Can I Boost My Alternator To A Higher Output?

Like today, I occasionally get this question, as some people need more output due to extra loads put on the alternator by adding other electrical equipment to a vehicle that did not come factory installed.

This is no simple question to answer, as too many variables enter the equation.  First I'll address what happens to the unit by ramping it up.  Most alternators are pre-designed to put out the maximum amps for their size, though some of them can vary and accept a small increase in output.  Also, alternators are only able to maintain constant output at 75% of their rated capacity, which means it you have a 100 amp unit, you can only expect to use 75 amps 100% of the time.  Taxing the unit for more will result in premature failure!

The real problem lies in the ability for the unit to put out a charge, in all the engine speed ranges or RPM"s.  If you have an OE 100 amp alternator, that was installed in a vehicle it was designed for, it will function properly at engine speeds from let's say 400 RPM's at idle to 3,500 RPM's top engine speed.  Now let's say you need to raise the output from 100 amps to 160 amps output out of the same unit, and for this purpose, let's say the parts are available to do such a boost.  What happens is, by putting the heaver windings to increase the output, you will also move the RPM range.  Since the unit wasn't pre-designed to run with these heavy components, you now have changed the range, to where it may only charge from let's say, 1,000 RPM's to 4,000 RPM's, so at idle speeds, it will drop out and no longer charge.  You will also have to run the engine faster to get the output you want.

For most applications, parts are not available to rebuilders to boost most units beyond factory specs.  Each unit is unique to the application and very few or none of the parts on these units are interchangeable anymore.  There are companies that build OE style units into high amp output units, usually for stereo amplifier applications, but these units are usually short lived as most consumers of these units, tax them to their limits.  For this reason, I personally do not market or sell these units, nor will I stand behind them either.

An exception to this, is in older units that came in a large amp range.  Take the Delco 10SI for example.  It was produced by Delco in the amp ranges from 37 to 63 amps and has been boosted to about 80 amps without any problems either.  So let's say you have a 45 amp unit and want to boost it to 72 amps, the parts are available to do the job and you won't be pushing the design limits.  It will still function properly in all the RPM ranges!

One other minor problem worth mentioning is if you increase the output, you will also tax the engine horsepower a little bit, which may decrease the gas mileage some too.  Is the trade-off worth it?

Bottom line is if you need a large boost in amp output, you need to use a larger alternator that is designed for that output.  If you need a large output alternator, but only have room for a small unit, then don't expect the unit to function at idle speeds and don't expect it to have a very long life if you tax it close to it's limit!

Alternators, Models & Design, Part 3, Delco 12 SI

The Delco 12 SI became the later, ramped up version of the 10SI.  This unit was used from the early 1980's to the late 80's.  Notice, I don't use specific years because some models used several different units during the crossover years. 

Though all the parts are somewhat interchangeable with the 10SI and a few with the 10DN, there were differences.  I've seen all the parts interchanged as some point or another and though they will work, they don't fit properly.  The 10SI's maximum designed output was 63 amps, but has been rebuilt to much higher outputs.  A big majority of the 12SI's were at 78 amps output and some were designed to give 94 amps.

Externally the main difference was the enclosed fan.  Behind the fan, the drive end housing had a raised ring that the fan meshed with.  This gave it more efficient cooling and I should mention that most all external fan alternators, cool by drawing air in from the rear of the unit, through the unit and out the fan, instead of pushing it through from the front!   There is also a larger cooling area on the back of the unit over the rectifier.

The 94 amp unit has cutouts on the drive end housing around the stator windings and it also has double angled cooling fins on the rectifier.  This makes it easier to identify when working on the unit.  Sorry, I have no pictures of the 94 amp-er.

Internally, the rotor and stator are thicker than the 10SI units and the rectifier has an extra grounding and mounting slot or hole.  I've had problems swapping a 12SI stator, putting it in a 10SI unit because the stator leads are not quite long enough and must be stretched and put in a bind to get connected.  They have broken in service, so I seldom do that anymore and only if necessary!  Hookups and testing are the same as on the 10 SI, so refer to that section for that information.

Alternators, Models & Design, Part 2, 10SI Variations

The Delco 10SI marine units have a screen behind the fan inside the case and a screened cover that covers the rear half of the unit.  This is to make the unit explosion resistant so if there is a spark inside the unit, it won't ignite the gas fumes that accumulate around the engine on inboard units.  The unit pictured below is a 'one wire' unit, but they do make a plug with two wires sticking out of it for regular wiring.  There is also a grounding tab and bolt just above and to the left of the battery post.

 
There is even an external regulated version that allows you to use the 10DN plug like the alternator on the previous post-part 1, where the regulator is removed and a terminal block is used in it's place.  Notice the spade terminals go in a different direction than in the regulated units. 

Other internal variations include regulators with 6, 8 and 24 volt settings and a reverse rectifier for a positive ground system.  The 24 volt works best with a stator and rotor that is wound for that output.  There are some units that have three spade terminals and the third one in the offset position is a stator tap, which is usually used to run an electric tachometer.  Another variation of this uses a post that goes into the hole labeled 'R' beside and above the battery post.  This can be added to any 10SI unit except marine!

Another variation uses modern produced housings for replacing other brand units.  The first one is a Prestolite replacement.  This unit also has screens for marine purposes as a lot of these particular units were used on boats.  This type of mounting may replace some foreign brand tractor units also, though lining up the pulleys can be a problem.

And this unit below is used for replacing Motorola units which are usually found on old tractors and equipment!