How to build an ITB manifold
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!
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
when you set them up use a straight edge to keep all 6 runner openings in a line, it doesn’t matter if there’s a bit of a kink or miss alignment at the join as the whole thing will need lots of porting out at the end to make the transition down from the new 40mm flange
next you need to make up a holding JIG, i use a piece of RHS and drill and tap 4 holes in it to bolt the manifold on. Of the 12 holes in the flange i use holes 1,4,9 and 12.
then with the flange bolted to the JIG and bolted to the mill table the runners can be milled flat
Next is an important step, this is done AFTER milling the runners. Take out the middle bolts (4 and 9) and loosen off the other 2, then slip 2 shims 1mm thick directly under runners 3&4 then gently tighten the end bolts down until the tip of the flange just touches the JIG, this puts a 1mm bow in the whole thing. This is to compensate for the distortion caused by the welding.
Then sit the flange plate on where its the best average fit, clamp at the ends to pull it onto the runners and get some good tack welds to each side of each runner.
Then weld away!!
make sure all your surfaces are raw and clean, keep it bolted to the jig the whole time just with the two end bolts and the 1mm shims in place. Try to avoid welding the same side of all the runners first, I usually weld top, bottom, top, bottom and so on then go back and do the other sides etc. This is where you will curse leaving those fuel rail mounts there!
I should note the DCOE bolt pattern is 90mm spacing and the m20 bore is 91mm spacing so when you make a flange, space the holes so they go 90mm 92mm, 90mm, 92mm etc. You can find an AutoCAD file of the whole plate here in my downloads section.
Unbolt it from the JIG, check it with a straight edge it should be pretty good but if its still a little bowed all it will take is a gentle bump with a big hammer and some blocks of wood… be very careful!!! it wont take much, try to get the head face as perfect as possible.
That’s the fun part done next is the tedious part, the runners need to be hogged out to match the flange so get comfortable with the die grinder and start cutting, keep a little dish handy with some WD40 or even Kero mixed with engine oil works well, dip the cutter in now and then to keep it cutting really well, stops it clogging up. Try to get the transition as smooth and gradual as possible from the 40mm hole down the the head face flange.
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
Last thing to do is tap all the holes in the new flange for the studs, give it a clean and a coat of your favorite color!…. sorry no pics of last step.