On speed density cars will adjusting the fuel pressure change mixture? If I increase fuel pressure to richen the a/f will the computer compensate?

 

I assume you're still trying to figure out your Jeep Cherokee..... although it would be good to include that info if that is the case. Speed-density is a method of calculating engine mass air flow. It does not set a standard for how the ECU will react to changes in fuel pressure..... what is valid for a 93/speed-density LT1 will not necessarily hold true for a Jeep Cherokee. The A/F control structure is unique to the way the system was designed for a specific engine, not for all speed-density applications.

If the ECU has a closed loop control sytem, the ECU is going to compensate in closed loop for the increased fuel pressure. It may not compensate at WOT, depending on the control strategy and the logic for selecting long term fuel corrections. I doubt you can richen the mixture in closed loop, and to meet the NOx specs during emissions testing, your engine will be tested under conditions that are exclusively typical of "part load/part throttle" closed loop operation.

Richening the mixture will reduce NOx, assuming that the reason there is too much NOx is that it is running lean. Richening the mixture will drive up HC and CO, so if you aren't real low already on HC and CO, you may just find you solve the NOx problem, and now have other problems. Reducing the coolant temperature would also help to control NOx, and hurt HC and CO.

 

Thanks for the detailed answer. Yes, I did ask this question to get information for my Jeep. The problem is that cylinder pressures are higher than spec (should be 130-150 psi, I have 175) and my shop is running out of answers to getting this car to pass emissions testing. I was just trying to explore any possibility of lowering cylinder temps and thought richening the mixture might do the trick. Someone has suggested running some top engine cleaner through it so I'll run that past my mechanic next.