Moses Ludel’s 4WD Mechanix Magazine – How-to: Tuning the Carbureted Jeep Inline Six Stroker Motor
Important: Many 1972-90 Jeep inline six models still require periodic emission control inspection. To avoid failing an emission inspection, make sure that the carburetor and ignition system comply with local, state and federal requirements….
3) Clifford Performance does offer a carburetor manifold for the 4.0L engine (cylinder head). This manifold accommodates a variety of carburetor designs and also some of the aftermarket TBI systems. Depending upon the application, a Clifford exhaust header is recommended or required.
Carburetor CFM Requirements
More horsepower requires a larger capacity radiator (horsepower = BTUs). Similarly, more horsepower demands more fuel and air flow.
In the carbureted era, an engine’s displacement determined the “size” of the carburetor. Regardless of whether a one-, two- or four-barrel carburetor, the venturi or bore flow must match the engine’s displacement.
Years ago, Jack Clifford and I were talking inline sixes and their carburetion needs. Jack shared a simple ‘rule-of-thumb’ for choosing a carburetor: For an engine to run at 4,000 rpm, the CFM (cubic-feet-per-minute) flow should equal the engine’s displacement figure.
Allowing for a higher ceiling than 4,000 rpm, a 258 Jeep inline six requires a stock carburetor around 350-375 CFM. 400 CFM is enough for ‘in the day’ build-ups of a 258 inline six, running a Clifford 264H grind camshaft and a header.
Today, Clifford Performance recommends a Holley 390 CFM four-barrel or a properly jetted Weber 38-series (shown above) for engines running a mild cam and header. Clifford provides base settings for a Holley 390 CFM four-barrel.
Here, for a 258 six run to 5,000 rpm*, the carburetor needs 373 CFM. Again, a 400 CFM carburetor would be plenty for a built-up 258! (405 CFM would handle a 280 cubic inch stroker motor running at 5,000 rpm.) For margin, a 500 CFM carburetor could sustain a stroker 4.6L or 4.7L inline six—balanced and running a warmer camshaft—at 6,000 rpm.
In most cases, I recommend a four-barrel design, as bigger two-barrel types are more difficult to tune. A trail Jeep 4WD’s inline six can run on the four-barrel’s primary bores—most of the time.
Fuel Efficiency and ‘Gas Mileage’
Suffice to say, a larger displacement engine does require more fuel flow ability. This should not be confused, however, with “using more gas”. A properly built stroker motor should not be fuel-hungry.
Arguably, driven in the same way, a Jeep 4.0L built to a 4.6L stroker inline six might actually gain fuel mileage. With a mild camshaft and compression, the torque peak rpm drops to a much lower point. The engine produces usable power and higher manifold vacuum at low speeds.
The 258 boasts a torque peak in the 1600-2000 rpm range! This is much like a diesel engine’s torque rise, making the 258/4.2L inline six a world class ‘stump-pulling’ gasoline engine. The 4.6L stroker inline six, built properly, should produce considerable torque at a reasonable (i.e., lower) and useful rpm.
These stroker engines, especially for combined off-pavement rockcrawling and highway use, should be built for a quicker torque rise and high torque output at lower rpm. This means use of a milder camshaft and lower compression ratio.
Summing Up the Carburetor Options
Even for a moderate (240-260 horsepower) stroker build-up, fuel flow needs increase. At 280 cubic inches, a carburetor of 400 CFM is plenty.
When selecting a carburetor, I prefer an OEM design with a proven use in an engine of similar displacement. A pristine carburetor, with its original jetting, metering and tuning components intact, is an optimal find.
1) Rochester’s 2G, 2GC or 2GV from a Chevy 283 or early 307 V-8 application would make sense. The 283 or 307 Chevrolet V-8 passenger car or light truck carburetors, with their original jetting, would be well-suited to a 4.6L inline six.
2) The 273/318 Mopar V-8—A very effective two-barrel for the 273/318 was the Carter BBD. (This was not the feedback BBD design used on the later 258 Jeep six!) Though a two-barrel, these ’60s era 273/318 BBD carburetors would be a possibility for the stroker inline six. Carter’s #4113, #4114 and #4116 are typical truck versions within this model group.
3) 304 I-H V-8—These 2300 Holley two-barrel carburetors actually work well. (The 266 V-8 application, if you can find one, is excellent for a stock 258 six.) The 304 V-8 version could work for the 280/4.6L stroker inline six. This carburetor must be original, not a “rebuild” or cobbled mix of Holley pieces. Available in manual and automatic choke varieties.
4) 289 and 302 Ford V-8—Autolite or Motorcraft 2100 series carburetor from the mid-‘sixties to 1971. The 2100 was also used by AMC on 304 V-8 Jeep engines to good success. The Jeep 304 V-8 version, not the 360 application, would be correct here.
OEM four-barrel carburetor options:
1) General Motors’ smaller V-8 engines like the 305, 262 and even turbo versions of the 231 V-6 were often equipped with Rochester Quadrajets during the late smog era. A non-feedback, late ’70s carburetor from this group would closely match with the 4.5L to 4.7L stroker’s needs.
2) OEM Carter AFB (below) and AVS four-barrel (above) carburetors were popular on many G.M. and Chrysler V-8 engines of the Muscle Car Era. You want a unit designed for small displacement V-8 engines like the performance 273/318 Mopar applications. As a yardstick, avoid any carburetor tuned for an engine with more than 318 cubic inches.
3) There are a variety of “generic” performance carburetors from Federal-Mogul (Carter label), Holley and Edelbrock. When a four-barrel carburetor fits V-8 engines of 350 cubic inches and bigger, this is not what you need for a 280 cubic inch inline six!
Footnotes: The Ignition System
Carbureted 4.6L stroker motors (or the 4.5L and 4.7L bore variations) can run a conventional electronic distributor. There are aftermarket units available from MSD, DUI and others. Some comply with emission control standards—check before buying!
If you run an aftermarket or modified carburetor system, the best ignition is an aftermarket HEI or electronic type. By the early ’80s, the Jeep inline 258 becomes a closed-loop, emission compliant fuel and spark system that involves a Motorcraft distributor and a Carter BBD carburetor.
The stock Jeep Motorcraft distributor will work as a conventional distributor if you are on a tight budget. In my Jeep Owner’s Bible and other Jeep books, I describe the use of a Motorcraft distributor with a Chrysler five-pin module from the early Mopar electronic distributor era. This eliminates the failure prone Motorcraft module and also the Jeep ECU module function.
Wiring the Motorcraft distributor to a five-pin module and eliminating the ECU interface, this electronic, breakerless distributor offers both centrifugal (mechanical) and vacuum advance mechanisms. The advance curve is mild and suitable for a street-and-trail driven Jeep 4WD application.