Cast-Iron Exhaust Manifold Improvements

Cast-Iron Exhaust Manifold Improvements

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Chevrolet, G.M.C. & Buick Speed Manual
Chevrolet Inline Six-Cylinder Power Manual, by Leo Santucci
Chevrolet, G.M.C. & Buick Speed Manual
Chevrolet Inline Six-Cylinder Power Manual, by Leo Santucci

    Cast-iron exhaust manifolds produce less power than a properly designed set of tube headers. In addition (and assuming no variables), there are 2 very different factors involved in comparing manifolds to headers.
    The most obvious is pumping loss (“back pressure”), you’re just not going to fix this.
    The optimal cam for manifolds will have a slightly earlier exhaust valve opening point to increase the time to blow down, which also change the exhaust CL and LSA.
    The higher average backpressure also means that the overlap should be reduced, typically by increasing the LSA a few degrees up to 116 or 118 (see the Buick 455 engines).
    These 2 changes go in the same direction but for slightly different reasons, and the total will vary with the engine build.
    The other factor is the “local port pressure” right at each individual exhaust valve but only during overlap.
    This is only distantly related to backpressure. It's very sensitive to reversion waves, and it's possible to have 4-5” of backpressure in the pipe but only +1“ or even vacuum, at the port during overlap, or the reverse of +1“ in the pipe and +5“ at the port - that’s what causes those weird spikes in the torque curve.
    These pressure changes can be more easily controlled with headers, since the pipes are separate and can be tuned for length, although there are things that can be done to manifolds to help suppress positive pressure returns.
    All Mopar engines were installed with cast-iron exhaust manifolds. The castings varied over the years, but the castings from the late 1960’s to early 1970’s generally offer the lowest restriction and best flow of the possible choices (the obvious exception being the Max Wedge/Ramcharger 413/426 1962-64 items, which are now priceless). These are the first selection if the many advantages of iron manifolds are to be retained:

    »    Original appearance for restoration or “sleeper”

    »    Lower cost (at least, in some cases) - and you only have to buy them once

    »    Long life expectancy - don’t rust out in 2-3 years

    »    Low maintainance - no bolt tightening

    »    Lower noise level

    »    Lower underhood temperature

    »    Maximum ground clearance

    »    Good starter access

    »    Retains exhaust cross-over and heat riser operation for cold-weather use

    »    No master cylinder problems (temperature, clearance, etc.)

    Although (for example) the early 340 manifolds fit all LA type heads, they do not fit all chassis, such as the narrow 1960-66 A bodies. The “383 Magnum”, 440 GTX, TNT, R/T etc. B/RB manifolds don’t fit any A body directly, and may have clearance problems on some other chassis

    Tim Hage of Streetmaster Performance reports: “‘Magnum’ B/RB manifolds fit 1963-66 (early) A bodies on B (low deck: 361, 383 & 400) motors only (when used with the Streetmaster’s spool-type B/RB conversion engine mounts, which limit engine movement). You may have to clearance grind some bumps off the manifold, but they do fit without fender cutting.”

    From Pete Holbrook: “My ’64 Polara has, at this time, a set of the Magnum manifolds on its 440. Installation problems include overheating (melting) of pushbutton shifter cable on driver’s side and the necessity of raising the motor on passenger’s side in order to slip the manifold onto the head studs. I am going to install a set of the Hedman “Shorty” headers with the Ceramic coat to keep the heat down.”
    My assumption, based on his comment, is that the Magnum manifolds will probably fit all 1962-65 “B” bodies, including Fury, Belvedere, Satellite, Polara, Coronet and 1962 Dart, which are very similar in design.

    Jeff of Tucson AZ reports: “The max wedge exhaust manifolds, completely unmodified, allow you to put the 383/400 engine directly into the 1963-66 A bodies with only trimming the length of the shock top mount bolt and moving the heater motor ½”. It's a perfect fit and allows in the chassis exhaust. They also allow the RB 440 to fit with a spread of ¾” on each wheel well, easily accomplished with a porta-power and a welder. The 440 goes right in and fits great; it's a beautiful swap, I've built 5 of them over the years and I do know what im talking about.”

    Please: click here to make a comment if you have any experiences to share:!

    The best early LA manifolds are the early 340 castings. Please: click here to make a comment if you have casting numbers and applications for Magnum, Road Runner, GTX, R/T, TNT etc. B/RB manifolds:! The raised casting numbers are visible on the manifolds, as shown:




Casting Nº

Right (passenger’s)


A & E body


Left (driver’s)


A & E body


    Even though there are the best stock parts, they are still inferior to a tube header in terms of power. However, all iron manifolds can be substantially improved by porting. This means enlarging the entry openings (where the manifold bolts to the exhaust ports in the head) to or slightly beyond the gasket size, and re-contouring the passages as deeply into the manifold as you can reach.
    The first step is to get the correct tools. The preferred tool is a die-grinder, Dremel, etc. equipped with a ¼” carbide or abrasive bit. An air die-grinder is fastest (also most likely to cause mistakes). An electric die-grinder is fine (Sears Craftsman Industrial model is an excellent buy for the money), just a bit slower. A Dremel is going to take longer due to the smaller motor. The best bits are carbide (don’t bother with “high-speed steel”, etc. as they will quickly become dull), and are available in a wide variety of shapes. I use a ¼” cylindrical bit for roughing out work. They’re expensive, but unless you drop it, it’ll last a long time and cut through iron like cheese. For small curves, radii, etc. you may find abrasive bits helpful; I use ¼” ball & ½” cone shapes. “Cratex” and abrasive cartridge rolls of various sizes are useful for finishing work (use low speed for these). You will not need finer polishing material than 320 grit, as a satin-smooth finish is adequate; if you can, a mirror polish reduces heat absorption. For best control, keep the speed down - this prevents the tool from bouncing and digging in.
    You must wear eye protection when doing this work, for obvious reasons. Be sure to use ear protection as well (shooting muffs, etc.). Wear a dust mask, abrasive dust is hard on the lungs.
    Begin with a clean gasket surface. Cover it with a layer of duct tape; use a rolling pin, etc. to make sure it’s firmly attached. Take an exhaust gasket of the same type as the manifold (used, in good condition OK), and very carefully position it on the manifold, being certain to line up the bolt holes all around. Now, trace a line following the inside port surface of the gasket on all 4 ports. Cut through the tape along this line using the point of an Exacto, utility knife, single-edge razor blade, etc. Carefully peel back the tape inside the port opening, and discard. The remaining tape will offer good protection against accidents. If you prefer, use Dykem, etc., coating the cylinder top surface as per product directions. Spray paint may be substituted - lightly spray the top surface from directly above. Wait 5 minutes, then remove the gasket. In the case of Dykem, you now have a line separating the area needing work from the area that will be left “as-is” for now. If paint is used, only the painted area should be cut. You may still find it useful to apply duct or masking tape to the gasket surface to protect it from accidents. The area inside the gasket is not the absolute limit to the modification, but it’s a safe place to stop.
    Remove metal with the carbide bit, using smooth strokes, from the edge of the existing port opening. Don’t try to get the whole bit into the port - a 45° angle is fine. Work right to the edge of the tape (or paint, etc.). Do the next port; don’t worry about the passage itself yet.
    Once 1 manifold is done to the tape line, do the other one.
    When trying to decide how extensive to make your openings, remember that it is easier to remove metal than to put it back!. If not sure, stop at the edge of the gasket marks. The most frequently made mistake is doing the 1st few ports to the max, and not having the patience to do the remaining ones to match it. Any porting at all is a big improvement, and the first metal you take off has the greatest effect.
    After you’ve gotten this far, you can try extending the new size deeper into the passages. Some will be easier to do than others, and some will obviously need more work. Be careful to note where the bolt perches intrude into the port shape (LA manifolds especially) - you can’t remove all the metal here, or it may crack when tightened. Just smooth the metal leading to & from the boss as much as possible. remove any obvious casting flaws, parting lines, etc.
    The longer-stemmed bits allow you to go deeper into the passages. Keep the speed down, as the length will cause chattering when pressure is applied.
    After doing all 8 ports, carefully inspect all passages for gouges, roughness, etc., and re-do problem areas. Are all port openings done equally? If you’re satisfied here, this is a good place to stop. Blow out the manifolds with an air hose, remove the tape or paint.
    Another factor is that the most active gas flow leaving the port is at the roof (top of the port opening). Raising this a bit aligns the new opening more closely with the actual gas flow.

    An easy way to make another minor improvement, if space permits, is to move the manifolds out away from the port by 1/2” or more (add a 2nd gasket and longer studs or bolts). This moves the restrictive angle that the gas must follow farther away from the port, and increase total port & manifold internal volume - every little bit helps here.
    An easy spacer is an extra header flange or two, but be sure that the transition from the flange to the manifold is not restricted. The flange may be slightly larger than the port opening in the head, but must not step down entering the manifold. Do not taper, blend, or bevel the extra flange to act as a transition between the port and the manifold. If possible, slightly enlarge the inside of the manifold opening to produce a reversion step (as above). A 1/16” step is generally possible and will help. If there is not much room, the greatest step should be at the roof.
    Many successful cars have been built with iron manifolds. The famous “Flying Green Brick” used highly-ported 340 hi-po manifolds for several years on a 450+ hp LA stroker motor.

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