iv taken a look through the search and didnt find anything relevant.

i was toying with the idea of setting up underpulleys in my car, but iv got a custom sound system. is there an recommended alternator that will pump out the same power as a stock one while being underdriven?

recommendation? ideas?

 

Get the FD alternator. Do a search for installation instructions. It's a 100A unit, vs. the stock 70A (which is really a 50A) unit.

 

Underdrive pulleys are a waste of time and money on a street car, the power gained is only at high rpm and tiny. These are only useful on race cars that are searching for every last hp and spend most of their time at high rpm. Spend the money making your stereo louder!

 

OK, I just found this web page showing how much power it takes to drive an alternator: http://4wheeldrive.about.com/gi/dynamic/offsite.htm?site=http%3A%2F%2Fperformanceunlimited .com%2Fdocuments%2Falternatorfact.html
So if a Series 5 80A alternator takes 1.8hp to drive and you put on a 20% underdriven pulley, you've gained 0.36hp. If you do the water pump as well you might gain 1hp total. I don't think you'll notice!

 

Series 5 80 amp Alternator onto Series 4:

http://brfoundation.com/RX-7/alternator.htm

Series 6 100 amp Alternator onto Series 4-5:

http://brfoundation.com/RX-7/alternator2.htm

 

How underdrive pulleys give you more power:


(taken from unorthodoxracing.com)


Weight loss is the most critical factor in HP production, each lb. of mass taken off the crank/eccentric shaft is worth approximately 2.7 HP and the figure increases with forced induction, nitrous, VTEC and increased RPM's. Only 15-20% of the gains seen from our pulleys comes from the underdriving. We achieve the maximum gain for each application/model by the singular use or combination of reduced diameter ("underdriving") and weight loss versus the OEM crank pulleys. This principle allows us to maximize the HP gain without causing any adverse effects in the general function of the vehicle.


The diameter of each of our crank pulleys is reduced specifically (from 20-30%) to the vehicles accessory needs. In other words we maintain all accessory minimums: air conditioning efficiency, power steering feel, and voltage (minimum allowable voltage 12v) even at idle with everything electrical on in the car. There are no adverse effects from the use of our pulleys whether Ultra S, Ultra R, or Ultra SS pulley sets are used. Bolt them on and forget about them, then just follow your vehicles normal maintenance schedule.

 

I have the single rp pulley with alternator pulley.. Havent put them on yet but I know it will give more than 1hp. Its a noticible difference from what I was told by people who have done it....I just cant get that center bold off.

 

That sounds like someone trying to sell a product...
I still maintain the power gains will be minmal, notice there were no actual power gains quoted by Unorthodox, just a rule of thumb with no data.
Mazda sized the pulleys so everything was being spun at the most efficient speed as often as possible. To slow everything down 20-30% and expect them to run as well as before is just a bit naive.

 

stock pulleys are pulling the alterantor at a set speed to insure all accesesories are getting the power they need. now if you get an alternator that puts out 20% more power, and spin it 20% slower.....

same power output to accesesories. they dont post up hp gains because ever car is different. just like headers give you a rule of thumb increase in HP.

and no its not some massive HP boost, its an improved responsiveness with a minor HP increase.

people who have done it notice a difference. not huge, but noticeable

 

You've forgotten the 1st law of thermodynamics: ya don't get nothin' for free! If you spin a 100A alternator 20% slower, you’ll consume the same power as an 80A alternator, but you’ll only make 80A! All you’ve achieved is throwing away money on a new alternator and pulley for nothing. Just buy the S6 alternator; your stereo will love you for it.

 

umm ya thats the point.....

your using less energy to get the same power out of your alternator.

80A alternator spinning at full speed producing 80A
100A alternator spinning using 20% less energy to sping and still getting 80A

it frees up power. that un-used 20% is used to move the car.

 

No, no, no!
All an alternator does is convert mechanical energy into electrical energy. You can’t get more energy out than you put in! If you could you would be a very rich man! The FD alternator is capable of producing more current than the FC one but you still have to put more hp in to produce it, about 0.4hp in fact.

 

maybe im missing something here....

how does underdriving a 100A alternator 80A use more HP than an 80A alternator?

your spinning it slower(to lower its output), thus using less mechincal energy to spin it, thus freeing up that energy to move the car, but still producing to stock 80A.

end result being that you are essentially spinning an 80A alternator(actually 100A) at 80% capacity and still producing the same power output.

in this you are freeing up a net 20% of the power required to spin the original 80A alaternator

"You can’t get more energy out than you put in!"

i think your under the impression the alternator is making the extra power. the extra power comes from the engine spinning the alternator 20% slower. thus producing the side effect of improving performance a small bit.

 

if you spin the 20% bigger alternator 20% slower, it will produce the same amout of elec power, and rob the same amount of HP. the only gains would be it would last longer at higher rpm then a stock one which doesn't like to go over 4k rpm

 

Please understand I'm not trying to start a flame war or anything silly. I'm just trying to clarify basic, set-in-stone physical laws that you clearly don't understand!
Lets go back to basics:

Electrical power out = Mechanical power in x efficiency (~85% for an alternator)

Electrical power = Voltage x Current

So at 14V, it takes 1.8hp to generate 80A and 2.2hp to generate 100A

There is nothing you can do to change these rules! Since both alternators output at the same voltage (regulated), the only way to increase current is to supply more power. The 100A alternator has more windings than the 80A one. As I said, this gives it the capability to produce more current, but it also makes it harder to turn.

Remember alternators are rated at a particular rpm (not sure what it is). If you spin an alternator rated at 80A faster than this rpm, it'll output more current, but will require more hp to do so. If you spin it slower, it'll output less current, but will require less hp.

I hope you understand, coz this is getting frustrating!

 

Huggybear, I think what you are missing is shown here:

It doesn't matter what the alternator is CAPABLE of supplying, the car is going to draw the same current it needs no matter what size the alternator is. The only reason to put a larger alternator is if the existing one is not CAPABLE of supplying the current for an amplifier or extra lights. Lets say the car is drawing 60A. If you put a larger alternator, the car will still draw 60A even though the new alternator is CAPABLE of supplying more current. Putting a bigger one on and reducing the rpm does not change the fact that it takes the same hp to produce the 60A with either alt. There is no power saved by driving the larger alt at a slower rpm, the engine supplies the same hp to get 60A with either alt.
By the same logic, if you need a certain hp to do 60 mph in a two rotor car, putting a 3 rotor engine in and running at a lower rpm does not change the hp required to go 60; but you are now CAPABLE of going faster. (Much faster )
-John.

 

I replaced my Stock Alternator(80A) with a rewired Ford Contour alternator (200A), because i needed an alternator that could charge 2 yellow tops. And I didn't notice any performance change.

Check out www.egrproducts.com if you need a high current amplifier for ur stereo needs. It was around $400 - $450 for that amplifier and it was well worth it. If you run 2 batteries, don't forget bout the silenoid.

Good Luck

 

maybe im just missing how alternators work. im not a genius....

if its going to draw what it needs regardless of what alternator is in the car, why does running an aftermarket stereo system with an underpulley system cause problems running the stereo(at idle and such)? iv been told this from multiple sources. ie: dont use underpulleys if your running a power sound system.

its not going to get enough power is it?

im not sure how it works, but i was wondering if it was possible to get an alternator that will still get enough juice to the system while the car is running, but still allow me to take advantage of an underpulley system.

that was all i was asking about originally.

and no, im not trying to be difficult, im just trying to understand this, so dont take it personal if i question you about what you guys are saying. i appreciate the responses you guys have given so far. im just learning.

btw thanks for the link LoSRx7

 

" It was around $400 - $450 for that amplifier and it was well worth it. If you run 2 batteries, don't forget bout the silenoid. "

I meant $400-450 for that ALTERNATOR

Check your amplifier(s) to see their current (amps) draw.
Before I had a PPI PC4100 amplifier that used 80 amps and i had no problems with the stock alternator, just minor dimage at night. Never dropped below 12 volts. When you start dropping below 12 volts, then you need to upgrade your power supply. I would first go with a Yellow top, and then a capacitor. 1 Farad Capacitor = 1000 watts.

PM me if you have any questions or try my AIM name "LoSRx7"
Good Luck

 

An alternators output is directly related to it's speed. At idle they put out stuff all. If an alternator puts out 80A @ 3000prm, then it'll only put out 20A at 750rpm, barely enough to run a decent amp (plus it's trying to run the car). An underdrive pulley will only make this worse.
Please believe me when I say an underdrive pulley will reduce the ability of your electrical system no matter what altermator you run. You can't beat physics. The "advantages" of running an underdrive pulley are simply a lot less than you think and are not worth the money and effort for your application. Spend the money on something useful, like a grunty alternator.

 

Listen to NZConvertible.
Also,

 

"Unless you have excess capacity with the existing alternator, enough that it will not be missed when you reduce it's capability by underdriving it, there is no advantage. "

if thats possible, thats exactly what im looking for. based on what people have said here though, im getting the impression its impossible. is that correct?

 

Go to my website... I have that on my 'vert. It makes a huge difference on the amount of voltage put out.

www.geocities.com/rx7_ragtop

Click on the "my car and mods" link, then look for how I did that swap. It's an easy swap and well worth it.

 

We know that's what you're looking for, but as you have the stock alternator and a large stereo, you would not have any exxcess capicity to spare. It's not that it's not possible for any car, underdrive pully's it just isn't likely to help you. With amplifiers and lights at night and the heater fan running (which I imagine happens on occation in Winnipeg , you could probably use more capacity (larger alternator, as rx7_ragtop has done and NZConvertible suggested), not less.

-John.