Ok maybe I'm just dumb but I've got my TBI LO3 like so many of us and I was thinking about increasing my fuel pressure. I've seen many posts and technical articles about this and its relation to air/fuel ratios and lean/rich stuff. I've got an SLP y-pipe, random cat, and a Flowmast cat back, all that xtra HP and air out should've leaned out the motor right, no. I've got an open element with the K&N xstream top, all that more air in should've leaned me out too right, no. I checked the output voltage from the O2 sensor and used the field diagnostic test mode in the ALDL to test the a/f ratio and it is dead on. I've read that my ECM has the ability to learn and adjust things like the a/f ratio to keep everything in check. So if I increase my FP it shouldn't have any effect on the a/f ratio once the ECM figures out what I've done, right?? My TBI is totally stock, never did any mods to it, I don't have a chip. Also my car is relatively fast, a 15.5 at 88 with 2.73s in it! Any info about this at all would be greatly appreciated.

Frik J Maser
1992 RS LO3
Full exhaust minus headers
Full suspension minus lowering
All emissions equip intact and functioning
15.5 at 88mph Cecil Cty Dragway April 2002

 

The computer can compensate only to a certain extent. If your air/fuel ratio is dead on, don't mess with the pressure at all. When you start to go lean, then mess with it. You have allowed your engine to breath a little bit more, but not enough to where it's throwing the computer off. Even if you install headers, you should still be ok. Once you change the intake manifold and/or cam is when you will need to adjust the chip and fuel pressure.

 

Thanks
"Don't fix what ain't broke"

 

You need to think of fueling at two different levels, one is part throttle and the other is wide open throtttle. Your O2 sensor checks for part throttle mixture are fine but at WOT they wont work. At WOT the ECM does not use the oxygen sensor. This is because the oxygen sensors that came stock in our cars are really just switching devices. They very accuratly tell the ECM if the engine is running at either above of below 14.7:1 air fuel ratio. Yes, you can monitor your voltage durring a WOT run but if it says 600mv or 900mv all you really know is that you are richer than 14.7:1. You need a wide band oxygen sensor to accuratly monitor air fuel ratios. These sensors will report the actual air fuel ratio, not just act as a rich/lean indicator. Be cautious of some of the aftermarket air fuel ratio units. Many of these use the same type of sensor your car uses and are not accurate.

If you want to do some real testing to see if you need more fuel I'd suggest doing the adjustable fuel pressure modification to your car in addition to installing a fuel pressure gauge. You can then do WOT testing and measure the before and after results of your fuel pressure changes. You can use something like a G-Tech or the race track. Make sure you get repeatable results before you start and keep as many things the same as you can... use the same lane at the track, use the same strech of the road in the same direction with the same weight if using the G-Tech,engine temps should be close, etc.