I was having some cooling problems with the new engine, but the dual Spal fans took care of that. However, I'm wondering if my alternator can keep up now. When the fans kick on the idle slows down considerably. Without the fans on, it seems to idle around 800, then they kick on and it goes down roughly 150-200 rpm. Is that normal? The fans and the msd ignition are probably the two biggest draws.
The alternator is a stock replacement for an 86 LG4 without AC. I'm concerned I'm over taxing the alternator and asking for trouble. Any thoughts?

Thanks.

 

Yes it is normal.

The power to run the fans has to come from somewhere. The alternator, or anything else for that matter, doesn't just somehow miracle up energy out of nowhere. All it does, is convert it from one form to another (in the case of an alternator, from mechanical to electrical, and the motor converts it back). That's one of the laws that we all have to follow in THIS universe, is that neither matter nor energy is ever created or destroyed; only its form changes, or in some cases, matter converts to energy or vice-versa.

In fact, you'll notice that the idle speed drops even when you just turn on your headlights, for the same reason.

Check the temp of the housing of the alternator after the engine has been running for a little while without the fan, and then again after the fan has been running for a couple of minutes. As long as the fan doesn't heat it up too much, maybe 20° or so, it'll be fine.

 

The start up load for the fans is also significantly higher than the running load for the fans. You'll probably notice that your RPMs drop noticeably for the first few seconds after the fans are switched on then rise back up a little. This is also normal.

 

The typical start-up draw for a set of fairly big fans like yours is probably around 40-50A... running is probably between 15-25A. Thats a pretty big draw, and will bog down the motor at idle. Its nothing to worry about though, as long as your alternator and battery are in decent shape.

 

Sofa - Thanks for the refresher on the 1st law of thermodynamics...hahaha.

I will check the housing of the alternator, that seems like a reasonable test.
I just got the battery checked, so its up to par. The alternator is fairly new, not many miles on it, but it doesn't have a very high amp rating.
I guess everything seems normal, thanks guys. I think I was pretty surprised at how powerful these new fans were and freaked a bit. If your standing right in front of the car when they are on you can feel the air being pulled in.

 

Yeah, it can be easy to overlook the basics sometimes.

I guess I should also have mentioned, we probably don't want matter being converted to energy anywhere in the vicinity of our cars... most of us, most of the time, anyway...

 

yea, Fusion might be a little dangerous...

However, it was possible in back to the future....and if movies have taught me anything....

 

Is teh engine carbed or EFI? If you still have an ECM, you can program in an idle RPM adder for fan anticipate.

As for the alternator, near peak output is typically at about 1,800-2,000 engine RPM with stock sheave ratios. Output at idle falls off. A larger alternator will have a larger output at lower RPM, but will still take horsepower to turn. If the engine is carbed, there probably isn't much you can do about it.

 

Yea, its carbed for right now. I want EFI, but I just can't afford it yet and I really need to upgrade a lot of the stock stuff still, rearend, brakes, etc.

Doesn't sound like its going to be much of a problem, just have to make sure the thing doesn't over heat.

 

Could always wire in the throttle kicker from an A/C equipped car to bump the idle with fans on. I assume you don't have one unless the non-A/C '86 used it with the power steering low pressure switch?

 

Whats your ignition timing at idle? Sometimes adding a little bit more initial timing (assuming safe to do so) can help a problem like that. Mine was like that last summer when I only ran with 6* at idle, but it wasn't nearly as much of a drop when I bumped the idle timing up to 10* when it got cooler outside. (why the timing was set low to begin with is a whole 'nother bag of dishes...)

 

Yea, base timing is about 15* mech and 25-30 with vaccum advance plugged in. The engine was just started a couple weeks ago, cooling was an issue, so I had to address that first. Everything right now is "close enough to make it run". The idle mixture, timing, spark plugs, jetting etc. I haven't fine tuned yet at all. However, I'm talking it in to get sub frames and an exhaust welded on tomorrow. I'm going to have them weld a couple o2 bungs on the exhaust so I can fine tune with my wideband o2 stuff.

As long as the alternator doesn't fire itself or something and leave me stranded, I'll be happy...