M Body General Discussion

  1. CM360

    CM360 Well-Known Member

    Likes Received:
    Sep 18, 2011
    Anyone use them on a M?
  2. Duke5A

    Duke5A Well-Known Member

    Likes Received:
    Apr 6, 2018
    Can't say if they fit or not, but if you have a set sell them to the resto crowd. They command a pretty penny and really don't flow that much better than any other manifold.
  3. BudW

    BudW Moderator Staff Member

    Likes Received:
    Mar 4, 2012
    Oklahoma City
    Both sides of the ’68-70 (or ’71) 340 HiPo manifolds (casting number 2863545 and 2863552) will fit any FMJ – but will caution that they will not pass the emission required states (cities, etc.). Note: the ’72-73 (and maybe ’71) 340 Exhaust manifolds look more like a slightly bigger version of 318 manifolds.

    The passenger side will require a 90’ oil filter adaptor to work – but (not sure of exact percentage) most FMJ’s already have one (if car was built with a 3-cat converter system).
    mp 5249624ab.jpg
    340 Rt side.jpg
    With exhaust pipe installed, an oil filter just won't quite go on straight.

    The drivers side exhaust manifold goes up and over the steering gear – which looks out of place in an FMJ vehicle – but fits and works fine.
    70 340-4.png
    You can see how the exhaust manifold covers the steering gear, in this photo (which was swiped from internet).

    I agree with Duke5A that if you have a set, they might be worth more money to the restro crowd – for some of those owners go wild over prices.

    There has been a couple of flow tests using those exhaust manifolds which say there is not that much advantage using the 340 HiPo manifolds – but logically, my brain hasn’t grasped how that can be.
    The only thing I can think of is the driver’s side must be where the restriction might be at (maybe) – for the passenger side is as straight of a shot a person can make.

    Flow tests show the Magnum exhaust manifolds flow almost as well as the 340 ones do. They also do not look as out of place (speaking about the driver’s side 340) as much in an FMJ – but the only Magnum manifolds that fit are the ’93-98 318/360 Jeep Grand Cherokee ones (casting number 53009376, 530093778 and not sure of the 96-98 Right side).
    The Dodge Truck/pickup, Van, Dakota and Durango with 318/360 Magnums (any year Magnum) extend rearward too far, causing the manifold and firewall to want to occupy the same real estate.
    318 Manifold L 76-80.jpg
    A typical Left 318 Manifold
    Mag L 93-98 GC.jpg
    Left '93-98 Jeep GC 318/360 Magnum
    Mag L 97-03 PU.jpg
    left '97-03 pickup (all but GC). Other years are similar.

    Personally, I highly prefer quiet over performance – and I really like both . . . a lot, but a cast iron manifold will always be quieter and will require less maintaince over headers - thereby my choice (over headers).

    I have a pair of the before mentioned 340 HiPo exhaust manifolds which I’ve owned sense ’83. The Left one has been braised where it was once cracked and Right one has the exhaust heat riser flap removed (poor weld job on the holes). I also have an ’94 Grand Cherokee Left side manifold (cast number 53009477) so if I ever decide to something for small block (FMJ), will use the Left side Magnum and Right side 340.

    FYI: the 340 manifolds mentioned in first paragraph, have 2-1/4” (57 mm) exits.
    The GC Magnum manifold, mentioned in paragraph above this one, the exit size is 2-1/8” (54 mm).
    I have measured the 318 manifolds before – but can’t find my notes, but if I recall correctly, the exhaust manifold exit size on FMJ 318’s varies between 1-3/4” to 1-7/8” (44.5 to 47.6 mm).

    340 Manifold pr 68-71.png