Cast Iron Exhaust Manifold Improvements
10-29-2012, 01:08 PM
#1
TECH Enthusiast
Thread Starter
Cast Iron Exhaust Manifold Improvements Cast Iron Exhaust Manifold Improvements
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.
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.
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.)
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. 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 !
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.
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.
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.)
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. 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 !
10-30-2012, 10:37 PM
#4
TECH Enthusiast
Thread Starter
Good info, but where did you copy and paste it from..?
Here?
http://victorylibrary.com/mopar/ci-man-tech-c.htm
Here?
http://victorylibrary.com/mopar/ci-man-tech-c.htm
