Intake and Exhaust Design:


The web has a lot of websites with information on the design of intake systems.  They have formulas for figuring runner length based on volumetric efficiency and desired torque curves.  Most of the design of the intake for my Subaru engine came from discussions on the AirSIG e-mail group.  AirSIG is long since disbanded, however there is some stuff still posted on web sites around the net.  Maybe someday I will scrounge it up and post it here.

Update 10/31/07; This site seems to keep moving. This is the information that I used to calculate the size of the intake manifold plenum.

Basically I discovered that a two-liter plenum with 12" runners would suffice to put the maximum torque output around the 5400 rpm range.  The runner length measures from the back of the valve opening inside the head to where the tube attaches to the plenum and is based on a 1.5" id tubing.

The plenum is constructed of .055" 5053 aluminum and is constructed from four pieces.  I had them punched and bent with the holes for the throttle body and intake runners included.  After welding the box together I made a jig to hold the plenum in place on the engine block.

I utilized the stock intake flanges for the injector mounts and throttle body mount.  To do this, I used a hacksaw to cut them from the original engine manifold.  Then I trimmed them to fit the tubes and throttle body opening.

Once this was done, I bolted the injector flanges to the heads and began shaping the aluminum tubes to fit between them and the plenum.  I wanted the manifold to have some flex built in to prevent cracking so after cutting the tubes to fit, I cut them midway between the bend and the plenum.  I purchased some 1.5" id, black fuel delivery hose from NAPA and cut it into 2.5" lengths to reconnect the tubes.

Once all the parts were fabricated, I made a jig from plywood to bolt the flanges and plenum to.  Then I had the assembly TIG welded together.  I had the welder add a couple 1"x1.5"x.25" aluminum plates to the plenum in case I needed to drill and tap additional holes for vacuum lines or gauges.

in1.jpg (90124 bytes)   in2.jpg (95244 bytes)  in3.jpg (92172 bytes)  in4.jpg (42985 bytes)


The exhaust was more a matter of what will fit and what will be easiest to fabricate.  The first attempt at building the exhaust incorporated a pair of 1100 CC Kawasaki motorcycle mufflers hanging in the breeze under the plane.  In theory, this was a great idea.  After I fabricated and hung it under the plane, I realized how hideous it looked.  I guess at that point I was trying to be cheap.  The mufflers were from E-bay, and they were cheap.  It looked like I had a pair of bombs slung under the belly.  The worst part was when I ran the engine and discovered that the mufflers made no real difference in exhaust note.

With the failure of that brilliant plan, I designed a system that would exit within the cowl exit area and basically look like any other Glasair with the single exhaust exiting the cowl under the leading edge of the wing.  I purchased all of my stainless tubing from Burns Stainless.

The manifolds start with the flanges, shown below (see also the .pdf drawing next to the picture), add two pieces of 1.5"id mandrel bent 304 stainless sliced and welded to a piece of 2"id 304 stainless.  This was attached to a stainless steel automotive flex coupling.  After the coupling the pipes take a 90 deg bend to come together in line with the cowl tunnel bump where the stock exhaust exits.

A piece of  2 1/2" mandrel bent stainless, sliced appropriately and welded together brings the two together in the cowl tunnel.  I incorporated a slip joint in the left side collector to relieve strain on this Y-joint.

The 2 1/2" tube continues down the tunnel and exits in a 90 deg down turn under the belly panel.

I was quite surprised when I started the engine in this configuration.  The noise level seemed lower than with the two mufflers.  It was only slight, but seemed better.  I will have to wait and see what people think when I actually fly it.

ex1.jpg (87052 bytes)  EJ-22 and EJ-25 SOHC Exhaust Flange  ex2.jpg (91148 bytes)  ex3.jpg (29017 bytes)  ex4.jpg (45504 bytes)

Updated 10/31/07: After final fabrication on the fixed trike conversion my beautiful single exhaust wouldn't fit. I did what any artist would do when he finds his creation doesn't work; I sawed off all useful components and made a new, dual, exhaust system.