Lower Pulley or Upper Pulley or Both

[LavaSurfer]

I've talked to a few shops and have searched, and I see over the years there have been different opinions. Currently my 2012 ZL1 is stock except for a Vmax ported TB and a Roto-fab intake. I have the supercharger off at the moment to swap the isolator so now is the time to press off the upper pulley. A few shops I've spoken with said to go with just an upper (2.46) while others have said just do an ATI balancer and install a 10" lower. Both of these seem to be pretty close to each other as far as pulley ratio and would accomplish nearly the same result. At the same time, I'm also doing LT's, a KPM fuel pump, and injectors as it's only a matter of time until I'm looking at camshafts, etc. I'm thinking that since I have everything off, I should go ahead and do both and just order a pulley combination that is going to net me a pulley ratio somewhere right around the same to make future changes easier.

What would you do?

[VRZL113]

If the blower is already off the car... I'd do the upper.

[tartagliazl1]

Be careful with a lower 10” i have A 9.55” lower and it will rub the 2 cooler lines that run below the pulley. I had to zip tie them down to avoid this. Also if you go that big of a lower you’ll need a new idler puller to make room for the swap. I would just go 2.5 upper and call it since the supercharger is off anyway. How much boost are you trying to run? What about cooling mods? You spin that supercharger that fast all you are doing is building heat.

[LavaSurfer]

Originally Posted by tartagliazl1

Be careful with a lower 10” i have A 9.55” lower and it will rub the 2 cooler lines that run below the pulley. I had to zip tie them down to avoid this. Also if you go that big of a lower you’ll need a new idler puller to make room for the swap. I would just go 2.5 upper and call it since the supercharger is off anyway. How much boost are you trying to run? What about cooling mods? You spin that supercharger that fast all you are doing is building heat.

When I plug in the pulley combos I've been recommended I'm getting pulley ratios in the 3.1 - 3.3 range which would put the blower speed right at 20,000 - 21,000 at 6.5k. From what I can tell If I get a 9" lower, then I can fine tune everything with upper in the 2.6" - 2.8" range to stay in that blower RPM/ratio range. That way I wouldn't be going too small on the upper to run into belt wrap/slippage issues or too big on the lower to run into too many issues there.

As far as cooling I was planning on running the stock IC pump, reinforced brick, and I think I'm going with the Dedicated Motorsports 2-gallon IC res and heat exchanger. Later down the road an Innerchiller might be in the cards but that would be after a cam, blower porting, etc.

[ZMEnow]

Originally Posted by LavaSurfer

I've talked to a few shops and have searched, and I see over the years there have been different opinions. Currently my 2012 ZL1 is stock except for a Vmax ported TB and a Roto-fab intake. I have the supercharger off at the moment to swap the isolator so now is the time to press off the upper pulley. A few shops I've spoken with said to go with just an upper (2.46) while others have said just do an ATI balancer and install a 10" lower. Both of these seem to be pretty close to each other as far as pulley ratio and would accomplish nearly the same result. At the same time, I'm also doing LT's, a KPM fuel pump, and injectors as it's only a matter of time until I'm looking at camshafts, etc. I'm thinking that since I have everything off, I should go ahead and do both and just order a pulley combination that is going to net me a pulley ratio somewhere right around the same to make future changes easier.

What would you do?

This was quite a debate many years back, but after discussing with Jannetty racing as he was a firm believer you are much better off changing the lower vs the upper as the upper is prone to slip. I have been quite happy with the 125hp package which is the 18% lower for over 10 years, but yes you do have to do a relocation kit for the belt.