Ok this is a step by step showing how an m20 ITB manifold was built, but this same theory and techniques can be applied to almost any engine. It takes a bit fabrication skill but nothing too difficult and i have discovered plenty of little tips to help out. First thing to do cut the flange off an original manifold, keep it as long as possible i like to aim for at least 70-80mm and try to keep the cut as parallel to the face as possible dont worry about where the runners are heading at this point.
Disclaimer…. if your going to use one of the blades shown below be aware that fabricators have a nick-name for them… meat axe! they are very affective for cutting alloy but use leather gloves and be warned!
Next I trim back the fuel rail mounts so i have room to weld, welding the flangs on is a bit tricky even with the mounts trimmed right back so it really is no problem to take them off all together this makes the welding much easier. Then all you need is 2 little tags to bolt the rail back on i will show this in the next photos. For this one i am leaving them on
Here are the tags you will need if the mounts are removed.
and here’s the rail bolted in place with them, they go under two of the nuts holding the manifold on
depending on what model manifold your starting with some need the end runners fixing because they start heading up too soon so first i cut a couple of runner pieces from the discarded manifold, just trim and square them up roughly by eye.
To go off on a tangent. Look at the size of the runners at this point, we are about 100mm from the head and they are only about 35mm!!!
Think about this for a second the OEM manifold is still good for some great power numbers and its only 35mm, most other brands of ITB manifold are 45mm at this point, thats nearly double the cross sectional area this can have quite a devistating affect on the critical air velocity and performance!!! This also highlights why 40mm ITB is plenty big enough for even very tough m20's.
Ok onward… then trim the outside two runners right down short and weld on those little straight pieces
Lastly it goes back on the JIG and into the mill for a light skim, if everything was done well then it usually takes less than 0.5mm for it to clean up.
Flip it over and give the head face the lightest possible skim too just to make sure it will seal up well