MILEPOSTS Garage - The Online Classic Car Magazine Powermaster AlternatorImproving classic car charging system performance is not as easy as it might seem.

New high performance alternators are great at highway speeds, but they don't charge well at normal idle speeds
Everything from Hood Ornaments to Tail Lamps
Return to Index

Vol. 4, No. 4
December 16, 2007

Classic Car Charging System Upgrade
by Andrew Angove

Image: MILEPOSTS Garage

For those of us who like to drive our classic cars as much as possible, there never seems to be enough opportunities to do so. Weather is normally the #1 factor for me. I won't drive mine in the rain, or if the streets are still damp from rain. Ice and snow are obviously out of the question, as are a couple of weeks afterwards, since my community uses a salt/sand mixture on the roads when they get slick, and we all know that those two ingredients mixed together aren't kind to old cars.

Normally hot weather doesn't bother me too much, my car has a rebuilt engine, new water pump, radiator, hoses, etc., so it keeps its cool pretty well. I did get the A/C retrofitted and updated, so that I could also keep my cool while driving the car on hot days. Nights are OK as well, traffic isn't as bad and all those imperfections that are so obvious in the light of day disappear at night. Suddenly your classic car looks almost perfect. To quote Laura Branigan in her hit song Self Control, " the day, nothing matters, it's the night time that flatters..."

While I've attempted to keep my car as factory correct as possible, there are a few areas where original isn't so great, so restrained modifications have been made in an attempt to correct faults. One of these areas on my car is the carburetor, I'll just say Motorcraft 4300 and leave that story for another article. Anyone who is familiar with this particular carburetor will immediately know what the faults are and were with that piece of machinery. The electrical system is another, and that is what I'd like to address right now. Specifically, the charging system. Most specifically, the alternator.

My 1969 Thunderbird was factory equipped with a 55 amp alternator. It did a pretty good job keeping everything supplied with the power necessary for it to operate correctly, but had a difficult time keeping up with even moderate demands at idle. So I thought a higher amp alternator would be just the thing. My car has a lot of electrical gadgets on it that need juice to operate. The sequential rear turn signals are one of the big users. Combine those with brake lights, front cornering lamps that are as bright as a low beam headlamp, a couple of relays that make all the lights work in sequence, and you are discharging the battery at idle, even with few other accessories on.

If you happen to be sitting at idle at night, with headlamps on and the radio playing in addition to the turn signals, it's worse. During those hot daytime excursions, with A/C on it becomes even more of a challenge. And at night with both A/C and headlamps on? Well, let's just say the needle on the AMP gauge is very definitely on the discharge side of center.

In keeping with the good old American adage "bigger is better" if 55 amps in 1969 was sufficient, 85 amps in 2007 should be more than adequate, right? And 100 amps even better. But wait, you can get 120 amps, or 140 amps. That 140 amp alternator ought to really keep things powered up, even under a big load, right? Well, it sounds good but the reality is that many of the new higher amp alternators don't charge any better, or even as good as, the factory original 55 amp ones did. And that's the catch.

I will add that the original '69 T-bird charging system had one other thing that was cause for concern: the solid state voltage regulator. Ford got a bit fancy with a couple of their high end models in the late sixties and early seventies, and equipped them with an integral regulator, that actually attached to the alternator itself. They did as they were intended: controlled the voltage and reduced the amount of under hood wiring and connectors. But when they go out, they normally fail in the full voltage mode, which means your battery gets full alternator voltage all the time. The result? A cooked battery. You can actually hear them boiling. And voltages as high as 16 or 17 volts aren't out of the question. Of course, you have no notice prior to the regulator going out, and they are getting hard to come by, which can mean some downtime if one goes out.

So, there are a couple of pretty good reasons for upgrading. I checked into some of the after market alternators, and decided on a Powermaster unit since I'd heard mostly good things about them. They make a 140 amp one that is a direct bolt-in for old Fords, and it's advertised to provide 80 amps at idle. I thought 80 amps would be more than enough for anything I might have running, so I went for it. When it arrived, it had a tag hanging on it that stated it had been tested at the factory, and was putting out well over 140 amps at speed, and 89 amps at idle. Impressive.

The alternator looked to be well built and designed, a quality item overall. They are available in various finishes, including chrome if that turns you on. I went for the black powder coated finish, since I'm not a "lots of chrome under the hood" kind of guy. There was some initial confusion as to how the alternator should be hooked up, as the '69 Thunderbirds have a junction block between the battery and starter solenoid. If you follow the wiring hook up directions provided with the alternator, you've done it wrong. The directions will work for 99.9% of the cars on the road, but of course the Birds are a different species, and don't like to follow along with the rest of the flock.

After calling Powermaster's tech support, I was lucky enough to get a guy who took the time to understand my unique situation. He verified that the alternator output wire needed to go to the junction block instead of the starter solenoid. That would allow my factory amp gauge to still operate, and would keep the wiring mostly as it was when it was built.

I will first state that this alternator is awesome at speed. Regardless of what electrical accessories you have on, you can depend on 14.2-14.4 amps. That means your lights are bright, your blower motor is spinning as it should putting out tons of air volume, and your battery is staying charged. With 140 amps available, it can handle anything you throw at it. High end sound system with a powerful amplifier? No big deal. Subwoofer? No problem. You have power to spare, as long as you're moving. And that's the catch. There's always a catch, isn't there?

This high power alternator doesn't charge any better at idle than the original 55 amp alternator did. It might not even be charging as much. But did Powermaster get the 89 amps at idle specification? Did I get a bad one? Unfortunately, no, this is exactly the way they work. Alternators are tested at a specific rpm. They consider 4,800 rpm to be normal speed, and 2,400 rpm to be idle. This is at the alternator pulley. Before I go further, let me remind you that this alternator is designed and advertised as being correct for Fords from about 1965-1986. That covers a lot of different engines and configurations, but alternators really haven't changed all that much over the years.

The important thing to remember here is that the output of the alternator is taken at its pulley, which due to the difference in size between it and the engine crank, is normally spinning faster than the crank. Powermaster refers to this as the "street ratio," which they recommend to be 3:1. That means the alternator pulley makes three revolutions for every one revolution at the engine crank. So, if you take a typical Ford idle specification, we'll say somewhere around 600 engine rpm, at a 3:1 ratio, the alternator is at 1,800 rpm, which is 600 rpm below Powermaster's idle test speed.

That might not sound too bad, since you'd still expect decent output at this speed, but there is a performance curve, and it's pretty flat until you get to the 2,400 rpm range. In fact, at 1,800 rpm this alternator is only putting out about 38 amps. At 1,100 rpm, its doing good to give you around 5 amps, if even that. And considering that the factory peak output was 55 amps, 38 amps wouldn't be too bad at idle. But there's more.

My car has a Thunder Jet 429 V-8 under the hood. The same engine it left the factory with, and a common engine used in Fords and Mercurys during this period. The larger 460 is basically the same thing, and was used in even more Fords, Mercurys, and Lincolns during a period spanning 1968-1978, and even later in some trucks. To say it's a common installation in Ford products is pretty accurate.

My car has a 6" crank and a 2.84" alternator pulley. That gives you a ratio of 2.11:1, well under the 3:1 street ratio mentioned by Powermaster, which means the alternator pulley is only spinning at about 1,420 rpm at idle with the car in drive. Powermaster's published graph doesn't even start until 1,200 rpm, so it's pretty safe to guess that output at this speed is negligible at best. So, I did a bit more research and discovered that a smaller diameter pulley on the alternator would spin it faster, and increase the output. After talking to several respected companies, I determined the best I could hope for would be a 2.585" pulley. Due to the limitations established by the depth necessary for fan belts, and the diameter of the alternator shaft itself, this was as small as you could physically go. With a 2.585" pulley, the ratio would change to 2.32:1. Better, but still not good enough. At factory engine idle, this setup allows for just 1,392 rpm. Even bumping up the idle to 650 rpm, you're only at 1,508 rpm, still almost 900 rpm slower than the requirement, and keep in mind when the car is in gear, the idle drops considerably from that point. In gear, with the idle set to factory specifications, with the smaller pulley installed the alternator pulley is turning at 1,160 rpm, not enough to hardly even put a blip on the output meter.

I called Powermaster to talk to them about this, as I felt somewhat misled by the 80 amps at idle they advertise on their website. The tech guy I got this time was not anywhere near as helpful as the first one, and I was told that there was no alternator in the world that could charge at those speeds. Yet that is what they are advertising. If you're going to say an alternator is for 1965-1986 Fords, you should consider how many of those cars will not be capable of spinning your alternator at idle fast enough for it to charge.

In looking through the 1969 Ford Shop Manual, the highest factory idle speed I saw was 675 rpm. Considering the ratio, this would still likely be far less than what would be needed for even modest output at idle. And that was my beef. They were advertising this with a 2,400 rpm idle spec, when none of the cars it supposedly fits would be able to provide that type of rpm at idle. The tech guy told me to change to a smaller pulley. I told him that it was physically impossible to go small enough to provide a decent alternator idle rpm, given the reasons I mentioned earlier. Then he said to go to a larger crank, or put a stall converter in my transmission and up the engine idle speed. None of which are reasonable suggestions, considering the cost involved. And if this is necessary, they should probably tell you this up front, BEFORE they sell you the alternator, wouldn't you think? Plus, the second tech guy was just plain unpleasant on the phone. Not the type of guy for that job at all.

So, the bottom line here is don't think a high amp alternator will fix all of your low voltage problems, because it likely won't. What it will do is provide you with all the power you need at normal speeds, and charge your battery back up very quickly once you get underway. I have a Sears Die Hard International battery in my car, with a much higher reserve capacity than the stock battery, so it can handle relatively high discharge rates for short periods of time, and be fully charged within less than a minute once the car is moving again.

I'm still going to install the smaller diameter pulley on the alternator, just to see how much of a difference it will make at various idle rpms. My car runs best when the idle is set at 600 rpm in Park. At 675, it almost acts as if it wants to diesel a bit on shutdown, and although normally it doesn't, it has a couple of times. 650 would probably be best for overall alternator output performance and engine performance.

Would I do it again? I'm not sure. I do like the performance of the alternator under all conditions except for idle, and I feel I may be able to get it close enough where it performs acceptably at low speeds. Just be aware of this when you consider an upgrade. It seems like it should be an easy fix to a problem, but as with most things there's more to it than you think.

Update: This article was originally written for publication in the Spring of 2007, but was delayed since the charging issue was continuing to be addressed on my car. Since the article was written, an idle stop solenoid has been added to my car. This solenoid bumps up the idle rpm into the 700-750 rpm range, which is enough to produce adequate charging voltage at idle, even with a modest electrical load on the car. Voltage does continue to drop at idle, but not as quickly as it did, and it doesn't drop as low as it did, either.

The idle stop solenoid is wired to the ignition so that it only operates when the key is "ON" which means you can still use the accessory position without engaging the solenoid. When you turn the key off, the solenoid allows the throttle plates in the carburetor to shut completely, preventing run on, or dieseling.

With the engine running and the transmission in gear, the car does creep forward a bit, but it's not bad. The higher idle speed doesn't appear to impact fuel mileage to any degree, either. A bracket had to be fabricated to install the solenoid, but other than that it seems to have been an easy installation.

While I still believe Powermaster needs to be more clear in the performance limitations of its alternators, mine is now performing at an acceptable level at all engine speeds and electrical loads. It's great that they have a model designed especially for our older cars, but there's more to it than just bolting it on, plugging it in, and driving, if you expect it to charge as advertised. I'm glad I made the change, but I wish I'd known up front exactly what was involved.

If you have advice, tips, technical ability, or just know a secret or two about old cars, and you'd like to contribute, click here and tell us about it. We'll help you write it, and give you the credit for it! It's the perfect way to help out your fellow enthusiasts in the old car hobby.
MILEPOSTS Garage Index | Automotive Mileposts

Recent Updates | Main Contents | AUTOPOSTS Forum

Vintage Car Care | Parts | Books | Magazines