. Most alternators won't handle the extra RPMS your throwing at it, a 33% OD is a lot. I'd get that underdrive pulley in the works, as it is your risking a stator explosion @ high RPM, your lucky just the bearings/internals have been letting go.

Dubai944

Yes it seems logical it's just excessive RPM. I also was running a new BMW electric power steering pump which is an amp hungry beast, and a new gearbox cooler pump, but I think the Bosch is well within it's limits in terms of output required.

HJK

Did your gearbox have the oil cooler oe or you build it in?
H

Dubai944

No, it's an S2 box. The rear floor is cut out of my car and I run an external filter, cooler and tilton pump.

Cochezz

On our BMW touring car www.amicoracing.com I use a big *** pulley on the alternator, the car doesn't even charge at 1000 rpm, but that fine becuase the idle is around 1700rpm. Not that its much but spining the alternator any faster than you need to is a waste of HP.

What model is the pump off? I'm looking at using PS this year, our driver finds the manual steering beats up on him.

Thanks

Dubai944

It's a BMW Mini PS pump.

Cochezz

What year mini? the first gen or second.

How did you wire it I thought those had to be run by a control module?

Thanks this is very helpful.

Dubai944

Well, it was a new part from BMW late last year, so I presume the latest version. There is no external control module, just two plugs on the unit. The large 2 pin plug takes a high current wire through a 100amp external fuse directly to the battery and the other to earth. The smaller 3 pin plug takes 12V through a 15amp fuse from the board and also connects a trigger lead to the D+ (charge light) alternator circuit. The third pin is for the mini ECU and I didn't use it. The pump stays off until the engine is running which it senses through the alternator output. All seems to work well.

Cochezz

Dubai944 if you could provide me with any sort of part number for the pump and a wiring diagram sketch you would be my personal hero for a day!