I need to get a dual belt alternator pulley (air pump has been removed). While shopping I've come across underdrive pulley sets and non-underdrive. Some sites selling the non-underdrive ones say their's are better for street applications than underdrive. Can someone explain the pros and cons of underdrive.

thanks

 

any accessory thats underdriven of course will see less rpms than if it were stock. ie an underdriven alternator will pump out less amps. an underdriven water pump will pump less cc's. keep in mind that if u install an underdriven main pulley(the one off the eccentric shaft) it will under drive all accessories. but in return you get a few horsies and a faster revving engine kinda like installing a lighter flywheel. in my opinion its almost not worth it.i would only underdrive the power steering and maybe the alternator but never the water pump.

 

About 80-85% of the hp increase from "under-drive" pulleys comes from the reduced weight rather than the under-drive ratio. The primary purpose of under-driving is to keep the belt-driven accessories in their correct operating range on race engines that spend the majority of their time in the high rpm range. If your car is driven on the street, I recommend that you just stick with the stock ratio so the water pump and alternator work properly at idle and low rpms.

Under-drive Pros: Keeps the belt-driven accessories (primarily the water pump) from over-speeding on race engines.
Under-drive Cons: Reduces the water flow of the engine at idle, and can cause the alternator to knock off-line if there is a high electrical load (the FC RX-7 is notorious for this).

 

The FC already has a low power alternator that barely supplies everything that needs juice. An underdrive pully means you'll get even less power to the alternator. Most people should stick with the stock pullies, unless you like the risk of being stranded at night waiting for someone to jump your car.

When new, the stock alt puts out ~30A at low revs and 70A at 3500+rpm. The headlights alone draw about 10-15A. There's also the blower, the rear defroster and the other various lights (put together). Each of those 3 also draws a large load. So during the day its not much of a problem, but you could have issues at night.

Oh yeah, a slower water pump likewise means higher temps.

 

Mainly, the underdrive pulley is meant to help with waterpump cavitation that occurs above 6500 rpm's. I noticed a huge difference after installing mine 2 years ago because the heater hose that connects to the block just below the oil filter stopped blowing off when I was pegging the rev limiter while doing burnouts.

For an N/A application, I would highly suggest one. As for a turbo application, I would advise against it

 

Anyone running the one from Unorthodox Racing?
In terms of quality, I'm wondering how it compares to the one from RX7.com.

Here is a brief write-up with some good info:

http://grassrootsmotorsports.com/news/2006/05/

Check it out!

 

I could be wrong, but IIRC they discontinued them for our cars some time ago.

Also, I beleive they're all about the same.

 

Thanks for all the feedback, very helpful

 

AAhhh so this is why 2nd Gen drivers use 3rd Gen alternators? I'll have to do some research on swapping to a 3rd gen alternator. I have a custom stereo using 2 amps now, and once I finish building my sub-enclosure a 3rd amp.

 

Yeah, I've been shopping around. They have 3rd gen, but not 2nd.

 

The stock alternator isn't really made to handle anything more then factory. If you plan on running lots of electronics and ****, you should check into a 13BRE, 3rd Gen alt, or a 20B alt. You can find then in the for sale section or even the 3rd gen one from a Napa store.

I'de recommend it for you.

 

Thanks for the heads up. Any idea what goes into changing the wiring for a JDM S5 13B T? I assume it's not a direct plug in swap?

 

The only dual alternator pulley I can find that isn't underdrive is with Rotary Innovation, here's a link.....
http://www.rotaryinnovation.com/new_products1.htm

Also, if I'm swapping to a 3rd Gen alternator, should I shop for a 3rd Gen pulley? I assume I'd still want a 2nd Gen pulley...right?

 

related question, what should I expect to pay for a used FD alternator, I've never priced them.

 

switching to an FD alt is fairly common, and yeah people do it for the extra output. I'm sure you could do a search. There is in fact a wire that you need to switch, unless you have an s5 (1989+). I just happened to see the diagram in a recent thread. You might also need to swap the pulleys, I dunno. Just do a search until you find the info you need. I would guess that the price is roughly the same as any other alt, since you can still buy remans in auto stores.

 

Morning bump

Anyone know of any non-underdrive dual alternator pulleys?

I tried a quick search for FD alternator swaps a couple days ago, but will give it more time and effort over the weekend.

 

http://rx7cz.net/shop/

 

You'll need a second gen pulley because the third gens don't use v-belts, however the new pulley will have to be bored out to fit the new alt's larger shaft.

 

i just wanted to add my experience with my underdrive pulley. I purchased it for the extra timing marks, nothing else. I do regret buying it and will be going back to stock sized pulley soon.

The under drive pulley sucks at idle. My system keeps a low voltage right after I start the car, and it negatively effects the idle. Also, when the system starts to charge, it puts extra strain on my FD alt and causes it to squeal the belts until the system is charged back up to 14+volts.

I dont have many electrical toys in my car. Right now I dont even have a radio. And I run the stock fan.

IMO, any HP you think you are gaining form an underdrive pully, are sucked back up by the charge systems inability to charge effeciently.

I would only recommned under driving a total track car.