I scored a killer deal on a BBK UD pulley and correct belt at a swap meet. After putting it on my 93 Z28, the alternator (which is a new Duralast Gold unit) would just barely make 12.0 V at idle, no load. If the headlights or fans turn on, forget it.

I then bought an ASP 520004 alternator pulley. The correct belt length for this combo is 57.8" (Dayco 5060578). I now get roughly 13.2 V at idle, no load.

Another interesting thing I found is the stock pullies turn the alternator 3.08:1 in relation to the crank. With my new set-up, the ratio is 2.86:1.

Bottom line, you must have an alternator pulley on a 93 if you want to charge at idle. Hope you all can benefit from my findings.

Stock Crank Pulley: 7.52" OD
BBK UD Crank Pulley: 4.77" OD
(36.5% difference in circumference)

Stock Alt Pulley: 2.44" OD
ASP 520004 Alt Pulley: 1.67" OD
(32% difference in circumference)

 

Something like a cam that requires a raised idle would negate the need for an overdrive alternator pulley.

 

yep my stuff is wayyyyy under driven.........but with a 1400rpm idle i got 14.5 Volts all the time

 

Yeah, most people don't realize the underdrive pullies require a higher than stock idle and therefore get a bad rep.

 

Wouldn't a 1400rpm shift from park to drive break some ****?

 

nope the accumulation in the valve body is tuned for it also a 5300 converter softens it a bit........gives a little chirp going into reverse but its no big deal lol

 

Not on his car lol Hes one of the fastest for a reason.

SON OF A! First shbox and now you.....I was also gonna guess a 5k stall, so I was close lol.

 

yep 5300 but its still a lockup converter so i street drive with no problem......plus the 4.56 gear minimizes the effects of the converter on the street and the OD and lockup allows me to cruise down the highway at 80 and run like 2900rpm all day long.......



the under drive balancer was one of the best mods i made to my car personally........i was having problems frying alternator bushings becuase of excessive RPM........i havent had an issue since i went underdriven.......also the aluminum hub/shell ATI under drive balancer dropped 7lbs off the snout of my crank!!! thats big for rotating weight....on a car that idles at 700 though i woudln't run one.......reason why i wont put one on my bolt on LS2 gto and thats it.......its my daily......i want full voltage at idle while im in traffic with the radio blasting and AC on.....

 

Nothing new.
This was discovered some 15+ years ago when people would post up about their battery dying 4 to 6 months after installing an U/D pulley and before an O/D pulley was developed.

 

I tried bumping the idle from 800 to 1200 RPM, and only got a 0.2 V increase.

 

What kind of horsepower difference are we talking about...for all this work and what seems to be a few bucks as well

 

About 5rwhp/Tq

 

if that.

 

Those of us who have tried pulling the belt entirely at the track have found a .1 gain. The underdrive pulleys are generally something like a 30% underdrive so simple logic suggests a .03 gain from and underdrive pulley, but you are adding a slight amount of rotating weight and spacing the damper further from what it dampens with the piggyback pulleys. IMO if someone were to bother with an underdrive doing it with a new damper like the ATI is the ticket, lighter, underdriven and tuned for the higher rpms we push once modified.

 

yep......the gain would be very very minimal and IMO not worth the money or aggrivation on a stock/mildly modified car...........the more rpm you are twisting the more it becomes relevant to the power of the engine also the weight is huge.........i dropped nearly 7lbs off my balancer setup going to an aluminum ATI unit.......on a 5500rpm stock car this will help........on a 8000+rpm motor it helps alot.......

 

The weight savings is nice but so is tuned for the right rpm and SFI approved. The underdrive is nice to keep accessories alive for a setup like your's but let's be honest the underdrive is NOT a primary reason you choose the balancer, it is just a small added benefit to all the other features you needed more.

 

exactly the underdrive was just a small benefit........1st off its an SFI approved balancer which i needed per NHRA rules.......also its 7lbs lighter than a standard balancer........it offers much better dampening ability vs a stock balancer and at the rpm i turn it not only keeps my alternator alive with the underdrive but it is perfectly safe at 8000+rpm....

 

7 lbs lighter? how heavy is the stock balancer?

 

I believe 7lbs includes the weight savings of the aluminum hub offered with some of the ATI units.

 

Isn't the BBK pulley just a "piggyback"? In other words, you install the BBK, then put the stock balancer back on and just hook the belt to the BBK pulley. Your actually adding weight and driving the accessories slower and probably not getting much HP gain on a <450HP motor? Which is why the BBK is so cheap?

Not to hijack, but I researched these pullies a while back and I'm about finished with my build, so I'm going to be deciding what to do with the balancer/pulley and I need some help with this. I bought a new keyed aluminum hub a few months back when ordering parts. I would really like a setup that I can put a timing light on just to confirm the PCM is getting an accurate reading. I don't know how much I would benefit from an underdrive so I don't mind staying with the stock balancer, but isn't it subject to turning even with a keyed hub?

Suggestions?