By Nigel Field                                                                                                                       manifol3.rtf    4 Oct, 96 


The stock manifold on the EA-81 has many drawbacks. It is restrictive to flow, and is heated by the coolant which reduces available normally aspirated power. It is heavy at 7 lbs, bulky, and has an awkward physical carburetor interface. A much improved manifold can be easily constructed from carbon fibre by the average builder. This article will attempt to describe how this may be accomplished. Carbon is used for its high modulus and low weight to give an acoustically solid structure. You will first make the head interface flanges from steel, and then design and construct the manifold itself. Although not complicated, construction is a multi step process completed as a series of small evening tasks. 


The first parts to make are the cylinder head manifold and cooling interface adapter flanges. No these are not carbon and they are the toughest parts, but still easy for a homebuilder. There are a number of design options available. The simplest method requires basic tools but some brazing or welding is required which unfortunately is unavoidable. 

The adapter plates will provide 1.5 in diameter stub tubes to couple with the carbon manifold. As the head ports are slightly smaller than this, the ports will require some slight enlargement. Using a 1 inch sanding drum on a drill gently open the head ports by grinding away aluminum on the inside face keeping the hole round. Try not to remove too much on the coolant passage side to leave as much clearance as possible for the coolant tubes to be added later. Check constantly with a piece of 1.5 inch stainless exhaust pipe until the ports are the correct size and match the pipe. Transition any ridges inside to make a smooth opening. There is a cast ridge running vertically into the port that will interfere with this process so grind it away at the opening area only and transition into the port. I'm not sure exactly what it does but all the experts say leave it in.

Now using a cut up file jacket, make a template of the bell shaped interface at the head port to include the 3 mounting bolt holes and surrounding flat gasket face area. Using the template fabricate two plates from .125 minimum stainless or 4130 steel. Drill the three mounting holes to 1/4 inch to later take 1/4 X 28 cap screws. Cut out the induction hole using a series of closely spaced drill holes and then a 1/4 round file unless you have access to a lathe, then turn them. Fit snugly to 1.5 by .035 inch stainless exhaust pipe. Mark and cut out the curved coolant slot using a drill and file. Fabricate two 1 inch long induction stub tubes from the 1.5 in pipe and temporarily fit them into the induction holes in the flange. Next make coolant outflow tubes from 3/4 in 4130 or stainless about 4 inches long but whatever will best fit your installation. Cut these tubes at about a 30 degree angle up from the horizontal to fit over the coolant holes in the flange. Now trial fit everything on the head ensuring all holes and ports properly align and the coolant tubes clear the bolt holes. When satisfied mark the locations and then braze or weld the pieces together. If you don't have OA welding equipment try a friend, or farm it out. The welding will probably warp the plate slightly so using an inverted belt sander, hold the plate nice and square against the belt and grind it flat. Rotate it occasionally to avoid uneven grinding. To prevent hose blow off, make a raised end on the coolant tubes by wrapping 2 turns of solid copper wire about 22 gauge near the end and solder. Now etch and paint with your favourite epoxy paint or enamel.

There is insufficient room to use standard bolts to attach these plates so the heads will need preparation to take allen head cap screws. The heads are metric so get some old bolts of the right thread and cut off inch lengths of thread and screw these into the six mounting holes until flush or just slightly recessed with the head gasket face. First cut small slots in one end so you can turn then in with a screwdriver. Install these with JB Weld high temp epoxy available at any auto parts store. After cure drill and tap to 1/4 X 28. Purchase some 1/4 X 28 X 3/4 Allen head cap screws for mounting. Fabricate two gaskets from good quality gasket material to match your new adapter plates and bolt them on using Permatex gasket compound. The black rubber impregnated compressed paper stuff works best. Do not use cork or any other very soft material as it will squish out and leak when you torque the plates down. OK that was the worst part now on to the composite stuff.


You will make the inner core and fit it to the engine, then complete the various layups of carbon, then dissolve out the core, then fit attach tabs and the carb mountings, and finally finish the outside of your manifold. You first need to have the heads on with head gaskets and at least two bolts per side snugged up. This will a ensure proper fit of the manifold. 


The core is made from fine grained builders insulation Styrofoam SM in three pieces, the plenum and two runners. Do not use the expanded bead popcorn foam as it is too rough. The design and shape of the plenum depends on your particular application and carburetor installation but basically it is a cavity which feeds the two runners. It serves as a capacitor or storage tank if you like, to help even the flow through the carb caused by the vacuum pulses. Mine is a down draft 2 barrel mounted on top of a 7 X 4.5 X 2 in plenum. For low profile applications a side draft carb could be used so design your plenum accordingly. Try hard to keep it symmetrical and watch out for the steering effect of the throttle plate(s) which tend to direct raw fuel to one side at part throttle. This will cause uneven mixture and all the associated headaches of roughness, burnt valves ad nauseam. My throttle plates open fore and aft so it remains symmetrical. Throttle plate symmetry is very important. 

Cut your plenum core with a band saw or hack saw. Sit it on top of your engine in exactly the position you want it. Shim it up underneath to give at least 3/8 inch clearance from the block to allow for its eventual thickness and some running clearance. Now plan your runner shapes to provide a gentle curve from the plenum though a 90 degree down turn to mate with the top of the stub tubes on your head adapters. Make them by cutting foam out in two dimensions with a slightly tapered square cross section. In other words they are bigger at the plenum than at the heads. This keeps the velocity lower in the middle but provides more mass to help push the new charge in. Now trial fit the runners to the plenum and adjust as needed until you are happy. Use a sanding block to trim fit everything. Glue the runners to the plenum with a dob of 5 min epoxy or hot glue in the centre of the joint not the edges because its hard to sand. Then re-fit everything and push the runners gently onto the stub tubes to leave a slight impression sanding mark. Now start to shape the core by sanding the corners to a nice radius and working your way down the runners. Leave them basically square at the plenum and transition to perfectly round at the ends where they meet the stubs. They should be the same as the inner diameter of the stubs seen by the impression marks that you made earlier. Foam seems to sand easier in one direction only. Use light pressure, take your time, and trial fit regularly to check progress. The core will be a little rough due to the grain of the foam, this is OK. Any chunks that get torn out need to be filled with wax or something that wont stick to epoxy. It will not take long however to get a perfect Styrofoam core matched exactly to your application.


The manifold is made of three plies of 282 2.7 oz bi-directional carbon re-inforced with extra plies at the plenum. The preferred laminating resin is PTMW Aeropoxy or Epolite 2184 and 2410 hardener as these have good high temperature properties and excellent peel strength. Any good laminating resin will probably do however. To avoid a mess and bumpy overlaps the layups pretty much have to be done in stages with a cure in between. Start with the plenum top or bottom and lay on 2 plies of carbon with the weave at 45 degrees to the long axis. Lap about 1 inch onto the base of the runners and fully up the sides of the plenum flush with the other side. Use gloves and conform it with your hands. Carbon is very stiff initially and takes about 1 to 2 minutes for the resin to soften it, then it conforms just fine. Now do the runners with 1 ply and overlap onto the plenum by to 1 inch. Wrap the small ends around the core best you can and allow an overhang which will be trimmed later. While your all sticky, layup some mount tab construction material on saran or plastic sheet about 4 inches square, 3 or 4 plies of carbon or glass. After cure transition the edges with a hard block and 40 grit sanding belt cut up, or paper ensuring you don't damage the exposed core. Then do the same layup as above to the other side overlapping the first layup on the sides of the plenum and runners. Use wet resin rich layups, shiny bit not dripping as pin holes will leak air. When cured it will be much easier to handle so transition sand all the edges and do it all over again trying to move your overlaps slightly to avoid a big bump. When you are finished you should have 3 plies on the runners overlapped on their sides to 6 plies, and 6 plies on the top, bottom and sides of the plenum. If you are mounting your carb to the plenum add a 3 or 4 ply pad over the mount point for added strength and to allow for sanding flat. When cured have a last look and if you are satisfied then place in your oven pre heated to 150 deg F and let soak for 30 min then gently raise the oven to 170 F and soak for another 60 minutes. This will raise the creep temp of the epoxy to about 220F. 

Now trim the head ends of the runners flush with the foam core. Locate and cut out your carb inlet hole(s) with a dremel and dental burr or hole saw. Take a chisel and dig out as much foam as you can reach from all openings. Dissolve out the rest with MEK or lacquer thinner. Gasoline is not recommended as it turns the foam into a thick goo which won't completely dissolve and makes a real mess. Pour it full of boiling water with a little dish soap to remove any wax if you used it. Now slosh the inside with epoxy thinned out with a little MEK or alcohol, let drain and cure. This should make the inside nice and smooth. Repeat if necessary but a slightly rough plenum area helps vaporize and mix the fuel. The runner ends at the heads need to be very smooth as this is the point of highest velocity, so sand the inside curves best you can with small strips of paper on your finger. 

Mounting tabs(s) are next. Mount your nearly finished work of art on the engine and locate where you wish to secure it. Mine has one tab only on the rear which ties into the inner of two bolts on the cover plate on the former oil filler neck which is long gone. I fill my oil through a small fitting I made on a rocker cover breather pipe. Use the construction piece you laid up earlier and cut it into a tab about 1.5 inch wide at the junction of the plenum and tapered to about 3/4 inch at the bolt. Tack it in place with a dob of 5 min. When set make a flox fillet both sides at the junction and layup 2 plies carbon both sides overlapping onto the plenum. Cure, trim and drill out the bolt hole. 

Block sand the carb mount pad until flat then drill the holes to mount your carb. Install AN-3 bolts from the inside up through the holes to make studs. Rough up the inside of the bolt heads and coat them with JB Weld to hold them in. If your carb has big mount holes as mine does make some spacers from aluminum pipe or whatever to make a nice fit to the AN-3s. Add a vacuum port if you like made from small pipe. Drill a corresponding hole somewhere in the plenum and secure it with JB Weld epoxy. OK almost done. FINAL FINISHING

To make your new manifold a true work of art it needs a nice finish. Final wet sand all bumps until presentable, then paint on a thinned coat of epoxy and let cure for a few hours until a firm tacky then bake at 170F until cured. This will fill most of the weave and look pretty good, but for a really nice look, wet sand it again and apply another epoxy coat as before. I took mine one step further and sanded it with 320 then sprayed on a clear coat of Dupont Chroma Clear. It looks like a mirror but this wont give you any more power.


The transition to the stubs is made with a 2 inch length of 1.5 in ID silicon hose and worm clamps, just like Lycont do it. The carbon should just touch the top of the stubs so there is no heat transfer and the manifold will run much colder than ambient due to the latent heat of vaporization of the gasoline. The worst case is after a 5 minute shutdown and re-start where the manifold gets heat soaked from the engine, but it wont get soft or collapse and once running will rapidly cool.