258 intake
elwood blues
Always Off-Roading Jeeper
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- 1978 CJ7 4.2L Comp Cam & valve train, MC2100 carb, MSD ignition, Dana 30 front, T18 trans, Dana 20 txfr, AMC 20 rear.
I'm going to start by completely disagreeing with what others have said. An AMC straight 6 is not a tractor engine, nor was it ever derived from one. The AMC straight 6 is a derivative of a 1950's Nash flathead 6 design. It was completely redesigned in the 1960's specifically for AMC cars and came in both 199 cubic inch and AMC 232 i6 cubic inch varieties. Later AMC 258 i6 / 4.2l cubic inch versions only had larger bore and stroke to the original design. There was an even larger 282 cubic inch that was not sold in the US.
Even the last year 2006 4.0L Chrysler L6 was still based on the original 1986/87 fuel injected version designed by AMC.
Depending on the year of the engine, max horsepower is achieved at anywhere between 4000 and 4400 RPMs, and max torque is achieved at between 1800 and 2200 RPMs.
To get to your question, the stock intake on all AMC version straight 6's is an open plenum design, one big hole for all air/fuel to go through. The point of the dual plane is to split the primaries and secondaries of your 4 barrel carburetor. Manufacturers wouldn't make them if they didn't work, and have engineers that tested them to work.
If you plan on going with a 4.0L head on your AMC 258 i6 / 4.2l block, all of that is irrelevant anyway, you'd want to stick with the factory fuel injection that came on the 4.0L and not switch it back to a carburetor. The factory aluminum intake manifold on a 4.0L head is a far superior design to the factory cast iron AMC 258 i6 / 4.2l intake manifold any day. Plus the fact, as least as far as I know, there are no aftermarket intake manifolds for a 4.0L head. There are a few different places that make headers for the 4.0L head.
So, I guess I'll bounce it back to you, what do you plan on doing? Running the 4.0L head, or sticking with the stock head? If you stick with the stock head, then we can go further down the "can a AMC 258 i6 / 4.2l run a 4bbl carburetor" rabitthole...
