[george kujanski]

i hadn't thought about it,but you're right Lpete. Any additional load causes the RPM to drop. if there was any additional HP margin, adding the load wouldn't cause the speed to drop. It's at equilibrium, and more A/F mix is required to compensate for the additional load to bring the speed back up.

George

[ponjohn]

Originally Posted by BILTIT

My Mark8 fan draws 130A momentarily on startup and settles out to 42-45A on full speed. I too used to blow 30A fuses and relays. I upgraded to a 70A relay and 60A fuse and all is good. I have since switched to a fan controller which slowly increases the voltage to help ''soft start'' the fan. The 12SI upgrade is one which i have done aswell.

A fan controller would help some as most of them start the fan at half speed and ramp up the voltage as required per motor temp. An upgraded alt. never hurts either when running additional electrical loads.

30 amps at what voltage? The fan will draw more current at less voltage.

This is my argument: IF the accessories are being inadequately fed by means of a poor ground or poor supply path the motor/alt will load prematurely, trying to over-compensate.

Sixty amps MAY be enough for what he intends to do; you'll never know this until you start by determing IF the supply path AND grounds are reliable.

Don't forget, we all paint everything, paint is a bad conductor, paint the rad support, paint the firewall, paint the underbody, powder coat the frame....with no solution OR awareness of what is being done to the conductivity of the ground.

So far I am not convinced this is a normal situation without exploiting the weakness of his charging system. Especially with a large CI engine and an overdriven alt.

J

[BILTIT]

30Amps? Did you mean 130Amps? It was at 14.3 volts.

I agree with everything you have said EXCEPT

''The fan will draw more current at less voltage''

Ohms law says this is impossible.

http://www.the12volt.com/ohm/ohmslaw.asp (for those unaware of ohms law).

Example:
If the fan has a resistance (load) of 0.5 ohms.

Amp=14V/0.5ohms=28A
Amp=12V/0.5ohms=24A
Amp=9V/0.5ohms=18A

[65gto69]

Originally Posted by george kujanski

Anything you can do to get the idle vacuum up at idle will help by making the engine more efficient at idle. Vacuum is a good indication of loading. How about Rhoades lifters?

Perhaps you should contact Old Man Taylor for more info...he has a bracket car. Perhaps he can make some suggestions.

George

George,

Cam is a flat solid lifter cam, can't use rhoades.

I will see if I can get his attention, I know he frequents this board, I just have to find where he is.

Mark

[BILTIT]

As Ponjohn has pointed out, all connections and wiring should be checked to ensure it is not causing additional load on the alternator before upgrading.

I also found that if my carb idle mixtures were not set just right, it would cause the engine to idle at less than optimal efficiency and drop a fair bit of rpm when the fan was on and running.

I assume you are using relays with all the additional electrical components?

[65gto69]

John,

I will get a volt meter this weekend and check the readings for the alt. and other stuff.

I will post next week what I come up with.

Mark

[LPete]

Originally Posted by ponjohn

30 amps at what voltage? The fan will draw more current at less voltage.

This is my argument: IF the accessories are being inadequately fed by means of a poor ground or poor supply path the motor/alt will load prematurely, trying to over-compensate.

I don't think I understand what you mean "load prematurely" and "over-compensate". The alternator is carrying the entire electrical load, as long as the voltage is above battery voltage. The amount of power the engine uses to turn the alternator is basically proportional to the electrical load (power produced), not current draw.

Also, the amount of power the alternator can put out at a given RPM is fixed, so if the electrical load exceeds the alternator output it really shouldn't load the engine any more.

It seems to me that a lower-output alternator would slow the idle down less, interestingly enough. After the alternator output has been "used up" the battery would start supplying power. Not that this is a good thing.

I'm not disagreeing, I'm just trying to understand what you mean.

[LPete]

Are you using manifold vacuum to the advance? It seems far-fetched, but if the timing is advanced with the fans on, and the vacuum drops enough with the extra load to kick out the vacuum advance, it would slow the engine down a lot.

If it is hooked to manifold vacuum, you could try disconnecting the vacuum advance and resetting the idle speed, then see if the speed still drops a lot with the fans on. If that helps you could go with ported vacuum to the advance.

Just a thought.

[BILTIT]

The power output at a given rpm is fixed IF the field current/voltage is maxxed at that rpm.

If the electrical load is increased, the alt. load on an engine will increase until the field current/voltage is at its max then the output is fixed.

Just clarifying.

The amount the alt. loads the engine at idle is a function of when and how quickly the alt. output increases (field voltage/current is increased), not the max output of the alt.

[65gto69]

Originally Posted by LPete

Are you using manifold vacuum to the advance? It seems far-fetched, but if the timing is advanced with the fans on, and the vacuum drops enough with the extra load to kick out the vacuum advance, it would slow the engine down a lot.

If it is hooked to manifold vacuum, you could try disconnecting the vacuum advance and resetting the idle speed, then see if the speed still drops a lot with the fans on. If that helps you could go with ported vacuum to the advance.

Just a thought.

LPete,

I am running mechanical advance, no vacuum advance.

Mark

[george kujanski]

Improving electrical connections is always a good thing and should be done, but in this case with the given observations, I believe it is not the root cause.

Perhaps if there are poor connections in the fan motor circuits, the initial load on the electrical system is high for a longer time, i.e. the motor does not come up to speed quickly enuf. This instantaneously will load down the engine, reducing the output current from the alt, system voltage drops and making fan strartup more difficult.

Perhaps. I believe there's just too much loading on the electrical system. The engine speed will drop unless the throttle is opened to bring it back up. As you use more energy the engine needs to supply it and the only way to do that is to open the throttle more.

If my suspicion is correct, improving the charging system with a larger alt that will put out more current at idle will worsen the RPM drop since the alternator loading on the engine will be higher.

Things to remember: you don't get sumthin' for nuthin' and energy you use has to come from someplace plus waste. If you have a device that needs 10 HP for example, you need to produce at least that much because nothing is 100% efficient.

George

[ponjohn]

Originally Posted by LPete

I don't think I understand what you mean "load prematurely" and "over-compensate". The alternator is carrying the entire electrical load, as long as the voltage is above battery voltage. The amount of power the engine uses to turn the alternator is basically proportional to the electrical load (power produced), not current draw.

Also, the amount of power the alternator can put out at a given RPM is fixed, so if the electrical load exceeds the alternator output it really shouldn't load the engine any more.

It seems to me that a lower-output alternator would slow the idle down less, interestingly enough. After the alternator output has been "used up" the battery would start supplying power. Not that this is a good thing.

I'm not disagreeing, I'm just trying to understand what you mean.

What I am trying to illustrate is:

His alt may be able to supply the needs of his car without problem, on paper with room to spare (headroom).

Throw in the unknow resistance from faulty connections, improperly sized wire, poor grounds, paint on paint and this load my now appear to be 75 amps ( over compensating)... not the calculated current draw.

Biltit mentioned ohm's law- you;re forgetting the fan requires a certain amount of wattage to run. IF either factor of the equation changes the other factor needs to compensate. A motor is not a steady resistance, motors are not rated in ohms but in watts.

SO: IF the Voltage is low current is going to TRY and increase (to the point where the source can no longer supply the correct voltage or the supply the over-current situation) to compensate for the low voltage. Over current or more understandable, low voltage, situations burn accessories up. They are rated for x current at x voltage. When voltage drops current rises and now wires burn along with windings.

Remember, wire is rated at like 600 volts no concern for us, if the wire used for an accessy is rated at 30 amps and the accy draws 25 amps at proper voltage. IF the voltage drops and the current increases to 35,40,45 amps the wire is going to burn.

I run a mk8 fan as well, I went to dc control unit which manipulates the dc signal to fan making the apparent efficiency rise.

Believe me, I have a lot of work (electrically) to do to my car as well. I get dimming lights at night when the headlights are on. In my case i run the 94 amp Si.

My power accy's are: msd, fuel pump, electric water pump, electric fan. I don't believe this would add up to or exceed 94 amps. I have work to do on the grounding system and supply wires to each assy.

This is important stuff that most people could care less about.

[george kujanski]

Biltit: there's one other thing that happens. if an alt is supplying an output at a given speed and that load is lower than its rating at that speed, the output voltage is regulated at about 14.0 and life is good.

To what you said, the alt is maxed when the field current and voltage is at the max. Alternator field voltage is derived from the system voltage; once you overload the alt, system voltage drops towards battery voltage and the max available field voltage drops, further reducing the alt output.

This whole scenario could be several things, but bottom line, adding electrical loads require more energy. The only way the engine can provide the increased energy is to open the throttle, otherwise the speed drops, voltage drops, and the lower voltage reduces the electrical power to the loads until the engine reaches equilibrium again.

That's what it looks like to me.

George

[65gto69]

How much electrical power do these devices draw (if you know):

Accel 300+ CD ignition system (box and coil)

Holley electric fuel pump (blue)

Electric fan 1800 cfm (I guess this can vary per make and motor)

These are the 3 new added devices I am using since I dropped in this motor. Prior to this motor, I had a 400 30 over, street/strip cam, points distributor, accel super stock coil, mechanical fuel pump, everything else basically factory, harness, remanufactured alt. (currently using), same battery but new currently. This setup ran fine this way.

New current setup with new items above, electrical problems????

Mark

[george kujanski]

ponjohn: you are missing one thing, tho. for a resistive load, as the voltage goes down, the current does not go up. volts = current x resistance. If resistance stays the same, reduce the voltage and you REDUCE the current. Has to be.

If you were correct, we could supply the entire world with current by reducing our supply voltages to zero. Doesn't happen.

George

[65gto69]

Do you guys think an out of box Holley 850 carb 4781 is too small for this motor and what I have attached to it????


Mark

[BILTIT]

No, 850DP is just right. Thats what i am running now (had a 3310 before).

[LPete]

It is true that with a motor, if voltage drops current will increase (as long as the current is available). When the motor spins slower the back EMF (that limits current flow) will be lower, and current will increase.

I believe the alternator in this case HAS to be maxed out at idle. I'm guessing he has at least 40 or 50 amps draw with everything on. Like George mentioned, good electical design is good practice, but I also think this isn't the root cause.

I wonder if there's any chance there is a little mechanical advance at idle that drops out when the engine slows down (dropping idle further). Checking the timing with and without the fan on would eliminate this as a cause.

[george kujanski]

65: the additional fan has got to be the trigger that causes the cascade fault. If you have the ratings on it, please post it.

Again these fans draw a LOT of starting current. My opinion is once you switch it in, you abruptly load down the engine and it responds by slowing down.

Same thing as when you engage the trans, xcept the trans is a added mechanical load. The engine sees both loads as a mechanical load because it has to drive the alt mechanically. The additional electrical load is reflected back to the engine thru the belt drive but even greater than the electrical load due to the fact the alt is only about 50% efficient

You lose 400 RPM with the trans, the added electrical load will have an effect also. Got to be.

Perhaps this engine is also running at a lower idle vacuum (less efficient at idle).

george

[Jagtec1]

Put an internally regulated 12SI from an '85 (I think) Riviera, with AC. Add a voltmeter, and go.

My RPM drops somewhat with my fan on also...so what? It doesn't matter. Been to the track yet? I'm curious to see what your new setup runs!! Gonna be quick.