Mine is 3418623 but I don't know that it is original to the car. Outlet inside diameter is 1 7/8". Is there a correct right side 1972 manifold with the larger 2 1/4" diameter?
Nope!! 70 &'71 and up were 1 7/8". If you not going 100 original, I'd swap it for a 96/97/98 Jeep Grand Cherokee passenger's manifold.
And the matching left side manifold is 2.25", is that correct? Is there any right side 2.25" manifold that exits straight down in the center like 3418623? I think I have seen some dyno numbers with different manifolds, but I can't find them. What does the smaller outlet translate to in terms of HP or performance?
There are no dyno numbers for the JGC manifolds (that I have seen). The JGC is 2 1/8" which is as close as you're going to get without spending lots of money. The individual port are pretty good size also. The JGC does not exit in the center like the 623. It exits a little more towards the rear where there is plenty of room/space. You can easily grind out the JGC to more than 2 1/4".
I usually go for performance over originality but that is not the case with my current project so the Jeep option is out. I might consider an earlier 340 manifold if it is similar in appearance but if the asymmetric exhaust size is what the car came with, then that is likely the way I will go.
The 68-70 340 passenger side manifold looks more like a header. Very different than the 71 and up models.
The 68-70 numbers are left side 2883553 and right side is 2863549. But they go for $ 600.00 a pair now days
What's the deal with the L1 and L2 designation near the part #s on the 1970 manifolds? And, I've seen a pair that both have the L1 marking.
Don't pay 340 prices for the 3418623 manifold. C bodies used that manifold from 71-73 on the 360's. Fairly plentiful.
They're hard to give away because of shipping costs.....I have one hanging outside, it's used as a counterweight!!
71-73 left side manifold had a 2.25 inch outlet, right side 1 7/8 outlet. But the pipes on both sides were only 1 7/8 (or 2 inch?) for the first couple of feet, then fed into 2.25 inch pipes. Not only that, but the driver's side pipe was indented for torsion bar clearance. As for performance, I'm surprised someone hasn't posted the Mopar Muscle (I think) dyno testing of a 300 hp crate Magnum engine with different exhaust. According to them, the best 340 exhaust manifolds were only marginally better than LA 318 manifolds, and headers were only maybe ten hp better. To that I would just cite my own experience: Back in the late seventies, I put a set of cheapie headers on my 73 Duster 340 that was stock except for a 71 intake and a 650 spreadbore Holley plus the factory heat riser was removed from the stock passenger side manifold. I didn't replace the rest of the exhaust system, which was completely factory original down to the original mufflers. The headers were worth seven tenths of a second in the quarter. Actually they were worth more, but now it had so much more low end torque that now I had a traction problem that I had not had before. Shifted at 5200 with the stock manifolds (and there was no point in going any further), shifted at 5800 with headers. I had similar results more recently when I replaced the 68-70 340 hi-po manifolds with TTI headers in my 67 360 Barracuda with a slightly hotter than stock cam, Edelbrock RPM intake and 750 Holley, plus in this case I also replaced the 2.25 inch muffler shop exhaust with TTI 2.5 inch mandrel bent X pipe. Again, a huge gain in low and mid range, plus significantly more top end. So much more power down low that I couldn't give it full throttle in low gear without going sideways until I started running drag radials on the street. I doubt that replacing the OEM 72 passenger side manifold with the early hi-po 340 manifold would really make much difference. At least, it didn't on my 65 Barracuda when I made that switch. My two cents. Your experience may differ.
The cheap headers (Black Jack, I think) I put on my 73 Duster were 1 5/8 inch. The TTI's I put on my 67 Barracuda back in 2012 or so were 1 5/8 to 1 3/4 step headers.
