Moses Ludel’s 4WD Mechanix Magazine – How-to: Fuel-and-Spark Modifications for the Jeep 4.6L Stroker Inline Six

Fuel and Spark Needs for Jeep 4.6L Inline Six Stroker Motors!

Induction system of 4.0L MPI engine

The 4.0L inline six with a hybrid 4.2L crankshaft has become popular—and practical as well! The ease with which this engine can be built makes the “stroker motor” a must for Jeep owners facing an engine rebuild.

Jeep’s relationship with the 4.2L/258 and 4.0L inline six-cylinder engines bridges a span of 35 years (1971-2006). The 4.6L inline six hybrid is as likely to show up in a ’72-up CJ as a 2006 TJ Wrangler. Then there’s the 4.0L XJ Cherokee, with literally millions of units built between 1987 and 2001, each a candidate for the 4.5L-4.7L engine build!

This page provides an overview. There are two additional pages that detail the tuning needs of carbureted and fuel-injected 4.6L inline six stroker motors. This information also applies to the 4.5L and 4.7L Jeep inline six stroker build-ups.

4.2L six with stock Carter BBD two-barrel carburetor

  The 4.2L is the carburetion era, while all 4.0L inline sixes have multi-point fuel injection (MPI/EFI). In theory, the 4.6L long-block can work with both carburetion and EFI/MPI. Most builders, however, opt for the benefits of electronic fuel-and-spark management (EFI) for greater performance and distinct off-pavement drivability gains. This includes conversions to aftermarket throttle body injection (TBI) and the Mopar Performance EFI/MPI Conversion Kit for the 4.2L sixes.

  Note: The Mopar Performance MPI/EFI Conversion Kit is patterned after Jeep OEM mid-’90s 4.0L induction and spark systems.

Stroker Motor Tuning Demands

     Whether the engine is carbureted or fuel injected, “stroking” the 4.0L block to 4.6L six begins with a 0.030″ clean up re-bore to a new bore size of 3.905″ (originally 3.875″). Add to this a change in the crankshaft’s stroke length from 3.414″ (the stock 4.0L crankshaft) to 3.895″ (the retrofit 4.2L crankshaft).

To determine the engine’s new cubic inch displacement, let’s do the basic math. Where practical, we’ll round off numbers.
Begin with the formulas:
Bore Area = Pi times the bore’s radius squared
Volume of each cylinder = Bore Area times the Stroke length
Total Displacement for a six cylinder engine = Cylinder Volume times 6
Plug in the numbers for a 4.6L stroker:
3.14 (Pi) X 1.9525 (bore radius) X 1.9525 (bore radius) = 11.97048
11.97048 (bore area) X 3.895 (stroke length) = 46.62315 (volume per cylinder)
46.623 X 6 (number of cylinders) = 279.738 cubic inches (stroker 4.6L engine’s total displacement)

     So, we’ll round off and call this a “new” 280 cubic inch inline Jeep six! The original 4.0L inline six was 242 cubic inches. A 4.2L is 258 cubic inches. This is a 22 cubic inch increase over a stock 4.2L and 38 cubic inches larger displacement than a 4.0L!

Mopar Performance 4.7L Stroker Long-Block retrofit engine to replace the 232, 258, 4.2L and 4.0L inline Jeep sixes.

Given this displacement increase, these stroker motors add horsepower and torque. A stock MPI/EFI 4.0L will put out from 177 to 193 horsepower (depending upon the year and whether an “H.O.” design or not). The 258/4.2L six, in its final emission-laden and carbureted form, eked out a meager 116 horsepower. In stroker 4.5L to 4.7L layout (dependent on the re-bore size), the stroker motor will produce 240-300* horsepower!

 *Note: The horsepower spread represents a variety of stroker build designs. Various bore sizes, camshaft choices, compression ratios and cylinder head designs determine final horsepower output. In the Tony Hewes HD video interviews, we discuss two practical builds for reliable street-and-trail usage. View these videos before beginning your 4.6L build-up.—Moses Ludel
See the pages covering carbureted and electronically fuel-injected tuning details for the Jeep 4.5L, 4.6L and 4.7L inline-six stroker motor build-ups!