Introduction
The Mazda 323GTX was available with one engine, the 1.6L DOHC turbo, B6T. 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.
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.
More information on Mazda engine history and an interchange guide can be found at www.SoloMiata.com
The Crankshaft
The crankshaft used on all Mazda 323GTX's and early 1.6L Mazda Miatas has been known to fail at the crank nose. Since the Miata sold quite a few more units in the first few month as the GTX did in 2 years, it was easier to spot a problem on the B6T in the roadster. A new crank design (big nose crank) was used in the 1991 1/2 - 1993 1.6L Miata. There have been reports of broken crankshafts by about 1% of Miata owners. The problem is confined to 1990 and early 1991 cars that use the same crank design as the GTX. Difficulties are related to the crankshaft nose, keyway, cam drive sprocket, or pulley bolt. Failures are often precipitated by the removal of the pulley bolt and cam sprocket to facilitate front crankshaft seal replacement. In the past, it was recommended to replace the front seal during timing belt service as preventative maintenance. Now, it would seem prudent to leave the pulley bolt undisturbed provided the seal is not leaking.
In some of these scenarios, difficulties may not be immediately detectable. Usually a worn keyway will result in a poor running engine as the crankshaft loses its position relative to the camshafts. It is difficult to diagnose because the relative positions of the timing mark on the pulley and crank position sensor on the back of the intake cam do not change. That is, the timing light reports ignition timing is OK. Some owners experienced poor performance as the slipping sprocket allowed relative crankshaft and camshaft timing to wander. If the crankshaft nose breaks off, this is easy for a mechanic to diagnose. The car will not run and the mechanic finds the pulley assembly laying on the lower splash shield. In all examples, repair is the same; replace the crankshaft.
It is possible to seem fine today and have a broken crankshaft tomorrow. It can happen at any time, regardless of mileage. That said, it appears that failures are more likely after the pulley bolt has been carelessly replaced and usually within a few hundred miles. Although sudden and catastrophic failure is possible, many victims detect degraded engine performance or a noticeable pulley wobble.
More information on the crankshaft wobble can be found in Lance Schall's article in the garage section of Miata.net
I had this problem with my 1991 and 1990 Miata, as wel las my GTX. The 1991 Miata just suffered from the wobble, and the keyway on the crank nose was worn out. The 1990 Miata and GTX had the crank nose actually break off. The '91 Miata has been running for over 35,000 miles on the new 1993 engine, the 1990 Miata received a V8 swap, and the GTX is getting a 1993 Miata block.
Solution
Since I had experience with a bad crank nose in my 1991 Miata, I was already aware of this problem before looking for my GTX. Since I knew I would be rebulding whatever GTX I bought, I purchased one with a broken crank nose. This was good and bad. Good because the car was cheap, bad because it was disassembled by the previous owner. The GTX has a lot of non-typical parts that can be difficult to put back together if you were not the person to take it apart. The car was in rather good condition, and only had 86,000 miles on it. I paid $1,400 and towed it to storage. When I replaced the 1.6L in my Miata, I used an engine from an automatic Miata, they came with 9.0:1 compression vs. the manual 9.4:1, and the cams were better suited for a turbo application. This engine has been dyno tested to over 280rwhp in my Miata. I decided to install the same engine in the GTX to get higher compression, and more power off boost. I am using the GTX head to retain the stock distributor and cams.
The new short block was delivered with 46k miles on it, but was sent to the machine shop for inspection. It was determined that new rings and bearings were all that were needed, along with some porting on the stock head. The stock exhaust manifold was ported to remove some casting marks on the inside. Before the bottom end was assembled, a few modifications needed to be made to the Miata oil pump. The GTX pump is used on the short nose crank, my new engine was from a 1993 Miata and came with the large nose crank. The oil dipstick tube is located in the pump housing on the GTX, while on the Miata it is located in the pan. Not to worry, the Miata pump was produced with a plug in the same loaction as the dipstick in the GTX pump. The plug was removed with a quick tap, and the GTX tube inserted.
Another item not to be overlooked is the crank sprocket and balancer. When I bought my '93 engine with the 'big nose crank' I did not ask about these part, and they were not included. After getting a price from the local dealer, I decided to buy then used. It's not a big deal, but if you're buying from a private seller, there is a good chance he removed these parts and would not typically send them along unless you asked for them. If you are installing you new engine without a rebuild, it is a good idea to replace the front and rear crank seals, and use a new crank bolt when you reassemble the block.
Head
While I was shopping for my new engine, i was able to used the grinding station at the local machine shop to smooth out the intake and exhaust ports on the head. Nothing radical, just renmoving the casting marks and increasing the radius around the valves. I had new valve seals, valve springs and guides installed before I was finished with the work on the head. With the machine work done on the head and bottom end, it was time to bolt them together. I decided to call up the fine folks at Flyin Miata and order a metal head gasket and a set of ARP head studs. FM is the place to go for turbo Miata parts and service, and some of the good stuff for the 1.6 Miata can be used on the GTX. I have not had any bad experience with the Mazda head gasket, but the GTX would be seeing my boost then what I have run in the past and the metal head gasket seemed like cheap insurance for twice the cost of OEM. The head studs are another insurance item. I didn't know how many times the stock head bolts were removed, and I didn't want to waste my new head gasket. The ARP studs are expensive, but this combination should be pretty bulletproof in keeping the head were it belongs. With the studs torqued down, the rest of the parts were installed.
Turbo
I decided to use the stock turbo, rebuilt of course, since I was only planning on running 12-14psi with the Miata engine. The same engine in my Maita puts out 175rwhp at 6psi, 12psi should be fine for what I'm using the car for. I had the turbo rebuilt locally for around $300. I did this before finding the GTX group on the Yahoo site. If I had to do it again, I would have went with a popular turbo upgrade for a little more money. The stock turbo will be fine for my application, but if I'm rebuilding or replacing something, I generally try to upgrade at the same time.
Clutch
When it came time to pick a clutch for my GTX, I had some concerns about using the Centerfore Dual Friction (CFDF) unit a lot of GTX owners were using. Some owners say it holds fine up to 16psi, while others had complaints at stock levels. I installed one in my Miata before the turbo was installed and followed the break in period as instructed. It would slip at 6 pounds of boost. The CFDF was replaced with a FM clutch that has held over 280rwhp on nitrous. The FM unit I'm using is their organic disk, but they no longer sell them. FM recently switched to ACT as their primary clutch supplier. If products from ACT are good enough for FM to sell (they stand behind and test everything they sell) I was confident the ACT clutch and pressure plate for the GTX would fit my needs. I bought all the parts from a local speed shop that had them in a few days. Since ACT didn't have a 'kit' for my application, I had to order the pieces individually. I ordered a pressure plate, clutch disk, pilot bearing, release baring and alignment tool.
Intercooler
Before my engine project began, I knew I had to replace the stock intercooler with something larger. The stock unit just isn't large enough to cool the intake air when turning up the boost. The SAAB unit on my Miata was considered, but it too was not large enough for the type of boost pressure I wanted to run. I found a Starion/Conquest intercooler at the local junkyard and brought it home for $50. These unit are not known for great efficiantcy, but the price was right and they are a great impovement over the stock GTX intercooler. I wanted to mount it in front of the radiator, and behind the stock GTX grille. Since I already removed the A/C from the car, the only thing that had to be removed was the hood latch. The A/C was removed to make room for the larger downpipe, and hood pins will replace the latch. Both pipes on the Starion IC had to be modified to fit in this loaction and mounting tabs were welded on. The pipes and end tanks were then powder coated, the cooling tubes and fins were left bare.
