Alternator Upgrade: Higher Output Needed

[DRTE30]

Greetings,

I have a 1991 318is that has had some additional electronic components added to it.  What concerns me, is that I feel that the original 60watt alternator just cannot cut it for what I have going on in in the car.  Since taking ownership, I have swapped the clutch fan for a two-stage electric Volvo fan, rear view mirror w/reading lights, aftermarket deck, amps totaling 1000watts RMS, yellow top battery, OEM fog lights and a nice little M Technik illuminated shift knob.  When the engine is at a normal driving RPM everything seems fine, however at idle when the system is under a bit more stress to charge the battery, any of the above devices running at once or at the same time causes the RPM speed of the idle to dip repeatedly.  I don't even turn up my stereo all that loud and it will cause an RPM dip.  I can literally hear my subwoofer punch, then see the RPM needle dip 300RPM.

All of the amps are properly grounded with very big wire, and the runs are short.  Likewise, the power cables are sufficient enough.  Lastly, all of the chasis ground cables have been replaced and contact areas cleaned.

There seems to be a bit of misinformation on the internet regarding upgrading the alternators for the M42 with another OEM unit.  What I want to find, ideally, is the highest wattage alternator (maybe E36 140watt Bosch?), Bosch, and it needs to practically bolt right in with little modification to the pulley wheel and/or bracket.  Any suggestions?

[DesktopDave]

I'd think first about a larger capacity battery.  It does bolt right in, no mods needed.  A good AGM like an Optima might be a long way to solving your problem.  Also be sure your grounds are spotless!

I'd be pretty sure a later M44 E36 alternator would bolt right in.  Many others might fit.  Use an impact wrench and a three-leg gear puller to swap the v-belt pulley onto it.  Be aware that a larger alternator will make a dent in your fuel mileage...

[Slowered318]

First you should do all your tests and check for parasitic battery drain, should be 35 milliamps or less when parked. Maybe trickle charge your car overnight and see if you still have the same idle problems.

A good size AGM battery should handle the erratic current draw from your stereo and take some strain off your charging system. If you have the 65A Valeo alternator then I would consider a 80A-95A Bosch to charge the battery quickly. Even full size transport trucks typically use 160A or 200A alternators to run all the lights, accessories (some have fridges and microwave oven) and charge four or more batteries, I think 140 would be overkill for your car and could possibly stall out the M42 LOL!

I still get a slight idle dip when the electric fan turns on but no light dimming problems. I'm using a 80A Bosch and D34 Optima Yellow Top, a D31 should fit with some modification to the battery tray. I would really like to run a light weight battery, possibly the Deka ETX30L or Odyssey PC680 but I often let my car sit for days/weeks. Can't wait for the price of Lithium-Ion to come down, need more Prius's on the road.

[DRTE30]

Thanks for the thoughts.  I completely forgot to mention that I have a HUGE yellow top in my car already.  In regards to the grounding points, on some locations I've used Penetrox, which I use for work.  This is an anti-corrosion fluid used for electrical connections in electrical panels.  I really don't see how this would hinder the flow of electricity for my grounds, as the coatings I do are quite thin.  If anything the fluid has increased the surface area contact between the grounding strap and the chassis.  Perhaps it's time to complete the other grounding areas.  I also forgot to mention, that when the stereo is on, and the headlights/fog lights are on, I do not have dimming.

With the alternator, I am convinced that this is the next step to do.  I have read online that some people have had great results with the E30 318i alternator as this is a 80amp, some with the 325i alternator as this is 90amp, but also, some have had mixed feelings.  This is also hard to sift through the nonsense.  Perhaps these people have already done other measures to ensure their charging system is tip-top, while others know little to nothing and the first thing they upgraded to the system was addressing the alternator.

Due to the increased size of my stereo system from stock and additional axillary pieces, I would rather install something over 120amps right off the bat and be done with it.  Plus I'm in college, so I don't have a lot of money laying around to do this twice and pay for parts twice.

Perhaps I'm worrying too much?  I simply don't think it's good for a charging system to be under stress from a HUGE stereo system and other components on at the same time.  This poor little car was never designed for such things; surely it cannot be good in the long run.

[Slowered318]

I'm pretty sure you can adjust the idle a bit with a custom ECU or by playing around with the AFM. Problem is that most alternators don't generate much power at idle. Back in my SPL competition days we would jam the throttle body open so the car would idle up around 2,500 rpm. Even with 4+ batteries that would help because the we didn't play music through the subwoofer, just a constant tone or "burp".

You could also address what's causing the problem. For example you could use a class D or T amplifier and you could instal a more efficient electric fan. I'm not sure the Volvo fan specs but my 16" aftermarket fan draws about 175 watts, most "high performance" fans are upwards of 250 watts and that's overkill if you have a proper fan shroud. I think Hayden makes a 17" dual speed fan that's 130/270 watt, this would work great if you mount it properly and wire it up to the 80/88 switch.

[DRTE30]

I have to check out your suggestions when I have a free moment.  In the mean time, I found an E34 Touring with the alternator I was looking for- it tested fine and I got a screaming deal on it.  Before it goes in, I'll test the things you mentioned.

As far as the stereo goes, my sub amp is class d, and my small drivers are powered by a class AB and the entire system is 4ohm.

[Slowered318]

I think one of these should handle your power needs.  http://www.tirerack.com/accessories/detail.jsp?ID=118