Project FD RX7 Restomod: Part 13 - Porting the 13B REW - Page 2 of 8

Stock 13B REW Ports FD RX7 The 13B REW engine features two different size ports. A smaller primary port (left) that is located on each side of the center iron, and a larger secondary port (right) that resides on the front and rear iron of the 13B. Note the roughness of the port and how it does not match the port opening very well.

Rotary 13B intake ports On the block of the engine, the corresponding primary ports on the center iron are skinnier, while the outer secondary ports are larger on the front and rear iron and feed the larger secondary port opening inside the engine.

There are a few main types of porting that is done to the rotary engine: Street Port, Bridgeport, and Peripheral Port.

STREET PORTING

This is the most common and mild porting option that results in moderate performance gains with little to no compromises in gas mileage, drivability, or noticeably increased exhaust volume. Street ports generally feature a mild port cleanup with a small increase in size while staying “within the lines” of the oil and corner seal tracks. However, there is a pretty broad spectrum of variability from “Mild” to “Extended” and “Large/Aggressive” street ports.

Mild Street Ports typically clean up the port shape and open up the upper area of the port slightly to delay port closing and improve air flow and higher rpm gains. Extended Ports further extend the upper and lower area of the port, improving power at higher rpm. Aggressive Street Ports increase the port size further while advancing the port timing which greatly increases power in the midrange and spools turbos much quicker. This also increases overlap between the intake and exhaust, and any time you increase this overlap, low RPM performance and idle quality suffers and requires a slightly higher idle. Sub 1,000rpm idles can still be achieved with most street porting.

“Race-Port” is what some engine builders call an even more extreme street-style port with more overlap and drivability issues.

BRIDGE PORTING

Bridge porting increases airflow into the combustion chamber by cutting a thin slit or “eyebrow” above the main port while leaving a strip or “bridge” to support the rotor’s corner seal as it passes over the port.

This is considered the first stage of “race porting” and provides massive gains in high RPM airflow and power. As overlap increases from more aggressive bridge porting, idle quality quickly suffers and the infamous “brap, brap, brap” exhaust note becomes more pronounced. Because of this, the idle rpm must be increased to prevent the engine from dying, low rpm response and performance starts to suffer, and the exhaust volume becomes noticeably louder.

13B RX-7 Rotary Half Bridge Port Bridge porting falls into two main categories: Half-bridge and Full-bridge. Half-bridge Port adds the additional “eyebrow” ports only to the outer secondary intake ports in the rotary’s side housing/iron, while the primary ports on the center iron is just street ported. This somewhat gives you the best of both worlds by minimally affecting the low rpm performance and improving high rpm airflow.

Full-Bridge Porting adds “eyebrow” ports above both the primary and secondary intake ports, greatly improving high rpm flow. This is popular on naturally aspirated rotaries, which tend to use very long Bridgeports that significantly advances the port timing and overlap, produce more high RPM power, but are often only recommended for race cars due to their compromises like requiring much higher idle RPMs and worse low-rpm performance.

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