How much HP can be attributed to an UD pulley and what is the downside?

 

From what I've read, typical HP is usually 2 - 5 HP. Perhaps more in some cases. Again, most dynos would have a hard time definitively showing this increase due to margin of error and other factors.

As for the downside, I don't believe there are many. The PS pump and other devices still perform without any notable loss. Total cost for the belts, HB bolt, and pullet average around $250 - $275.

I guess this biggest draw back is digging through all the mess to get the old balancer out and the new in. There are always opportunities to cross-thread a bolt here or there.

Typically, this mod is done in conjucntion with a cam change as all the stuff has to come out at the same time. Just a thought.

As we speak, I am in the process of doing a head, cam, pulley job - and loving every bloody knuckle along the way.

 

Thanks. I'm doing a 415ci LS3 and looking for any issues with using a 25% UD pulley vice a standard size pulley.

 

I used a ASP Underdrive Pulley (25% UD) and have no issues. The only minor issue was the lights would dim slightly when stopped a stoplight at night (stock idle with an A4). Once I installed the new cam, I raised the idle speed to 800 rpm and no more dimming. The alternator overdrive pulley can help with the dimming issue, if needed. The crankshaft can be easily stripped when installing the new pulley so, be careful.

 

same experience here.

Another advantage is for people who do a lot of performance driving and cars with aftermarket gears, the UD pulley actually drives the accessories at their original intended speed.

 

Apparently there are few issues and no controversy with an under drive pulley. Had I asked the same question about gears or Z51, there would be a thousand comments one way or another

 

9 rwhp time and time again.

When you do a head cam job to a car the idle rpm is usually raised and if it is increased 20% it makes up for the speed lossin the rotation. Off idle obviously has no power loss as the alternator is spinning above the minimums needed for full charge.

I have one now and my idle battery voltage is 14.2v.

There are no downsides to an UD pulley on a corvette. Experiences with old trans ams and Z28's dont apply and should be kept in the past with the cars they didnt work well on. The only car that may have an issue is an auto with a stock cam which idles at 550rpm in gear with a 3 year old battery and a 1000watt amplifier.

 

Thanks, Guy. Was hoping you would chime in.... Makes me feel better about it.

 

Its a great mod and for the 200-250 it takes to add it in it would be a bargain if it gave you only 5hp....but its 9 as shown by tuners over and over.

 

I haven't done the UD pulley mod, yet, so here's a dumb question.... what is the best way to drop the rack and pinion to get it out of the way?

 

Unbolt it and the hydraulic lines and slide it to the passenger side enough to access the bolt. It isnt nec. to remove it completely. It will rise somewhat to permit access. If a cam swap is in order it comes completely out by rotating it and removing it trough the passenger wheel well.

Take a few dig pics of how the power sterring lines are oriented n the driver's side so you recall how they go back in.

 

 

I want to qualify by saying I am poor, lonely battery salesman, but if you can get 9hp just by changing a pulley, with NO DOWNSIDE, why wouldn't GM build the car that way instead of spending 10s of thousands trying to get more hp?

I am not disputing the claim, because dynos don't lie. I just don't know, that's why I am asking.

Also, according to my team of battery and electrical engineers, just because you are putting out voltage, doesn't mean you are putting out amperage.

 

please forgive my ignorance, does a new belt come with the pulley and or where would get the proper belt for the pulley, esp. if one goes with an ud on the crank and an od on the alt.

 

I heard it's 9-15 rwhp

 

Underdrive and efans on a trailblazer ss gives you 15-18 rwhp.

I sel tons of these to TBSS guys.

BTW -I sell the ASP pulleys for Vettes as well - $239.00 with bolt.I have 2 on the shelf as we speak.

 

Good point, Zig. I got a spare S-belt and SC belt from LPE when I did my SC. Put it in the cubby in back in case I ever broke a belt away from home. Plan to do the same thing with the new motor.

 

If you raise the idle of the car the alternator is spinning as fast as it was with the stock pulley at the slower idle rpm so no there is no difference between the old and new amperage. Further, if you use an overdrive pulley on the alternator and an underdrive pulley on the crank there is no amperage difference. Not sure why you posted here but just for UD pulley facts: My cousin's 2002 trans am still has the original battery in it. He has an UD pulley on it since 8k miles when I installed it and it now has 112k miles. No issues to this day with any dimming or electrical issues. No there is no downside to a pulley swap. I think GM should use it as stock. But, I do have a theory! I think since they use the same pulley on so many vehicles that may have an issue with the power, its possible that using the same one on the corvette keeps costs down.

Why dont they grind the cam with 208 on the intake side?
Why not mold the TB with the same shape as a ported one?
Why dont they put in a 160 thermostat?
Why do they release the car running so rich from the factory?
Why do they use a restrictive air filter?
Why do they have the air cleaner draw hot air instead of a cool air duct through the shroud?

They have to stop somewhere.

 

Ok, so you cranked up the idle and increased the alternator speed, I'll buy that, but then I think you have an emissions issue, so I understand why they don't do it.

Thanks!

 

On my C5 with head/cam ect and yes UD pulley, 900rpm idle, the station attendant said it was the cleanest running car he ever tested w/sniffer.

Understand that the UD pulley can come with an OD pulley for the alternator so no added idle speed is needed ( I have installed them for cars with the stock idle and no OD alternator pulley without issue). Why are you so insistant on finding something not right? Sometimes in life you win.

Its a gift mod.

One more fact. When you do this mod it is usually in addition to a cam swap and no cam that makes any real change in power can use the stock idle rpm. Its a tech section and people come here to discuss mods and ultimately do them. The ones that stand the test of time and 1000's of installs are worth doing and I dont know anyone that skipped this mod when doing a cam. I see no one complaining about not passing emissions, voltage, current, ect. One more thing. When you get an emissions test done, it isnt at idle rpm.

I have never seen a single person remove one for it having a downside. If you had a stock idle, automaic trans, and had dimming due to huge stereo amps being drawn, you get an OD pulley for the alternator and its done.