The MEFI 4 Ram Jet 350 harness comes with a 50-amp Maxi fuse in a water-resistant holder. The old MEFI 3 harness just had a short wire that you had to extend to the battery. We’ve heard of some folks who forgot to fuse this wire on MEFI 3 installs and paid the price of a dead controller because of a current surge due to a shorted wire.
A MEFI system has two of these plugs, a J1 and a J2. This MEFI 3 plug set has open connector points. The MEFI 4 plug set uses all of the connector points. This is not the only difference. Comparing the MEFI 3 and 4, most of the connections points are used for different feeds. For example, on the MEFI 3, the J1-4 connector is the ECM ground, but on the 4 it is for the Emergency Stop switch. Another example: On the J2 connector, the MEFI 3 uses the J2-19 connector for the TP Sensor 5-volt Reference, and the 4 uses this for the 12-volt ignition feed.
The New MEFI 4 controller harness has three separate ground wires; the MEFI 3 harness only had one. These wires are marked with different ground locations to attach them to. The extra and different ground locations allow the new controller to gather and possess more data and to process all data more accurately.
If your Ram Jet doesn’t already have a serpentine belt accessory drive setup, consider installing one while you have everything apart to install the MEFI 4 upgrade harness. A serpentine belt system is more efficient, and the belts last longer than older V-belts. GMPP offers two accessory drive kits that fit the Ram Jet 350 and other small-blocks. PN 12497697 has everything you need to convert a vehicle without air conditioning.
Open Loop VS. Closed Loop
In an open-loop EFI system, a single programmed fuel curve is used for all conditions. The ECU takes in data from the few sensors the engine has, and guesses at the correct air-to-fuel ratio (AFR). It then adjusts the AFR as close as possible, using the limited fuel map (program) it contains.
With a closed-loop system, the computer constantly monitors input from several sensors. These include one or more O2 sensors, an intake or manifold air temperature sensor, and others. It samples more data more often, and constantly makes adjustments to the AFR, timing, and other factors. The fuel curve and timing can be adjusted over a much wider band than with an open-loop setup. The result is an engine that produces more power, gets better mileage, and doesn’t make your eyes water from a rich condition, or knock and overheat from being lean.
Green Mode Break-In
The MEFI 4 controller comes programmed with what GMPP calls a “green mode” to control the rpm for the break-in period. This period is as follows: From startup to the end of the first hour, engine speed is limited to 4,000 rpm. For the second hour the limit is 4,500. The third hour is limited to 5,500. When converting to a MEFI 4, your older engine will be limited during this break-in period even though it is already broken in. After all, the new brain doesn’t have a way of knowing the motor isn’t new too. So don’t worry. Sit back for a few hours of easy driving and then enjoy the new power of your MEFI 4 upgrade.
Improving the Breed
The Ram Jet 350 is a true plug-and-play, fuel-injected crate motor. This Chevy small-block originally came with a MEFI 3 controller that only operated in open loop. This resulted in an engine that ran rich, underperformed, and made your eyes water at idle. The new engines come with a MEFI 4 controller, which allows the engine to operate in a more efficient closed loop. If you have a MEFI 3, you can convert the controler and harness to MEFI 4 with GMPP’s upgrade kit (PN 12499116).
Painless Performance Parts offers aftermarket harnesses for GM, Ford, and Jeep vehicles that also utilize the MEFI 4 controller. These controllers utilize a wideband O2 sensor and are great for those who are not running a Ram Jet engine. They also offer calibration tools and programming software for the MEFI controllers. Contact them if you are a DIY type and want to do some fine tuning.
GM Performance Parts
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