13BT vs. 13B-REW: (Last revised Aug. 14, 1997)

There are two types of 13BT blocks, one from the 87-88 Turbo RX7 (FC3S GTR) and one from the 89-91 Turbo RX7 (FC3S GTX). While the older 13BT GTR block differs a lot from the 13B-REW block, the GTX block is some what a compromise in between. Most of the changes made by Mazda engineers are refinements to address the problem areas of the previous designs. For comparision purposes, the 13B-REW motor described in this article is the one from the FD3S, not one from the Japan-only Mazda Cosmo.

One significant difference on the 13B-REW is the areas around the dowel pins on the side housings. They are reinforced and ribbed with thicker castings. This is a major problem area on high output 13BT engines, as they tend to crack at high power levels and is the main reason that the 13BT blocks are not reliable beyond 400HP. Most competent rotary rebuilders will look for this before rebuilding a 13BT motor. The 13BT GTX blocks are said to be more reinforced than the earlier motors but not as significantly as the newer 13B-REW designs. Before the introduction of the 13B-REW motors, some engine builders experimented with brazing extra metal around the dowel pins area on the side housings of the 13BT motors.

The port sizes on the 13B-REW are extremely larger than the 13BT's, especially the primary ports on the intermediate housings. This is accomplished by increasing the height of the side and intermediate housing where the ports are located and relocating some of the coolant passages inside the housings. The larger ports increase the duration and "lift" of the intake stroke, allowing more air to be ingested into the combustion chamber. While the intake ports are larger, the intake runners of these motors are still about the same size. The 13BT intake manifold has a surge tank plenum which is found to create too much pumping losses because of its large volume. The 13B-REW intake manifold is a "Dynamic Pressure Intake System" which eliminated the surge plenum tank and has opposed-facing secondary intake runners. This new design enhances intake flow by taking advantage of the strong pressure waves inherant in rotary engines. The throttle bodies on the 13BT are 3x45mm, while the 13B-REW are 1x45mm + 2x50mm.

  • Intake Runners: Upper, Lower
  • Plenum: 1, 2
  • Runners: 1, 2
  • Contrary to popular beliefs, the 13BT's and 13B-REW share many internal components. The 13B-REW rotors are interchangable with the 13BT GTX rotors, although they don't have the same part number and thus are not the same part. But they do have the same compression ratios of 9.0:1 and they weigh the same. The CR of the 13BT GTR rotors are 8.5:1. I believe the later model rotors are manufactured from a more accurate milling technique while the older ones are left partly casted. This is evident in the bathtub faces of the rotors.

    Almost all the internal seals and springs are the same parts in the 13BT and 13B-REW. This includes the 3-piece cast-iron 2mm apex seals which are found on all post '86 rotary motors, both turbo and non-turbo. With the exception of the side seals and corner seal springs, all the other rotor parts of the two motors such as oil seals, rings, springs, side seal springs, and corner seals, are the same parts. The newer design corner seal springs are reccommended for use in all rotary rebuilds.

    The eccentric shafts for these motor seem to all look the same, but the 13B-REW shaft has a larger oil bypass/crank pulley bolt. The newer motor surprisingly maintains the same warm-up oil-bypass pellet as the older motors. The 13B-REW motors does have larger front-end thrust bearings than the 13BT motors. These larger bearing components require the use of a different front counterweight. The larger thrust bearings allow better loading and thus a tighter torque setting of the pulley bolt. This design will reduce the chance of the eccentric shaft bending at the front section, which is sometimes a problem on high-output 13BT motors. The large thrust bearing is probably also designed for use with the pull type clutch assemblies found in the FD3S.

    The front cover of the 13B-REW has one more attachment bolt which prevents the gasket from blowing out, which is a problem on the 13BT motors. These front covers are interchangable between the two motors as the bolt patterns are the same (except for the one extra bolt on the 13B-REW). The 13B-REW motors uses crank trigger sensors mounted on the crank instead of the crank angle sensor (CAS) on the 13BT. The crank trigger sensors allows a better accuracy resolution because of its wider teeth spacing. Also, torsional flex on the crank angle sensor shaft causes inaccurate readings. The two triggering systems has similarities in the numbers of triggering teeth. The 13B-REW's crank trigger has 12 syncro teeth and 1 home tooth, while the 13BT's CAS which spins at half speed, has 24 syncro teeths and 2 home teeth.

    The rotor housings on the 13B-REW motors are refined for reliability. Since the motor is designed for higher standard output, the wear surface on these housings has a carbon-graphite coating which is said to exhibit 32% less friction than the 13BT motor's fluorocarbon resin coating. The new coating actually allows less oil to be injected into the combustion chambers, thus the 13B-REW requires only two oil injectors instead of four. In addition to this coating, these rotor housings has the water passage machined around the spark plug areas to increase cooling at the spark plug tips. This modification is similar to the ones done on race motors.

    The stationary gears on the 13B-REW are factory hardened. They are interchangable with the 13BT's, provided that the 13B-REW's thrust bearings and front counter weight be used. Inside the stationary gears, the standard main bearings on the 13B-REW motors are the multi-holes type and are locked into place by an anti-rotation set screw. This design, similar to the "3 window-type" bearing is pioneered in the earlier LeMans and IMSA motors. The holes allow extra oil to flow around the oil groove thus providing a thicker oil film for the eccentric shaft to ride on. The 13BT has standard bearings with no oil holes. Oil pump capacity and pressure are also increased in the 13B-REW motors. The oil pumps are not interchangable. The 13B-REW oil pressure regulator is interchangable with the 13BT motors and is a reccommended upgrade when power output is increased. Also, a new oil pan design on the 13B-REW motors helps to minimize air induction into the oil strainger. A thinner pan with a inner bulge maintains oil level to keep the oil strainer submerged in oil. The 13BT GTR blocks use mechanical oil metering pumps, while the 13BT GTX and 13B-REW blocks use electronic oil metering pumps.

    The water pump of the 13B-REW is probably the only reverse rotation water pump assembly found on any Mazda rotary engine. It is driven by the back of the serpentine belt. The water pump housing assembly on all the rotary engines are interchangable, but the appropriate front cover/pulleys/etc... must be used.

    Fuel injectors used in the 13BT motors are 4 standard Nippondenso top feed rail injectors rated at 550cc. Depending on production dates, the 13BT motors had both low and high impendance ratings. The 13B-REW motors has 2x550cc injectors for primary and 2x850cc injectors for secondaries. This higher fuel capacity is for higher power levels. These injectors are side-feed rail types with high impedance so it can operate cooler. They are not interchangable with the 13BT injectors.

    The engine mounts locations of the 13B-REW are under the rear side housing, while the mounts on the 13BT are located on the intermediate housing. Because of this, interchanging the motors between the FC3S and FD3S chassis is a very involved project.

    The tranny bellhousing bolt patterns are the same on the 13B-REW and 13BT motors, thus the trannies are interchangable with the appropriate clutch/flywheel assemblies.

    In summary, because of these and many other refinements, the 13B-REW is a much more potent powerplant which can easily handle high HP output beyond for which it is originally designed for. Many of these refinements are shared in the designs of the 20B-REW and 13B-REW of the Mazda Cosmo.

    Any questions or comments? Please send email to pko@mindspring.com


    Disclaimer: The preceding article contains information the author has collected based on observations of the Mazda powerplants. The accuracy of information is maintained as truthful as possible. However, the author is not responsible for typo- and technical errors.

    Copyright ©1997, Paul Ko, All rights reserved.