tuning and headers [Archive] - GrandAmGT.com Forum

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Saber326
10-23-2007, 09:53 PM
im not looking to do much else to my car but i was wondering if i put on headers, a larger downpipe, CAT and larger exhaust if i would need to get a performance tune. im not really sure because the computer considers everything going INTO the engine (except o2 sensor for feedback). the only difference here would be less backpressure. i think less exhaust will go out the intake valve and more out the exhaust during valve overlap than stock levels. this will increase pressure pulses in the intake runners and cause the engine to run leaner(higher AF). and at the same time, less exhuast gas will remain in the cylinder after each cycle and raise o2 levels in the exhuast gas. not really sure if my stock o2 sensor would work properly with my stock ecu anymore since the pressure will be lower (due to larger pipe), and therefore less oxygen per unit volume which would provide feedback to the ecu to lean out the micture (higher af). the o2 sensor feedback is based on the stock exhuast header/downpipe configuration. therefore, anything modified before the o2 sensor (before reguarding the direction of exhaust flow) should give the computer readings lower than actual. so therefore would it make my af higher because it thinks too little excess oxygen is in the exhaust gas? has anyone done this without a tune and noted any differences in AF ratio? i would really appreciate it if someone could answer all these questions and explain the actual effects it would have on each of these aspects i discussed, or if my theory on each of these aspects is correct. i dont like doing modifications to my car without knowing every single aspect of the engine it will affect lol. im already running lean and i cant do anything that will make it run leaner without a tune to richen it up.

eric99gt
10-23-2007, 10:40 PM
Ummm drumking....stfu with the search thing. What is he supposed to search for? It's not like he asked if these rims will fit. To begin the search to your answer, I would throw out that most people who have similar mods to the ones you have mentioned have noticed considerable gains with a tune. Honestly even a stock car gets better with a tune.

silvergtjrad
10-24-2007, 05:26 AM
Your car will run fine but a tune will make it that much better. What other mods are on your car? Get in touch with mark from MP racing, he can do a somewhat "custom" tune via mail. His user name on here is lvemy3100

Saber326
10-24-2007, 11:39 AM
i have cold air intake, irridium plugs, tb coolant bypass, and my exhaust consists of the stock exhaust with the muffler removed and a straight pipe put in its place with a resonating tip. those are the only performance mods ive done. when i changed the spark plugs, the outer electrode was pretty white an i really dont think i can run any leaner. again, my question is: can i do this without making my car run leaner? i dont really want to spend extra $200 on a tune cause the new exhaust system will already cost me a pretty penny. so i was thinking on either getting a tune or getting headers. cause if i have to get a tune when i put on the headers ill just go with the tune alone. if i go with the tune, ill probably just buy hptuner cause in like 9 months im going to get a c4 corvette and i can use hp tuner on that as well.

Sprucegagt
10-24-2007, 11:54 AM
You can't change the cruise AFR. It will run about 14.7:1 (average over fluctuations) but can swing a little higher to 14.8 or 14.9. It's nothing to worry about for idle and cruise conditions. Now don't confuse the look of the electrode with AFR during full throttle conditions. During full throttle you need to see the base of the insulator not the electrode.

http://www.dragstuff.com/techarticles/reading-spark-plugs.html