44mm Carburetor on Racing Beat Upper Manifold and Mazda OEM Lower Manifold.
48 DHLA Carburetor on Racing Beat Upper Manifold and Mazda OEM Lower Manifold.
Racing Beat Holley Kit (PN 18045) (Less air cleaner - Carb reversed on manifold).
Ahh yes.. The lovely 73 Mazda RX-2.
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Michael Ferrara is a nice guy. He also happens to be the publisher of DRAG Sport magazine. Five months ago, he asked if I would be interested in contributing to this new publication and I agreed. Then, not long ago, Michael sent me an email suggesting a topic for this issue's article. The suggested article would involve dyno testing a street-ported 13B 6-port engine with a variety of carburetors. Some further conversation revealed that DRAG Sport already had a freshly-assembled '88 6-port 13B that was street ported by Rotary Power of Gardena, California. This engine was destined for Michael's own '73 Mazda RX-2 (just one of the perks of being the publisher). I agreed to the project, but I did have my reservations.
Our Previous Experience
About 10 years ago, we here at Racing Beat tested a street-ported 13B 6-port engine with a 50mm Mikuni, a 50mm Weber and a 48mm Dellorto carburetor. The results we obtained didn't warrant production or further development of the carburetor kits or the templates for porting. For years, we advised everyone interested in street porting a 13B to seek out the 4-port housings.
In any case, I was stuck with a 6-port 13B with a street-port that was a complete mystery to me. On top of that, I had also been asked to try three different carburetor and intake combinations. The first combination would be a Mikuni PHH 44mm sidedraft carburetor; the second would be a Dellorto DHLA 48mm sidedraft carburetor; and the third would be a Holley 600cfm four-barrel carburetor. What fun! The Holley is not easy to work on for development work since many of its tuning components are not easily replaced. Enough complaining, it's time to get to work.
Theory of the Street Port
Before jumping into the dyno results, it's important to understand "streetable" porting in general. Porting is especially critical with respect to the rotary engine's intake ports. There are essentially two families of 13B engines, 4-ports and 6-ports. Either type can be "street-ported." Thus, the port area and port timing can be changed by grinding (see Figures A, B and C). The equivalent modification in a piston (reciprocating) engine is accomplished by porting the cylinder heads and changing the lift, duration and timing events of the camshaft. On a rotary engine, all of this is accomplished with a grinder. In a 4-port engine, it is typical that all four intake ports (two on intermediate housing and one on each of the end housings) are enlarged and shaped to the same size. However, on 6-port engines it is impossible to get the two ports on the intermediate housing to match in size and shape to the ports on the end housings. There just is not enough material on the intermediate housing to allow the port to be opened as large as the ports on the end housings. Trying to match these intermediate ports to the end-housing ports will result in breaking through to the water jacket and rendering the intermediate housing useless. With these limitations in hand, many engine builders start making poor choices. Because these engine builders want to go really fast, they enlarge the top of the upper port in the front and rear housings, regardless of the fact that these ports are already larger than traditional 4-port street ports. This process is then followed by enlarging the ports of the intermediate housings as much as one dares without breaking into the water jackets. This process leaves the porting seriously unbalanced between the intermediate and end housings. When the 6-port actuators and valves are removed from the engine (as they had been in this case), the problem becomes even worse.
Closing the Intake Event & 6-port Actuators
We have compiled Figure D to offer a direct comparison of the "intake" closing events of various porting methods. As shown, raising the top of the upper port in the "6-port Street Port" view can get pretty extreme, especially when you consider that the ports in the intermediate housing can't match the timing of the 6-ports end housings. Using the 6-port valves and actuators helps the situation when not at full throttle. However, at full throttle the unbalance remains.
Regarding the 6-port valves and actuators, our testing has shown that the factory 6-port valves open between 4000 and 5000 rpm at full throttle. Below 4000 rpm, power is optimized with the valves closed. Above 5000 rpm, power is best with the valves open. Between 4000 and 5000 rpm, there is no appreciable difference with the valves open or closed.
For more detailed specs, dyno sheets, and comparions pick up a copy of Issue #3 at newstands and tuning shops nationwide!
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