Just looking to see if I'm in the right ball park in my figures. I have been trying to find a calculator that gives BASIC rpm to boost estimates and can't find one. I know many factors effect that such exhaust sizing & etc., but again I'm only looking for ballpark figures. Also this on a vq40, and is the reason for looking at the smaller crank pulley vs your more common 5.75" pulley. The vq40 stock CP is pretty big and heavy in comparison and I've had an undedrive pulley on for years as a result. Without going into the debate of underdrive pulleys, just really trying to get help on basic impeller rpm vs boost estimates. I've written Vortech several times and no response from them yet.
V3 estimated outputs per rpm calculators & my guestimates at 6500rpm-
Stock Vq35 setup---5.75" CP + 3.33" SCP = 43,000rpm = 7-8 psi
ALT. CHOICE--------5.25" CP + 3.33" SCP = 39,300rpm = 5-6 psi
ALT. CHOICE--------5.25" CP + 3.12" SCP = 41,900rpm = 6-7 psi
ALT. CHOICE--------5.25" CP + 2.87" SCP = 45,600rpm = 8-9psi
I'm trying to play it safe for now and stay in the 5-7psi range & somewhere in the 350whp ballpark. Also curious why most people change the SCP vs the cog pulleys. Wouldn't the cog pulley change vs SCP be less the likely to have belt slippage? Or is belt slippage not too common even with the 2.87" pulley?
Also one last question. Would a dual 2.25" full exhaust, shorty headers, no cats, magnaflow mufflers, and magnaflow Tru-X pipe be adequate with my 5-7psi and 350whp range goal. I was thinking it might take a tiny amount of topend away, but also as a result give me better low-to-mid range which might be more benefit on a 4100lb truck. That is my current exhaust setup now and my way of think sounds like a reasonable choice even with a Vortech.

 

psi doesnt mean anything.

tune to the 350wh range and adjust pulleys to meet that goal.

you are going to want to start with the 3.12 and go from there.

 

Actually the 3.12 is what I was leaning towards. I quess I can start with the stock 3.33 is see what it does after a tune and on the dyno. If it's still lacking, then I could just bump to the 3.12 and so on.
Is it usually a good idea to retune with each pulley change? I'm guessing that answer would be yes. Thats why I'm trying to get it close to begin with since my tuner(Uprev) is several hours away and I'm trying to prevent the multiple trips back & forth.
Is the pulley easy enough to change on site at the tuner in case I want to swap them there?

 

I've heard a few guys on here were able to swap the pulleys out without removing the bracket, but that was not my case.

 

Thats what I was afraid of.

 

it might be possible with the smaller pulley's but definitely not the large ones. FWIT, i couldn't get my 2.87 off without removing the blower. If you can get tools behind the plate to remove all but one of the 4 bolts holding the blower to the plate and swing the blower slightly out of the way you could swap that pulley.

like str8dum1 said, it is best to start with 3.12 if you are wanting around 350hp. 3.33 won't get you there.

 

That's Crazy, all that work to change out the Pulley.....

I just Loosened The Jackshaft Housing, it let the Jackshaft move back, pulley came right off.......If I recall, I think I remove the Cog Pulley first to give me more access room to the jackshaft Housing Screws....3.33 to 3.12 to 2.87

 

So looks like the 3.12" is a smart first choice for initial goals. Thanks for the input guys. Do you think my exhaust (described in 1st post) be adequate in that HP/boost range?

 

ah that's right! I tried to do this and one of the bolts stripped out but i couldn't. Heck, it might have even been you that told me to do that, Tim.

So ya, what's he is saying, the jackshaft is the thing that is on the backside of the pulley you want to change. There are 4 bolts holding it in place. Take those 4 bolts out and then unbolt the 3.33 pulley you want to change and then pull the jackshaft out to release the front pulley. Should work great.

Exhaust will be fine. I'm running dual 2.25" for ~600hp with no issues. You can run 350hp with stock exhaust without issues.

also, about belt slip. it's common on 3.12 and 2.87. Just tighten your belts really tight and if it continues to slip you might need the GTM pulley mod to add some pressure to it. 3.12 should be minimal slip and with properly tightened no slip.

 

Cool, thats what I was hoping to hear. Appreciate the the answers guys.