Performance Testing: Dyno Results Before and After 13b Porting on 13b-rew Engines

Performance Testing: Dyno Results Before and After Porting on 13B-REW Engines

Performance testing is a critical aspect of understanding how modifications affect engine output. For enthusiasts working with Mazda’s iconic rotary platform, dyno testing provides objective data that separates genuine gains from speculation. In this article, we will explore the dyno results before and after porting on the 13B-REW engines. These engines, known for their compact rotary design and high-revving capabilities, are a staple in the automotive performance community. We will cover the fundamentals of the 13B-REW, explain what porting involves, detail our testing methodology, present baseline and post-porting results, and discuss what these numbers mean for real-world driving and racing applications.

This guide draws on extensive experience with rotary engine development and is intended for builders, tuners, and anyone looking to maximize the potential of their 13B-REW. Whether you are building a street car, a track weapon, or a drag racer, understanding the impact of porting is essential to making informed decisions.

Understanding the 13B-REW Engine

The 13B-REW is the second-generation twin-rotor engine from Mazda, produced from 1992 to 2002. It succeeded the 13B-RE and brought sequential twin-turbocharging to the RX-7 FD3S. Unlike conventional piston engines, the 13B-REW uses a Wankel rotary design where triangular rotors spin inside an epitrochoidal housing, generating power through continuous combustion cycles.

Key specifications of the 13B-REW include:

Displacement: 1.3 liters (654 cc per rotor) – equivalent to approximately 2.6 liters in a four-stroke piston engine when considering power cycles.
Rotor configuration: Two rotors with a 12A-type housing geometry, later refined for improved sealing.
Induction system: Sequential twin-turbochargers (primary and secondary) designed to reduce lag and provide broad power delivery.
Compression ratio: 9.0:1 (factory) – relatively high for a turbocharged rotary, allowing strong low-end response.
Redline: 8,000 RPM stock, with aftermarket builds often safely revving to 9,000 or more.

The 13B-REW’s unique characteristics make it simultaneously beloved and challenging. Its smooth power delivery, high rev ceiling, and distinctive exhaust note are offset by a reputation for apex seal wear and high oil consumption. Porting, as we will discuss, is one of the most effective ways to unlock the engine’s true potential while addressing some of its inherent airflow limitations. For more details on the rotary cycle, refer to Mazda’s official history material or technical resources at Mazda's rotary technology page.

What Is Porting?

Porting refers to the modification of the intake and exhaust ports of an engine to improve airflow. In rotary engines, porting is fundamentally different from piston engine porting because the ports are machined directly into the housing side plates (irons) rather than into a cylinder head. The shape, size, and timing of these ports dictate how efficiently the engine breathes at various RPM.

There are three main types of porting for rotary engines, each offering different performance characteristics:

Street porting: A moderate enlargement of the intake and exhaust ports, typically increasing port area by 15–25%. Street ports retain smooth idle characteristics and reasonable low-end torque while improving mid-range and top-end power. They are ideal for daily drivers and street-focused builds.
Race porting: More aggressive than street porting, with port area increases of 30–40%. Race ports shift the power band higher, sacrificing some low-end response for significant top-end horsepower. Suitable for track cars and weekend racers who keep the engine above 4,000 RPM.
Bridge porting: The most extreme form, where additional “bridge” passages are cut into the housing to allow extra airflow at high RPM. Bridge ports can increase peak power dramatically but often cause erratic idle, poor part-throttle response, and increased emission issues. Typically reserved for dedicated race vehicles.

For this test, we opted for a street port with a slight bias toward the upper end of the street port range (approximately 25% increase in intake port area). This choice balances drivability with measurable power gains, making it relevant to the largest audience of builders. A detailed guide to port shapes and angles can be found at Atkins Rotary’s porting resources.

Dyno Testing Methodology

To assess the impact of porting on the 13B-REW engine, we conducted a series of controlled dyno tests using a Dynojet 224x dynamometer. The engine was a stock 1994 13B-REW with 50,000 miles, fresh oil, new spark plugs, and OEM sequential turbos. No other modifications were made besides the porting itself. The same dyno, same ambient conditions (75°F, 30% humidity, sea level), and same fuel (93 octane pump gas) were used for both baseline and post-porting runs.

Testing procedure:

Baseline runs: Three full-throttle pulls from 3,000 to 8,000 RPM, with correction factors applied according to SAE J1349 standards. The engine was allowed to cool between pulls to ensure consistent intake air temperatures.
Porting modification: The engine was removed, disassembled, and the intake and exhaust ports were modified by an experienced rotary machinist using a template for street port dimensions. All irons were port-matched and finished with 600-grit abrasive to ensure smooth surfaces. No changes were made to the turbo system, fuel delivery, or engine management.
Reassembly and re-testing: After reassembly, the engine was run in for 200 miles of mixed driving before returning to the same dyno facility. Three more pulls were performed under identical conditions to ensure repeatability.

The dyno used measures wheel horsepower, which we then corrected to estimated flywheel horsepower using a fixed drivetrain loss of 15% (typical for the FD3S RX-7). All results presented here are flywheel values for easier comparison with factory ratings.

For more on dyno testing best practices, refer to Dynojet’s automotive testing guide.

Baseline Dyno Results

Before any modifications, the baseline dyno results for the 13B-REW engine showed a maximum power output of approximately 248 horsepower at 6,900 RPM and a peak torque of 212 lb-ft at 5,400 RPM. These numbers are consistent with a healthy, factory-tuned 13B-REW in good condition. The torque curve peaked early and then gradually declined, typical of the sequential turbo system’s behavior as the secondary turbo takes over.

Important to note: the factory rating for the 13B-REW in the FD3S RX-7 was 255 hp (JIS net) for the US market, so the measured 248 hp suggests the engine was slightly below spec, possibly due to mileage and the age of the turbos. However, this provides a realistic baseline for a well-maintained engine before porting.

We also recorded air/fuel ratios and boost pressure. The baseline run showed a peak boost of 10.0 psi, with the primary turbo providing boost early and the secondary progressively bringing in the remainder. Air/fuel ratios hovered around 11.5:1 under full load, rich but safe for the factory tune.

Porting Process

The porting process itself took approximately 12 hours of machining time plus additional time for disassembly and reassembly. Key steps included:

Disassembly and inspection: The engine was stripped completely, rotors and housings removed, and all iron plates cleaned. We verified that there were no pre-existing seal issues or housing wear that could affect the porting results.
Intake port widening: The intake ports on the front and rear rotor housings were widened from their stock dimensions by approximately 3mm in the direction of rotation, following a street port template. The port opening timing was slightly advanced by machining the leading edge of the intake port.
Exhaust port reshaping: The exhaust ports were opened by about 2mm on the trailing edge to improve scavenging. No changes were made to the auxiliary ports (the small secondary ports that help low-RPM operation).
Polishing and smoothing: All port surfaces were hand-polished with increasing grits up to 1,000 to reduce turbulence. Special attention was paid to the port entries and exits to avoid sharp edges that could cause flow separation.
Reassembly with new seals: The engine was reassembled using fresh apex seals, side seals, and corner seals to ensure a proper seal and to eliminate any variables related to worn seals. The oil metering pump was recalibrated to factory spec.

The porting was conservative—no bridge ports or race port aggressive timing. This ensures the findings are applicable to most street-driven rotary builds.

Post-Porting Dyno Results

After completing the porting modifications and the 200-mile break-in period, we returned to the dyno for testing. The results were a clear improvement across the entire RPM range:

Peak power: 302 horsepower at 7,400 RPM – a gain of 54 horsepower over baseline (an increase of 21.8%).
Peak torque: 237 lb-ft at 5,800 RPM – a gain of 25 lb-ft over baseline (an increase of 11.8%).
Power band: The engine now held over 280 horsepower from 6,500 to 7,800 RPM, whereas the baseline dropped off after 7,200 RPM.

The air/fuel ratios remained safe between 11.2:1 and 11.8:1, indicating that the stock fuel system and turbochargers could still supply adequate mixture despite the increased airflow. Boost pressure increased slightly to 10.5 psi peak, likely due to reduced backpressure from the enlarged exhaust ports allowing the turbos to spin more freely.

Perhaps most striking was the change in the torque curve shape. Instead of a sharp peak at 5,400 RPM followed by a decline, the post-porting curve showed a fatter, flatter torque plateau from 4,500 to 6,500 RPM, with torque staying above 220 lb-ft for a broader span. This translates to stronger mid-range pull and less need to wind the engine to high RPM for passing power.

Performance Comparison

The comparison of dyno results before and after porting highlights the effectiveness of the modifications. Below is a side-by-side summary:

Metric	Baseline	After Porting	Change
Peak Power	248 hp @ 6,900 RPM	302 hp @ 7,400 RPM	+54 hp (21.8%)
Peak Torque	212 lb-ft @ 5,400 RPM	237 lb-ft @ 5,800 RPM	+25 lb-ft (11.8%)
Redline Power	220 hp @ 8,000 RPM	270 hp @ 8,000 RPM	+50 hp
Max Boost	10.0 psi	10.5 psi	+0.5 psi

While peak power gained the most percentage-wise, the real-world benefit is the broader torque curve and the engine’s ability to sustain high power deeper into the RPM range. For track driving, this means fewer gear changes and more time spent in the sweet spot. For street driving, the improved mid-range makes the car feel noticeably punchier without needing to rev to the moon.

Factors Affecting Results

No two dyno tests are identical, and several factors can influence the magnitude of gains from porting. Recognizing these variables helps set realistic expectations for your own build:

Initial condition of the engine: A worn engine with poor compression or weak seals may show smaller gains because the porting cannot compensate for sealing losses. Conversely, a fresh engine with good seals will respond better.
Tuning: Our tests were conducted with the stock ECU and fuel system. With a standalone ECU and proper tuning, the gains from porting could be further optimized, potentially reaching 330–350 hp with the same turbo setup. The factory tune is conservative and leaves room for improvement.
Turbocharger match: The 13B-REW’s sequential turbos are designed for stock port sizes. After porting, the turbos become the limiting factor. Larger single-turbo conversions or upgraded twins would extract even more from the ported engine, but that was beyond the scope of this test.
Porting quality: Not all porting is equal. A novice job with rough surfaces or incorrect timing could hurt performance or even cause drivability issues. Always use a skilled rotary specialist with proven templates.
Calibration of the dyno: We used a consistent Dynojet for all runs, with environmental corrections applied. Differences in dyno type or correction factors can make results incomparable with other published data.

For more in-depth analysis of rotary porting theory and empirical data, Rotary Aviation’s technical articles provide excellent supplementary reading.

Conclusion and Recommendations

The performance testing of the 13B-REW engine before and after porting demonstrates a notable enhancement in power and torque across the operating range. With a conservative street port, we achieved a 54-horsepower peak gain and a significantly broader torque plateau, all while maintaining factory drivability and reliability. These results confirm that porting is a valuable modification for those looking to maximize the performance of their rotary engines.

For enthusiasts planning a similar upgrade, we recommend the following:

Assess your engine’s health first. Perform a compression test to ensure the apex seals are in good shape before investing in porting.
Choose a port type that matches your usage. Street port for daily drivers and occasional track days; race port for weekend warriors; bridge port for dedicated race cars only.
Invest in a proper tune after porting. While the stock ECU may cope moderately well, a standalone ECU with a dyno tune will unlock the full potential of the increased airflow.
Consider supporting mods such as a larger radiator, upgraded fuel pump, and better intercooling to handle the extra heat and demand.
Work with a rotary specialist who has experience with the 13B-REW. Incorrect port dimensions or poor finishing can ruin otherwise good housings.

Porting remains one of the most cost-effective ways to increase power on a rotary engine. Combined with proper tuning and maintenance, a ported 13B-REW can provide thrilling performance that respects its legendary heritage. Whether you are chasing lap times or simply want more grins per mile, these dyno results prove that porting is a worthy investment.

For further reading on performance modifications for the 13B-REW, including turbo upgrades and fuel system changes, consult resources like the RX-7 Club’s technical forums where real-world builders share their dyno sheets and experiences.