Daveinnola

I need to change the HB. Undecided wether to go stock size or underdrive. Thinking of going 25% under. I have a 2001 z06. Being a manual helps. And I don't do a lot of city driving. Mostly highway and backroad cruising.

Mr. Black

The only time I would ever consider an underdrive crank pulley would be dedicated drag racing. All other times there is not enough gain to justify the losses.

493lszosix

I’ve ran the powerbond 25% under drive for years no issues, 04z

Fastbird

Drop an underdrive on it and call it a day. Plenty of guys with years running underdrive pullies with no negative effects.

Mr. Black

Didn't mean to imply that they don't work or cause massive problems. I know that's not the case

I myself don't see the point in running one for such little gains. I'd rather find 3-5hp somewhere else.

Fig_

I run a 10% under drive and get about 13 volts with all loads on, 14 with loads off. I noticed a difference in the way the engine revs I really like. If it was a daily driver I might have stayed with stock size.

Daveinnola

Thanks for the insight. I think I'm gonna do a 10%. If I do 10% under do I need a longer belt.

Fig_

Im running a 77 inch belt, oem is closer to 80. Got a 76 inch In the mail today I think will fit better.

J-Rod

I just went the opposite way. One of my cars had an underdrive pulley on it that came apart (old ASP unit). I ended up putting an ATI back on with the stock size. That being said, I never really saw any issue with the underdrive unit in terms of accessories.

Daveinnola

Just a follow up. I finally got around to finish the work on my car. I went with the Summit racing 25% underdrive pulley. I also put on a set of speed engineering headers and new Magnaflow exhaust. Finally got it finished and tuned with HP tuners. All I can say is WOW. Like having a new car. Sitting at a red light idle is 750 rpm. No problems with A/C or alternator. I can idle just fine. The engine is definitely more snappy. Also replaced plugs and wires while I was under hood and other general maintenance. I'm at 64 k right now. Good for many more miles.