Project Miatabusa Part 11 – Shorai Lithium Iron Battery (and Lots of Plumbing)

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Fuel Plumbing Fun 

Most port fuel injection systems, car or bike, run at a standard 43 psi of fuel pressure. But 43 psi relative to what? NA Miatas have a manifold-referenced fuel pressure regulator that keeps the pressure drop across the injector (from the fuel rail to the air inside the manifold) at 43 psi. NB Miatas have a returnless system that runs a constant 43 psi relative to the outside air, regardless of what’s going on inside the manifold.
Turns out Hayabusas have also played the fuel pressure game both ways, depending on their year. I’m not sure which years of Hayabusas switched, but I do know the engine we’re using was designed for manifold referenced fuel pressure. It’s pretty important that you get this right. At wide-open throttle, both systems have the same fuel pressure, so peak power won’t be impacted, but at lighter loads, there can be up to a 10 psi difference between a manifold referenced fuel system and a constant pressure system.
Cars with manifold-referenced fuel pressure have a feed line, a regulator on the fuel rail, and a return line. Constant pressure cars have only a feed line, and the regulator is back in the tank. Bikes on the other hand, have their regulator in the tank either way. The reason is simple. The tank is so close to the intake manifold on a bike that it’s no problem running a manifold pressure signal to the in-tank regulator. As a result, the Hayabusa can have a dead-end fuel rail with no return line, but still run manifold-referenced fuel pressure.
So how to get that manifold-referenced fuel pressure to the the dead-end Hayabusa fuel rail in a Miata?
Project Miatabusa fuel pressure regulator radiator and oil cooler plumbing
I solved this problem with this simple block I machined from a discarded chunk of billet fuel rail.
Project Miatabusa fuel pressure regulator radiator and oil cooler plumbing

This mini fuel rail has a feed port, an output port, and a return that flows through a stock Miata fuel pressure regulator. Either of the brass ports can be the feed and the Miata regulator will keep the fuel in the mini-rail (and the fuel going downstream) 43 psi higher than the manifold. If you happen to have a Hayabusa engine that wants a fixed 43 psi, this same system will work, just don’t connect a vacuum line to the fuel pressure regulator and it will reference to atmosphere.

Project Miatabusa fuel pressure regulator radiator and oil cooler plumbing

I mounted the regulator rail to the brace on the back of the bellhousing adaptor. This will be the only Miatabusa with a separate regulator rail like this. Future adaptors will extend high enough to bolt directly to the cylinder head, making the whole assembly more secure, and the port for the fuel pressure regulator will be machined directly into the bellhousing adaptor.

Project Miatabusa fuel pressure regulator radiator and oil cooler plumbing

Tiny detail downstream of the regulator: The fuel feed faces the wrong direction. This could be solved with a little loop of fuel hose, but there’s a more elegant solution.

Project Miatabusa fuel pressure regulator radiator and oil cooler plumbing

Unscrew the rail and it’s actually a series of plastic injector mounts with connecting fuel pipes.

Project Miatabusa fuel pressure regulator radiator and oil cooler plumbing

Turns out the feed pipe is directly interchangeable with the connecting pipes, so you can assemble this fuel rail however you want.

Project Miatabusa fuel pressure regulator radiator and oil cooler plumbing

Simple! This feed line is also designed for an O-ring sealed hard plastic fuel pipe, so there is no barb to hold on a conventional fuel hose. Once you remove the grey plastic doodad, there is a steep hump that some 5/16 fuel hoses will actually push over, provided you talk sweetly and use lots of lube. If you manage this, put the hose clamp on the far side of the hub and you’re set.

I used 300-psi GoodYear fuel hose, though, and it was too beefy to stretch over such a big hump, so we had to use a brake line flaring tool to put a little hump on the end of the pipe. Without the hump, a fully-clamped 5/16 fuel hose could still be pulled off by hand pretty easily.

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