Project Miatabusa Part 11 - Shorai Lithium Iron Battery (and Lots of Plumbing) - Page 3 of 5

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.

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.