Atomic EFI: Jeep 4.2L Inline Six 'How-to'
At the 2011
SEMA Show, 4WD Mechanix Magazine video cammed new products for Jeep 4WD and Ram truck viewers.
We sought out the most innovative and useful products in the market.
on Jeep 4.2L/258 inline sixes has been a concern since the 1980s. At the SEMA Show, MSD’s hottest new item caught
everyone’s attention: “Atomic EFI” debuted, a retrofit four-throat fuel injection system that suits a broad range
of engine types and driving environments—including popular Jeep® 4WD
A bolt-on EFI
retrofit system, MSD's Atomic EFI has potential for AMC/Jeep six-cylinder inline and V-8 engines—plus many Mopar
V-8 engines! This installation enhances a trail running 4.2L Jeep inline six.
the MSD Atomic EFI unit houses many of the system's features. The precisely metered fuel injection package
includes the multi-component throttle body, a power module with wide-band oxygen sensor and a handheld
controller. To keep the package compact, the throttle body mounts the ECU at its side.
uses an array of sensor signals, similar to factory electronic fuel injection. A throttle position sensor (TPS),
manifold absolute pressure (MAP) sensor, engine coolant temperature sensor (CTS), the wide-band oxygen sensor and a
fuel pressure sensor are each part of the package.
Eliminating concerns about available fuel volume, the fuel delivery system features four precision, 80 psi
injectors. These injectors use a reliable stainless steel ball-and-seat metering method for flow control and
maximum internal sealing.
Avoiding the need for fittings that can leak, the built-in injectors use internal fuel rails. For complete
fuel atomization, Atomic EFI uses annular booster rings at the bore tops, which assures optimal fuel
mixtures. The result is a sleek, self-contained EFI unit.
When we first saw MSD’s Atomic EFI fuel injection at the SEMA Show, MSD
staff talked enthusiastically about the new system. They spoke of potential applications, mentioning Jeep engines
Note: Dan Hiney is the co-organizer of the
'Wheelers for the Wounded Rubicon Super Event'. Last year on the trail, I watched Dan's fine driving skill get
overshadowed by BBD carburetor woes—common to 1981-90 Jeep CJ and YJ Wrangler 4WD models. Months later, MSD's
Atomic EFI won a SEMA Show Award, and here wasa solution for Dan's Jeep 4.2L six-cylinder
engine...Dan Hiney and I collaborated on this ground breaking project. I did the research, Dan spun the wrenches
and took photos. We constantly compared notes. With just preliminary tuning, Dan tackled Sierra Trek's infamous
Fordyce Trail, intent on thoroughly testing our work. Findings reveal that Atomic EFI solves a host of issues for
Jeep 4.2L inline six-cylinder engines. Here are the details, exclusively
On the weekend of September 8th and 9th, the magazine covered
the "2012 Wheelers for the Wounded Rubicon Super Event" in HD video...Dan Hiney drove while I filmed the YJ
Wrangler—crawling effortlessly over the same rocks where the Jeep stumbled and stalled last
year...See the trail HD video and hear Dan Hiney's comments about theMSDAtomic EFI under the hood! For additional
information and Atomic EFI specifications directly from MSD, click here for access to the official MSD Atomic EFI website!
sanitary Jeep 'FSJ' Cherokee with an AMC/Jeep 401 V-8 and MSD Atomic EFI is among the proven, larger
displacement applications. Flexibility (100-525 horsepower using the same EFI unit and supply pump) inspired
us to do the 4.2L inline six-cylinder project. (Photo courtesy of MSD.)
universal throttle body unit bolts directly to a common square-bore carburetor intake manifold. Equally well
considered, Atomic EFI throttle linkage adapts to many carburetor and cable types.
See the HD
video clip from our 2011 SEMA Show coverage—below! The video can be opened at 1080P mode and full
For maximum durability, a backcountry must, the 1.75-inch diameter throttle inlets ride
in precision roller bearings. This provides a smooth pedal transition, highly valuable when crawling your 4x4 in
rocky, steep terrain!
Tuning has been simplified. The Atomic EFI’s TPS features a
self-calibrating, non-contact sensor—one less calibration detail when setting the system up. For electrical
circuitry, MSD incorporates the latest CAN-bus technology. This creates a single, reliable connection between
components and feeds accurate information to the throttle body-mounted electronic control unit
The MSD Atomic EFI package includes a remote-mounted power module and well-constructed
wiring harnesses. The weather-tight connections are an off-road must!
addition to the power module, the installer or tuner can set values for MSD Atomic EFI fuel injection
functions. This is the hand-held monitor and programmer.
the EFI components, wiring and fuel delivery components have been installed, initial programming takes only
five minutes. There are six questions to answer about your specific engine.
Following base tune, the engine can be
monitored with the programming device in real time. This provides engine data for fine-tuning or
troubleshooting the system. MSD's Atomic EFI even features a removable micro-SD card to record and store
data. This is a valuable tool for precise tuning, analysis or troubleshooting engine
The precise fuel delivery and self-contained nature of the MSD Atomic EFI
is impressive. A cure for carburetor woes, the MSD Atomic EFI Master Kit P/N 2900 deserved a Jeep 4.2L inline six
Compact design of MSD Atomic EFI
is clear in this application. Our project adapts the system to a stock 4.2L AMC/Jeep inline six two-barrel
project, we picked a stock Jeep 4.2L inline six in a 1989 Jeep YJ Wrangler chassis. The Jeep came equipped with the
BBD carburetor and a complement of '80s emission controls. Our target goals: better performance, improved fuel
efficiency and a cleaner tailpipe reading.
Makes Carburetion Obsolete
From an emissions
standpoint, EFI eliminates a fundamental carburetor issue: venturi effect. When a column of air passes
through the narrowed-down venturi area(s) of a carburetor, a low pressure zone or vacuum results. This pulls fuel
out of the float bowl via metering jets and passageways, into the carburetor’s venturi throat(s). Here, fuel mixes
with moving air to achieve a fixed proportion of air/fuel (AFR).
venturi effect is independent of throttle plate position, fuel continues to flow for a short interval under
deceleration, even with the throttle closed.This is not a function of engine manifold vacuum—it
is a manifestation of venturi effect.
is a brief, rich fuel supply to the cylinders, which raises major challenges for tailpipe emissions. An additional
obstacle is the conventional carburetor's fixed-jet air/fuel ratios. Despite electronically actuated fuel metering
(like the Carter BBD carburetor's "Sole-Vac" system), tailpipe emission standards call for drastic
is non-existent with EFI. When the throttle closes, the TPS, MAP and other signals switch off injection of
fuel—electronically and in milliseconds. There is no uncontrolled flow of fuel to enrich the exhaust gases
and raise emissions concerns.
EFI has played a vital
role in meeting U.S. E.P.A. and California (CARB) clean air tailpipe requirements. Through continual
electronic adjustment of the air-fuel ratio (AFR), EFI engines produce more
horsepower, gain fuel efficiency and live longer—with reductions in cylinder wear, valve
trouble and sludge.
requirements have progressed steadily since the 1960s. Only electronic fuel-and-spark management and catalytic
exhaust treatment devices could keep up with U.S. EPA Clean Air standards and California's CARB
to clean air, EFI has spurred a second muscle car era over the last two
decades. Worldwide, high horsepower and torque from smaller displacement, high-tech engines has become the
norm for cars and trucks. The Jeep 3.6L V-6
Pentastar engine is a good example.
EFI instantly sets
optimal air/fuel ratios for any driving condition. Pump gasoline burns best at 14.7:1 air-to-fuel ratio in a warmed
engine. This optimal burn or “stoichiometric” also reduces tailpipe emissions at cruise speeds. When accelerating
at wide open throttle (WOT), gasoline engines do require fuel enrichment; EFI/MPI can instantly provide precise,
enriched AFR simply through software mapping and sensor feedback.
This MSD Atomic
EFI wide-band oxygen sensor replaces the 4.2L OEM oxygen sensor. The Jeep OEM sensor sent feedback to a
microprocessor that adjusted fuel metering, spark timing and several smog control functions. The aim was lower
tailpipe emissions—not improved performance.
For OEM electronic fuel-and-spark
management systems, engine tuning is the constant, onboard computer adjustment of air/fuel ratio and
spark timing in response to engine loads, driving demands, altitude and engine temperature. Computer software
mapping meets performance goals and tailpipe emissions demands.
(Illustration courtesy of MSD, zoom-in for details.)
schematic shows full-function Atomic EFI with optional electronic spark timing control. If your engine type
has an MSD 'Pro-Billet' distributor offering, you can add the Pro-Billet distributor to the EFI package and
gain spark timing control benefits.
MSD Atomic EFI can be
used as a fuel-and-spark control system. A modified two-pin Pro-Billet MSD
distributor with an MSD 6-series digital box enables Atomic EFI to control spark timing. Functions include
idle timing stabilization for a smoother idle plus the ability to set the spark timing degree range and
rpm limits, essentially building the spark timing "curve".
*Note: A Ready-to-Run distributor eliminates the need for a 6-series box. If considering a 'Ready-to-Run'
ignition distributor, consult with MSD about whether the Atomic EFI spark timing control option will
interface...The 'Pro-Billet' is available for AMC/Jeep inline sixes and V-8s, and the Pro-Billet does work with
both the Atomic EFI spark timing control function and the 6-series digital box.