There is little doubt the Miata (MX-5 in some markets) is a brilliant car. Conceived in the US after the demise of the also inexpensive - but antique - open-topped-MGB, its weight-to-power, handling and overall driver 'feel' make it one of the most tactile and enjoyable cars ever created. It's a popular track car, too, especially in the US.
There is a growing number of MX-5s appearing on Australian racetracks, too, thanks largely to the ability of enthusiasts to source cheap second-hand cars from specialist car importers. Known in Australia by the sometimes-confusing term 'Japanese import' (all MX-5s are imported from Japan!) these cars are sourced as accident damaged or high-mileage cars in Japan and imported into Australia solely for off-road/competition use. Australian road registration of these cars is forbidden (they come into the country largely tax-free and being domestic Japanese models, don't always meet Australian safety regulations) but they make an excellent basis for a cheap and cheerful track car.
Bill's 'Bugger' (translation: 'Damn!') MX-5 is one of these. Stripped of door glass and most interior components, and items such as electric headlight units, Bill's car is equipped with a roll cage (including B-pillar cross-car brace), racing seat, harness, dedicated track suspension (with sub-frame braces), good brake pads and semi-slick tyres. Although some track cars get smothered in cash, Bill's MX-5 is a budget track machine, running standard engine internals with its Japanese road mileage of 61,000km on the clock.
The only mods to the driveline are a locked diff, an aftermarket Microtech MTX8 management system (fitted for its 'tuneability' in anticipation of further engine mods), a barely-muffled cat-free larger exhaust on the standard exhaust manifold and an Australian-made Lochie Stewart high-flow air cleaner assembly that draws intake air from the plenum below the windscreen. The car is prepared by Scott at Rev Doctor Motorsport (Hornsby, NSW) and tuned by Dave Flood at Redline Motorsport (Seven Hills, NSW)
After his first few rounds of fun, Bill wanted his standard engine upgraded with more cam and compression. That could be easily achieved with a modified cylinder head on the car's standard short block. Rather than modify the head fitted to the car, it was decided a second head should be bought - that way, a head could be built beforehand and easily swapped onto the car without having a 'dead' car lying around for any length of time. So Scott sourced a second-hand (Jap import) complete 1600cc engine. That way, another short block could be built up gradually with strengthened components and dropped into the car, under the high-comp head, at the end of the season, giving Bill a gradual power-up as he became familiar with the car.
However, Bill had a coolant leak on the track one day - the result being a thoroughly barbecued engine. The head swap had now become a complete engine swap!
Back in the Rev Doctor workshop, it was discovered the engine had become so hot that the ends of the plug leads had melted in their wells, sealing the spark plugs into the head forever. During the strip down, the pungent smell of cooked synthetic engine oil was almost sickening. And it was possible to see the warp in the head - you didn't have to measure it with straight edges or feeler gauges! Although there was no evidence of bore scoring, it was decided the pistons would have been too suspect to consider re-using. A fresh bottom end was required - and in a hurry!
Attention now turned to the 'spare' short block that had had its head removed for upgrading. Its mileage was unknown, however, turning it over by hand revealed its bores looked in reasonable shape with no discernible wear. Rather than being stored and later built up as a hi-po unit with all-new pistons and rods, it instead had its bottom-end stripped of everything in anticipation of a freshen-up. Alarmingly, when the sump tin came off, it was found to be missing its oil pickup pipe despite being described as a 'runner' by the seller. It's lucky the engine was not simply plonked into the car after the first engine was cooked (briefly considered before Murphy's Law was remembered - if something can go wrong, it will!) as it would have lasted only seconds before seizing.
The bores measured up with no taper or wear - these Mazda 323-based MX-5 engines rarely give trouble in that department - and the crank measured within specs. The block was honed and the pistons given a thorough inspection before everything was reassembled with new rings and bearings. The bottom end, stock as a rock, is generally regarded as safe and reliable to its factory limit of 7500rpm year after year.
The new cams (from Australian cam manufacturer Ivan Tighe) are catalogued 725C and have raw specs of 400-thou lift and 232 (at 50) duration. Designed for greater output at higher revs, they're not sold as or intended to be road cams. With that sort of lift on a fresh billet (rather than a regrind that sees the extra lift added as a function of a smaller base circle, rather than more 'nose' on billets such as these) the edges of the lifter bucket bores required relieving to give clearance to the lobe of the cam.
The head, as originally intended, received a lot more attention. With everything disassembled and referring to the cams' lobes, Scott spent several hours with a die grinder carefully relieving the lifter bores to provide the necessary clearance required to allow the cams to spin without fouling. The intake and exhaust ports, too, were given a quick clean up - but with the Mazda factory casting so clean, there was little to be gained from a relatively modest, low-comp 1600cc engine.
With the head modified to suit its high-lift cams, it was sent away for servicing (new valve guides fitted, gasket face machined 20-thou for slightly higher compression and heavier valve springs fitted to the original valves) in anticipation of being fitted with its new cams and dropped onto the stock bottom end.
Once the head was assembled with the Tighe cams and the bottom end freshened, the engine was pieced together using a factory Mazda head gasket. Of course, the oil pickup pipe was scrounged from the 'cooked' engine. The cams were dialled in using the original engine's vernier adjustable cam sprockets (fitted so that Scott could play with timing changes using the original cams).
The management system required remapping to suit the new cams' up-top appetite. This was Dave's task, using his Dyno Dynamics rolling road and several hours of key-tapping with an exhaust probe in the pipe of the Mazda. The Microtech MTX8, like many Australian-made management systems, is easily tuned on-the-run with a lap-top computer. Capable of supporting either throttle-position (TPS) or manifold air pressure (MAP) sensing for load (or a combination of the two for cars with idles that are too lopey to get a steady signal from a MAP sensor, yet require good part-throttle driveability), Bill's MX-5 retained its previous MAP-based system. Note there is no airflow meter; a feature of the standard Mazda factory system. The car runs on street-sale Shell Optimax 98RON premium unleaded fuel.
The result? Around 20 percent more power where Bill wanted it - up top. The power peak has moved up the rev scale, too. After a couple more rounds of the PSCRAA championship in which he competes in NSW, Bill reports he's cutting seconds off his lap times thanks to his car's new found rev-ability that allows him to keep the MX-5's corner speeds up. In fact, it's caused him to have a rethink about his suspension setup he's running!
Looking at the dyno graph, there is a horrible dip in the curve that would make this MX-5 drive like a pig at low speed - like on the road. But, as it's a track car and seldom sees below 5500rpm or 100km/h when it's racing, it's always operating above its mid-range compromises where little time has been spent mapping the management system. Note that the car is now a lot grumpier at idle and down low. In fact, a TPS may be added to the management system to help smooth the low-speed driveability, such as manoeuvring around the pits.
Next? There is still a bit more power and a lot more drivability to be found with the current setup. The intake and exhaust both require opening up (larger, shorter-primary extractors and a less resonant exhaust system and possibly a larger throttle body) but the intention is for the cooked bottom end to be stripped, bored and built with strong pistons and rods with higher compression. Speed's just a matter of money - how fast do you want to spend?
Ivan Tighe Cams
Rev Doctor Motorsport