Now that the 105 amp has been installed and is functional, we need to take a look at another aspect of it to make it a reliable modification.
In this thread we have seen a lot of experimentation. We've sliced and diced the pulley, fan, housing, wiring and mounts. It looks like we are winning the battle to install that fat alternator.
Are we done?
Not quite.
Looking at the graph from earlier in the thread:
I can see that an alternator shaft speed of 2000 rpms translates to 65 amps output.
(roughly)
2K-4K is the alternator 'sweet spot'.
Let's take a look at this possible real world example:
If your alternator is putting out 65 amps at an engine idle of 1,000 rpms, it is turning twice as fast as the crankshaft (2,000 rpms).
(Assumes an underdrive pulley on a DOHC with a stock alternator pulley)
You might want to increase the alternator pulley size to slow down that alternator.
This may not apply to a TBI or NA 1.0L.
With a DOHC, drivers often spin them up to 7,000 rpms during a spirited drive.
That means your modified 105 amp alternator is spinning at 14,000 rpms - which is off the charts.
Heheheh.
Those of you who are sticklers for theory:
Leakage inductance accounts for the fact that after 4,000 rpms your alternator isn't putting out any appreciable additional juice.
Put in simple language: there's no real advantage spinning those alternators over 4K. They do most of their work below those speeds.
Also, synchronous impedance is the fancy term explaining why alternators put out so much juice at low rpms.
Don't take my word for it; check out synchronous impedance and leakage inductance.
I may have those terms backwards. Don't worry about it.
Alternators are designed to put out juice quickly and at low speeds. That's about all you need to remember. All that theory? Leave it to the engineers.
Bottom line?
If this modification works, but the alternators fail prematurely, perhaps it is time to think about a larger diameter alternator pulley.
If it works and you are unsure how fast your alternator spins, don't leave it to chance.
The engineers who designed the OEM systems know their engine and alternator rpms.
You should too!
And if the alternator chart only goes to 12K, there's a reason.
A larger diameter alternator pulley ON THIS MOD would have several benefits.
-lower alternator speed (larger alternator pulley)
-less engine drag (increased mechanical advantage)
-better belt grip (more surface area offered to belt)
come to mind off hand.
Other members could explain the difference in force required to spin an alternator at 7K and 14K.
I can assure you, it is not TWICE as much force, but more.
You boys who have been modding these juice machines (alternators) would have to take one apart and provide the MASSES of the
-rotor
-pulley
-fan
-any other rotating parts
so we can help with the calculations.
The general idea is to have the alternator spin as slowly as possible while doing the most amount of work. It may seem trite, but at high rpms, the forces of alternator drag increase exponentially.
XRW44 is a bit more into the theoretical end of these alternators than me, so he'll probably be able to suggest proper diameter pulley diameters, depending on the crankshaft pulley diameter.
Besides, he and Jersey practically OWN this thread.
Remember those old pyrometers?
Here's an idea for a new dash gauge:
(for the man who has everything)
Alternator Temp/RPM.
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