Michael Ferrara (aka the Publisher of DRAG Sport) is at it again. After the last article (three different carburetors on a street-ported 6-port 13B), he asked us to go one more step with the engine - fuel injection. Shortly thereafter, we received a TWM Induction throttle body complete with fuel rail and pressure regulator, four 750cc/min Bosch fuel injectors from R.C. Engineering and a Haltech F10 ECU (Version 7.08) with harness.

The Throttle Body
The TWM 50mm throttle body (p/n 2900-5002) had the bore spacing and bolt pattern to fit in place of a Weber (or Dellorto or Mikuni) sidedraft 2-barrel carburetor, so it fit right on the manifold we used previously. However, there is one thing to note about the TWM throttle body: the overall length of the throttle body was either 4.82 or 5.38 inches (depending on the air horn selected), while the Dellorto and Mikuni we had used previously were about 6.65 inches overall. As you will see later, overall length does affect the power output. This was demonstrated best with the TWM throttle body when we tried both lengths (25mm and 50mm air horns) with interesting results. As you see in the pictures, two injectors, along with the fuel rail and pressure regulator, are all part of the assembly. TWM also offers the throttle bodies with four injector bosses.

The Injectors, Fuel Flow & Horsepower
The RC Engineering injectors are Part # PB8X-750, flow checked at 750cc/minute at 43.5 psi. Actually, at this point, we had determined that the cart was quite some distance ahead of the horse. You should first calculate what components you need, then order them. Of course, there are very knowledgeable people who can advise you on component selection, but they often don't have all the information, and that can lead you astray.... My point here is that you might assume that the best way to run the engine would be to run the engine on four injectors - two in the throttle body and two in the intermediate housing where Mazda normally mounts their primary injectors. Trouble is, the fuel wouldn't be distributed uniformly in the air. True, all the fuel still goes into the appropriate rotor housing, but the charge may well be "stratified", that is, not mixed uniformly, and therefore not burn smoothly (see Figure 1). For this reason, I was interested in the possibility of running only two injectors in the throttle body and none in the intermediate housing. So we will take a moment and look at the mathematical situation. Could we only use two of these injectors and still have enough fuel to feed the engine?

Learn more about our results including detailed schematics and charts inside Issue #4 on newsstands and in tuning shops nationwide.