Just a though, but as I'm running a Nippon Denso alternator, can I not temporarily boost my available horsepower by switching off the "live" to the alternator while on wide open throttle?



At maximum draw, the load on the engine for a typical alternator would be:
14.4V x 60A / 55% efficiency = 1571W or 2.1 HP at the crank

I know the alternator would be damaged if run with no load, but with no activation then this would not be a problem?

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Too many cars, and too little time.

I would think it a bad idea, the back emf from the alternator coming off load produces a high voltage spark depending on how much load was on the alternator at the time. This will be enough to blow any of the diodes.

It could be prevented with a designed suppression circuit or particular diode fitted backwards across the switch, but its probably not worth the trouble.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

A supression circuit would be easy with a big audio cap, say 1 farad. Soak up anything
A couple of extra kg's tho.

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Dean Hutton's first car was a Mini.

For a race car, so no extra grams, never mind kgs.

Thanks Mick, I thought there must be something more to it.
Never though of the collapsing field. Will just stick with the smallest possible alternator then.

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Too many cars, and too little time.

First off, I know almost sweet nothing about electrics....but I always thought
an alternator was only underload or cost horsepower if there was a current draw (all the lights on) but if there was no power draw there was no or at least very little horsepower loss.
So this is wrong?

yes you are right, but the battery and alternator will work together to find a voltage that is suitable to both of them (about 13.7 volts)

As soon as the engine is running, the ignition will draw a consistant load (about 5 amps for a normal mini ignition system), this will drop the system voltage a little which the alternator will work to correct by providing as much current as is needed to put the voltage back up to its prefferred level.

This means that the alternator will supply the current to the engine directly . As long as the egine is running their will always be a certain amount of current being drawn by the ignition system.

Consider the battery is a like a tank of water with a small leak out the bottom. The guy working to keep the tank full (the alternator) will at first not notice the leak which will allow the tank's water level (voltage) to go down, he will then over compensate by putting too much water back in raising the level a little too much. After a while the guy on top will work out exactly how much water is leaking and fill the tank with just enough to keep the water level consistantly correct.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

Thanks Mick.

I discussed this topic with a friend about doing this to his race car last year. One problem we came up with would be the voltage to the coil, with the alternator connected you should get, as others has said, around the 13.7V range. If you were to disconnect the alternator in theory it would drop to 12V but with the 5A or so load you would well in the low 11V range.

The ignition coil is basically just a multiplier, so the lower the input voltage the lower the output voltage and so a smaller spark will be produced. So if you were to drop the input to say 11.5V then you are only getting about 84% of the spark you would get at 13.7V

This may not make any difference on a low comp motor, but high comp race or boosted motor you would probably find that you had to close your plug gap to let it fire high in the rev range.

Nathan.

Just charge your battery before race day and remove the alternator completely.

Will save kilograms and horsepowers. The amount of running we do on a race day, the battery will easy handle it, especially that yellow cap of yours!!

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68 Mini
70 Moke

WOFTAM......... Put a bigger pulley on the alternator and water pump.....

I use a light weight alternator and a light weight sealed battery under the passenger seat. I also have a special plug that I can plug one of those jump starters into for cold morning starts at the track. Once the car is warm I do not use the jump starter for the rest of the day.

In addition to this I have an alloy switch box that has ignition, fuel pump and starter motor switches which allows me to crank the engine and get full oil pressure before I start it.

This is a full race car setup; the light weight battery would not be good for a daily driver – any very expensive.

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1964 Nb Race Car - big bore short stroke & lots of revs - Sold
1969 Mk2 Cooper S - First Car

Ditto that plus the sealed light weight gel battery .....

I had alternator problems awhile back and thus running on battery alone...
OK for most short races if fuily charged but longer ones - yes - started getting miss fire under high load (eg flat out in 4th down main straight Eastern Creek)

... difference in performance between operating alternator vs non-operating alternator ... non-discernable

Does anyone have a pic of these lightweight alternators?

Would be interesting to see, as mine is a great big A+ job which must weigh quite a bit.

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68 Mini
70 Moke

Can't find a good (or current photo) - but here is from when I first bought my car...

Alternator is from a Charade of some sort... and a lot smaller than your typical Lucas one

Only mod was to machine (single) mounting leg thinner... (can just make out behind spark plug lead)

Darren, I might have a spare one for you, but you would need to change the pulley as most of the Swift ones run a multi ribbed belt.

Size comparison with a standard mini alterator

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Too many cars, and too little time.