Wow... Alternator = 5hp

[wastintime]

Hey everyone,

I'm going to Sebring this weekend for a DE, so of course right before hand my car has the only serious problem I've ever had with it.
Unfortunately, I won the bet, it was the alternator, but I've never seen an alternator fail like that... the shaft simply snapped in half! and not even on track, I was in 4th gear cruising at 55mph on my way home.
The funny part is we also noticed the bearings felt like crap, so jokingly we threw it on the dyno, but... one new alternator later, the car is repeatedly dynoing at 5hp higher than it had been previously. Lol, I'm sure that's worth at least 2 seconds a lap right?

[924RACR]

Dang, dude - you can't keep giving out all those speed secrets like that!

On a related note, can anyone tell me where I can get some ceramic ball-bearings for my alternator? This dinky little 2-Liter is starting to get to its limits...

[cgarr]

Imagine how much HP more you could get if you had a variable pitch fan on these engines

[wastintime]

*sigh* and after all that the RSA still isn't making it to sebring The "new" alternator was borrowed from a friend's C2, I kinda had to give it back, and the new one was never shipped... oh well... the WRX is fun too... I just wish I'd had time to get it aligned

[304065]

What you mean is that the worn-out bearings on the old alternator were costing you five horsepower. As to the exact horsepower drain caused by the alternator, that would require back-to-back tests with it switched on or off. Energy is energy, though, right? So a C2 alternator puts out 1610 watts, which is 2.16 horsepower. But considering that most alternators are around 55% efficient at turning mechanical motion into electrical energy, the real number is 3.92 hp.

So race with the lights off!

[CliffBrown]

Originally posted by john_cramer
What you mean is that the worn-out bearings on the old alternator were costing you five horsepower. As to the exact horsepower drain caused by the alternator, that would require back-to-back tests with it switched on or off. Energy is energy, though, right? So a C2 alternator puts out 1610 watts, which is 2.16 horsepower. But considering that most alternators are around 55% efficient at turning mechanical motion into electrical energy, the real number is 3.92 hp.

So race with the lights off!

John

How about adding a suggestion to make sure and use a fresh battery that is at full charge before the race to keep the duty cycle low for the alternator.

Would it be getting too extreme to disable the alternator during track time and running the car off the battery charge only?

[304065]

Cliff,

You should always keep the battery charged, but that's not the main reason: the main reason is that if you let the battery voltage fall, say to 12.25V, that's only 25% charge, and the alternator may burn up trying to bring the battery back.

An alternator disconnect would probably be more trouble than it's worth: I can just imagine forgetting to flip it back on and having the batteries go flat after trying to run all the systems off battery alone. Also, bringing the alternator on and off line abruptly isn't good for any part of the electrical system.

[wastintime]

Lol, actually, the spec miata that I ran for a season had an alternator disconnect switch that was worth a couple of horsepower... sometimes it was definitely worth it.

[78targa]

Wouldn't it make more sense to run without the battery and shed the weight? The alternator is providing the power to run the car. Isn't the battery only there to start it?

Of course you wouldn't want to have to try and restart the car...

[Dave at Pelican Parts]

The battery is also there to tell the alternator what voltage to regulate to. With no reference voltage, the charging voltage could easily go to "way too d--n high". The battery also serves as a way to smooth out all of the (potentially large) voltage spikes in the alternator's output. Which is why you do not really want to test your alternator by disconnecting the battery; it can kill some of the electrical parts on the car.

--DD

[78targa]

Dave, yes I agree with all of that. Without a battery you don't have an energy "sink". I believe there are some ways around that though.

Personally, I think that both the battery and the alternator together add a safety margin you would want on the track. Still, it is doable...just not advisable.

[304065]

The Circle-Track guys run 16V batteries with 12V accessories (OK, the system voltage is 14V) precisely so they don't have to run a power-robbing alternator for the short sprint heats.

Given that the alternator in our cars is also the shaft for the cooling fan, it doesn't make a ton of sense to try and engineer around it, unless you are going the flat-fan route, but even then, the ignition box draws probably 10 amps, wipers (we race in the rain here in the Northeast) draw about the same, and other electrical accessories such as cool suits, helmet blowers, data loggers, air-fuel readers and video cameras all consume some current.

The REAL problem with cutting out the alternator in a 911 is that when you shut off the field, the idiot light will come on. The problem is that you then don't know if the fan belt has broken. In stock form, an otherwise working charging system is a good indicator that the fan is still turning, and the electrical load is not that great if you aren't trying to run a big load.

[Eagledriver]

The HP drain from the alternator is minimal. If your battery is charged and you have the lights off you are probably drawing less than 100 watts (9amps). 1HP = 750 watts, so you can see you are drawing less than 1/4 HP even if the alternator was only 50 percent efficient.

The big draw is the fan. I'd be you are losing 10 HP or more at 6000 RPM. This is why some racers use smaller crankshaft pullys.

-Andy

[wastintime]

Lol, well, the WRX made a lot of 911s cry... Great weekend at Sebring... especially since I've never been there before... Turn 14 is a blast!