RPM drop and electric fans!

[Ricks70Duster340]

Thank you!!

That AAW wiring kit is a good one. Concurrent with that installation on my car I moved my battery to the trunk and upgraded the alternator to a 100A power master. I used a 1/0 cable for both the plus and neg battery runs. I ran the neg cable to the pass side cyl head and ground cables to different places to ensure good grounds. That poor 70A alternator you have is surely too small but having good grounds will help your overall electrical system.

 

[goldduster318]

Just do the calculations:
Power (watts) = I (Current) x V (Voltage)
1 hp = 746 watts.

Let's just say your current is 40 amps, and your voltage drop is high, so you have 12.5 V, and your alternator is extremely inefficient, so its only 50% (should be closer to 70%)

40 amps * 12.5 V = 500 W.
500W/0.5 = 1000W (it's divide because its the load and not the generation)
1000W/746W = 1.34 hp.

That alone shouldn't drag the engine down much at all. If the rotors are trying to stop the rotation of the alternator, that would honestly be the only reason that would make sense but I find that to be somewhat unlikely.

I'm able to run an EFI ECU, Aeromotive 340 lph fuel pump, Contour fan, headlights, and wipers and keep the battery charged using a 78A Delco 12SI alternator. The stock style alternators have very low output at idle but that shouldn't mean that the engine drops in RPM so much. Of course my battery is still under the hood. It would pull the idle down somewhat but 50-100 rpm would be realistic and temporary until they are sped up.

Is the timing retarding when it drops below a certain RPM? Like vacuum advance hooked to manifold vacuum, springs are very light so they start opening even at normal idle speed, etc?

 

[Sr4440]

If this were mine, i would just connect an idle stop solenoid to the throttle and have it raise the idle every time the fans came on. Search: 34 results found for "idle stop solenoid"

 

[72bluNblu]

If this were mine, i would just connect an idle stop solenoid to the throttle and have it raise the idle every time the fans came on. Search: 34 results found for "idle stop solenoid"

That’s a great idea!

But I think the 400 rpm drop is excessive and that should be sorted out first.

Not knowing what fans are on there exactly it’s hard to say, like on my car my contour fans pull less in the low speed that comes on first than what they’re capable of at their high speed, but I would expect the idle rpm drop to be more like half what the OP has. There will be a drop, the alternator is loading the engine more with that amperage draw from the fans, but 400 rpm seems excessive.

 

[Ricks70Duster340]

That’s a great idea!

But I think the 400 rpm drop is excessive and that should be sorted out first.

Not knowing what fans are on there exactly it’s hard to say, like on my car my contour fans pull less in the low speed that comes on first than what they’re capable of at their high speed, but I would expect the idle rpm drop to be more like half what the OP has. There will be a drop, the alternator is loading the engine more with that amperage draw from the fans, but 400 rpm seems excessive.

The solenoids do not have the power to increase the throttle by themselves. They only work when first energized then when the user opens the throttle it will keep the idle higher until it turns off.

 

[RustyRatRod]

The solenoids do not have the power to increase the throttle by themselves. They only work when first energized then when the user opens the throttle it will keep the idle higher until it turns off.

I was gonna say it, but I'm glad somebody did. lol

 

[goldduster318]

That’s a great idea!

But I think the 400 rpm drop is excessive and that should be sorted out first.

Not knowing what fans are on there exactly it’s hard to say, like on my car my contour fans pull less in the low speed that comes on first than what they’re capable of at their high speed, but I would expect the idle rpm drop to be more like half what the OP has. There will be a drop, the alternator is loading the engine more with that amperage draw from the fans, but 400 rpm seems excessive.

I moved to ECU control and a high/low switch for my Contour fans this year. So basically the computer only knows "fan on" or "fan off" but the speed is selected afterwards. They now can cycle from a stop on high if that's what I wish, have done it during Power Tour and the Dream Cruise and you barely notice. I measured the inrush with the engine off and its 65.2A.

The battery does act somewhat like a capacitor so its proximity will reduce the voltage dips and spikes during transients.

My car has EFI so it will adjust to get the right idle speed but it would take a second or two to find it again. Its not very noticeable.

It would be possible to add a very large capacitor to blunt transient effects IF its only an instantaneous problem.

 

[FDNY340]

@Mattax and @crackedback are right on.

What model SPAL fans are you running?

Assuming that those fans are powerful enough to handle the cooling duties of your engine, they could easily be pulling 30 amps between them even after the initial big amperage hit to start them. Your 70A alternator might not even be putting out 20A at idle- you see the problem.

A second battery is just a band aid, it won't solve the issue. What you need is an alternator that puts out more amps at idle than what your fans AND other accessories put out.

If you have a brand new American Autowire wire harness, you're probably already running a single wire alternator (standard config for AA) and your system can handle a lot more amperage. I have a full AA harness, run a dual electric fan set up (Ford Contour) and use a 180 amp alternator. Like crackedback said, I don't need 180 amps, probably ever. But at idle that alternator is putting out enough amps to run my fans without discharging the battery.

And if your battery is in the trunk (mine is too), if you're pulling from the battery to keep the car running when the fans are on you have to pull a significant amperage from the battery though all the wire to the front of the car, which loses a ton of efficiency. Charging the battery is a low amp draw, so that's not a big deal with the battery in the trunk. Cranking the starter pulls a ton, but, that's a short period of time. The fans running is a large draw for a long time, and that trunk mounted battery, even with large cable, is going to take some work to pull amperage from.

Thank you Blu

Running 2
SPAL Automotive USA 30101500 Spal Electric Fans | Summit Racing

Besides the fans, also running Magna Fuel Pump and a remote tranny fluid cooler with the MSD etc.....
I didnt have the room for the Contour fan set up..... I wish I did!!
I did have to adjust pulley size initially to speed up rotation for the Alt and water pump.
Sounding more and more like undersized Alt....


Sounding more and more like undersized Alt....

 

[RustyRatRod]

Thank you Blu

Running 2
SPAL Automotive USA 30101500 Spal Electric Fans | Summit Racing

Besides the fans, also running Magna Fuel Pump and a remote tranny fluid cooler with the MSD etc.....
I didnt have the room for the Contour fan set up..... I wish I did!!
I did have to adjust pulley size initially to speed up rotation for the Alt and water pump.
Sounding more and more like undersized Alt....


Sounding more and more like undersized Alt....

View attachment 1716483612

View attachment 1716483613

That was my thinking. You said it's a 72? Amp?

 

[FDNY340]

That was my thinking. You said it's a 72? Amp?

70 Amp
45Amps at Idle

 

[RustyRatRod]

70 Amp
45Amps at Idle

Yeah I think that's the issue.

 

[George Jets]

Or just get the proper regular size battery that is up to the task. New not 1 to 3 years old.

☆☆☆☆☆

 

[FDNY340]

Yeah I think that's the issue.

Next 2 sizes are 80 at idle or 120amps at idle. Which would you go with.

 

[RustyRatRod]

Next 2 sizes are 80 at idle or 120amps at idle. Which would you go with.

I ASSUME you've upgraded the wiring. You said you have an AAW harness, right? Then I would choose the 120. Of course I am also assuming you have an adequate charge wire (it looks BIG) and it's also fused.

 

[FDNY340]

I ASSUME you've upgraded the wiring. You said you have an AAW harness, right? Then I would choose the 120. Of course I am also assuming you have an adequate charge wire (it looks BIG) and it's also fused.

Yes, Yes and Yes!!
Thanks RRR!

 

[crackedback]

Where are you grabbing power for the fans? If you are pulling it from the battery and running wire all over hells kitchen back to the fans, that is not an efficient approach.

JMO, since the fans are likely to run primarily when the engine is running, source power right off the alternator charge stud. You have a short run to the fans. Mount any relays for the fans on that pass side side apron area. On high amp draw items, you want the power runs as short as possible.

Ground the fans direct to neg battery terminal or to the radiator support. Have AT LEAST an 8 gauge ground wire, 6 would be better, from Battery neg to radiator support if grounded there.

 

[92b]

Is the timing retarding when it drops below a certain RPM? Like vacuum advance hooked to manifold vacuum, springs are very light so they start opening even at normal idle speed, etc?

This above. Does the engine have a good strong idle? How much manifold vacuum does it have at idle? Wouldn't hurt to check timing like gold duster suggested. Also check the idle mixture screws to see if the mixture is good. These checks won't cost any money. I have had engines on the edge of lean or rich that don't have a strong idle and were easily drug down by a Iight load.
There are also various electric and vacuum idle up devices that have been used in conjunction with air conditioning equipped cars before fuel injection replaced carburetors.

 

[George Jets]

Here is a neat trick, turn your key on then go put a snap-on wrench on the Alternator pulley. If the wrench sticks to the pulley (magnetically charged) then you know the Alternator fields are energized before you even start the engine. Showing you the battery had no reserve and is lame and needs to be replaced.

The battery should carry the electrical system on its own, when the battery gets drawn down after the fans have been on for a while > that is the time for the Alternator to kick in and refill the battery back to a full charge.

The battery is the reservoir.


☆☆☆☆☆

 

[72bluNblu]

Thank you Blu

Running 2
SPAL Automotive USA 30101500 Spal Electric Fans | Summit Racing

Besides the fans, also running Magna Fuel Pump and a remote tranny fluid cooler with the MSD etc.....
I didnt have the room for the Contour fan set up..... I wish I did!!
I did have to adjust pulley size initially to speed up rotation for the Alt and water pump.
Sounding more and more like undersized Alt....


Sounding more and more like undersized Alt....

View attachment 1716483612

View attachment 1716483613

Ok, so a couple of things. First, it's the alternator. This is what I run, I think I said 180 amp earlier but it's a 165A. With the AAW harness and relays you shouldn't have any issues running this, although it looks like you'd need to look up the V-belt version, mines for a serpentine.

1974 PLYMOUTH DUSTER Powermaster 8-58529-103 Powermaster Street Alternators | Summit Racing

I also ran the 100A version of a Powermaster alternator with the contour fan set up, before I had the AAW harness. It was still set up with relays for the fans though.

The other thing is, I think you're at the absolute minimum for CFM with two of those fans. That's less than 2k CFM between the two of them. I don't know if that will be enough. I don't think it would be enough for my set up and the way that I use it.

Or just get the proper regular size battery that is up to the task. New not 1 to 3 years old.

View attachment 1716483632


☆☆☆☆☆

Here is a neat trick, turn your key on then go put a snap-on wrench on the Alternator pulley. If the wrench sticks to the pulley (magnetically charged) then you know the Alternator fields are energized before you even start the engine. Showing you the battery had no reserve and is lame and needs to be replaced.

The battery should carry the electrical system on its own, when the battery gets drawn down after the fans have been on for a while > that is the time for the Alternator to kick in and refill the battery back to a full charge.

The battery is the reservoir.


☆☆☆☆☆

It's just not the battery. His alternator isn't putting out enough juice to feed those fans, and with the battery in the trunk that means he has to pull that extra juice all the way from the trunk. The alternator goes to max draw, the battery has to get involved, and that's the 400 rpm drop. He already has the relays, it's just a matter of feeding them enough amperage. If it was just the battery, his fans would kill it and he'd be complaining about a hard start/slow crank issue in addition to the issue with the RPM drop that he has.

 

[Sr4440]

Let's ask a few questions about the engine setup. 1. Does it have an aftermarket cam? 2. are you running vacuum advance and is it connected to manifold vacuum? The reason I ask if you already have low manifold vacuum, and you add a small load, the vacuum drops and the ignition retards (due to lower vacuum) engine slows down more. Then the vacuum drops and the ignition retards (due to lower vacuum) engine slows down more. Until it stops 400 rpm later. Need a vacuum and voltage reading while the event is taking place to really understand what is happening.

 

[FDNY340]

Where are you grabbing power for the fans? If you are pulling it from the battery and running wire all over hells kitchen back to the fans, that is not an efficient approach.

JMO, since the fans are likely to run primarily when the engine is running, source power right off the alternator charge stud. You have a short run to the fans. Mount any relays for the fans on that pass side side apron area. On high amp draw items, you want the power runs as short as possible.

Ground the fans direct to neg battery terminal or to the radiator support. Have AT LEAST an 8 gauge ground wire, 6 would be better, from Battery neg to radiator support if grounded there.

Ran number 6 power and ground to the passenger apron to supply a 6 bank fuse panel which powers the fans and MSD and 2 step. The relays are at that location also.

 

[FDNY340]

Let's ask a few questions about the engine setup. 1. Does it have an aftermarket cam? 2. are you running vacuum advance and is it connected to manifold vacuum? The reason I ask if you already have low manifold vacuum, and you add a small load, the vacuum drops and the ignition retards (due to lower vacuum) engine slows down more. Then the vacuum drops and the ignition retards (due to lower vacuum) engine slows down more. Until it stops 400 rpm later. Need a vacuum and voltage reading while the event is taking place to really understand what is happening.

Yes aftermarket Lunati cam with no vacuum anything. One of my issues is also no vacuum ports on manifold or carb to take a reading from. Have a 1" spacer that I may be drilling out soon to tap a port into it!!

 

[crackedback]

Ran number 6 power and ground to the passenger apron to supply a 6 bank fuse panel which powers the fans and MSD and 2 step. The relays are at that location also.

From where? Alt or Battery?

I agree with other that you may have timing dropping out when in gear, which creates a waterfall effect. If it happens only when the fans are running, may not be the issue.

 

[FDNY340]

From where? Alt or Battery?

I agree with other that you may have timing dropping out when in gear, which creates a waterfall effect. If it happens only when the fans are running, may not be the issue.

From Battery

 

[crackedback]

From Battery

You are running that energy from the alternator, to the battery and back to the relay location. I know from alt to starter relay is around 8 feet. You are running that power probably ~20 feet. That length creates unnecessary voltage drop to the end user items (fans).