Intake

The stock intake manifold is not a good choice for aircraft use. It consists of two large aluminum castings with a number of ports and runners for things like EGR that we don't need in an aircraft. This makes it both unnecessarily heavy and way too tall for use in a Cozy (and most other aircraft).

Fig. 1: Clearance Test

Fig. 2: The LIM is a beast

In the photos above, note the large number of accessory ports and runners - basically, everything in the casting that isn't the four main runners. Also, note how ridiculously high the entire intake system is. The horizontal UIM section (which is just sitting on top of the core in these photos) is designed to bolt onto the studs/flange at the top of the vertical LIM section. It really needs to come down 6” (or more, if you didn't raise your turtleback).

Fig. 3: Cutting it might work

Fig. 4: Just enough clearance

Although the UIM can't be easily reused, the LIM does make a great flange upon which to construct the rest of the intake. It has ports that already match the engine block, it brings those ports smoothly up into more-or-less circular runners, and it has provisions for the fuel rail which will save us some time.

If you're building a Cozy, cowl space is particularly tight. I lifted by turtleback 2” to obtain the room I have here. For any aircraft, do a dry fit of all components before proceeding. If you need to lower the manifold even more, you can ditch the LIM and construct your own manifold from scratch.

First, trace an intake manifold gasket onto a 1/4” sheet of aluminum. You may want to add 1/8” or so to the outside line to give you enough meat to round over the edge and to allow for the saw kerf. Then cut out this shape and drill/grind out the port holes and mounting holes.

Next, cut two 1-3/8” dia and two 1-5/8” dia 90-degree tubing bends to whatever length you want to provide clearance between the engine block and the intake runners. You might want to install the primary injectors in the block if you plan to use them, so you can see what clearance they need, and think about where the secondaries will go.

Without a turbo they can go anywhere convenient, like on the outside. With a turbo, you'll want to keep them away from its heat, so allow extra clearance to mount them on the inside, as the stock LIM does. (You could also mount them in the horizontal runners over the top of the block, or even near the intake manifold, just be aware that this increases the risk of flooding due to fuel not fully vaporizing and pooling inside the runners.)

Put the ends of the runners in a vice and squeeze them until they're oval and roughly matching the shape of the holes in the mounting plate you cut. Tweak the fit by grinding, bending with pliers and pry tools, etc. until you have a “good” fit. It doesn't need to be perfect, but the closer it is the better the final result will be. Then have this welded (or weld it yourself, if you have the tools/expertise to weld aluminum.)

For fuel injection ports, buy aluminum fuel injector “bungs” from Burns Stainless and other suppliers that are already drilled and shaped to match a standard fuel injector. Braze or weld them onto a suitable location on your new intake runners, taking care to make sure they're in a perfectly straight line. Then buy a section of fuel rail stock (Burns Stainless sells this, too), and drill it to match.

If you put a bit of time into it, you can set up your intake almost any way you want to match the space you have available. Be sure to provide plenty of support to cope with stress and vibration. Study the photos below and note where Mazda put gussets on its LIM casting - in most cases, from flat sections around the mounting holes out to the runners themselves. It would be a good idea to include similar support in your own intake.

Sizing the intake manifold is a black art. I've described below the rationale that I used. In the process of constructing this manifold I must have read the theories of every expert available on manifold and plenum design, size, and shape. I even started to create a bibliography until I realized it was longer than this text! As a result of the research process, I've learned the following sage advice that I will now pass along to you:

1. Intake tuning is voodoo.
2. Very few people actually know what the hell they're talking about.
3. The ones that do disagree with one another, and couldn't possibly describe it to YOU, you peasant, because the physics is too complicated. Generalizations / simplifications are nothing more than starting points. “Put it on a dyno and see. Even the automakers do that.”
4. No two experts do the same thing. Most of the experts have done several things, and all “seemed to work”.
5. You're on your own. Get to it, and good luck.

It's no wonder this step of the process is a stumbling block for most builders. But if you read between the lines you learn three more CRUCIAL things:

1. If you follow a few simple generalizations you could probably make tupperware and plumbing drain pipe work. It might not be “optimal” power but it would run just fine.
2. Most of the black art relates to NA engines. Stick a turbo on there, and most of the black art goes out the window. Even tiny changes in boost settings will wipe out any tuning you're going to do. Focus on volume and proper airflow.
3. Power ain't your problem. True story: John Slade gave me the honor of managing a takeoff in his 13BT-powered Cozy. I did what pilots everywhere do - I smoothly applied full throttle. Imagine my surprise when he pulled the throttle back to something “more reasonable”. Cooling cooling cooling. Your one overwhelming concern is going to be how to dump the heat you produce, and you are NEVER going to have enough - or you'll have so much scoop and radiator that you'll be doing only 160kts even at 300hp.

I initially wrote this guide around brazing, because I wanted to do the fab in the shop to allow for trial-and-error fitment and tweaking, and had neither welding equipment nor experience.

I think brazing is a perfectly acceptable solution, but I hit a few problems. First, I was never happy with my torch, and when I started looking at spending $30-$60 for a decent one plus welding rods plus the extra cleaning and prep work… It turned out to be easier to go to the guy down the road.

Second, the sad truth is that I never really mastered brazing well enough to trust it for my intake. I think I'd be fine putting a mounting tab on the radiator or plugging a hole. But the intake has linear FEET of joint lines if you account for all the runners, and they're really close to each other. It takes practice to manage the heat so that you can braze one runner right next to another and get good joints on both.

The assembly starts with the flange on the engine. The UIM is junk, but the LIM makes a great flange for this fabrication because it already has all the runners in the right spots and the right sizes, and it gracefully brings the oval housing ports out into nearly-round pipes.

Start by finding the lowest line you can establish across the LIM where the intake runners are still circular, plus a half inch for margin (see below). Then rough-cut the LIM in half. This is a fair job because the LIM is very thick, and has a few steel freeze plugs in it that eat bandsaw blades for breakfast. I had the best luck using a Sawzall with a metal-cutting blade.

Fig. 5: Cut line (look closely)

Fig. 6: Band saw, nom nom nom

This cut line allows you to use the stock secondary injector rail, which will save fabrication time and cost. The tradeoff is a taller intake. If you lifted your turtleback 2” as I did, this will just fit. If you need the clearance and don't mind making your own secondary fuel rail you can cut the LIM another inch or two lower and they will still be mostly round and easily weldable.

Fig. 7: Test fit

Fig. 8: Plenty of room

Here you can see my cut LIM mounted for some trial fitting. If you look along the cowl line you can see the ruler I've mounted to simulate the cowl line. It's hard to get a sense of perspective from the picture, but this arrangement allows for a good cowl line provided it's shaped to just touch the intake.

After finalizing the new size of the LIM, clean up its face with a belt sander or grinder. It doesn't actually need to be perfectly smooth, but you'll save some welding setup time and get a cleaner finished part if you do. Now would also be a good time to use a cutoff tool / die grinder to clean up the LIM and remove the EGR runner as shown below.

Fig. 9: Starter holes drilled

Fig. 10: Unused runner

Fig. 11: Unused runner removed

Fig. 12: All gussied up

You can spend a little or a lot of time here depending on your goal. I wasn't going for a chrome finish (my main goal was to minimize weight) so I left some areas a little rough. If you like a brighter look and you're using a used LIM like this one, a wire brush does a great job of shining it up.

Fig. 13: Top view

Fig. 14: Runners line up

As you can see, the finished cut line puts the runners all in an even line - not a requirement, but very nice to have for the next step. The runner on the right (the rear side of the engine when it's mounted) has a bit of a bump the welder will need to fill or dimple out, but this isn't hard.

The runners are fabricated from mandrel-bent aluminum pipes, which are available just about everywhere. I'd strongly recommend getting an extra piece or two, despite the price. Mistakes happen, and it's less tempting to do a hack job if you have extra material already handy.

The stock intake has larger primaries and smaller secondaries. I found 1-5/8” x 0.065” (16-gauge) tubing worked well for the primaries, and 1-1/4” x 0.065” tubing worked well for the secondary. You can mix and match thicknesses and sizes, just be aware that thinner material is not as strong so you'll need to add support gussets in critical spots.

First, cut two large and two small straight runners to about 18” in length. These will pass across the top of the engine, and are cut longer than necessary to help with fitting later. Then cut 90-degree bends as required to yield two large and two small stubs about 6” long. The bottom side of each bend that will be welded to the LIM should be cut very low, a few degrees before the bend straightens out. This will make the runner angle downward as it passes across the engine, and give you more clearance for the intake plenum on the spark-plug side.

Fig. 13: Runners welded, note the angle

Fig. 14: LIM installed with runners

When you do the final fitting before taking everything to your welder, I recommend installing the secondary fuel rail and injectors to make sure you have the necessary clearance to install them. You can turn the injectors so the connectors face sideways to get a centimeter or so of extra clearance for installing the rail. (Remember, the injectors will be in the rail when you install it.)

Fig. 17: Plenum side, note the angles

Fig. 18: How the plenum will work

Note that the runners should spread apart as they cross the engine. This provides clearance for the oil filler neck and dipstick. We'll be cutting the oil filler neck down later to reduce its size - for now, just leave a 3” diameter gap between the runners near the filler neck.

I have yet to post pictures of the rest of the process, but it's fairly obvious by now:

1. Another set of 90-degree bends are welded to a 5” tube that will serve as the plenum.
2. Those bends are welded to the runners.
3. The excess lengths of the bends inside the plenum are trimmed off. You could optionally install horns for better airflow, but I chose to keep it simple.
4. A flat plate is welded on the front end of the plenum, and a throttle body is installed on it.
5. A hemisphere is welded onto the other end to seal it off.
6. A pair of stiffening strips is welded across the runners to support them.
7. A pair of support brackets are welded onto the assembly to support it.