CIS Injection Plenum Fabrication
The system I'm adapting for my type4 engine is a plenum designed system with a single progressive throttle body. Those who are trying to extract the maximum horsepower possible from an engine spend a lot of time designing a plenum which will work best for their application. In this application, I am ONLY worried about having smooth usable power up to 4500 RPMs from a stock 2.0L. My main concern was to be able to use the stock throttle cable in the stock location with the least bit of modifications to it.
Here's a few shots of what I came up with. The stub tubes are 1.45" ID seamless aluminum tube. I put a flare on the inner side and will weld it as seen in the 3rd photo so that it will get clean airflow to either side. I kind of fashioned these like velocity stacks on carbs for the exact same reasons.
Here, I've got all but the rear side welded up on the basic plenum. So far, so good.
The throttle-body I'm using is a progressive type unit from a late 70s, early 80s Audi 4000. I needed to modify the throttle lever so that I could use the stock barrel nut. Another consideration that had to be made was the amount of pull on the cable needed to actuate the throttle to "Wide Open Throttle" (WOT). This is very important if you ever hope to put the pedal to the floor. Ideally, you want to be able to be at WOT just prior to reaching the bottom of the pedal stroke, just like with a stock carb set-up. The answer was simple; I would remove the throttle arm fro the throttle-body and replace it with a modified throttle arm from a stock Solex carb. Well it worked and basically fit with little modification. I had to cut off the return spring tab as well as the idle adjustment arm with a hack saw and file the edges down smooth. Then I put a slight bend in the arm so that the barrel nut wouldn't hit the secondary butterfly actuation link. I then had to place one washer underneath this arm prior to installing the arm to give just a little more clearance.
I had to transfer the mount holes for the throttle body to the face of the plenum. Once they were drilled, I used 4 small pieces of "gang channel" to anchor the throttle body to the plenum face. I also added a boss for the cold start injector and a vacuum line (in case I ever need one).
Here's a couple photos of the finished plenum with the throttle body and cold start injector installed. To ensure I don't get any un-wanted vacuum leaks, I fluorescent penetrant tested ALL welds to ensure they were air tight.
I now needed some flanges to connect the plenum assembly to the plenum runners. I searched for some aluminum flanges, but none were to be found. So I was forced to make my own. I made these from 1/4" 6061 T-6 aluminum. I started by tracing with a scribe around one of the steel flanges I got from C.B. Performance. I made three since I had the extra material and, well. . . I tend to mess things up :-) Since I'm a firm believer in the "use what you got" philosophy, and don't have any real metal boring tools, I cut the center hole with a 1.5" hole saw (yes, the wood type. . .:-) ) and cut the basic shapes on a band saw. They were finished on a disk sander.
Now I needed some runners. For these, I used a new set of 1.5" J-tubes since they were the right diameter and have smooth mandrel bends which are non-restrictive. Below are some shots of the runners taking shape.
Once I had the J-tubes cut and welded to make a 90 degree "L" shape (as seen in the last photo) and the aluminum flanges welded (as seen in the 2nd to last photo), I could begin to set-up the runners for final assembly.
The flanges which were factory welded on the J-tubes needed to be ported out since it appeared that they were MIG welded (MIG welders make big sloppy welds) on the inner side and were restricted down to almost 1 1/4" from the original 1 1/2" ID. After porting, these were re-welded (TIG this time) on the outside so that there would be no restriction.
I began by determining the height of the plenum and cutting the runners so that they were the right height. Then I slipped one of them into the manifold flange, positioned the plenum w/ stub tubes mounted above, and made a rough marking of the tubes for the first cut. Once these are cut, it's easier to make minor adjustments to the angles in the tubing. Once I had everything just right, I held everything in place as it was tacked in place. Then with the plenum in place, I repeated this again for the other side and tacked everything in place again. Then everything was completely disassembled and finish welded using a TIG welder.
NOTE: If you use back-up gas (Argon) on the inside of the tubes, the welds will be as smooth and clean as the outside. This is important seeing that you don't want to add any restriction on the inside.
Here's shots of the plenum parts:
Here's a shot of the finished plenum, runners and manifolds mounted to the engine. I'm going to have this powder coated in the near future once I figure out what color I want it.
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Revised: September 07, 2003 .