Thread: High Altitude GTP

I have read countless articles on SC pulley replacement and am ready to do my own. I run my '05 GTP exclusively at high altitude. Where I live (Denver) the normal air pressure is 12.1 to 12.2 psi. For those of you at sea level, you're at 14.7 and the other 90% or so of you are at about 1000 ft above sea level, which gives 14.2 psi atmospheric pressure. I'm planning some upgrades, but would like some advice. Right now I'm totally stock. I was thinking about running a 9-inch open cone, Autolite 104's and a 3.4 inch pulley with a tuned Overkill PCM. Here's the rub: I believe the normal amout of boost from the GenV in the 05+ (better airflow than 04) is about 8psi. That would bring the manifold pressure at sea level to 22.7 psi. Changing to a 3.4" pulley would add about 2.5psi boost, bringing the pressure to about 30psi, roughly. Since I lose 2.5 psi from the get go due to altitude, would a 3.4 pulley bring me back to sea level conditions with a stock pulley? If that's the case, should I tell Overkill that I'm running a stock pulley to get a proper tune since the air pressure to the manifold would be about the same as a stock pulley at sea level? Should I choose an even smaller pulley size? For reference, I rented an N/A GP when I was in Boston and it felt like it had more "juevos" than my GTP Comp G at home.

Thanks in advance for any advice.

But what I'm getting at is that a 3.6" pulley doesn't even get me to the intake pressure that you guys run stock. If stock seal level manifold inlet pressure is 22.5psi (14.7atmospheric+8boost), and the car is designed to run there all day long with stock headers/exhaust, why do I need to do those mods if I only intend to run at 22.5psi even though it takes a 3.4" pulley for me to do it? From my point of view, I'm not trying to run any more boost than a stock GTP at sea level. Do I really even need a PCM tune? I need a bigger inlet because I have to physically take in more air volume to compress to get the boost. I guess if I'm taking in more, I have to exhaust more? Not sure if I'm explaining that well.

To be honest, the only reason not to try the 3.6, then go to 3.4 assuming all goes well is that I have heard that the pulley is a PITA to pull off without the proper tool. So in order to save about a $35 tool rental the second time and about $75 on a pulley that I won't use, I am trying to be smart up front. The truth is there may not be a lot of experience with my current situation. If that's the case, I will just have to try it and see, then report back my results. I was really just hopping someone could verify that my math/logic made sense or could point out where my thoughts were flawed.

FWIW, has anyone read the MAP sensor or put an absolute pressure gage on? The boost/vacuum gage is good for letting you know how things are operating, but absolute pressure seems like it would be interesting to know on occaision.

Again, I appreciate the responses.

Originally Posted by ColoGTP

To be honest, the only reason not to try the 3.6, then go to 3.4 assuming all goes well is that I have heard that the pulley is a PITA to pull off without the proper tool...

So true! I have pictures of my original one; since the right tool isnt available here I had to improvise. It was rendered completely useless to say the least; I'm SO glad I was switching to the modular system which saves me a ton of headache now. It's for sure the way to go if you're gonna be swapping back and forth at all.