alternator wiring

[Carlos]

Was having trouble with battery/alternator wiring, not wired to alternator at moment but was going to put this connection back in on a switch so I can turn it of during racing and on for rest. Had thougt of putting it through the brakes so only draws under braking but will probably just put it on a dash switch for know. Im not electricaly minded so was wondering which side of the battery to link it to. have found the alternator connection point but wasnt sure wether this should go to the pos or neg side of the battery? sorry if its a stupid question!
cheers

[Mackmot]

if its attached its taking power away from the engine. If its not connected then you are just wasting that power.

You need to get a nice thick wire and connect it to the positive side of your battery immediately and forget the switch.

[Carlos]

Cheers Mackmot. Thought it would draw more from the engine if it was charging the battery apposed to just running blank, which I presumed because I have heard of people wiring through brake light, you live and learn! Good job I checked because I would have wired it to the neg side. Will forget the switch idea.
Carlos

[graham bahr]

a alternator thats mechanically connected and turned by the engine consumes such a small amount of power that you wont even be able to meassure it, one that is connectted electrically aswell will consume more engine power, the actual amount will depend on how much electrical load you have and how much charge there is in the battery, so if your battery is charged and only a few amps are needed to keep things running then again the power loss through the alternator will be low, lots of electrical load and a fairly flat battery you may be loosing 4-5bhp, personall i've never been able to meassure much difference, and having raced without alternators i would say its not worth the hassle of keep changing and bench charging batteries not to meantion worrying about if the car will restart incase of a mid race spin or stall.

personally i would now only consider running without a charging alternator in an ultra lightweight hillclimb car or very low powered race car such as a 2Cv

[Mackmot]

If you had a really powerful road car alternator of about 130A charging rate at 14v then you would only be losing 2BHP maximum.

[1200Datto27]

The only thing that I could think of people doing with this "wiring to the brake light" is if the alternator has a electronic clutch on it, and it only engages when the brake light circuit is powered, so that you are essentially using regenerative power.

I ran my racecar without a alternator for a while, and it was nothing but a hassle.

[greenamex2]

On the 'power loss' front the figure generally a lot less than most people think, particularly on high revving fuel injected car.

A battery under load will produce around 12.5 volts. A decent alternator running at speed will produce over 13.5 volts.

That's an extra 8% of power for your coil to then enable it to produce a bit better spark.

It's also an extra 8% of power to keep your fuel injection pump running at maximum pressure reducing the chancing of any temporary mixture leaning out as the fuel pressure regulator catches up with transient changes

Also most EFI equipped cars will start to increase 'dwell angle' and injector opening times as voltage drops, causing further load on the battery and messing up that very expensive dyno developed mapping you had done over the winter.

I would also bet that most batteries are producing a lot less than 12.5 volts under load by the end of a race whereas an alternator equipped car will still have over 13.5 volts to play with.

Back to the original question, what most touring car teams do is fit a switch to the accelerator that turns off the alternator at full throttle only.

[1200Datto27]

My Battery would start at 14.2 volts, and after about 30ks of running, would be around 11.2, and that was with a fully charged, new Odessy dry cell unit.

What i have done with the new car, is have an ECU installed that has enough functionality to allow me to control the alternator with it, and I have it so that the alternator is off over 35% TPS, unless the battery voltage is lower than 12 volts, in which case it stays on upto 85% tps until the battery reaches 12.9 volts. I haven't ever had it go into this state, as even with the 35% setting, just due to braking, it is on for around 30% of the lap.

[StephenRae]

Does the belt driving the alternator use up any significant power? Also while I'm here how much power is used by a belt driven radiator cooling fan say 12" diameter?

[1200Datto27]

A lot more than is required to run an alternator to provide eletrical power to the fan. electrical thermo fans only run when needed, ie, low speeds or stopped, when it doesn't really matter what load is placed on your engine, where as a engine driven fan is always running, wether you are doing 1 or 100mph, and is producing the greatest amount of drag where you do not want it to, at the top end of your rev range.

[bigted]

Carlos, dont even think about not using an alternator as i refuse to push start you when it wont refire!

[Mackmot]

Proper cars dont have fans, just ducting through their sidepods:P

[zefarelly]

I use a dynamo pulley (bigger) on my alternator to slow it down otherwise it fries diodes . . .next up is to make a bracket for a minature nippon denso alternator I found

[StephenRae]

Originally Posted by Mackmot

Proper cars dont have fans, just ducting through their sidepods:P

I agree, but how about a single seater with the air dragged in from a low pressure area by an electric fan then you would be taking the power with you and should be faster without ducts!

[R59]

As others have said. Don't run without an alternator unless you are using something like a 2CV where 2bhp of loss for charging is like losing 10% of your engine power!

I ran my 1600 Nova without one for a few races in 2002. Big mistake. Though the battery was tip top, it would drop to around 11v during the race, and the ECU would suffer as a result, and so would engine performance.

The main reason for not running one was the new ECU required a toothed wheel which took over the space where the alternator pulley went. After some machining, I managed to get one back on there. The car worked much better.

There are some nice little 55A alternators around which will do the job nicely. That's enough for a little race battery. And wire it directly to the battery positive. Your isolator is in the positive line, and once disconnected, the engine stops. The load remains on the alternator, which won't fry the regulator.

As has also been said, if your engine is a high revver, get a bigger pulley on it. The higher revs will create a even higher voltage going into the regulator which can cause premature failure. Modern alternators can cope with far higher revs than old ones. Let's face it, you've got cars like V-tec Civic's which rev to 14billion, and RX8's which rev more!

Rob.

[Al Weyman]

I actually did this on a Camaro race car n fact t is stll in place as I am rebuilding the car, I had it on a dyno (rolling road) and you could defnitely see a drop of about 4bhp when I kicked in the alternator much to the disbelive of the dyno operator. Interesting point here though as a mate of mine who used to run in a short circuit Sierra formula where the rules forbade alternators (???) went to enormous lengths to secretly fir one including a pully off the main drive shaft as he reconed the battery drain during the race effected the power. He may well have been right (he won the series) and if I had let the battery run down a bit on the dyno in my test may well have seen a loss of power.

[Pug620]

Carlos

If you are staying in stockhatch then the rules do not allow this it must be standard!!


Cheers

Marty

[Carlos]

I didnt even realise it wasnt connected untill recently. I have always charged the battery fully before each race and was running a standard type battery, it wasnt untill I changed to a smaller red top battery I realised it wasnt re-charging. Iknow I should have checked when I bought the car but to be honest never thought of it. As I have never had problems in the past I had thought about charging during non race running but have sacked the idea and have just reconnected it. thanks for all your comments, all very helpfull.
Carlos