Project FD RX7 Restomod: Part 13 – Porting the 13B REW

Semi-Peripheral Porting (“Semi-PP) are becoming increasingly popular, where a small peripheral port is added to the aluminum housings and the side ports in the engine irons are retained.  This is extremely effective at improving air flow and power at high rpm, even more than Bridgeports.  Increased overlap from Semi-PPs create the “brap, brap, brap” exhaust note and exhaust volume is also increased. There are companies who are offering intake manifolds with extra runners that feed these ports and bolt right up to the engine.  Semi-PPs are rumored to have been around since the 70’s as a means of cheating the rulebook in road racing and sneaking more air into the engine through the intake manifold.

Semi-pp’s are cut through the water jacket and require epoxy like JB Weld to seal the water jacket and the port tube in place.  Fortunately, this is on the cold side of the motor so the change in cooling should not impact the block that much.  Due to the high rpm air flow of Semi-PP’s, this port is used a lot in boosted applications to flatten the torque curve well past 8-9,000rpm and are considered more effective than Bridgeports.

One major advantage of Peripheral Ports and Semi-PPs is that the center of the apex seals is exposed to cold air as it passes over the port opening.  This briefly gives the seals a break from constantly touching the hot housing, which reduces the temperature and greatly improves the reliability of the apex seals.  Some apex seals that have durability problems tend to thrive and surpass other apex seals in reliability and performance when used in Semi-PP applications that better cool the apex seal.

Many consider this an extreme port that is not good for street driving due to the belief that overlap is increased, having “zero” low rpm power and response, jerky part-throttle drivability, and a poor idle.  However, not all Semi-PPs are made the same and the diameter of the port, and the location/angle of the port affects flow and timing of the additional port.

PORTING VARIABILITY

Rotary engine porting can be a controversial topic because there are no standards or numerical defining parameters for different port types, the difference between a “Mild” or “Aggressive” Street Port, a “N/A” vs “Turbo” port, and the difficulty of measuring changes to the port duct shape, which plays a crucial role in the performance of the engine.  The three main port categories that we mentioned (Street, Bridge, Peripheral) are just fundamental outlines of how and where the housings are modified to improve airflow.

Generally speaking, things that are not well-understood, defined, or easily measured are often considered an “art” or “black-magic” and those with decades of experience testing different iterations tend to not freely share their hard-earned knowledge and secrets.  Rotary engine porting, piston engine porting, and camshaft design are prime examples of this.

While it’s easy to measure fundamental aspects like the port shape/timing of a rotary (which are rarely discussed), or the camshaft lift and duration on a piston engine; it is the nuances in the design and execution that are not easily measured, often overlooked, and misunderstood which tend to play a far larger role in the peak power, powerband, character, durability, drivability, and idle quality of the engine.  Because of this and the countless variables in how the port duct can be shaped; the end result of a port job can vary greatly from one engine builder to another.

When it comes to porting templates, one size does not fit all and bigger is not always better.  Templates that are sold online are not always the best, most advanced, or ideal for various applications.  A given intake porting template will not always be efficient or ideal for both a stock and modified exhaust port.  The relationship between intake and exhaust port timing and overlap is crucial and it is imperative that they work together as a system.  Very minor changes in not only overlap, but the shape of the port and the rate the port opens and closes also plays a major role in the overall character and performance of an engine.

Intake and exhaust templates should be sold together and matched for a specific application and goal, but the vast majority of companies who sell templates online do not do this, have vague descriptions of the port shape, do not list port timing numbers, and leave customers to their own devices without much direction.