This week's half step toward 11,000 rpm Miata madness is a good solid head start toward making a workable intake manifold. Starting from virtually nothing, the fertile mind and ghettofabrication skills of Miatabusa braindaddy (that's the technical term for someone who father's an idea) Alex Vendler produced a believable plan and a compelling prototype using the tools and supplies scrounged from the hardware store, a costume shop, my garage, the local sex shop, and the sidewalk in front of my house. It was an entertaining weekend…
One of the trickier bits of converting a motorcycle engine to car duty is coming up with an intake manifold that will fit under the hood. In its 2-wheeled home, the Hayabusa's air box (which serves triple duty as a cold-air intake, air filter housing and intake manifold plenum) sits on top of the engine. The intake port is beautifully straight and hits the intake valve at a nice, high angle that promotes good high-rpm breathing, but even with the air box removed, this layout puts the air horns perilously close to the hood.
Our first simple attempt at hood clearance was simply to turn the two center trumpets around so they bent down instead of up. It actually looked like this might work, since the shorter outer horns give a bit more hood room.
Using the volume of the stock air box as a guide, Alex roughed out a cardboard mockup of a plenum to check clearance.
It was close, but not only did the hood object by about an inch, the outer horns were getting their breathing restricted by the roof of the plenum. This idea simply wouldn't do.
We aren't the first to do something silly like this with a Hayabusa, so we checked out some prior art for inspiration. The Hayabusa-powered rally Starlet I spotted in New Zealand last year simply used 4 silicone elbow hoses in place of the factory air horns. This is temptingly simple, and it negates some of the hood clearance concerns because the silicone can rub on the hood without really damaging anything. Ferdinand Suzuki carefully engineered those staggered air horns, though, and we're pretty sure we don't want to mess with the runner length that much. The Kiwi Toybusa technique will be kept as a last resort.
Another possibility was spotted on this Hayabusa-powered go kart dressed in Honda Z600 skin. Sadly, I didn't really photograph this car in much detail, as I spotted it years before this project came along. It appears that the builder added some aluminum elbows between the head and the throttle bodies. This lengthens the intake runners just as the silicone hose idea did, but it also moves the injectors away from the ports. The Hayabusa's injectors are tucked under the throttle bodies, aimed down the ports. WIth this layout, they're now pointed at the outside of the bend, spraying on the runner wall. At full wail, this probably won't matter at all, but at part throttle, this wall wetting will lead to inaccurate and inconsistent fuel mixtures. Might as well have carburetors at that point.