Under Driven 200 Amp Alternator: A Bad Idea

FastDriver · Nov 8, 2017

Here's an interesting tidbit for those that, like me, are "electrically challenged." Buying a higher-output alternator to compensate for spinning it slower doesn't work! It actually makes it worse!

My car revs out to 7000 RPM, and as such, it would normally overspin an alternator and damage it. My UD crank pulley helps prevent overspinning, but with an 800-900 RPM idle, I've found that my 200 Amp alternator is only putting out 11.8 volts. I called PA Performance and Rick there explained, politely in barney style, that, and I'm paraphrasing, I'm a moron for doing all that stuff. A 130 Amp, if it's enough current, will actually put out more voltage for me. He said I should probably idle it at 900-950, and I'll be getting plenty of voltage.

I'm not sure if my Weldon pump, Engine Management, high output coil, etc... are drawing more amps, but I'm guessing a 130 will be plenty. So, I've got one on the way. Will update when I install it.

Chris

FastDriver · Nov 8, 2017

I may have gotten in too much of a rush on that, because now I've got another idea. I just ordered a 2" and a 2.1" overdrive pulley that I'll install on the 200 amp alternator. I held a ruler up to the crank pulley and it looks like it's smaller than 5". I'm guessing 4.75" or 4.875". I'll get a more accurate measurement when I pull off the belt.

PA performance aims for 2000 rpm alternator shaft speed at idle. Even if I'm at a really low 800 rpm idle, A 2" alternator pulley puts me at 1900-1950. If it doesn't get the voltage up enough, I'll try the 2.1.

If I set rev-limiter to 7k rpm, the 2.0" should spin the alternator to 16,625-17,062 RPM, which is really pushing it. I could drop the rev-limit more or install the 2.1 which brings me to 16,250 at worst case. I can feel more confident pushing the limits now because if anything goes badly, I'll have the 130 amp as a ready replacement.

Chris

Noobz347 · Nov 8, 2017

FastDriver said:
I may have gotten in too much of a rush on that, because now I've got another idea. I just ordered a 2" and a 2.1" overdrive pulley that I'll install on the 200 amp alternator. I held a ruler up to the crank pulley and it looks like it's smaller than 5". I'm guessing 4.75" or 4.875". I'll get a more accurate measurement when I pull off the belt.

PA performance aims for 2000 rpm alternator shaft speed at idle. Even if I'm at a really low 800 rpm idle, A 2" alternator pulley puts me at 1900-1950. If it doesn't get the voltage up enough, I'll try the 2.1.

If I set rev-limiter to 7k rpm, the 2.0" should spin the alternator to 16,625-17,062 RPM, which is really pushing it. I could drop the rev-limit more or install the 2.1 which brings me to 16,250 at worst case. I can feel more confident pushing the limits now because if anything goes badly, I'll have the 130 amp as a ready replacement.

Chris

Just a couple of minor details:

Running the engine up to 7000 rpm for the length of a quarter mile (or less) should not significantly impact the alternator. Consistently over driving it would be an issue.
If the battery is up to snuff and charged, even holding at 7000 rpm would not be a huge deal.

If the alternator is discharging fully to a weak or drained battery at 7000 rpm then it's likely you might see some fireworks or blow your (properly selected) in-line fuse.

The point is that you shouldn't be overly concerned with short pulls to or above 7k unless you expect a really high discharge rate for some reason.

FastDriver · Nov 8, 2017

Rick seemed more worried about mechanical damage, citing centrifugal force, than electrical discharge. And in his words all it takes is once.

Bullitt347 · Nov 8, 2017

I have a 200 amp alt on the race car. Engine spins to 8k+ rpm. I have a 4" dia alternator pulley on it. No issues. Charges 13.5 volts at an idle, but my idle speed is close to 1,400 rpm lol.

FastDriver · Nov 8, 2017

What's your crank pulley size?

FastDriver · Nov 8, 2017

You didn't mention your crank pulley size, but I guess it doesn't matter given you are spinning at almost twice the RPM I am.

Bullitt347 · Nov 8, 2017

I have had a 7.8" (stock), an 8.5" and now a 9.5" dia 8 rib crank pulley.

FastDriver · Nov 8, 2017

The stock replacement pulley set LMR sells come with 5.8" crank pulleys. So, err... are you talking about another pulley, off on the crank pulley size, or do you have the mother-of-all_crank_pulleys?!

Bullitt347 · Nov 8, 2017

Hmmm, thought the stock pulley was 7.8" Thought the Cobra had a 6" pulley. Been so long since this car was stock. IDK, my smallest pulley is 7.8"

FastDriver · Nov 8, 2017

Haha. Ok. Well that explains the 4" alt pulley

Mustang5L5 · Nov 9, 2017

Upper limit is really more mechanically limited. The bearings and misc parts tend to get sketchy over a certain rpm. Hit or miss if it happens. It either fails or holds a bit longer.

I run a standard 130A alt, with 4.5" crank and 1 7/8 alt pulley. Idle rpm is maybe 700, and I have full 14+ volts at idle.

Too lazy to do math right now lol

FastDriver · Nov 10, 2017

Ok, broke out a better ruler, and it looks like my crank is actually a 4.5". It measures about 4 7/16" across the outermost edges of the crank pulley and the drop from the edges to where the belt rides is about 1/8". So 4.5 is pretty close to the truth. So, with a 2" alt-pulley and 850 rpm idle, that gets me 1913 RPM on the alternator, which is 95% of recommended. Close enough, I think. Bumping idle to 900 puts me just over at 2,025 alt RPM. At 7000 RPM, Alt-shaft-speed is 15,750, which stays under the "redline" of 16k. Perfect! This should work well.

Mike, I appreciate all of your help. So, here's your math, even though it sounds like everything is working for you, as is: At 700 rpm idle (that seems low), given 4.5" crank and 1.875" alt pulley, you're spinning the alternator at 1680. To get to 2000 alt-idle, you'd need 833 Engine idle. Assuming stock limiter of 6,250 rpm, you're only spinning the alternator to a max of 15k RPM. You wouldn't hit the 16k alt-rpm max unless you spun your motor to 6,667 RPM.

Mustang5L5 · Nov 10, 2017

Now theres some math and then some. Pretty good spreadsheet to use.

FastDriver · Nov 10, 2017

I do that for everything. I've got a bunch of car math spreadsheets: Injectors, turbo calcs, wheel calcs, engine displacement, gears/tires/speed... Hell, I've even got a 1/4 mile dragstrip simulation I made in college and have tweaked a few times since, lol. Welcome to any of it that you want. -Chris

mikestang63 · Nov 11, 2017

FastDriver said:
I do that for everything. I've got a bunch of car math spreadsheets: Injectors, turbo calcs, wheel calcs, engine displacement, gears/tires/speed... Hell, I've even got a 1/4 mile dragstrip simulation I made in college and have tweaked a few times since, lol. Welcome to any of it that you want. -Chris

sharing is caring

Mustang5L5 · Nov 11, 2017

So walked out to the car and did verify that I also do have a 4.5" crank pulley where the belt rides. I'm also using an ASP 1 7/8" alt pulley like mentioned.

Idle RPM? Unsure. I do have a hand held tach I could use, but my idle is being managed by the IAC so whatever Fords ECU wants to set idle to is where it's at.

My alt is a take-off 94-95 3G I installed 20 years ago. With my SN95 Efan on, I still manage to output over 14 volts at idle from the alt. So it appears that I still manage to meet the min requirements of alt RPM to generate full power.

I wonder if there are posted specs for various Alts as to max RPM and min RPM to generate 14v+. I thought I read 16-18k was a max limit for most.

This is the sort of info that needs to be considered in addition to alt rated max amp output.

Edit: according to the coil cover decal, 5-spd idle RPM is 625-775 rpm

FastDriver · Nov 11, 2017

I'm surprised at the idle RPM. I didn't remember it being so low. Haven't had one running like that in at least 10 years. 20 years on the same alternator is impressive! Go Ford parts!

The fact that you're making 14+ that way is in line with what Rick at PA Performance said about swapping to the 130. Now, I will say that when my car was idling cold, it made 14V, too. Once everything warmed up, it was putting out 11.8.

I 100% agree about posting the operating range of the alternator. I kind of intuitively knew it had to spin a minimum speed, but I never even considered max speeds before. Like most, I just put it on and expected it to work.

Stock idle at 625 with a 5.8" crank and based on a stock pulley I measured at 2.17 yields only 1,671 alt-rpm. That's really low, but then the stock alternators were only what, 80-ish amp units? Maybe they just didn't need a lot of speed. Now, 775 would be pretty close to 2,100 alt-rpm. Interestingly, the stock rev-limiter would peg the alt out at 16,705 RPM.

mikestang63 said:
sharing is caring

Happy to share. Any regulars that want any of my spreadsheets just need to PM me an email address. They were just meant for me and may not be the most user friendly, though.

Noobz347 · Nov 11, 2017

So uh..... What's theoretic max speed of an alternator supposed to be?

FastDriver said:
I'm surprised at the idle RPM. I didn't remember it being so low. Haven't had one running like that in at least 10 years. 20 years on the same alternator is impressive! Go Ford parts!

The fact that you're making 14+ that way is in line with what Rick at PA Performance said about swapping to the 130. Now, I will say that when my car was idling cold, it made 14V, too. Once everything warmed up, it was putting out 11.8.

I 100% agree about posting the operating range of the alternator. I kind of intuitively knew it had to spin a minimum speed, but I never even considered max speeds before. Like most, I just put it on and expected it to work.

Stock idle at 625 with a 5.8" crank and based on a stock pulley I measured at 2.17 yields only 1,671 alt-rpm. That's really low, but then the stock alternators were only what, 80-ish amp units? Maybe they just didn't need a lot of speed. Now, 775 would be pretty close to 2,100 alt-rpm. Interestingly, the stock rev-limiter would peg the alt out at 16,705 RPM.

Happy to share. Any regulars that want any of my spreadsheets just need to PM me an email address. They were just meant for me and may not be the most user friendly, though.

Post them as resources with use instruction under the discussion tab.

FastDriver · Nov 11, 2017

Max speed of the 3G alternators from PA Performance, based on their website is 16k RPM.

Under Driven 200 Amp Alternator: A Bad Idea

I was uncomfortably high & wearing a helmet

I was uncomfortably high & wearing a helmet

Stangnet Facilities Maint Tech... Er... Janitor

I was uncomfortably high & wearing a helmet

I have been doing it wrong this whole time

I was uncomfortably high & wearing a helmet

I was uncomfortably high & wearing a helmet

I have been doing it wrong this whole time

I was uncomfortably high & wearing a helmet

I have been doing it wrong this whole time

I was uncomfortably high & wearing a helmet

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I was uncomfortably high & wearing a helmet

That is…until I whipped out my Bissell

I was uncomfortably high & wearing a helmet

That is…until I whipped out my Bissell

I was uncomfortably high & wearing a helmet

Stangnet Facilities Maint Tech... Er... Janitor

I was uncomfortably high & wearing a helmet

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