Q:  What is the difference between the Zenki versus Kouki 13BT engines?

We'll try to run down the difference between the 1986 - 1988 Zenki FC3S Turbo 13BT versus the 1989 - 1991 Kouki FC3S Turbo 13BT engines.  Although some of the criteria can be applied to non-turbo 13B engines, this list is specific to 13BT turbo engines.

Zenki:  182bhp
Kouki:  200bhp
The power differences are due to higher boost, higher compression ratio rotors, better turbo system design, and different intake manifold design in the later Kouki FC3S Turbo.

Rotors / Compression Ratios:
Zenki:  8.5:1
Kouki:  9.0:1
Due to the slightly higher compression ratio rotors, the Kouki FC3S Turbo 13BT does make more power.  The Kouki rotors have machined recesses that allowed for production tolerances to drop down to ±0.06 CR; the earlier Zenki rotors were just casted, and only managed a tolerance of ±0.3 CR.

Internal rotor reinforcement ribs were thinned from 3mm for Zenki rotors to 2mm for Kouki rotors.  Due to the thinner castings and tolerance machining, the Kouki rotors are slightly lighter.  The Kouki rotor ring gears are also ion-nitrided for strength.
(SAE Technical Paper Series #900036 - "New Technology Employed For the Latest 13B-Rotary Engine")

Rotor housings:
Spark plugs positions are significantly different between the two rotor housings.  The Kouki 13BT spark plug positions are positioned lower.  It is questionable on whether if this is advantageous or not.

A knock sensor bung appeared on the Kouki 13BT rotor housing above the trailing spark plug hole, as the knock sensor moved from its old position on the center iron on the Zenki 13BT to the front rotor housing on the Kouki 13BT.
This knock sensor bung is one of the key exterior features of the Kouki 13BT engine.

Stationary gears:
Kouki stationary gears are ion-nitrided for strength.  Both front and rear stationary gears are treated.  If it very hard to differentiate the two unless the units are brand new; "nitrided" parts are typically a dark gray when brand new.

Intake ports:
The intake ports on the rotor housings are identical for secondaries but are different for primaries.  Amazingly enough, the Kouki FC3S Turbo 13BT primary intake ports open 10² later than the Zenki FC3S Turbo 13BT primary intake ports.  This means for stock primary intake ports, the Zenki 13BT intake ports are larger.  Port specs are available in the Mazda FSM.

Intake manifold:
Mazda redesigned the Kouki 13BT intake manifold to be taller with longer intake paths.
Check out our Zenki versus Kouki 13BT intake manifold comparison page:
FC3S Pro:  FAQ - Zenki versus Kouki Upper Intake Manifold Comparison

Throttle body:
The basic design is identical for both throttle bodies.  There are subtle differences due to different TPS' and oil metering systems.  The Kouki 13BT uses a double TPS design that uses two TPS bodies for the stock ECU.

The Zenki 13BT uses a mechanical linkage to the mechanical oil metering pump, while the Kouki 13BT uses an electronic oil meter pump that is totally controlled by the stock ECU.  (See below)

Throttle Position Sensor (TPS):
The Zenki 13BT uses a single, narrow-range TPS.  This single, narrow-range TPS opens 100% once the throttle opens to about 1/3rd.  The Kouki 13BT TPS uses a double TPS arrangement that uses two TPS bodies.  One TPS is a narrow-range TPS that mimics the Zenki 13BT TPS.  The other TPS is a full-range TPS that maintains contact through the entire throttle travel.
The double TPS is one of the key exterior features of the Kouki 13BT engine.

Oil metering pump / system:
The Zenki 13BT uses a mechanical oil metering pump that uses a mechanical linkage that connects to the throttle body.  Thus, the more the throttle is opened, the more oil is injected into the engine.  The Kouki 13BT uses an electronic oil metering pump that is controlled by the stock ECU.  Depending on engine load and RPM's, the Kouki 13BT stock ECU will dictate how much oil is injected into the engine.  The oil metering pumps are physically different.  The Kouki 13BT oil metering pump is larger with several wires going into a pair of plugs.
The oil metering pumps is one of the key external features that differentiate the two types of engines.

EGR valve:
The Zenki 13BT engines all have EGR valves in the U.S.  All Kouki 13BT engines do not have EGR valves.  All J-spec 13BT engines do not come with EGR valves.  If the engine has an EGR valve, it's supposed to be located on the lower intake manifold.

Water pump / housing:
Although similar, the water pump flange was slightly different.  Therefore, the water pumps are not swappable between Zenki FC3S and Kouki FC3S.  Oddly enough, the water pumps are identical between turbo and non-turbo engines.  Thus, any 1986 - 1988 FC3S water pump is the same for turbo or non-turbo models.  The same can said for the 1989 - 1991 Kouki FC3S, as the water pumps for both turbo and non-turbo for those years are swappable.  The only difference between non-turbo verus turbo water pump housings is the coolant supply return from the turbo for 13BT turbo units.

The thermostat cover design is different for Zenki versus Kouki.  Zenki FC3S 13B uses a cast aluminum filler neck that has a 2-bolt flange; a plastic filler neck and cap was part of the thermostat cover.  The Kouki FC3S 13B uses a black, plastic thermostat cover with a 3-bolt flange; this thermostat cover had no filler neck or cap built into it.
The thermostat cover is one of the key external features that differentiate between the two engine types.

Turbo system:
About the biggest difference between the two engines is the turbo system.  Mazda had gone from a "Twin Scroll" turbocharger to an "Indepedent Twin Scroll" turbocharger.  The Zenki 13BT twin-scroll turbo used a flapper door on the turbo exhaust manifold to direct exhaust gases into one turbine passage or both turbine passages; the passages were asymmetrical.  The Kouki 13BT independent twin-scroll turbo uses two independent exhaust gas passages from the rotor ports to the turbo exhaust manifold into the turbo turbine housing; it uses two symmetrical passages in the turbine housing.

The "Independent Twin Scroll" proved to be a superior design in terms of performance, as the powerband was widened.  The Kouki 13BT turbo system design was also better for exhaust gas flow, as there were no abrupt turns or "ledges" that hindered smooth smooth flow.

The Kouki 13BT turbo also had a better wastegate system due to twin wastegate passages that made for better control.  This will be important when you go with a freer flowing exhaust system, as boost creep is minimized.

The compressor sections (compressor wheel and compressor housing) and center housings are identical for both turbos, so the differences are limited to the turbine housing and turbo exhaust manifold designs.  Even the turbine wheel is identical.  You can just swap turbine housings, and everything else will swap over.

To accomodate the different turbine housing designs, the turbo exhaust manifolds are significantly different.  The Zenki 13BT turbo exhaust manifold is significantly larger and heavier than the Kouki 13BT version.  The Zenki 13BT turbo exhaust manifold has two holes for the rotor exhaust ports on one side and one hole for the turbo on the exit; the Kouki 13BT turbo exhaust manifold will have two holes for the rotor exhaust ports, but it has two holes on the exit side into the turbo turbine housing.  By keeping the exhaust gases seperated until hitting the turbine wheel, the Kouki 13BT "independent twin scroll" is a superior design.
The turbo is one of the key external features that differentiate the two types of engines.

Questions?  Comments?  Send email to:  reted@fc3spro.com