258 upgrade

by George Zeman

    After about 1 1/2 years, my Eagle project has reached completion. The new motor is in and running!! While it is still your basic six-cylinder 2-bbl, it has some neat upgrades that may be of interest to others. OK, here goes.

    Car: 1982 Eagle LTD four-door sedan, 258/2bbl/auto/2:35 rear, all the LTD bells & whistles

    Purpose: To replace old and very tired (previously abused) motor with something better. Not a “racing” motor, but something that would be streetable, and reliable, reliable, and reliable. More power is also nice!

    Driver: My wife, Newassa


    Block: Out of an ’80 Spirit. This is the last year of the “unlightened” blocks, and the only one with the proper mounting bosses for an Eagle. Has the cast iron oil pump, and the 12-counterweight crank.

    Bore: +.040 for 265 cid. Balanced with new damper and flexpale.

    Motor Mounts: Red polyurethane from Quadratec. (Note: stiffer than stock, so more vibration is present, but only really noticeable at idle.)

    Cam: Crower “Baja Beast” #44915

    Timing Chain: Mopar True Roller

    Water Pump: Flowkooler high-output, fits right up, but Eagle owners be warned, the PS bracket mounting boss has to be machined about .125 to fit your belt tensioner bracket.

    Head: From an ’82 Concord, milled to achieve 9.0-to-1 compression ratio. Has SBC guides, springs and valves, 1.84” interior/1.5” exterior. New stock rockers and bridges and Ford 460 pushrods (the closest ones to the proper geometry, but the fulcrum flats still had to be machined a couple of thousandths).
    Please note that to mate an ’81-up head to an ’80-prior block, the head bolt holes have to be enlarged to 1/2” from 7/16”. Use stock ’80 head bolts on outside of valve cover, and BB Ford on inside, as the stock 80 ones are too long.

    Valve Cover: Aluminum from 4WD Hardware.

    Engine hardware: New stainless steel from MMS Exhaust, stock with new exhaust manifold and front pipe.

    Cosmetics: Block painted with proper “late” AMC blue, all aluminum parts and all pulleys/brackets, etc., bead-blasted and refinished. This thing looks sharp!


    OK, Still with me? Here’s where things get really interesting. I wanted to clean up the (really crowded) Eagle engine bay and improve performance and reliability, so I decided to scrap the entire stock ignition system and all of its related paraphenelia.

    I also went with the new HEI distributor from DUI in Memphis. It’s a little pricey ($350.) but it is hand-assembled for you, and custom-curved for your particular engine/car/gear combo. Easy one-wire hookup, and has the positive-lock male type distributor terminals. For this, I went with a set of Accel Spiral Core wires (for a 91+ 4.0) and Motorcraft plugs, opened up to .45” Looks great too!

    I wanted to get away from that cursed electronic Carter BBD, (though ours was actually running OK), and its dependence on all of those stock Eagle vacuum switches and now way-obsolete feedback computer. The choices were to either move forward to modern fuel injection or backward to a simpler carb design. Since I could not readily afford the former, I went with the latter.

    I have adapted a Motorcraft 2100 2-bbl to the 258 six. I got this idea from Pierre Bochamp at AMRacing up in Quebec, in a NAMDRA article, and through all this, Pierre was a great help. He even opened up the ports on my intake and smoothed the adapter to manifold transition for a very reasonable sum. Thanks, Pierre!

    The Motorcraft 2100 is the carb that most AMC V8s came with. It is cheap, brain-dead simple to rebuild or tune, and very, very reliable. Also, I have about five of them in my basement! It’s also a good performance upgrade for the 258, as it flows about 300 cfm, versus the BBD’s 200-220 cfm. Can’t lose!

    Using the stock ’82 aluminum intake and an Offenhausser carb adapter #5451 (about $40), the 2100 bolts right up! I needed a good 1/4” or so “insulator”-type base gasket, and the whole assembly is actually about 1/2lower than the BBD asssembly. So no hood clearance problems! I had originally wanted to use the stock 2100 phenolic spacer, but that won’t work on the Offy adapter, and using it on the base gasket would have increased the height by about 3/4”, so I left it off for now.

    The carb I am using is from my ’78 AMX 304, so should be jetted about right for a 265 with larger valves. Running great so far! No heat tubes, so I needed an electric choke, no problem, get one from any ’80s FSJ. Works great.

    The last hurdle was the linkage. I used two common 1 1/2L brackets from a local hardware store, doubled up for strength, massaged the holes a little to line up with the 2100 throttle plate, and added a ball end.

    I made the throttle rod out of two stock ones cut about 60/40, using the 60 part, threading both cut ends 5/16 x 18, and joining them with a 2” coupling and a couple of locknuts. Simple, elegant and looks factory!

    The stock BBD return spring bracket is easily modified to fit over the inside master cylinder mounting stud, and provides almost perfect distance and location for the stock 2100 return spring. Ya gotta love it.!

    Right now I’m running an Edelbrock “Pro-Flow” air cleaner, looks cool, but is only temporary until I get a real stock-style air cleaner made at my local fabricators. OK, that’s it!

    My engine builder estimates (conservatively) that this motor is good for 160 to 175 hp. That’s almost 4.0 territory and can be increased easily, say if I want to drop a Holley 2-bbl 500-cfm on it or something.

    I will say this, the old “huff and puff up any little hill” old lead sled is gone, this Eagle wants to rock!

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