Originally Posted by farna There is another way -- an adjustable MAP sensor. Try a google search. Basically it's a finely adjustable power supply inserted into the MAP power line. The MAP usually takes around 5V, but by SLIGHTLY increasing the voltage to the MAP, it will increase fuel supply a slight amount. This isn't a substitute for bigger injectors, but a fine tuning tool.
For example, on the Jeep 4.0L MPI engine, bored 0.030" over and using a 258 (4.2L, 0.44" longer stroke) crank, you determine that you need 24.6# injectors. No one makes them -- it's 24# or 26#. Most of us opt for the 24# then increase the voltage to the MAP to around 5.15V. That changes the reference voltage of the MAP and increases fuel delivery slightly -- across the entire fuel map in the computer. 26# would run slightly rich.
I don't think I'd go over 6.0V, and I don't know anyone who's running at more than 5.5V. I'm also not sure the engine will run if the voltage is below 4.5V -- the Jeep computer seems to sense a low voltage as a faulty MAP and shuts the engine off, but doesn't care about a higher voltage. Or maybe it's just turning it down that low is decreasing fuel to the point it won't run, but I doubt it.
You did mention the five speed in place of the auto, and chugging "under 2000 rpm". Try down shifting. The little engine isn't making enough power to cruise in fifth at 2000 rpm. If you get a vacuum gauge, you can see as you drive where the engine makes power. Basically you want to hold speed while producing the most vacuum for the best cruising speed and economy. With the auto trans it was turning a bit more rpm.
My 4.6L six in a 3700-3800 pound car doesn't make good power under 2000 rpm with an OD auto trans (0.70:1 OD ratio, 3.55 rear axle). According to my vac gauge, the engine will pull up to 25" of vacuum. Cruising at higher speed it pulls about 20" in OD, around 65-65 mph. If I increase to 70-75 mph, it will drop to 15". Accelerate or hit a hill, and it drops down to 10". At least in theory, the best economy is achieved at the highest vacuum reading. That means the throttle plate is closed more, or I'm using less throttle to maintain speed. If I drop below 1800 rpm on a level in OD, the vacuum drops considerably just maintaining speed or at the slightest hill. I can only maintain 15" or so of vacuum and have to drop to 10" on the slightest hill. That's around 55 mph. Out of OD I can maintain 50-55 mph without going over 2000 rpm and maintaining 20" of vacuum, and without falling under 15" just to maintain speed on an incline. With this in mind, I can safely determine that my torque peak is very close to 2000 rpm. You need to keep the engine near the torque peak for best power and efficiency, running to low rpm will actually decrease power and mileage.
I first ran 3.08 gears with the same car/engine/trans combo. The big in-line six had plenty of power to pull that ratio in OD, even down to 1500 rpm (it does now, of course), but vacuum was way down. The engine wasn't straining, but would "chug" a little on takeoff at that low rpm. So I installed the vacuum gauge, and then changed to the lower rear gear ratio (3.08 to 3.55), and stopped trying to run a steady speed under 2000 rpm. I used to shift around 1500-1800 rpm, but now wait until 2000-2200. Fuel mileage has improved noticeably, and the engine is much more responsive. Your little 2.8L V-6 probably peaks closer to 2500-3000 rpm. So try increasing rpm at cruise, and shifting at a higher rpm. With the OD five speed you may need a lower geared rear end. Most old cars with manual OD transmissions used as low as 4.11 rear axle ratios. This car originaly had a three speed OD trans that I ran with the 4.0L and a 3.31 rear axle. That combo worked great -- though it feels a lot better with the 3.55. I was looking for 3.31 gears when I ran across a good used set of 3.55 gears... close enough! |
02-22-2006, 02:04 AM
