2.0L Done and Alternator Question

[Henry Schmidt]

I have seen many 90 amp alternators fluctuate a high RPM because of worn bushes. It's an easy fix if you don't mind pulling an alternator out.
If it have an external regulator, test it.
ooops
I obviously misread the question. I thought you were asking "had I seen an alternator cutting out at full throttle?".
I guess if I can't read a simple question then I'm not as smart as I thought.

The answer is no I haven't done that experiment. I did however spit a crank pulley off at PIR and that hp increase was cool, until the engine started to smoke.

[911pcars]

"..Now let's change the subject: have either of you electrical gurus ever experimented with cutting out the alternator field at WOT?"

John,
I'm neither of the geniuses you asked, but if you look in a wiring diagram, locate and identify the field circuit leading to the alternator stator. You can insert a simple ON-OFF switch in series with this circuit. With the switch open (OFF), you will cut off current to the stator and thus alternator output. You can do this with an external VR; not sure about internal, electronic VRs.

Caveat. I haven't done this personally with a Porsche alternator, but it's an old trick for hot rod 'merican cars. Might save a few HP at WOT though. Check on a dyno to see if it's worth having the extra gear.

As Henry pointed out earlier, for racing all you need is enough charging capacity to power the ignition system, brake lights, instruments, a few sending units, some basic accessories (fan, relays, etc.) and a little extra to keep the battery happy. The battery's state-of-charge will dictate how much the alternator puts out beyond the calculated amounts. For example, if you start the vehicle with an out-board remote battery, the alternator doesn't have to work as hard (power consumption) to recharge the on-board battery.

Sherwood

[304065]

Hi Sherwood.

There are actually two ways the ROTOR (what you meant) gets its current-- from the black wire (DF), and when the voltage regulator doesn't sense any current output from the battery, from the blue D+/61 wire. I could rig up a simple DPST normally-closed switch to cut them both out at WOT, you are correct.

The question for Loren is, what happens to the charging system when you suddenly cut the field current, and then put it back in again when the throttle is retarded. It seems to me that that would put quite a shock on the diode board in the alternator.

The question for Henry was, ever tried it to get a couple extra HP? But evidently not!

Grady said that using an overdrive pulley for the fan will increase engine cooling, resulting in a net power increase over the additional drag of spinning the fan faster. It seems to me that spinning the fan faster will increase the alternator's output, so you wouldn't require as much field current to get the same output. . . so an adjustable voltage regulator that would allow you to dial back the output at low RPMS, might help in that regard. . .

[Lorenfb]

"Well Loren I'll bite, what is your expertise? I'm certain your not going to say "well I own one". "

I'll let you be the judge of that after asking a number of So Cal Porsche shops
as listed on my web site ( www.systemsc.com ) of the many I have dealt with
in the last 25+ years. Start by asking your neighbor, Strasse Auto.

[Henry Schmidt]

Check your ego!!! Perhaps the previous posts are some good insight.

[Carl_Boehme]

Internal regulators consist of solid state electronics. BiCmos electronic devices typically come in two temperature varieties -40 - +85C and -55 - +125C Older devices such as TTL integrated circuits 0-70C and -55 - +125C

An example voltage regualtor used in an automotive alternator is a MC33099. It is spec'd to operated in the -40 - +125C temperature range.

Alternators are known to require about 2W of mechanical power to generate 1W of electrical power. At full load a 35 amp alternator would rob about 1.1hp mechanical power from the rear wheels.
( 35A * 12V = 420W * 2 = 820W / 746W/hp = 1.1hp )

[Lorenfb]

Henry Schmidt
SUPERTEC PERFORMANCE
2951 Grace Lane #A
Costa Mesa, Ca. 92626
phone- (714) 429-1863
fax - (714) 429-1864
email - supertec911@earthlink.net
www.supertecperformance.com

Ego? What's that?

"An example voltage regualtor used in an automotive alternator is a MC33099. It is spec'd to operated in the -40 - +125C temperature range."

One of MANY parts used that can fail. Heat is NOT the electronics parts friend!

[304065]

Carl, thanks for posting that. It stands to reason that the alternator can't possibly be 100% efficient: 50% seems more likely.

So, 35A * 14V = 490 Watts of electrical power. Multiply that by 2, 980 watts, divided by 746 = 1.3 HP

That's a bunch. Carrera guys lose 3.5 HP!

[911pcars]

You will want maximum HP/torque available at WOT, yet how much power (voltage x current) does the electrical system actually require at WOT?

I say not much. Let's see: Ignition (3A?), sending units (2A?), fuel pump (3A?), maybe an electric fan (5A?), brake lamps (0A). And if the battery doesn't need charging (usually after cranking), then it's not much more than that.

Sherwood

[304065]

True, the load for a perfectly stock car should be pretty minimal. I use an MSD box (10A) and a different fuel pump (7-10A) and a pair of SPAL fans for the oil coolers (25A), all allowed by stock class rules. Gotta think about reducing that load.

[911pcars]

John,
Are those fuse specs? The actual current might be a lot less under "normal" conditions. If this is a race car, one could calculate how long the system can last last without charging, then plan to switch the alternator OFF if you have to. Not sure it's worth 1-2 hp though, but every little bit helps.

I just read that the new F1 Toyota chassis is 33 lbs. less than the old one. Other than the main focus of increasing chassis stiffness and revising suspension geometry, I bet that 33 lbs. cost them $$ dearly.

Sherwood