• Hey Guest!
    British Car Forum has been supporting enthusiasts for over 25 years by providing a great place to share our love for British cars. You can support our efforts by upgrading your membership for less than the dues of most car clubs. There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Upgraded members don't see this banner, nor will you see the Google ads that appear on the site.)
Tips
Tips

Electrical power drain pulling down rpms.....

Share this page

G

Guest

Guest
Guest
Offline
Curious, whenever my electric fan comes on, say at idle, I can feel the idle of the car engine being drawn down, like the idle
actually drops about 100 rpm. Weird. Now, I have a very stong gel battery and a 66 amp Delco-style alternator. Both the alternator and the fan are wired with the proper wiring and run totally independant of the stock wiring harness, as much as that is possible. This doesn't affect the performance, per se, but does cause me to wonder. Is this normal.

I have never had to use my automatic rpm-compensator yet (my right foot) to adjust the rpms and the engine is not in any danger of dying at idle. Just wondering what the techies would say on this.



Bill
 
Not a techy but sounds more or less normal to me. Certainly occurs on my TRs with their wimpy little generators... any major current draw will affect idle.
 
It's normal. Think of what your daily driver does when the ac kicks on.
 
[ QUOTE ]
When the AC kicks on, the idle speed adjuster motor increases the rpm a couple of hundred.
D

[/ QUOTE ]

Ah yes, the wonders of electronic fuel injection. Any load on the engine at idle is automatically compensated for.

Once we start putting things like electric fans and other load creating items on, you can expect to see a change in idle speed when they kick on and bog down the alternator. The original engine driven fan applied the load all the time, so you never noticed it. /ubbthreads/images/graemlins/thirsty.gif
 
Hello Bill,

how big is your fan?

I have never noticed that electrical load affects the idle on my car. I just went out and started it up, idling at about 500 rpm cold, put full lights on, about 250 plus watts and no change in idle. This is with a standard battery and a 17ACR Lucas alternator (34amp output) When you think that the load I quote is about 1\3rd horsepower, not much for beefy 2.5 litre engine, even at idle?

Makes me wonder what power an A\C compressor takes to drive it?

Alec
 
I seem to remember that they (Just who the heck are they anyhow and why have I never met one of they???)used to say that A/C would / could cut milage by up to 20% so the load and revs should calculate out accordingly
 
That's because "they" are all auditors for the IRS and NO ONE ever gets to really meet "them", just the suits that go out to represent their findings to the poor unwitting public.

Or perhaps you have not met the famous "they" of the Men in Black, who no one ever remembers meeting anyhow.

Back to the subject. In the good old "60's", when horsepower was dirt cheap and so was Sunoco 260, no self respecting performance guy would ever order A/C in a car because of the legitimate 10%-15% drag on the engine, with it on. Besides the extra load and belts, it was more of the extra 300lbs. of weight in extra insulation, compressor, condensor and evaporator, different from springs on left and right, etc.

Heck, no one ever got into a serious race with the A/C on, (at least on purpose) but I saw many a 442, GTO and SS 396 flip belts at high rpm and no one wanted that mess to deal with. Once one goes, they tend to take the others with them.
 
[ QUOTE ]

Ah yes, the wonders of electronic fuel injection. Any load on the engine at idle is automatically compensated for.

[/ QUOTE ]

Not just electronic fuel injection. Older cars with A/C had a vacuum actuator on the carb which increased the idle speed when the A/C was turned on.
I looked at a Herald with and aftermarket A/C system on it a while back. It had a little thingy on the standard SV carb which raised the idle speed when the A/C was turned on. I really wanted to buy it but was informed that I would also be sleeping in it /ubbthreads/images/graemlins/mad.gif
Yisrael
 
[ QUOTE ]

how big is your fan?





[/ QUOTE ]

Alec, my fan is a Spal 16" that pushes about 2400 cfm. It draws a lot, I think about 22amps (that may be incorrect).

I do know that at speed, when the fan is not powered, my "fan indicator light" (yes, I have one, ain't it cool) will glow. Dan Masters said that this shows that the fan is now a generator and is actually trickle-charging my battery while I drive. I wonder why I never have a low battery.

For anyone interested, check out spalusa.com for a complete lineup of Spal fans. They are truly awesome.


Bill
 
[ QUOTE ]
Hello Bill,

how big is your fan?

I have never noticed that electrical load affects the idle on my car. I just went out and started it up, idling at about 500 rpm cold, put full lights on, about 250 plus watts and no change in idle. This is with a standard battery and a 17ACR Lucas alternator (34amp output)
Alec

[/ QUOTE ]
Hi Alec,
When you consider that the 17 ACR alternator is only capable of putting out about 10 amps at 500 rpm idle, the 19 amp load is not able to load the alternator/engine any more than the alternator puts out (10 amps) so not much load to slow the engine.

At 750 rpm, the alternator output would be capable of meeting the full load of 19 amps & might slow the engine a bit. The alternator should meet it's full output capability at around 1500 rpm.
D
 
Now I see why you converted your alternator way back when. I checked their site and the max draw for your fan at 2300+ CFM is 22 Amps. But you are staying cool!!!
 
Hello Dave,

yes, I know and larger alternators will give proportionally more at that speed but it is a relatively small amount of power. I would not normally have my lights full on at idle, I was just interested to see what would happen as I have never noticed any drop in idle speed with electrical load. I too have an electric engine cooling fan but don't know off hand what its rating is. I'm sure mine is nowhere near the 22 amps that Bill's one may be?

The theory of a fan charging the battery must be false though, (unless you are Smokey Unick{Sp?}, I think he tried that once but it was deemed illegal?) for if the fan is connected it will be driven, when off it is not connected.

Alec
 
Back
Top