MG Sports Cars

I haven't selected an alternator yet and I'm not sure how much amperage I need. So I would appreciate some opinions.

I'm doing a 302 with a carburetor. The car will have the standard MGB electrics plus a radio/singe disk CD player, 4 speakers, VDO tach, electric water temp and oil pressure gauges (VDO), electric speedometer (VDO), electric fuel pump, electric radiator fan, MSD 6200 ignition box and blaster coil, and I'm using an AutoWire complete wiring kit.

What amperage do I need?
Any opinions on who makes the smallest most reliable one wire alternator with that amperage?
And a dumb question - If an alternator is mounted such that it spins in reverse will it still work?

Steve

I'd advise running the highest output alternator you can. What you need at the minimum is one that will supply the total electrical needs of the car in the "worst case" scenario and still have some reserve to recharge the battery. If the output is marginal then it just takes longer for the battery to recharge after a start and you risk the possibility of a low battery if you do a lot of short trip driving. To figure out the total load just add up the circuit fuses for the items that might be running at any one time. Lights (all of them as they all may be used at one time) ignition circuit, radio/stereo, heater, wipers, fuel pump, and fan. That number is going to be higher than the actual current draw as the fuse ratings are higher but the total will give you a good number to base your alternator selection on. You're probably going to come up with a total of around 40 to 50 amps, add in a bit of reserve for battery charging and then a 60 to 75 amp alternator or higher will do. There's no loss in running a 100 amp unit though, it'll just replentish the battery quicker even under full load.
While not the smallest, if you have room it's hard to beat the standard GM Delco alternator. Can be run one wire, and available in a lot of different outputs for a very reasonalbe price. As far as I know rotation shouldn't affect the output of an alternator.

I'm feeling my Cheerios... (Sorry Bill! Usually we agree on pretty much everything.) In my view, alternator amperage ratings are sort of like wattage ratings on cheap stereo systems: they're more about marketing than they are about functionality.

This topic came up when I was an electrical engineer at Volvo Truck, so I carefully measured every single electrical load on a top-of-the-line longhaul premium sleeper cab 18-wheeler (a Volvo "770", specifically.) We put a ton of fancy electrical gadgets on those trucks, including wonderful stereo systems with great big subwoofers, small refrigerators, etc. Many of our customers seemed to reason that those gadgets would require a larger alternator, and the tests proved pretty conclusively that the gadget loads simply don't add up.

Starter motors, fan motors, wiper motors, and forward lighting are real concerns. Your audio system, all your gauges and dashboard lights combined, your fuel pump, and your ignition system put together probably don't add up to the load from just one hi-beam headlight.

Although headlights ARE a significant load, marker lights, license plate lights, and taillights simply aren't. Brake lights, turn signals, and reverse lights are only on for momentary amounts of time anyhow.

As a general rule that applies to all modern OEM vehicles and I presume also to the Advance AutoWire harness, you CAN'T get any realistic estimate of actual circuit draws from fuse ratings because fuse sizes are selected to protect wire insulation from burning - and are therefore a function of wire gauge and insulation type only, NOT of actual appliance loads.

I would humbly suggest that there's no point in putting an oversized alternator on your MGB. Even the smallest little Nippondenso alternator off a Japanese subcompact will be overkill for most MGB hot-rods. The only common exceptions I see would be (a) cars with a cooling problem that run their electric cooling fans all the time, (b) cars with air conditioning because A/C fans are a relatively big load, (c) cars that run big fog or driving lights in addition to their regular headlights, and (d) cars with nasty parasitic loads that leak current away from the battery when the car is parked.

I very highly recommend installing and using a battery disconnect switch to completely eliminate issue "d" above and because its an excellent safety and security feature.

You didn't ask for it, but I'll volunteer a couple other little hints from my experience years ago. (1) There are big differences in quality/reliability from alternator model to model, and from manufacturer to manufacturer. (2) Alternator designs don't all work equally well at low rpm.

It seems that most vehicles these days come with A/C, electric fan/s and all the std high draw lighting components, so you could latch onto any small frame alternator (foreign or domestic) at your local wrecker and you'd have more than enough capacity.
Edit: Just for the record, mine is a Delco Model CS121, 61amp (converted to 1 wire).



Edited 1 time(s). Last edit at 05/18/2010 01:20PM by ex-tyke.

Headlights. If you run top end Halogen bulbs you may be putting out as much as 100 watts per bulb. This equates to a roughly 17 amp load for the pair. Factor that in.

JB

Hey Steve,
I'm going to split the difference between Bill and Curtis. I agree with Curtis that the amp draw of your vehicle won't be all that high and should be calculated based on current draw of the consumers not the fuse ratings. But I get Bills point on the big alternator. There is no harm in going too big as the alt will only put out what is required. Why not just use a Ford alt that fits your engine and pulley combo?
Nice and easy, bolts right up and can be repaired by anyone in the field.
While we're on it I would not recommend a one wire alt. These were designed for easy fitment and neater looking wiring. They don't work particularly well for daily use. You have to buzz the engine to get them up on charge and then most go off charge at low speeds (like when your rad fan needs it most). Voltage sensing is done through the charge lead and any voltage drop in this line will cause charging problems.
The alternator won't care which way it rotates but it's cooling fan will. Not a big issue but important in tight confines.
Cheers
Fred

P.S. for the items that don't have an amp rating just take the wattage and divide by 12 (volts)

What Fred said X2.

I like what Curtis said. Too often people opt for overkill with oil pumps, carburetors, camshafts and alternators. I used to hang out on the Corral board reading about Mustang hop ups. Knuckleheads there would buy some spendy March underdrive pulleys and then wonder why their cars didn't run right.

When they found out that the underdrive pulleys were; causing the car to overhead, and causing the electrical system to drain, they would fix the problems with aluminum radiators, lower temp thermostats, and..... wait for it..... high output alternators. What a racket those parts companies had. The standard alternator is still on my Mustang with the stock pulleys. It works well.

I have worked with engineers like Curtis. The car (and truck) companies don't just guess at the right answer, they spend money to research the solution, research done by smart people. Get the stock parts. If that _really_ doesn't work, you'll soon know.

My car is set up with a Crane ignitopn box, two MGB radiator fans, haligon lights (just sealed beams), electric fuel pump, and low powered radio. I run a Delco 60 amp alternator (3 wire) and it has proven to be adequate.

Note that if you upgrade the alternator using a mostly standard MGB wiring harness you will have to run a new, larger charge wire from the alternator to the starter terminal or risk an electrical fire.

X3 as fred said.I know some people with hot rods and have trouble in heavy traffic with the 1 wire cycling off. I think you have to rev it to something like 1500-2000 rpm every once in a while to get them to kick back on. Here in the deep south its a problem when running electric fans in summer. I use a ford 3g from a 90s taurus in my sunbeam v6 after my old mustang 2 alternator died. The upullem yards are full of tauri and theyre easy to istall, instructions all over the web.

Mostly for aesthetic reasons, we at the T-Bucket Factory in McKinney Texas, have been using GM 1-wire alternators exclusively with no trouble. Admittedly, a number of our buckets, mine included, by nature of what they are, have rather erratic cams, stall convertors, and don’t idle at 800 rpm, but most do; that said however, Robbie has a point. Though we have never had an alternator excitement issue due to low idle rpm, I have heard of it happening. I have a 1-wire on my GT now but will have a 3-wire plan in my back pocket just in case, besides, I already have the 1-wire and I tend to be frugal in today’s economy :-)


"P"


Update: A friend just gave me an almost new 3-wire alternator so I'm going with plan "B" just in case. Just because it hasn't happened to me doesn't mean it couldn't!!



Edited 1 time(s). Last edit at 06/11/2010 07:01PM by pspeaks.

That's a fact. Although I've never had a problem with a 1-wire dropping once it is excited, it won't begin to charge until the engine is revved up so if you just start and idle or just putt around the battery will continue to drain. It's just another thing to keep track of.

JB

We just had that conversation at the shop today Jim, and I think we're saying the same thing. As with any group just shooting the garage bull over Basil Hayden, I don't know how accurate it is, but someone who is known to be knowledgeable in such things said that if you just start a one wire car and let it stay at idle it will drain the battery, but once the rpm is high enough to excite the alternator it will stay excited and continue to charge if allowed to return to idle. Obviously if you shut it off you have to start all over again. We drive our buckets, and a fire truck, in parades lasting 45 minutes or more at almost idle with no ill effects. I’ve been driving my bucket for almost five years with a one wire and an Odyssey PC680 battery and have never had to put a charger on it except a trickle charger during the winter when as hard as I try it refuses to go outside. I really don’t think a one wire is all that big a deal, but if someone doesn’t already have an alternator, and hasn’t wired the car yet, buy a three wire. Like I say, I didn’t buy my three wire but that’s what I’m doing only because I have one.

PS: Let me qualify this, T-Buckets have virtually no electrical draw except the cooling fan; I don’t even have a radio, which I couldn’t hear over the headers anyway, so I can get away with a little PC680. I do NOT recommend, nor would I put one in my MG.

Great thread!

Delco Remy makes a one wire CS unit that triggers at real low RPM, I suggest anyone that wants to go with a one wire system to go with this unit, especially if you run fuel injection with ECM. Voltage stability is a must for proper ECM calculation, otherwise you could have an erratic idle among other problems. This CS unit usually put out around100 AMPS which is the norm for today’s power demand.

On a side note; lots of people have chased electrical malfunctions of some kind affecting drivability, check your alternator diodes. You'll be surprised at how many may fail the test. I remember changing an alternator on my Astro van that had me puzzled, not only did it charge my battery a lot better but my idle problem also went away; after checking the old alternator we found out that one of the diodes was bad.

My 2 cents

Jacques you reminded me that years ago I had a Buick Summerset with that problem and for about three weeks in a row I had to replace the diodes and rectifier in the Delco Remy. I was rather beside myself and about to replace the entire alternator, but my son had a 69 Mach-1 and rather than buy a new battery he would jump it off my Buick. After learning of this, and volunteering to buy him a new battery, my diode problem went away and he got to school on time.

Yeah, you'd think they'd build a zener diode into them to control the voltage spikes.

JB

I went with a 60 amp 3 wire Denso.

Very compact, neat setup. Looks like you're making good progress, Steve.