re: my LED posting, I figured out some time ago that idle problems on my D-Jet car when the lights were on were due to parasitic drag from the alternator. My question is: what's wrong with the stock alternator?

First off, source material. Pretty much everything I know about the charging system on our cars and how it works comes from this link on Richard Atwell's page:

http://www.ratwell.com/technical/ChargingSystem.html

The 914 stock alternator is rated at a maximum of 50 A at 2200 rpm. Like on other VW's, the alternator pulley is half the size of the engine pulley, so if you have the idle set to 950 rpm, and the voltage regulator is running at 80% duty cycle (maximum sustained duty cycle Atwell recommends), then at idle, the alternator should be able to supply 950 * 2 / 2200 * 50 * 0.8 = 34.5 A.

In my posting about the LED lights, I calculated that with the lights on, and using low beams, the stock incandescent lights pull 19.5 A. Additionally, the car needs current to run the ignition, the fuel pump, and to drive the ECU. I'm not sure of how much of a load this is, so I'm going to just take a stab at it and say 100 W, so at 12.8 V this would be about 8 A, for a total of 30.8 A. That would imply a duty cycle at idle of 71%, which isn't that bad, and leaves 9% to drive other accessories such as the heater and fresh air blower, radio, and wipers. Yet from my experience, even at this duty cycle, the drag from the alternator is enough to pull the idle down to as low as 500 rpm, making the problem even worse. I've looked at the voltage on the field coil of my alternator at idle with the lights on, and it looks closer to 100% duty cycle to me, not 71%, and that was after opening the air bleed screw to get the idle back up to 950 rpm.

In my case, by going to LED's and reducing the required current for lighting at idle from 19.5 A down to 5.0 A, the idle is almost unaffected by turning on the lights. For my car, clearly the parasitic drag of the stock setup is a problem.

Is my math and reasoning right here? For people who have installed 70 A and 160 A alternators, have you seen improved idle stability? Are some of you running D-Jet setups with the stock alternator and stock lighting, and have only 50 rpm or less idle drop when turning on the lights when the car is idling at 950 rpm?

Well just for a sanity check: 50A @ 14.5 Volts = 725 Watts.

1 hp = 745 watts.
If the alt is 60% efficient, then to deliver 745 watts, it needs to use 1.67 hp

So at Max output the alternator should suck ~1.7 hp from the engine.
So "in theory" the engine should barely notice.
But "in practice" we can tell when you apply a big load like turning on the headlights.

I am guessing that your EFI was more sensitive to the drop in 'Bus Voltage' than the actual parasitic drag on the engine by the alternator.
Likely it was a combination of both.

Oh, and for a dimmer for your dashlights look up "Buck Puck Driver".
You want the kind that is controlled by a voltage (potentiometer) input and not the PWM style.
http://www.ledsupply.com/led-drivers/buckp...t_rYaAqjc8P8HAQ

I can't really speak to the idle issue, but I would love to explore it more. I was able to find a drop-in 150amp alternator, but I did it mainly for the built in regulator and 'modern' technology, as well as having enough power for things like lights, stereo, and seat heaters.

But I had no idea the stock alternator was so close to it's max capacity. Seems like a temporarily installed ammeter would be helpful.

1.7 HP is a lot at idle.

At pure idle the engine is producing ZERO output HP. Every in-oz of torque will slow the engine or require extra fuel and air.

We have become so used to cars with electronic throttles, that we don't expect engine slump any more.

Sweet. Will check it out, I was just going to build a PWM circuit with a 555 timer and match it to the potentiometer range of the stock headlight dimmer.

I agree on the load calculations, it shouldn't be enough to drag the motor down at idle, but as many of us know, it does. But, I'm violating my prime rule of "personal computer repair", which has served me well in fixing about a billion problems - "why" is not as important as "how". It's more important to know how to fix a problem than why the fix works (e.g. "just reboot it"). After many false attempts to fix my "idle w/headlights on problem", I now know "how".

It does because it IS.

1.67 HP at 500 RPM is 17.5 Ft-LBs of torque. Not a small amount
Think of the work done by a 1.5 HP electric motor.

Good point of the no-load condition at idle.

Question is what is the no-load loading of the engine at idle? In other words, how much HP is it taking for the engine to spin itself and drive the alternator (without headlights) with the drag of the transmission in neutral? 2HP? 5HP? Less?

If one assumes 5HP to operate the engine at idle w/o lights on; with 20A (lights) @ 14.5V; power dissipated is 290W. 60% efficiency and 745W/HP, the extra load is .65HP or 13% of idle load. Yeah, the engine will slow down with lights on.

Just push the accelerator pedal down further

Mark, I remember you were working on a upgrade solution ... are you selling those now?

I'll be installing 914rubber's seat heaters and an audio amp in the '71.

Maybe it's my total ignorance of things electrical but is this really a problem? If your idle drops from 950 to 900 when the lights are turned on, is this really a problem? I've had the stock alternator for 14 years with two sets of driving lights and have never had much trouble. I might install a seat heater this year and I might then need to make a change to something with more power.

Imaging the starter (just less than 1 HP?) and what it would do to the idle rpm if it was engaged to rotate the engine the wrong way.

Lars S

I remember this being a pretty nice replacement if not somewhat pricey.

I purchased one of those 160A alternator kits from Pete Newman a couple years ago. I was planning to installing it on the 'BB', but I may end up installing it on the 1.9L motor, and sourcing another one for the 'BB' (if not too expensive).

What ye say, McMark ...

It does, under certain circumstances. If your engine idles fine at 950, but gets rough at 900, then it would be annoying. Or if you add more electrical load the idle drops further and then runs rough/stalls. If neither of these cases apply to you, then the stock unit is fine.


Yeah, I can make them for the people with increased electrical loads, and those who want to ditch the stock regulator setup. As Elliot said, they're not cheap. $350 for the 150A unit and there's about a month build time since the base unit is build to order, then my modifications on top of that.

That's not too bad, really

Remember that theoretical calculations don't usually take into account the fact that our wires and chassis (ground path!!) are old and most of the connections in the system are none-too-great. Both of which add extra resistance into the circuits, and lead to lower efficiency and more power dissipation.

--DD

Compared to what we spend on other upgrade components, I agree! A stock rebuilt alternator, a new voltage regulator, and a new harness are about $250.

So more power and reliability for $100?