By Randy Stocker
Like most auto manufacturers Mazda has a lot of commonality between engines. There are four basic types of 4 cylinder piston motors that Mazda currently produces for the US market. They are grouped into engine families, ?B?, ?F?, 'Z' and ?G?. The Miata uses the ?B? family of motors.
The ?B? family starts with a 1.3 liter SOHC. It was primarily used oversees but did come to the USA for a short stint in the Ford Festiva (other markets got a DOHC 1.3). The primary example of the early ?B? motors in the USA came here in the GLC as a 1.5 liter SOHC, later upped to 1.6 liters for the 323 in the mid 80?s (78x83.6mm). The 1.6 liter version has as large a bore as the block can accommodate and cannot really have it's displacement increased much more, in fact, enlarging the bore beyond +1mm is not advised. A DOHC head was designed for the 1.6 liter for the 1988 323 GTX turbo. Because of the severe duty that motor could see many enhancements were made to it for reliability reasons. A stronger web stiffened block and oil spray cooled pistons were among the changes. See SSS Automotive site from Australia for B6T tidbits.
The 116 HP Miata 1.6 DOHC motor from 1990-1993 is a normally aspirated version of that 323 turbo engine. It mainly differs from the turbo counterpart in higher compression pistons (to 9.4:1), lighter connecting rods and a lighter flywheel. This means that the NA version of the motor is quite over engineered for its applications. (how many NA motors do you know have oil spray cooling for the pistons). The automatic transmission version of the 1.6 DOHC engine has lower compression pistons to 9.0:1 and the camshafts have less duration. These changes were done for the automatic version to gain torque at a lower rpm and minimize detonation from the torque eating trans. The tradeoff is a less peak HP rating of 100. In 1991 a running change was made to the crank design from repeated failures of the pulley keyway. The pre-91 motors had a 22mm crank snout while the late 1991 and later had 27mm crank snouts. The 1.6 DOHC motor lived on in the Mercury Capri and XR2 until 1995.
1.6 pistons. oil squirters
The pistons that were available for the 1.6 B6P were the 323 GTX 7.8:1 with a heavy dish (left), the Miata automatic 9.0:1 with a moderate dish (middle) and the Miata 5-speed 9.4:1 with a shallow 1mm dish (right)
The rods used in the 88-89 323GTX turbo are beefier. They have a thicker beam, thicker small end and larger size bolts. They weight approx 580 grams (w/bolts and nuts) vs the approx 540 grams of the Miata rod. The GTX rods are no longer available new from Mazda and the Miata pieces supercede it.
In 1994 the need to meet emissions standards and to confront cries for more power in the Miata was answered by using the 1.8 DOHC motor that has been in the Protege since 1990. It is also a ?B? family member but has a longer bore spacing to accommodate the larger 83mm pistons. The stroke was also increased to 85mm (both the 1.6 and 1.8 DOHC have the same 221.5mm block deck height and 134mm head height). The 1.8 is the same design and is just as robust as the 1.6. It has the same rods, same oil-cooled pistons, same oil passages, same head design, same HLA's and same crank design. Unlike the 1.6, the auto trans version of the 1.8 was not changed at all.
Since the 1.8 is really just a stretched 1.6, most everything on the front and back of the motor will interchange between the them. This includes the cam angle sensor, coolant intake pipes, flywheel/clutch assembly, various covers and brackets, cam gears, water pump, and timing belt tensioner, etc. The intake manifold, exhaust manifold, motor mount brackets and camshafts do not interchange because of the bore spacing differences. The 1.8 ?B? motor has also seen duty in the 1991-1995 Ford Escort GT/LXE, 1990-99 Mazda Protege, 1991-95 Mercury Tracer LTS and Kia Sephia GS. (FYI, the 90-93 Escort GT and Tracer LTS 1.8 DOHC use the same throttlebody and flowmeter as the 1.6 Miata).
A SOHC BP 1.8 was used in the Protege from 92-94 and the short block is identical to the DOHC. The SOHC pistons when used in a DOHC motor produce about 8.2:1 CR. The lower half of the SOHC two piece intake manifold also makes a great foundation for a custom IRTB for the 90-97 BP DOHC heads.
Dished BP SOHC piston.
The head of the 1.8 ?B? was slightly revised with the stretch job by using larger sized intake and exhaust ports, larger valves, moving the cam angle sensor from the intake to the exhaust cam, and using a 4mm higher lift cam with shorter valve stem lengths and a larger base circle. OEM cams specs. For 1995 the valve springs were revised slightly stiffer. This allowed the thick seat shim (1+mm) that was used on the 94 1.8 motor to boost the installed spring pressure to be removed and was replaced by just a thin metal shim (about .005") to protect the head seat.
95 valve springs (left) are stiffer compared to 99+.
Starting with the 3/95 start of the ODB-II implementation (VIN 14193) the pistons were changed with a slight dome to increase the compression ratio to an actual 9.0:1. The '94-3/95 pistons were factory rated at 9.0:1 but actually was around 8.8.
94 vs 96 piston
The Japanese/Austr market version of the Protege got a turbo version of the 1.8, the 'BPT' equipped GTR and GTX. The very rare GTR recieved, amoung other things, a special web stiffened block cast 'BPII', a special cast '26' head with a flow vane in the intake port and sodium filled exhaust valves!.
In 1996 the peak HP rating of the Miata 1.8 motor was raised to 133 hp from 128. This comes primarily from the new for 1996 ODBII software having the ability to lean out the above 6000 rpm fuel curve. The slightly raised dome 9.0 pistons also continued.