Intermitant low power

short answer: Impedance mismatch.

Try a larger diameter pulley on the alternator. My _guess_ is the belt is slipping at WOT and the load for all the pumps an injectors is more than you would commonly assume. The battery simply can't supply the load at the full voltage.

The voltage at the battery does not change from idle to 2500 rpm.

That's good. Low speed, low mass flow, short injector cycles, low fuel pump power demand... in other words, a light load.

I can't imagine you're complaining about loss of power at low-fractional throttle positions??? You're experiencing loss of power at WOT (wide open throttle) when you're demanding max power... right? If this assumption is wrong, then replace the computer.

Hi tick-tock,

I'm confused. You say, "The battery simply can't supply the load at the full voltage".

Wouldn't you want a smaller pulley to increase alternator shaft speed, for more output?

Alternators are capable of producing full rated power when cruising at well below full throttle. That's because/why the manufacturers put small pulleys on them. 1800 rpm on the tach -> 3000 rpm on the alternator, and full electrical power to the vehicle.

When the engine is making seven grand, the poor alternator is trying to spin at 12,000! A couple of things happen. But most important the torque/power needed starts to increase really fast, and by virtue of the voltage regulator, most of that power is becoming heat in the core of the machine. It's a fait accompli that the alternator fan is pumping *huge* volumes of cooling air over the coils, and that all translated into a lot of HP not making it to the wheels :^(

What's worse is that the voltage regulator may be dissipating so much power trying to regulate the alternator output that it could be on the edge of acting as a crowbar on the entire electrical system... But it's more likely that it's just dumping the alternator power to keep from burning up.

So the larger alternator pulley means lower alternator RPM at WOT and less wasted power. As long as the owner is going to drive hard, the system will spend enough time to keep the battery charged. When the car is converted back into a grocery getter, that'll be the time to put the smaller pulley back on.

As for the battery, again we have to look at the average power it takes to energize the injectors at WOT. Lots of RPMs means lots of injector cycles every second. Lots of mass flowing into the cylinders (because of the blower) means lots of fuel, and that requires long injector cycles. Lots of long injector cycles means lots of electrical power into the injector electronics... Oh and the high fuel demand means the fuel pump is drawing more current too.

Perhaps the blower has a waste gate too? That would be another electrical load cycling on/off to keep the system in balance.

Decades ago Smokey Yunick wrote that when you try to make a lot of HP, you keep bumping up against the part or system that has the least surplus capacity to work harder. He also said you'll never know which part it is until it shows itself as the weakest link in the chain.

How many amp output is your alternator?

What was the voltage reading at the batt posts prior to the installation?

My guess is that you have increased electrical demand without increasing electrical output.

A while back when my truck alternator when out, in an effort to save a few bucks, I bought a 90 amp alternator to replace the factory installed 100 amp alternator. I saw a noticeable difference immediately when using head lights and AC. I could not believe the difference 10 amps made.

The stock alternator is a 130 amp. I have not checked to see if it can do that.

Actually the car had power issues of this sort before I installed the blower.

So, what's your problem?

And you're using the same word to describe two separate things. Electrical power and engine power.

Now, it sounds like the blower has nothing to do with the "power issues"? Is this power issue electrical power, horsepower or both?

Let's talk about performance (horsepower) and power.

Maybe I'm just thick, but you haven't described a new problem that I can see.

Help me.

is the supercharger engine driven or electric motor driven ?

from your description it sounds like the alternator cannot cope with the electrical load

"It will drop even further if the air conditioner is turned on. The voltage will drop down into the 12V range"

this statement supports this remeber the battery is there simply to start the engine,

its the alternator that then supplys the power for all the electrical loads.

and as the voltage drops from 13.8 to 12v thus confirming the alternator is not coping

It would help to know what kind of supercharger you installed. If it is a crankshaft driven type, the extra demand on your electrical system should be minimal. If it is an electrically driven supercharger, you may improve the situation by installing a higher capacity battery. I have installed several of the crankshaft driven types (Eaton) and never noticed an increased electrical demand.

The blower is a Kenne Bell, belt Driven by the serpentine belt on the engine.

The lack of horsepower is there throughout the rpm range. Yes, it's more noticeable at WOT, but just plain driving, it's there ( or not there ).

I have put a boost gauge on it and it reaches 6psi as it should.

When it is acting up, it's as if the blower was never installed.

I think I need to maintain 14.5 v system voltage at all times. Like I said, I have put three different alternators on it and this one is the best of the bunch. The others had even lower voltage. A pulley wouldn't make a difference large or small, because the voltage does not change with rpm.

Is it possible the alternator youve put on is faulty, ie the rotor brushes worn are favorite.

whats the amps out put of alternator, Use a clamp dc amp meter to find out.

Dont just go by the voltage, You can get alternators with 14 volts output but little or no amps, a amp meter is the best test for output.

Unless and until you measure the electrical *load* as a function of throttle position & engine speed, there is nothing else to be said.

Enjoy your wheels. :^/

P.S. 14.5 Volts is too high... prepare to $pend on new electronic$ very $oon.

You need to determine whether the 6psi boost is there when the power is down. If it is there, then you need to find out what is loading your electrical system down. You might also want to monitor your fuel rail pressure to verify it is maintained when your car is low on power.

Generally speaking, most car alternators are set to deliver a max of around 14.4 volts, this is 1.2 volts more than the battery can give (or take) so that a charging current will flow!!!

A fully charged battery in a good condition will measure out at around 13.2 volts.

Your car should be designed to accept voltages of around 13 volts (give or take 1 volt aproximately....what you say is both not needed and not normal either....

If the alternator is working correctly, once the car is started, the alternator will supply all the car's needs.....

You may have wiring problems that have not been found up to now.....

Suggest a little background research may help, In the understanding of the charging system.

But until you work out how much your system is using in amps by connecting amp meter to main out put cable on battery with all consumers turned on this will give you a base line to achieve.

then connect alternator after discharging battery for a while ie turn everything on for a few minutes then connect amp meter on out put lead of alternator and start engine, raise revs to about 1000 rpm and see what the alternator ins putting out with the battery discharged.

now for instance if all consumers on vehicle total 100 amps going out of battery with engine on but not running, then ideally when the engine is running you need at least 100 amps to cater for the electrical load, and some spare to recharge the battery.

if there is just enough output to cope with the load the battery will remain discharged until you turn something off leaving some spare capacity from the alternator to charge battery.

also if the alternator is running at full out put this will slow the engine down as the alternator consumes a lot of power somewhere in the region of 5 hp or more under full load.

try turning everything off start engine look at rev counter then turn all the electrics on and engine could drop 500 or so revs.

http://autorepair.about.com/od/glossary/a/howitworks_alternator.htm

http://www.google.co.uk/search?q=car+alternator+output+amps&sourceid=ie7&rls=com.microsoft:en-gb:IE-SearchBox&ie=&oe=&rlz=1I7DKUK_en-GB&redir_esc=&ei=lqUHTInaFIL-0gTNx7hl

how much power does a car alternator use.

An alternator typically takes about 1 HP (horse power) for every 25 amps of power generated. So, a 100 amp alternator will require about 4 HP at full output. Most alternators do not operate at full output for very long.

http://ecomodder.com/forum/showthread.php/how-much-energy-does-alternator-use-2737.html

http://www.tritonclass.org/mir/alternators.html

http://www.alternatorparts.com/FAQ.htm

http://www.alternatorparts.com/how_much_horse_power_does_an_alternator_make.htm

http://www.google.co.uk/search?q=how+much+hp+does+alternator+use&sourceid=ie7&rls=com.microsoft:en-gb:IE-SearchBox&ie=&oe=&rlz=1I7DKUK_en-GB&redir_esc=&ei=1qgHTP68JaG80gSCoYlp

Thanks for the help. I will check the amp output of the alternator. I find it hard to believe that three alternators are not producing the correct amperage. If there is an undue load the the car is creating, I can't imagine what that load may be. The alternator is supposed to be able to produce 130 amps. It seems that there would be some hot wires to the touch if it were running at full amperage all of the time. I don't have any electrical add-on's that would draw any more than what it's designed for. The only time that I'm demanding any more from it is at WOT.

When the voltage is low, the throttle responce is sluggish. At that time there is nothing turned on ie, head lights,air conditioner etc.

I will let everyone know.

it would be nice to know what year cobra as it makes me wonder what did you do with the evap system now that your engine no longer is making vacuum to vent the fuel tank I would guess your random loss of power is related to fuel level most never consider the fact that more fuel consumption means more air has to enter the tank and that going to be hard under any boost condition

It still has vacuum between the throttle plate and inlet of the blower. That is where all of the original vacuum lines connect for the evap system. The set-up is CARB legal.

Something to consider;

I believe the original wiring from the alternator to the battery is a particular gauge. Maybe 10 gauge. Stock wiring is not intended to supply that 130 amps for any long period of time. As stated in previous posts here is what I would do.

1. Get the total amp draw of all the accessories you use when you notice the power loss.

2. Verify the total output of the alternator you have installed.

3 If the load approaches the higher or maximum output of the alternator and this is for longer than what the alternator is supposed to produce you have simply exceeded what that charging system can deliver over that period of time.

4. Is there any current limiting provisions in the chips you have installed to prevent over revving?

5. I have seen loading only a 100 Amp alternator stall an idling 855 cubic inch diesel. That kind of output eats up horsepower.

6 What kind of air intake do you have ? Is it sufficient for the volume requirements?

If I got you thinking , that's good !

h

did you increase the vent capacity of the fuel tank and if the blower is under the throttle is there a pop off to prevent throttle damage on snap throttle close or back fire

Try connecting a good digital voltmeter to one of the injectors. Beat the crap out of the car, and when the pony stumbles, look at the DVM. If the voltage is below about 12 volts, load test your battery. You may have a weak battery that will not support the additional power draw.

IMHO, your power loss is not voltage to the injectors. I think you should be having this discussion with the people that took your money for the upgrades you made.

Something about a satisfied customer. Have you contacted them yet?