This week I found myself tied up with a bunch of family activities, and wasn't able to spend too much time in the shop working on the intake manifold. However, as it turned out my original plan for the manifold may not have been a workable one anyway, and I was able to get some computer design time on a new manifold concept, so hopefully next week I will make a dent in the work on the manifold.
One of the things about the sheet metal intake that I was rather unsure of was the correct length of the runners. My Pipemax program suggests a length of 13.9" for the total runner, from the valve to the runner entrance. This would mean a 10.9" runner, since the port length from entrance to the valve is 3". I had previously built another sheetmetal intake using this dimension, and it had not worked as well as the Hilborn individual runner injection manifold, but I wasn't sure whether this was due to the runner length or some other manifold design feature. The previous sheet metal intake had been a crossram design, and didn't have a lot of room above the runners in the plenum, because I was trying to keep the whole thing under the hood of the Galaxie. It did have the correct plenum volume, but the runners also had more taper than you normally see in a sheet metal intake. So, I was unsure about what feature of the intake had caused the substandard performance.
I have also seen other sheet metal intakes for SOHC engines with runner lengths that seemed to vary quite a bit. The first photo below shows a picture of the sheet metal intake on Mike Jackson's SOHC race car, and the runners appear to be in the 6" range. The second photo shows a tunnel ram intake with much longer runners:
After last week where at the machine shop another intake measured a total runner length of about 11", and this manifold was tuned for 7500 RPM, I really didn't know where to be for runner length. Bottom line was that I wanted to make an adjustable intake, where I could vary the length of the runner from about 10" to about 14", and get dyno data for each combination, in order to pick the best combination for the engine.
My first idea on this was to put a curve on the intake runners, as shown in the following drawing:
From there, I could stack aluminum plates with holes the size of the runners on top to increase the runner length, and then bolt a plenum box onto the top of the whole thing. This would have worked out great, except that it turned out to be pretty difficult to fabricate the curved runners. I had some mandrel bent 2 1/2" aluminum tubing to start with, but to get the taper I wanted on the runner from top to bottom I had to cut out a tapered section about halfway up each side of the runner and reweld the runner together, and also cut a slot in the top curved section, and spread the runner out on top, and then weld in a pie shaped piece. The tubing measured about .070" thick in the straight section, but down to .035" thick in the curve sections, making the welding and fitting rather difficult. I tried it with one piece of the tubing early this week, and it didn't go real well. If I was a better aluminum welder, maybe it would have been OK, but I figured I'd have a few hours in each tube to get them right, and I could definitely see screwing a few of them up and having to throw them away, so I decided to work on a different approach.
The design I came up with looks more like a conventional sheet metal intake, and still allows plates on top of the runners inside the plenum to lengthen the runners for test purposes. The drawing below shows the basic idea:
With this design and no extensions on the runners, the plenum volume is just about right (585 cubic inches), and the total runner length is about 10". The throttle bodies are mounted on the side of the manifold due to constraints in the vehicle. Adding four 1/2" extensions to the runners would make the manifold look like what is shown in the drawing below:
Runner length is now 12", and the runners are starting to take up some room in the plenum. Further, there is only a couple inches of plenum area over the runners, which is a concern. So, adding spacers to the flat plates at the top of the manifold will restore some room over the top of the runners, and add plenum volume, as shown in the drawing below:
This manifold is obviously not the ideal design, but it will allow me to test runner lengths and plenum volumes on this engine. If it works pretty well I will probably just use it on the car and if not hopefully it will point me in the direction of a better design that I can build, perhaps later in the summer.
Hopefully next week I will have some pictures of the runners and plenum mock up for this intake.
