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How-to: Tuning the Jeep Carbureted  Inline Six Stroker Motor!

Carter BBD carburetor found on stock 1981-90 CJ and YJ Jeep models.

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....

     1) The common stroker build uses a 4.0L block and 4.0L cylinder head (for fit and the improved combustion chamber design). If you build a stroker six for your CJ or an '87-'90 YJ Wrangler, first inquire about local emission standards and test requirements.

     2) Older (232/258) intake manifolds will not fit a 4.0L (low- or high-port type) cylinder head! If you must meet local emission requirements, it would be advisable to prototype your stroker six after a '91-up 4.0L engine with a high-port cylinder head. Meet the fuel-and-spark management system requirements with either a Mopar Performance MPI/EFI Conversion Kit or retrofit '91-up 4.0L MPI components.

     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.

     4) The OEM type BBD carburetor is best suited for a stock 258/4.2L six. (See the how-to on tuning and rebuilding a Carter BBD carburetor.) 

     5) For a stroker inline six, consider EFI/MPI—or at least an EFI/TBI retrofit. In addition to the emission-legal Howell conversion, MSD and CompCams now offer TBI retrofit packages.

     6) If your Jeep is intended for public road or highway operation, make sure that the carburetion or EFI retrofit system complies with all emission laws—before you purchase expensive parts! Be certain the vehicle will pass an emission inspection and test.

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. 

Clifford intake and header with a jetted Weber carburetor

     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.

     Another formula for carburetor flow at wide open throttle (WOT) is: 

CFM = (RPM x CID) divided by 3456

     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.

  *Note: How often would your trail-based Jeep 4x4 see 5,000 rpm? This is certainly a reasonable rpm ceiling for non-racing use.

     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.

     Note: In my experience, an engine with a quicker torque rise (lower rpm torque peak) will be more fuel efficient at lower rpm. Driven from an idle to 2,500 rpm most of the time, the stock 4.0L peaks its torque at too high an rpm (3500-3600 rpm range). This hinders fuel efficiency and the engine's "stump pulling", bottom end power.

     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.

     Advice: For four-wheeling on rocky trails, maximum horsepower is not as important as the available torque at lower rpm. High manifold vacuum at lower rpm, the option of using lower octane (i.e., affordable) fuel and steady rockcrawling performance are each desirable for a Jeep 4x4 driven off-pavement. If you build a stroker with high compression and a hot camshaft, expect low speed bog, loading up at crawl pace plus 'ping' or detonation issues—especially on lower octane fuel.

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.

     Note: Retrofitting a carburetor requires a manifold adapter from a source like Summit Racing, Trans-Dapt or Clifford Performance. Safe throttle linkage modifications and other hook-up details also apply. A Clifford or Offenhauser intake manifold should be used for improved induction flow with 350-500 CFM carburetors.

Trans-Dapt makes carburetor adapters like this 2300 Holley to smaller BBD-type base...Photo courtesy of Trans-Dapt.

Two-barrel possibilities:

     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. 

     Note: The Kaiser-era Buick/Jeep 225 Dauntless V-6 and Buick/Jeep Vigilante 350 V-8s each used this carburetor design.

Rochester 2G (shown) and 2GC were reliable. This is a stock Buick/Jeep 2G unit. Use a carburetor from an engine in the 283-307 V-8 range.

     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.

Stroker motor would work with Rochester carburetor and an adapter to Clifford or Offenhauser manifold—or at least the Carter BBD manifold.

     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.

     Note: Avoid electronic feedback versions (the M4ME types). Look for 4MC and 4MV types. These are spread-bore designs that can deliver exceptional fuel efficiency on their primary bores. The Quadrajet is also far less altitude sensitive than Holley and other designs. Install an aftermarket brass float when rebuilding these Q-jet units.

Carter AVS on a Dodge Dart GT small-block V-8

     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.

Carter AFB four-barrel from Muscle Car era

     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! 

     Note: If you are familiar with these aftermarket designs and their individual tuning features, stick with a four-barrel in the 390 to 450 CFM range. Match CFM to the horsepower, using the formulas in this discussion.

     Caution: Unless your stroker six is built to racing standards for JeepSpeed™ or other out-and-out desert competition, avoid using a 500 CFM "universal" two-barrel—like Holley offers in the 2300 design for restricted racing applications. Keep in mind that a two- or four-barrel 520 CFM carburetor will sustain a 300 horsepower engine at 6,000 rpm! This would not be a suitable carburetor for the milder 240-260 horsepower rockcrawler! 

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.

Prestolite breakerless ignition should be avoided. Later Motorcraft distributor is much more reliable. 

     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.

This is a Jeep/Motorcraft distributor, which can be used without the ECU.

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.

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