Step 3:

Bend and fit runner tubes, cut injector holes

Runners tubes:

The runners will be made of 1 1/4" electrical conduit or EMT for short. This stuff is cheap, easy to work with and depending on the manufacturer and their tolerances, you can bend it in a 1 1/2" tubing bender.

Note: These were made on a bender with a 5" die. EMT is galvanized and releases all sorts of nasty stuff when welded. One way to remove the galvanizing is to soak the ends in white vinegar overnight. A blow torch and good ventilation will also work.

Here's how it looks:

Start by cutting four 28" sections. Then cut one end of each one at 19 degrees and put any tubing seam on the short side of the cut. Notice the mark I made on the seam.

From the shortest point of that cut, mark 3", 17 1/2", and 25" and 28".

You want the runners to all enter the plenum at 90 degree angles. Because of that you need to twist the tubing between the two bends. Here's a top view that may help explain it. Note: if you look close you can see two bungs on the plenum. One is to sense intake pressure and the top may be for water injection. I made them from the holes I cut in the intake flanges.

At the 3" mark, bend the tube to 73 degrees with the tubing seam on the outside of the bend.

Now your going to need to get your hands on a protractor. I use a digital SmartTool but a regular protractor will work fine. Take a look at the picture below. I made a tool that clamps to the tubing and holds the protractor in place. Put that tool on the tubing and level it before you release the pressure on the tubing bender.

Now put the tubing on the 17.5" mark and twist it until you get to 9 degrees. Make two, but bend one up and one down. Now bend two more in the same fashion but twist them at 12 degrees.

Here's the finished tubes that are labeled for each cylinder.

Plenum Alignment:

I wanted the rear of the plenum to be very close to the rear of the engine. I also wanted it to be square to the engine. I took a piece of wide flat stock and clamped that to the case. Then I tacked a couple pieces of scrap 1/4" wide stock to it at about the height that the plenum was going. At this point I haven't welded on the plenum caps so those spacers are there to provide a little room. Then I put a 1/4" scrap on the shroud to space the plenum above it. A drop line was then used to center the plenum on the case. At the point where it was centered I tacked the plenum to the jig. Note: The threaded rod you see below was just so that I could measure the runners to be sure they were square to the plenum.

At this point the Intake flanges are made and ground to the right angle. The runners should fit right into the holes on the plenum. If you remember I marked at 25" and 28". The 25" mark should be pretty close to the plenum and you will have to cut some tube off the end. The rear runners were about 1/4" off but that was good enough for me. I allowed for having .75" inches of tubing inside the plenum. The reasoning is that it should allow for more air flow than if it was fit to the edge of the plenum. If you have the tools, then the optimum setup would be a bell mouth opening similar to a velocity stack. You can see what I mean on the SDS site: http://sdsefi.com/air12.html

Now tack it up and get ready for the injector bungs. Don't weld it just yet because you may need to change something depending on the injector bungs and fuel rail.

Injector Location:

Generally speaking the closer the injector is to the valve the better the throttle response. As you move the injectors father up the intake path the fuel has more time to atomize and your top end will be better.

Some people will aim the injector so that it sprays directly on the back of the valve. The reasoning is that when the fuel hits the valve, it cools the valve, vaporizes the fuel better, and mixes with the air better. In any case whenever possible you should put the injector after any source of air disruption (such as a butterfly or a sharp bend). If the fuel is injected before a disruption then it will be disrupted into the runner walls where it will tend to stick.

In my case I wanted to protect the injectors from tree branches so there was only one place to put them and that is about 3 inches above the flange on the inside of the bend. It is a tight fit and the exact placement will depend on what you use for a fuel rail and what you use for injector bungs. Injectors with hose barb ends will probably be easier to fit in that space. When you do a test fit, make sure you put the electrical connectors on the injectors.

Injector Bungs:

Below is one option. It's a 1/2 pipe thread to 5/8 compression fitting. It's important you get the ones with the flat top and not the tall top. When the nut is tightened down the compression fitting squeezes on the injector and holds it in place while it seals it. Some have used the fittings with a female thread. In that case you can weld a steel pipe nipple to the manifold and screw the fitting onto it.

I had a friend with a lathe and he made some bungs similar to these: http://sdsefi.com/air12.html. The bottom bungs are mild steel the top bungs are stainless.

Fuel Rails and Bung Holes:

At this point your going to have to play with things and decide what your going to use for a fuel rail. I chose to make the rails from stainless steel because I had the parts on hand and it will never corrode. As I mentioned before, injectors with hose barb ends would have been easier and I will use those on the next intake. Below are the some of the rail parts.

The mounted rail shows the tight fit. When fitting the bottom bungs it helps to have a jig that holds them straight and level. I tacked the long side of the injectors to a small piece of flat stock at the proper rotation and distance apart. From there it was a matter of putting the injectors into the bungs and holding the fuel rail on top to see where the holes would go and how much I would have to angle the bungs to fit the runners. I ground the holes with a die grinder slightly small than the bungs. After the final fit I welded them on and used a die grinder with a 4" shank to clean up the inside of the runner.

At this point you can weld the rail up. On the finished rail the fuel runs into the straight ends and out the elbows. Because the fuel runs out the tops, the system self bleeds out the air. The fittings for the brass pieces and the gauge was made from nuts that I re-drilled and tapped for pipe threads. I used 3/8 fuel injection line and fittings to the first rail, then 5/16 fuel injection line and fittings after the first rail. The "U" pieces are there to capture a nut so that I clamp the injectors to the intake.

At this point, weld everything up and paint it. Next will be the plumbing and flow testing.

[ Step 1: Fuel Tank ] [ Step 2: Flanges and Plenum ] [ Step 3: Runners and Rails ] [ Step 4: Plumbing and Testing ] [ Step 5: Computer Wiring and Tuning ]