I have an 04 S with the following mods:15% SC pulley, 2% crank pulley, Miltec Cat back, Alta CAI, Alta IC, Unichip, upgraded coil and wires, Densos. I did the IC, Unichip and crank pulley over 6 months ago. I have noticed the car runs much better than before the mods but one interesting "quirk" happens each tank full of gas (91 Chevron only). I fill up and things are ok until the guage reads 1/2 tank, then the power increases just like you throw a switch and add what feels like another 15 to 20 HP. This continues until I fill again and things go back to "normal" and the cycle repeats again. I know that as the tanks empties it would equal less weight, but this does not come on gradually, it's like I said, sudden and happens at 1/2 tank. I actually went in to a station yesterday with very little fuel and put in about 6 gallons. The guage read between 1/2 and 3/4 full. As soon as it moved down to 1/2, up went the HP. I am using the "B" map on the Unichip and have no pinging problems. The exhaust tips are clean, with very little sooting. Any ideas?

 

Is your butt dyno miscalibrated? haha

 

My car is completely stock...... and just a couple of days ago I told my friend how odd it is that at 1/2 tank, It feels like the power "jumps up".

 

Perfect title because yes, this indeed is very interesting.

 

never fill your tank more than 1/2 way and the problem goes away.

 

Well, when the needle is at half, there is actually alot less than half in the tank.

I would guess all it is is just a lot of weight shed, and it takes a certain amount of weight loss before it's felt on your butt dyno (just like when 10hp aint enuff to feel on butt dyno, but 12hp is).

Alot of people don't realise how much, weight makes a difference in little engined cars. Every pound counts - trust me. The difference between 3/4 tank and just 1/2 tank for me on a local race track here is 3 seconds off my lap time, which is HUGE.

If you take out your back seats, I am sure you will have a feeling of being 12hp faster too.

Another thing that gives the S a little boost is when it's cold out. Man, 12hp right there.

 

is it possible your gas cap is not venting well? or would that toss a code? try opening the cap after you've used your half tank and see if it hisses sorta. i doubt it'd be the problem but why not . mine runs better full!! go figure.

 

As I said, it's not gradual so using up fuel and getting lighter is not what I'm experiencing. I'm not complaining, just befuddled.

 

yeah on reflect the cap thing would get worse not better.