Im new to the site here so I may be way behind you guys, but have all of you with GTP's checked the adjustment on the bypass valve on your S/C. The factory guys do a poor job getting it correct and usually leave a lot of HP just spinning around instead of being turned into BOOST. I adjusted mine and noticed big gains in power, it made enough difference that my wife even said WOW when she drove it the first time after I worked on it.

Here's a link to what exactly a bypass valve is.
http://www.magnuson-products.com/bypass_valve.htm

My 2000 GTP

So .. how do you adjust it?

DF - 2002 GTP

I didn't think anyone cared, lol I also own a Lightning and thats where I got the idea from. You have to remove the plastic cover on top of the engine to expose the S/C, the bypass valve is on the front of the engine on the drivers side. What happens is the valve is normally open and when you get on the throttle it closes. If the valve is not set correctly it will hit the stop before it gets closed and leave the valve open and you arn't getting full boost. Go to this site and see how to set it on the L, its the same style Eaton S/C. Be careful and dont leave the lever off the stop or the valve can stick closed and it will not idle good at all if this happens. If you do not understand the directions on this site let me know and I will help, scroll down until you see Boost Act Adjustment. Its right after the pulley's. Hey I swear this works. Mine was way out of adjustment. You can see where the bolts make a mark on the plat and where you end up is how far it was off.
http://home.midsouth.rr.com/lightningpage/tips.html

Adjusting the bypass affects fuel mileage more than it does performance (it's tiny if actually there at all). Adjusted so that only a slight tip in of the throttle gets the boost coming on puts a pretty good dent in cruising mileage. On a cross country trip from WA to MI and back last year I had mine set pretty aggressive going to MI and noticed my mileage took a big hit. Wondered if it was something else so for the trip back I adjusted it so that it was a lot less agressive and mileage went back up where it had been. Avg mileage on the way was 23 compared to a previous trip where I got 29 over the whole trip. Set the bypass up like it was stock and my mileage went back up to that number.

RC

Ok GTP boost, I'll buy that but it was most likely using more fuel because of the added boost. The bypass valve is in no way connected to the fuel system except for the amount of air its pumping down the throat. More boost means there must be more fuel added to make more HP. If you keep your foot out of the boost the valve is wide open and that means there would be no difference in gas milage. After adjusting the valve I noticed a lot quicker throttle response so your in the boost quicker and I got to full boost much faster. I like it and I sure didn't buy a S/C GTP for the mpg. This thing is down right fun to drive now. Its no Lightning but its nice.

That's a given that it was using more fuel because of the boost coming on earlier, it enrichens the mixture under boost. I mentioned nothing about the fuel system. My point is that with it adjusted more agressively only slight throttle inputs get the boost coming up so it's harder just cruising to keep it off boost inless you live where it's very flat.

RC

I got ya buddy and I agree totally. And my point was I didn't buy this thing for mpg I bought it for performance and fun. I just assumed most every else that owned one did to.

I agree, it's just not worth the trade off to me when there is no performance improvement. Now this seems to be a topic of great debate on the various boards. I'm one of the first to have played with the adjustment having done so back in '98 and I've done it numerous times at different states of modifications. The reason for the difference of opinions might be because some were adjusted really conservatively at the factory while others more agressively. But on mine in moving it to as agressive a position as it will allow did nothing but make it suck more fuel in typical driving and my cross country trip gave me a good chance to confirm it under a mostly cruise control state where only small controlled throttle inputs are the norm. My around town mileage also suffered because I drive pretty conservatively most of the time on the street. I'd average ~17-19 mpg before adjusting it and with it set agressively my around town mileage went down to ~13-15 because every little throttle input had the boost coming on. My ETs never improved with it set agressively, and in fact my quickest is still with it set at the factory position. I'm not a casual racer either, my data is important so I only compare apples to apples and not apples to oranges which is a real problem with a number of the comparisons you see by people on the boards. What I mean is that if you don't have consistent and optimized data to start with to compare a change to then you can't equate your data afterwards and this is something you see in comparisons on the boards all the time. Then there's that ass dyno thing... that's really accurate.

The place where this adjustment might have the most benefit is for part throttle response. The autoX guys might find it useful, or might not.

RC

WOW your getting 17-19 on the road with it set mild. My 2000 gets 24-26 around town and set to the most agressive point I can get it. I don't know then, I can tell you this I normally run mid grade gas in the car and after I adjusted the bypass I had Detonation that I never had before and had to go to 93 octane to get rid of it. I can definetly feel the power difference in my car. Maybe just maybe yours was not out of adjustment nearly as far as mine. You never know until you check. I do know this, adjusting the bypass to get a little more boost was far easier than changing out a blower pulley and it was free.

There is NO WAY on stoplight driving that you're pulling 24-26 mpg. If so you're the first guy ever in the history of GP.net to get such figures. Oh, and adjusting this doesn't give you more boost, it just makes it more sensitive to throttle input. It doesn't even remotely do anything like a pulley change does. You're loosing credibility with me at this point. You had to go to 93 octane to get rid of KR? Well the engine requires that stock, it's not just a suggestion.

Ah, looked at your profile. Must be some secret squirrel fuel that none of the rest of us is privy to. I don't mean to sound like an ass but sometimes you get people flying in the face of everything you and others like you have learned over the last few years about something and it just doesn't go over.

RC

Im not even sure what you ment by what you just said, but I guess its time for me and my squirrel fuel to hit the road, sorry I even came to this site and messed up your day. Thanks for the warm welcome buddy. I came here to see what you all were doing with the GTP's, also my profile is clean son and Im not hiding anything from you or anyone else. All I know is your wrong about my mpg, this last tank I got 24.7, and who said stop light driving, I said around town. There isn't a stop light in sight of my town, its 30 miles to Mobile from my house out in the country.
OK if the bypass valve is hitting the stop to soon and not shutting all the way where is the boost going? Its going around and around, not into the intake and down the throat. If you put in down the throat its just like adding a pulley for .5 pound or less. So you would not be takeing advantage of all your boost if the bypass is cracked open when its on the stop. It gets more sensitive to the throttle because it shuts off more complete.
I live way down in the south right on the Gulf where it is Hot and Humid so I could always get a way with running mid grade gas here. In the winter the air get more dense and I always used 93 but the other day after the adjustment I had to use 93 to get rid of the Detonation.
So I'll leave you with this and go on my way. See Ya

You sure didn't mess up my day but don't compare apples to oranges. If you've got no stoplights where you live then how can you even begin to compare when I'm talking about around town driving. Your example of around town driving seems to be more an example of most peoples highway driving so why do you go off that you can't understand how it's so low? Go look at the EPA ratings. 18 city. Unless your city driving includes stops and is at widely varying throttle positions during drivng it's foolish to even call it city driving or in this case "around town". I can see why you did it though, to try to prove a point you didn't have.

You're not hurting my feelings flying out of here, you're just hurting your own education about your car and getting your feet back into reality.

By the way, how does hot and humid in your mind allow you to get away with running mid-grade? So there's more moisture in the air. You think that equates to water injection when the air temps are high like you encounter? Misguided.

By the way, try reading what I wrote again. Your profile is clean? What's that supposed to mean? I made the comment because your profile indicates you to be a "refinery operator" thus my comment about secret fuel. Sheesh.

RC

Don't worry my friend. The bypass valve is a highly arguemental thing here. Saying that you adjusted that is almost as bad as saying that you took out your MAF screen or are using water injection. Let me be the first to say WELCOME. I'm sure if you poke around a bit you'll find some places that will be more then happy to give you the good info you are looking for.

-Dan