Alternator woes

So at the last autocross I discovered that I can’t have an external fan on the alternator (cut the radiator hose).  So I gathered some parts to upgrade to a 3G unit.  Except apparently the female plug socket on the mustang alternator I bought is too small for the adapter plug.  

So until I sort this out what do we think of just removing the external fan from the 2G alternator?  It’s not a daily driver, just an autocross toy.  Will a day of dodging cones be catastrophic without an external fan?

shroud the fan in the area where it contacts the hose.

Put an electric fan behind the alternator.

What I would do if this car is trailered in is eliminate the alternator. I would get 2 deep cycle battery using one as a main and one as a spare. An alternator can rob up to 10% of your power. If rules allow the deep cycle can be put in a strategic location.

In reply to pjbgravely :

It’s an overpowered V8 Miata.  I don’t mind the HP loss.

i just don’t know if running the old alternator without a fan will nuke it in 2 miles or 2,000.

Without a fan, the alternator will get pretty damn hot very fast.  And when it gets hot, its output will drop to very little.  And then at some point, it'll just fry the thing. 

If you don't want to remove the alternator, you'll definitely need to find a way to give it some cooling airflow. 

As far as 10% power loss, unless we're talking a 300 amp alternator with a big fan at full load on a tiny engine at high rpm, there's no way in hell an alternator is going to draw anywhere near that much power.  On a 300hp engine, you'd be talking about 30hp.  Even if we assume half of the alternator's power draw from the belt goes to moving cooling air and other inefficiencies (leaving 15hp going to power generation), you'd need an 1800 amp alternator to draw that much power. 

Alright.  I dabbed up this awesome shroud out of some aluminum I had laying around.  Fan intact.  Thanks all.

I win.

rslifkin said:

As far as 10% power loss, unless we're talking a 300 amp alternator with a big fan at full load on a tiny engine at high rpm, there's no way in hell an alternator is going to draw anywhere near that much power. 

Low RPM.  Horsepower is a function of torque and RPM, so at low RPM, a given amount of watts generated will require more torque than at high RPM.

I used to use this "feature" to regulate the idle on my bridge ported 13B.  I could drop the idle from 1500 to 1100rpm by turning the fans on just from alternator drag.  That's about the point where the alternator would stop actually charging because of the 3" crank pulley.  So for most of the time it would idle up where the engine is happy, and then around town where I want to be a bit more civilized, I'll turn the fans on at traffic lights to cut the idle down.

In reply to Knurled. :

True, it's more significant at lower RPM.  But alternator load does drop some with RPM (less drag from the fan, etc. and peak output will go down at lower rpm).  Plus, if we're talking about removing an alternator for performance reasons, nobody is worried about what it does at 1500 rpm. 

Dirt track racers now run alternators.   Electronic ignition works better at  14.3 volts than on -12.

rslifkin said:

As far as 10% power loss, unless we're talking a 300 amp alternator with a big fan at full load on a tiny engine at high rpm, there's no way in hell an alternator is going to draw anywhere near that much power. 

This is why I said "up to 10%".  Yes it has to be a low HP engine at lower RPMs.  I had a 100 amp alternator stall out a 8 Hp engine. They take a lot more power to run than you think.

1200 watts (100 amps x 12 volts) is about 1.6 hp (before any inefficiencies), so yeah, I could believe that stalling an 8hp engine at low rpm (if the alternator was pullied to spin fast enough for full or near full output).  

If it were me...  I'd take the old hose cut a 4 inch section off then slide it length wise wrap it around the hose near the fan gaurd and zip tie it in place that gaurds going to eat something with vibration let it be a sacrificial section of hose.

Also you might take a section of brake line bend it along the hose axis then walk in to a real parts store and ask to look at there hoses and see if theres an off the self molded hose that will work.  I've had good luck in the past doing this.