Alternator Cutoff

Tech questions that don't fit above forums

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Q-ship
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Re: Alternator Cutoff

Post by Q-ship » Wed Nov 03, 2010 6:30 pm

Are you aware that you are talking about roughly 1 horsepower or possibly less?

If any power or ET gain can be had from cutting out the alternator, There is a problem somewhere else.

It's a matter of physics, not opinion.

velotec

Re: Alternator Cutoff

Post by velotec » Wed Nov 03, 2010 7:07 pm

Q-ship wrote:Are you aware that you are talking about roughly 1 horsepower or possibly less?

If any power or ET gain can be had from cutting out the alternator, There is a problem somewhere else.

It's a matter of physics, not opinion.
Well - let's see -
45 amps x 14.7 volts = 661.5 watts

What's the efficiency - guessing 33%

So input power is maybe 661 watts / 0.33 = 2005 watts

1 horsepower is ~ 746 watts so killing the alternator should save 2005 watts / 746 watts per HP = 2.7 hp

But a lot of the inefficiency is mechanical loss, which is constant with the alternator on or off so...

I have no idea. I'll just try it and see what happens.

cjperformance
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Re: Alternator Cutoff

Post by cjperformance » Thu Nov 04, 2010 8:15 am

even if you did gain 2.7hp by cutting the alt, you will lose at least this by a drop in ignition energy.

Anyway, I am surprised that your 105amp alt will not keep up with 45amp draw with the underdriven pulley, what size is your crank pulley, ie what drive ratio do you run?
I have run atl:engine @ 2:1, 2.5:1, and as low as 1.5:1 when running 7k+ with a 110/120 amp alt running twin thermos, 7al, big BG pump, elec water pump. No problem.
And on a street rod, 110/120 amp internal reg delco copy single wire alt @ 2.3:1 driven off of the diff yoke, 4.11 diff gears, same load as above plus headlights and even that keeps up and it wont begin to charge until 1300 tailshaft rpm. Gets .1 volt drop from alt stud to batt terminal, 6mm charge cable from alt to batt, 530CCA battery.

When you say wont keep up, im thinking you mean the battery dies off, or charge rate is inadequate or?? What exactly do you mean?
Craig.

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Re: Alternator Cutoff

Post by whitehendrix » Thu Nov 04, 2010 10:47 pm

it would be interesting to note the primary voltage to the plug coil(s)at he beginning and end of the run to chart voltage drop.. you figure theres a massive ratio there where 1 volt lost could equal thousands of secondary volts gone as well.. depending on the transformer ratio.. if h ad to guess.. what.. 50kV secondary possibly?
at 14.4 thats 3472:1, which will conversely equal the secondary voltage loss should it drop 1 volt..


catch 22 here..
"Whitey" AKA Jim
Electrical engineer, PLC programmer, machinist, fabricator, wheelie expert.

velotec

Re: Alternator Cutoff

Post by velotec » Fri Nov 05, 2010 2:16 pm

cjperformance wrote:even if you did gain 2.7hp by cutting the alt, you will lose at least this by a drop in ignition energy.

Anyway, I am surprised that your 105amp alt will not keep up with 45amp draw with the underdriven pulley, what size is your crank pulley, ie what drive ratio do you run?

When you say wont keep up, im thinking you mean the battery dies off, or charge rate is inadequate or?? What exactly do you mean?
The crank pulley is 7 3/4 inches. The stock alternator pulley, that's on there now, is 2 1/2 inches. The Moroso racing pulley is 5 inches.

Can't keep up means the battery voltage creeps lower and lower from round to round and the starter cranks slower in later rounds.

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Re: Alternator Cutoff

Post by cjperformance » Tue Nov 09, 2010 2:22 am

velotec wrote:
cjperformance wrote:even if you did gain 2.7hp by cutting the alt, you will lose at least this by a drop in ignition energy.

Anyway, I am surprised that your 105amp alt will not keep up with 45amp draw with the underdriven pulley, what size is your crank pulley, ie what drive ratio do you run?

When you say wont keep up, im thinking you mean the battery dies off, or charge rate is inadequate or?? What exactly do you mean?
The crank pulley is 7 3/4 inches. The stock alternator pulley, that's on there now, is 2 1/2 inches. The Moroso racing pulley is 5 inches.

Can't keep up means the battery voltage creeps lower and lower from round to round and the starter cranks slower in later rounds.
You need a solar panel! lol

Not a fix but can you utilise 2 batteries, keeping one on charge and swap them at some stage between rounds? Done this before on non alternator cars.

With the morosso pulley you should see plenty of charge from only 1400engine rpm. This is higher than you would generally see in staging lanes so this is likely where your problem lies, many starts, not enough charge, but on the track its should be charging fine.

If you could get a 3" alt pulley it would make just enough difference to be ok in the staging lanes and charge from aprox 1100-1200 engine rpm.

Or, try a better battery, what do you use now?
Craig.

velotec

Re: Alternator Cutoff

Post by velotec » Tue Nov 09, 2010 11:15 am

cjperformance wrote:
Or, try a better battery, what do you use now?
Optima Yellow Top

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Re: Alternator Cutoff

Post by cjperformance » Thu Nov 11, 2010 8:50 pm

What is your current charge rate?
Have you tested your alt to see if its capable of what it should be?
Craig.

velotec

Re: Alternator Cutoff

Post by velotec » Fri Nov 12, 2010 1:33 pm

cjperformance wrote:What is your current charge rate?
Have you tested your alt to see if its capable of what it should be?
I don't have an amp meter in the circuit. I have a volt meter in the dash - it's a good one.

The 12SI in there now was rebuilt a couple of months ago. It works better than it did before and better than anything else I've had in there. I raced last night - 6 laps. At the end of the night the battery was ~11.5 volts in the lanes.

I have a CS130 (or it maybe it's a CS144) built for 200 amps. I don't need 200 amps of course - I'm just hoping it will charge the battery at idle.

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Re: Alternator Cutoff

Post by wireman » Sat Feb 05, 2011 8:20 pm

HI,
new member here.are you running the output wire from the alternator directly to the battery? you should have a minimum 10 gauge wire(gxl or txl type not gpt).if you change to 8 gauge it will read higher at the battery .if your battery is in the trunk and you use a battery cutoff switch the output wire will go through the small posts(4 post switch) to the battery.if you are running the alternator output wire to a firewall junction for battery cable to starter that could be your problem.not sure as i don't have your wiring schematic.i have run an alternator for 20 years in a drag car without problem.you will always be better off with an alternator on the car.i have wired a lot of drag cars.if you pm me i will try to help you.as to previous post about alternator light.the light socket is not grounded(plastic),so if voltage is on both sides of bulb it will not light.just a simplified version.hope i was helpful.

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Re: Alternator Cutoff

Post by BCjohnny » Tue Feb 08, 2011 4:08 pm

What's the efficiency - guessing 33%
Sorry, not even close. Modern designs can be double this, over 70%. Even older ones can be around 60%. :wink:

So a 30 amp load, will draw around one horsepower. Therefore running an alternator is cheap extra power. As regards PMI, on a modern 50 amp internal fan machine, rotor radius/weight means this is negligible, as a sum of the whole.
"If an honest man is wrong, after demonstrating that he is wrong, he either stops being wrong or he stops being honest." Anon

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