When it was introduced in 1999 the Ram Jet 350, from GM Performance Parts, was hailed as the best crate engine to hit the market. It was the first production crate engine to be offered with plug-and-play fuel injection and 350 hp and 400 lb-ft of torque right out of the box. It’s no surprise that the Ram Jet 350 quickly became the engine of choice for Chevy V-8 conversions in Jeeps, Land Cruisers, and other rigs.
The only downside to the early Ram Jet was the open-loop-only engine controller, the MEFI 3 (multipoint electronic fuel injection, version 3). It seems that the brain of the system had some limitations.
GM Performance Parts addressed these issues around 2004. GMPP dropped the MEFI 3 controller, and now all new Ram Jet engines come with a MEFI 4. The unit is a true open-loop system. Just like your fuel-injected truck or car, the unit starts up in open loop and, after warm-up, switches to closed loop. With a closed-loop EFI system the computer constantly takes in input from several sensors, including the O2 sensor. 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 over heat from being lean.
For owners of Ram Jet engines with the old MEFI 3 controller, GMPP offers an upgrade kit. This kit consists of a MEFI 4 controller, a wire harness, an intake air temp sensor, and a heated oxygen sensor with mounting bung. This bung must be welded into the exhaust pipe, and the air temp sensor goes into the intake tube or manifold plenum.
The MEFI 4 only uses a single O2 sensor, much the same as the early OBD-I, throttle-body injection systems. Placement is best done in the first few feet of the exhaust pipe after the manifold. Jeff Abbott of Painless Performance points out that the Ram Jet MEFI 4 uses a narrow-band oxygen sensor and adjusts all injectors the same regardless of which bank the sensor is placed in.
You can’t simply take a MEFI 4 controller and plug it into your MEFI 3 harness. Most of the wires are in different places, and the MEFI 4 has a host of new ones. The inverse is true too: You can’t use a MEFI 3 controller with your new MEFI 4 harness. This means that your old harness and controller is not a backup option. It’s best to sell it to one of those boat owners who needs the MEFI 3 for an existing application. (MEFI 3 controllers are no longer made, so availability is limited.)
Installation of the MEFI 4 harness is actually quite simple. Most of the connections are the same as your original ones. There are several new connections that need to be made, but these are simple. You should be able to make the conversion in a weekend: one day to remove the old harness, weld in the oxygen sensor bung, and locate the air intake sensor mounting point, and one more day to install the new harness and MEFI 4 controller. One thing is for sure. If you liked your MEFI 3, you’ll love the new MEFI 4.
The Ram Jet 350 is essentially a ’97 Chevy Vortec motor with a custom cam and port injection EFI intake system. The engine produces 350 hp and 400 lb-ft or torque out of the box. It’s a perfect swap for Jeeps and other trail rigs. This is partially due to the smooth operation of the EFI in off-camber situations and at any altitude.
The Ram Jet 350 has been a popular engine for Jeep, Land Cruiser, and other motor conversion projects since its introduction in 1999. This small-block Chevy was the first production EFI crate motor with a plug-and-play harness and computer included. The MEFI 4 controller makes the Ram Jet even more versatile than the original engines with MEFI 3 controllers.
One of the key components of the MEFI 4 controller upgrade is the inclusion of an oxygen (O2) sensor. This sensor allows the controller to sense the fuel mixture and adjust it on the fly. This prevents the engine from running too rich or lean, improving performance, increasing fuel economy, and extending engine life.
The Ram Jet 350 MEFI 4 upgrade includes an IAT/MAT sensor. This sensor measures the intake air or manifold air temperature depending on where it’s mounted. On the Ram Jet 350 the sensor is mounted into the intake manifold air plenum. Also be aware that the parts bag says “IAT Sensor” and the harness plug tag says “MAT Sensor.” They are the same thing. (Note that the Ram Jet kit sensor has a 1⁄4-inch pipe thread and must be installed in a grounded location.)
The two MEFI controllers look the same. The only way to tell the difference is by the part number on the sticker on the back. A MEFI 3 (right) for the 350 says PN 12489488. The MEFI 4 (left) will either say PN 12499120 or 88962717 depending on whether it’s an older or later version.
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.
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