Intercooler Carbon Fiber Air Guide

TL;DR : adding performance and lightness

 

📜 Inspired by the mod seen last Summer on a Seven Sprint, a first aluminium air guide was built for my car when the engine was tuned. Over the two months I've been driving with it, OBDII data clearly showed an air intake temperature kept in check, very close to ambient. That first item did dissipate a fair share of the energy from the crash. It's bent but can be repaired, the plan is to send it to a friend that will use it on his Caterham 165.

🌡️ We have already driven over 2000km without an air guide in just the two weekends since I got the car back. Data showed intake temperatures in excess of 50°C, that's not ideal. That small experiment confirmed the added value of building a new one.

With the usual goal of keeping things as light as technically possible, I decided to use carbon fiber and placed an order for three 1mm thick 200x400mm twill sheets. I copied the original build and got a pair of 500mm 20x20x2mm corner beams, also in carbon fiber.

⚠️ SERIOUS WARNING : carbon fiber is a nasty and dangerous thing to work with. Personal protective equipment must be worn to work with it (gloves, some kind of mask or respirator, safety glasses). Preferably work outdoor if you use power tools.

If you want to build your own, the best is to do it with the car available for reference. While the aluminium one was 350mm wide, I measured that the length of the intercooler core is 300mm, and decided to make the new box that size.

I traced the bottom plate, then cut it down to 300mm using a steel hand saw. I laid it flat between the intercooler and radiator, with the back sliding around 5-10mm under the intercooler.

Rinse and repeat with the top plate, with the same dimensions of 200x300mm. This time it slides 5-10mm over the intercooler.  It was attached with adhesive tape.

I measured the height of the mouth of the first air guide, and checked that 75mm wouldn't interfere with the nose cone. I did cut a pair of 20x75mm cardboard strips and assembled everything with adhesive tape, got the nosecone back on the car, and confirmed there was no interference, the opening was correct and the top plate was following the inclination of the nose cone nicely.

Time for the sides then. The highest part is around 125mm on the intercooler side; the front is 75mm as previously established. Length is around 195mm, matching the distance between the intercooler core and front edge of the bottom plate. I finally got the Dremel out, fitted with an EZ-Lock metal cutting wheel. It proved surprisingly durable, with minimal wear for the whole project. The sides were cut from the third carbon fiber sheet, taking care of keeping a full 400mm length for the air scoop, to be built later. Long story short the first cut is a 125x400mm piece of carbon for the sides, and we keep a 75x400mm piece for the front blade/scoop.

 

The length of each side panel is measured, then cut from the dedicated 125x400mm piece. The factory-perfect edges are retained as the bottom edges. Height is double-checked on each side against the intercooler core, the incline down to 75mm is traced and cut. Once presented on the car, it appeared that the bottom plate and intercooler core face were not perfectly orthogonal. A final oblique cut of a few millimeters of the "125mm" edge was required. 

All pieces were finished (straightened...) and deburred with a metal file. The protective film was only removed after cutting and filing of all the edges. At this point I could test-fist "the box" using adhesive tape. I was left with very few carbon scraps as only a pair of 100x200mm rectangles, a pair of 50x195mm triangles and a 10x75mm rectangle are all that's left. With the longer 75x400mm piece for the scoop of course.

Corner pieces were cut to match the edges of the bottom and top plates (it was also close to 195mm I think). Bottom ones were trimmed a few millimeters on their vertical wall to match the contour of the "tank" (side) parts of the intercooler. The ones on the top have an oblique cut that also matches the face of the intercooler. Once again I took care of keeping as large leftovers of corner beams as possible, as they could be used as brackets to hold the front blade. The corner pieces were also deburred and finished with the file.

At some point it was fully assembled with tape once again, and the nose cone fitted back to the car for a final visual check from the inside.

As everything was going to be assembled using pop rivets, it was time to drill some holes. I had a assortment of black rivets in various sizes, and chose to go with 4x10mm. One reason for that choice is that the 3.2mm ones are a pain because I lack a suitable drill bit. With 4mm rivets, a 4mm metal drill bit did the job just fune. 

 

For the bottom plate I made sure to keep my rivets out of the way from the spaceframe (aka the car chassis) and radiator. The spacing looks balanced and pleasant, just a bit offset to the back. I did mark the left side of the bottom plate in-situ with a felt tip permanent marker, the holes would be 10mm from the edge. Position of those marks was mirrored to the opposite edge using the leftover 75x400mm piece of carbon as a ruler (aligned to the factory-cut side, it goes without saying). A small piece of wood was used as a backing against my foldable workbench (Stanley Fatmax), each hole center-punched with the automatic center punch, then drilled.

 

Position of the holes in the bottom plate were marked and drilled in the bottom corner pieces. And finally riveted right against the edges of the bottom plate, primary head of the rivets (nice-looking side) up against the visible part of the plate.

Holes for the vertical sides were first marked on the corner beams (on 20mm from each extremity, one perfectly centered, again 10mm from the edge of the beam), marked with the marker, punched, then drilled with the side pieces aligned in position. Rivets were installed from the outside of the beams to the inside of the box this time (same for all the others to come too).

The open box was presented to the car, with the lid still attached to the intercooler with tape, pictures show actually it's at that moment that I got the nose cone back for the final check. I wasn't locking the lower DZUS fasteners, I'd rather do it once to much than not enough. Anyway, the top corner pieces were aligned and holes for the top piece marked in-situ, again with a hole 20mm from each extremity, and one dead-centered. The corners were punched, and drilled with the top panel altogether. Once secured in place with rivets, similar holes were marked and punched  for the sides.

Drilling that last part was more involved and required a piece of wood to be hold in the vise. With the full box laying on that wooden beam, it was time to drill the final holes in the top corners and side plates at the same time. Thinking it through took more time than the execution. It was a rather quick job. The final 6 rivets were set, and the box was ready. As night was coming, I got the car back together and continued on the next day.

"How shall we attach this beauty ?"  you may ask. And ask you may. The air guide I was improving upon had three pairs of holes on the bottom, to secure it to the chassis using 3 zip ties. It honestly worked alright. You could certainly go with that. I designed a pair of clamps to be 3D-printed instead. I winged it one evening with a random default 20mm hole for the chassis member. I was very lucky that it was actually the correct size. A draft piece was printed to validate that the "C" clamp would be flexible enough to fit around the chassis, and I measured that I had 6mm between the tube and the bottom of the air guide. I did adjust the design with 7mm and went on with printing the final parts.

Time to drill the bottom of the box to fit those clamps then. With the box on the car, I marked the width of the chassis member on the side of the bottom corner beams as precisely as I could. Then moved to the desk to mark the center between those, mark a 20mm offset on each side of those marks as my clamps have a 40mm length, and finally a small cross for each hole, 10mm from the edges. Once again I punched using a piece of wood hold in the vise as a base, and this time drilled 6mm holes, leaving some wiggle room for the 5mm screws I would be using. Once the clamps were done printing, I pushed an M5 heat insert in each hole.

As I didn't like the idea of a direct contact with the radiator and the intercooler, I got a few meters of 3mm thick, 12mm wide adhesive foam sealing strip. On the third day, the whole (inside) top edge was lined, as well as the bottom of the front edge of the bottom plate, between the beams. On the beams I placed 2 layers of similar 1mm-thick tape. Just on the part that lay on the top of the radiator, not the full length, that is.

 

I did test-fit the clamps with the box to make sure everything was aligned, then clipped the C-clamps on the chassis, got the box back in place taking care of placing the foam seals correctly, and finally tightened the four screws and washers by hand. They were locked using the mini-ratchet and 4mm hex bit. Short screws are M5x12mm, long ones are M5x35mm. As it was getting dark, the nose cone and bonnet were put back on the car for the night, followed by the long nose shower cap. 

 

I moved on to the front blade/scoop. After some hesitation, I retained the idea of using the leftovers from the corner beams rather than designing parts to be 3D-printed. The easiest was to have the brackets attached from inside of the mouth of the air guide, rather than having them attached from the outside.

The 75x400mm carbon fiber piece was shortened to the width of the box (300mm - 6mm), with the long edge filed straight and deburred. I did also use the file to have rounded corners on each side of the front of the blade. Brackets were cut from the leftover corner pieces, cut to the same length, and only keeping the vertical wall for the inside of the box. That cut of the lower wall of the corner pieces was extended by about 10mm to keep them out of the way of the bottom of the box and top of the radiator. The edge against the radiator was lined with 1mm-thick foam seal.

Holes were marked, punched and drilled. Again with the corner piece and flat blade drilled together. The scoop assembly was riveted (this time using 3.2mm rivets) and positioned on the air guide to mark the position for the holes in the vertical walls of the brackets and side panels of the box. In the end they were aligned similarly to the ones of the blade assembly. They were punched and drilled once again resting on a piece of wood on the foldable bench, then the scoop was aligned on the box and the position of the holes transferred to the sides of the box from the inside. 

This time the piece of wood was hold against the outside of the box while it was punched a few times from the inside. As the cordless drill was narrow enough, it was possible to drill from the inside too.

As I wanted the scoop to be removeable in case I need to adjust its angle of make it narrower, a pair of stainless steel bolts nuts and washers were used on each side.

Overall that project took me 5 or 6 hours over the span of 3 days, not accounting for the design of the C-clamps that would add maybe an extra hour or so. I would be much quicker if I had to do another one, as most of the time was allocated to thinking rather than doing.

Regarding weight, the aluminium air guide was close to 990 grams, let's say 1kg with the zip ties. The new one was just over 350 grams without the front scoop and 3D-printed brackets. It's probably close to 400 grams overall. I've removed 60% of the weight, I'm happy with that result. One could shave some extra grams by using 1mm thick corner pieces, and even have them in 15mm rather 20mm. The front blade could be discarded, and you could use zipties to secure it to the car.

A very minimalist and low-cost approach would consist in a single sheet used to create a partition between the back of the radiator and the front of the intercooler.

About costs, I paid 46€ for three 200x400mm sheets and 45€ for a pair of 500mm corner pieces. Throw in the adhesive foam seals and let's round that up to 100€. In real life it looks like a million bucks. Too bad it's hidden so deep under the nose cone. The upside is that it will remain safe from UV radiation.