Carbon & Aluminium Intake Manifold

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I knew making the runners would be the most tricky part, so I decided to start with them. Ideally, these would be a continuous tube (i.e. not two halves bonded together) with a carefully defined cross sectional area and shape, blending from a rectangle at the port to a circle at the plenum. A good surface finish on the inner face is beneficial for minimum pressure drop, but I also want a good cosmetic finish on the outside as it will be very noticeable when the bonnet is up!

I devised a lost wax method of making these parts and set about making a mould for the 8 off wax plugs. Given the accuracy I needed, I decided the master pattern had to be CNC machined, so I spent some months designing and building such a machine. This will be used for other parts of the car, so it was not just a one-off for this plug!

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This was the first part I cut on the machine so there were a few teething troubles, but the PU tooling board is easy enough to repair with car body filler and re-machine. These are the lighter coloured regions in the pictures:

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A couple of layers of Pattern Coat along with guide coat and progressively finer grits of wet and dry, followed by polishing soon saw the finished master. This has shrinkage allowances added to it in order to compensate for the down stream processes

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The first attempt at a mould for the waxes was using an all plaster arrangement. This proved to be a dead end as the extremely porous plaster 'drank' the liquid wax before it could freeze! I therefore reverted to a silicone-plaster 'mother mould' technique using additive cure silicone as the tooling face. A plasticine dam was built around the part before pouring the first of two halves of the silicone.

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The box structure then allows the plaster to be poured. Note the black 'dots' are registration marks for the two halves of the plaster mould.

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The process can be repeated for the second half

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From here, I could then cast my first wax plugs. I moulded it over a piece of dowel to act as a handle during lay up

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After all this effort, I was finally able to lay up the runner itself. The schedule was 2 layers of gel coat, followed by 4 layers of 7.5 oz/sq.yd biaxial carbon sleeve and high temperature lamination resin. The stack was consolidated with a coated heatshrink before curing at room temperature. Once cured, I could post-cure the part, during which time the wax also melted out as planned.

There were a few issues with the first part - the finish on the wax wasn't great due to moisture still in the plaster (despite a week of drying time), runs in the gel coat (a little to liberal with the application) and trying to figure the best way of rolling the sleeve into place whilst getting the weave straight meant that the both the internal and external finish left a lot to be desired, but I am hopeful the next parts will be much better. This part was sectioned to check the lay up and there were some very small inclusions in the thickness of the wall (a hazard of not consolidating under vacuum) and a couple of air bubbles under the gel coat (I will use a roller next time!) but overall, I am pleased with the result so far - it certainly seems strong enough for my needs!

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Eight more to go then!