Alternator question- and power usage question

[imiceman44]

My '90 with the 6.2 came from the factory with a 12SI unit. When it finally died I installed a 140 amp CS144 version on it, exact same version that is used on gas engines. The adaptor plug and the basic modifications to the brackets were the same as if you were converting a gas engine from one style of alternator to the other.

I'm not sure about the regulator or engine rpm questions. Granted, the 6.2L has a lower max. rpm rating then a gas version engine, but you are running the same rpm range during normal operation whether it's a gas engine or the 6.2L. Think about it, the 6.2 equipped trucks came with the same transmission, transfer case, and gears to a gas truck which equals same engine rpm for all speeds. Even if the gas engines can run 1,000-1,500 rpm higher for the maximum rpm, it's not like those engines see 5,000+ rpm for any length of times during normal driving.

Not quite
The diesel trucks have lower stall TC's for a reason, they run on average at lower RPM's thus the lower Stall Converters.
PLus most of the time it's the idle RPM's you need for charging and idle is lower on diesels.

 

[6.2Blazer]

Not quite
The diesel trucks have lower stall TC's for a reason, they run on average at lower RPM's thus the lower Stall Converters.
PLus most of the time it's the idle RPM's you need for charging and idle is lower on diesels.

So the "lock up" torque converters on the 700r4 somehow make the rpm lower between a diesel and gas? What does the stall rating have to do with operating rpm once the vehicle is moving? Oh, and the typical alternator only puts out a small amount of their total capacity at idle.......hence the reason people need to "rev it up some" to keep up with a winch or other electrical accessories.

 

[kennyw]

I think the point is the factory was allowing for the lower RPM usage of the diesel by having the alternator start charging earlier. Though you don't need full output at idle, you want the alternator to keep ahead of the draw on the electrical system at idle to keep from draining the system. I'm not talking about aftermarket components like winches here though. Just talking about headlights, wipers, dash/parking lights... things the factory thought about.

Running an alternator from a gas rig will probably be fine because the driving above 1000 RPM will recharge the dual batteries and you should have enough reserve with dual batteries to run a long time if it isn't fully charging at idle.

 

[imiceman44]

So the "lock up" torque converters on the 700r4 somehow make the rpm lower between a diesel and gas? What does the stall rating have to do with operating rpm once the vehicle is moving? Oh, and the typical alternator only puts out a small amount of their total capacity at idle.......hence the reason people need to "rev it up some" to keep up with a winch or other electrical accessories.

Kenny in the post below you pretty much got it.
At a lower Idle GM wanted to get a little more of the power so it can keep up with the stock components.
And while driving they are almost the same but you do spend more time idling than you think you do, and you do want to be charging at least not discharging your batteries which by the way get drained pretty quick if you are starting in cold weather with no block heater.
And the glow plugs also draw a lot and the newer ( 85 and up) keeps cycling even after getting warm, when idling brings the temps below optimum.
Also a well geared diesel will spend most of it's driving around 2000 RPM.
A gas engine will spend most of it's time between 2500 and 3500 RPM.

 

[diesel4me]

Guess I'll rebuild mine if it croaks..

Strange,this is all news to me--I sold parts for 20+ years,and AFAIK GM only has one part # of regulator for the altenators like 10SI and 12SI's..they must use a different stator winding in the alternator to boost the lower RPM output. .
I will try to save mine if it ever acts up and put new brushes or regulator,or bearings if it ever craps out..but I bet 90% of us could swap an alternator from a gas motor in,and never know the difference..gas motors spend lots of time idling in traffic too,they don't discharge the battery(s)..

 

[6.2Blazer]

Kenny in the post below you pretty much got it.
At a lower Idle GM wanted to get a little more of the power so it can keep up with the stock components.
And while driving they are almost the same but you do spend more time idling than you think you do, and you do want to be charging at least not discharging your batteries which by the way get drained pretty quick if you are starting in cold weather with no block heater.
And the glow plugs also draw a lot and the newer ( 85 and up) keeps cycling even after getting warm, when idling brings the temps below optimum.
Also a well geared diesel will spend most of it's driving around 2000 RPM.
A gas engine will spend most of it's time between 2500 and 3500 RPM.

Okay, one of you guys answer these simple questions:

1. What is the standard idle speed for a 6.2L diesel?

2. What is the standard idle speed for a standard TBI 350 gas engine?

3. Explain your reasoning again for the lower idle speed debate?

The "driving around 2,000 rpm" vs. "2500 and 3500 rpm" is also a moot point since most alternators are putting out the maximum, or at least close to, output by 2000 engine rpm anyway. A 12SI alternator graph shows max. output by about 5000 SHAFT rpm, and with the size difference of the crank vs. alternator pulley and the associated gearing effect that would be pretty easy to do (crank pulley would need to be 2.5 times larger diameter than alt. pulley, correct???).

 

[Roz]

ahh yes, i would have to ask that as well, are the pullys the same size?

 

[imiceman44]

Okay, one of you guys answer these simple questions:

1. What is the standard idle speed for a 6.2L diesel?

2. What is the standard idle speed for a standard TBI 350 gas engine?

3. Explain your reasoning again for the lower idle speed debate?

The "driving around 2,000 rpm" vs. "2500 and 3500 rpm" is also a moot point since most alternators are putting out the maximum, or at least close to, output by 2000 engine rpm anyway. A 12SI alternator graph shows max. output by about 5000 SHAFT rpm, and with the size difference of the crank vs. alternator pulley and the associated gearing effect that would be pretty easy to do (crank pulley would need to be 2.5 times larger diameter than alt. pulley, correct???).

My engine runs at 550 at idle, my friends 6.2l runs idle at 500.
All the gas engines I've had idled at 800 plus.
And most of the time I run at 1500 RPM on my diesel, it always is at 2500 to 3000RPM on my SBC.
As far as the pulley size I never noticed a difference but then again it would be a minor change to smaller to give it faster spin so I might need to measure to give an answer on that.
It would make more sense to have a different size pulley than to have a different internal charging system since all it is is a shift in the RPM range.

My perkins straight 6 turbo diesel idles at 400RPM and tops at 1800RPM, driving mostly at 1100 to 1200 RPM so I put the 135amp unit in it and it works fine, this unit is an aftermarket which is supposed to put out more amps at lower RPM's and it's 12SI unit.

 

[6.2Blazer]

My engine runs at 550 at idle, my friends 6.2l runs idle at 500.
All the gas engines I've had idled at 800 plus.
And most of the time I run at 1500 RPM on my diesel, it always is at 2500 to 3000RPM on my SBC.
As far as the pulley size I never noticed a difference but then again it would be a minor change to smaller to give it faster spin so I might need to measure to give an answer on that.
It would make more sense to have a different size pulley than to have a different internal charging system since all it is is a shift in the RPM range.

My perkins straight 6 turbo diesel idles at 400RPM and tops at 1800RPM, driving mostly at 1100 to 1200 RPM so I put the 135amp unit in it and it works fine, this unit is an aftermarket which is supposed to put out more amps at lower RPM's and it's 12SI unit.

Nice try.........

Factory recommended idle speed for a 6.2L is 650 rpm. A TBI 350 is what, around 500-700 rpm?

As I've said before, there is simply no logical reason why an alternator for a gas engine would not work on a 6.2L diesel. Idle rpm is quite comparable, cruising rpm is comparable, and the rpm redline difference is moot because the alternator is already putting out max. amperage before you get to that point anyway.

That's great that your Perkins diesel is different and max rpm is only 1800 rpm, but that has nothing to do with the conversation regarding the difference between 6.2 diesels and SBC's. Just because two completely different engines run on diesel fuel does not mean you can simply group them together when comparing.

Ironically, you also mention that a 12SI alternator works fine on the very low rpm Perkins.............again, think about it. Max output for the alternator is around 5k rpm which would easily be reached with a simple 3:1 ratio in the pulley's (1,800 x 3 = 5,400). Even at the crusing speed of 1,200 you are turning 3,600 rpm on the alternator which would probably be at least 75-80% of the max output on a 12SI alternator.

 

[imiceman44]

Nice try.........

Factory recommended idle speed for a 6.2L is 650 rpm. A TBI 350 is what, around 500-700 rpm?

As I've said before, there is simply no logical reason why an alternator for a gas engine would not work on a 6.2L diesel. Idle rpm is quite comparable, cruising rpm is comparable, and the rpm redline difference is moot because the alternator is already putting out max. amperage before you get to that point anyway.

That's great that your Perkins diesel is different and max rpm is only 1800 rpm, but that has nothing to do with the conversation regarding the difference between 6.2 diesels and SBC's. Just because two completely different engines run on diesel fuel does not mean you can simply group them together when comparing.

Ironically, you also mention that a 12SI alternator works fine on the very low rpm Perkins.............again, think about it. Max output for the alternator is around 5k rpm which would easily be reached with a simple 3:1 ratio in the pulley's (1,800 x 3 = 5,400). Even at the crusing speed of 1,200 you are turning 3,600 rpm on the alternator which would probably be at least 75-80% of the max output on a 12SI alternator.

Hey I like your spunk.
I was not born yesterday, I didn't associate the perkins with the 6.2l I just threw that in there as an aditionnal case where we have lower idle.
As you point out, RECOMENDED idle is 650 on a 6.2l doesn't mean every truck is running that.
I had a truck that idled at 640 (mini tach).
Anyway if you read my post you notice I mentionned that if GM was smart, they probably worked on pulley sizes not the internals, but I had no way of proving either way since I didn't have both on hand to compare.
Anyway, this conversation has gone deep enough, I am sure the original poster got what he wanted.
Hey I like a challenge like that.


IceMan