Buying a Series 5 RX-7

The second generation RX-7 (the Series 4) was released in Australia during 1986 with a naturally aspirated EFI six-port 13B rotary engine. It offers improved handling, comfort and features over the previous generation but, due to a commensurate weight increase, performance from the 110kW atmo 13B is barely any better than the lighter Series 2 and 3 RX-7s. Shortly after, the turbocharged model was released in Australia, giving the new generation ‘7 the performance that its chassis deserved.

Then, in 1989, the updated Series 5 RX-7 arrived. The S5 is largely the same as the S4 but brings some welcome cosmetic and mechanical enhancements. External changes include subtle side skirts, front and rear spoilers, a revised front airdam, body coloured side strips and round taillight lenses. Wheels remain 16 x 7 alloys but are a more attractive mesh design.

Inside, the trim colour of the Series 5 is black rather than the multitude of ‘80s colour options found in Series 4s. It also offers more comfortable seats together with a more modern steering wheel and gear knob. Overall, these changes effectively modernise the basic design which first hit the Japanese market in 1985. The Series 5 RX-7 was a very expensive car back in 1989 so, accordingly, it boasts a long list of standard equipment. You get an anti-lock braking system, security alarm, central locking, electric sunroof, cruise control and CD player.

Under the bonnet, the Australian-spec turbocharged 13B engine rose from 133kW at 6500 rpm and 245Nm at 3500 rpm (in the Series 4) to 146kW and 265Nm at the same revs. This modest increase is largely attributed to the slightly larger turbine and a maximum of around 8 psi boost (an increase of approximately 2 psi).

The Series 4 RX-7’s turbocharger is a twin-scroll design with one of the divided turbine passages blocked during low rpm/load. This is said to improve gas speed and turbine response until the second passage opened at higher rpm/load. But, curiously, the Series 5’s turbocharger does away with the staged twin-scroll system and a larger turbine is used. A higher flowing internal wastegate and ECU controlled boost are also fitted to improve reliability by limiting boost at low rpm (where the engine is prone to apex seal damage caused by knock). Upping the rotors’ static compression ratio from 8.5 to 9.0:1 helps improve drivability.

Performance

While the 133kW Series 4 turbo did the quarter mile in mid 15s and 0 - 100 km/h in about 8 seconds, the 146kW Series 5 knocked a second from the 0 – 100 km/h time and did as well as mid 14s over the quarter mile (according to contemporary road tests). But a generous spread of torque means the S5 RX-7 is faster than its 146kW would have you think – although, while boost is available from 2000 - 2500 rpm, it isn’t really on song until 4000 rpm. From there, it does the business in typically smooth rotary fashion and provides a strong surge of power up to about 6500 rpm. The engine will happily rev harder but the standard turbocharger becomes a restriction over 7000 rpm.

The improved refinement of the Series 5 compared to first generation RX-7 means that it doesn’t feel quite as fast or dramatic when driven hard. On the plus side, the independent rear suspension and viscous LSD put the power down much better. It’s a much better overall package. On a good day (usually a cold night), it’s amazing how well a Series 5 goes for an unmodified late ‘80s car - Mazda engineers clearly got the torque curve and gearing right. But on a bad day (a hot day with the intercooler heat-soaked from sitting in traffic and the ECU presumably retarding ignition timing to keep detonation at bay) it’s questionable whether a 1300+ kilogram car powered by a small capacity turbocharged engine is really such a good idea...

Handling

If you’ve ever driven an early RX-7 (or an older RX) the first thing you’ll notice about the Series 5 is the steering is much, much better. The S5 has speed dependent power-assisted rack and pinion steering - at low speed it feels quite light and turn-in is sharp while higher road speed (above about 60 km/h) increases steering weight considerably. The steering doesn’t feel as sharp at higher speeds but that could be caused by the reduced power steering assistance or Mazda’s dynamic tracking suspension system (DTSS) which changes rear toe with increased cornering loads.

Overall, handling and grip are very good, although the RX-7 does prefer smooth roads – it doesn’t like mid-corner bumps. At speed, the car feels somewhat like a bigger version of the original and retains some of the agile feel of the early ‘7. But the car can become skatey in the wet, especially at the rear during the rapid transition onto boost. In the dry, there’s plenty of grip in reserve.

Ride quality is quite good although some jarring is felt over sharp edged bumps. This picturedcar’s standard suspension has relatively old dampers, so there may be some scope for improvement in the ride department. Interestingly, the Series 4 RX-7 has push button hard/soft dampers and relatively soft springs while the S5 has firmer fixed spring and damper rates. The S5 is noticeably firmer than the S4 and trades off some comfort for improved handling.

Common Problems

Poorly maintained or worn-out RX-7s almost inevitably come with electrical bugs. You’re lucky if they’re confined to interior electrical problems (like faults with the central locking, alarm, electric windows, dash lights or electric sunroof) because under-bonnet electrical problems can be the source of real headaches. The intense heat generated by the turbo rotary engine will eventually lead to faulty ECU sensors and vacuum-leak-inducing brittle and cracked hoses. Combined, these factors make it a real challenge to diagnose idle and drivability problems.

Oil and coolant leaks can also be a problem. Without topped-up and clean supplies of these two fluids, a turbocharged rotary won’t last long. It certainly pays to sort out any leaks. Leaks in the ABS unit are also common and this is an expensive fix.

It’s pretty widely known that blown seals can be a problem in rotaries, particularly apex seals. Some earlier rotaries use a 3mm two-piece apex seal which has proven very reliable. For the Series 4 and 5 RX-7, Mazda switched to a new 2mm three-piece apex seal which is said to provide better sealing. In practice, they’re less durable than the older seals and won’t stand up to lean-outs or excessive boost levels - that said, well maintained and unmodified 13B turbos commonly last 130,000 - 160 000km. A compression test by a reputable rotary workshop is the best way to gauge how much life is left in a rotary - better yet, a receipt for a recent rebuild from same will give even greater peace of mind.

A less commonly known source of problems is the engine water seals. Poorly maintained cooling systems can result in corrosion of the rotor housing near the water seals or failure of the water seals themselves. This can lead to internal coolant leaks which can spell the end of an engine. Pressure testing the cooling system or testing the coolant for traces of combustion gases are the best means of making sure all is well.

How well a rotary starts (both from hot and cold) and whether or not it blows smoke are also strong indicators of engine condition.

Modifying

Turbo rotaries are ridiculously easy to get extra power out of. The first step in the case of the S5 is to replace the factory computer with a good quality aftermarket ECU. Why? Well, the standard computer doesn’t just hold back power – it’s also the source of many reliability problems. The ‘rip out the factory ECU’ argument is further strengthened by the factory boost cut function which has a nasty habit of destructively leaning out the rear rotor when upping the boost pressure.

With an aftermarket ECU fitted, the next step should be a free flowing exhaust. This is all pretty basic turbo car stuff, but turbo rotary exhausts get very hot so stainless packed mufflers are the go - at least near the front of the system.

The top-mount factory intercooler is effective at near stock boost levels, but stop-start traffic causes significant heat-soak. A front-mount intercooler is probably a good idea if you do a lot of this kind of driving or plan to run a larger turbo and more boost.

With just the ECU, exhaust and slightly increased boost (to around 12 psi) power at the rear wheels should be up around 150 - 160kW (on a Dyno Dynamics chassis dyno). Be warned that the standard 2mm three-piece apex seals won’t hold up to much more than 12 psi, especially if the engine has done a lot of kilometres

If you’re seeking even more power, it would be wise to rebuild the engine and slot the rotors to use the more durable RX-4 type or aftermarket 3mm two-piece apex seals. Some porting can also be carried out at this point, but it’s worth keeping in mind that - particularly with big turbos - retaining some low rpm, off-boost torque is a good idea.

Living with the RX-7

As an everyday car, the Series 5 RX-7 is a much better proposition than older rotaries and is comparable to its contemporaries such as the 7M-GTE-powered Supra Turbo and Z32 Nissan 300ZX. That said, it does have a few drawbacks...

First, fuel consumption up to and exceeding 20 litres per 100km is not uncommon in urban conditions. A lightly modified Series 5 can return a best of 12 – 13 litres per 100km on the freeway. Second, as there’s no meaningful boost pressure below about 2000 rpm, it’s a bit of a pain to drive at low speeds and in heavy traffic. Third, its packaging means that any more than one passenger is a squeeze. Boot space is quite good and the rear backrest folds down, but the load area is not easily accessible.

How Much?

The 1989 – 1991 Series 5 RX-7 is often AUD$1000 – AUD$2000 more expensive than the similar looking Series 4 - but the extra cash is well spent! An average S5 costs approximately AUD$12,000 – AUD$14,000 and these are mostly ‘grey’ imports which might lack features such as an electric sunroof (as found in Australian delivered S5s). And don’t forget those common drawbacks of grey imports. Australian-delivered S5s in excellent condition can sell for up to AUD$16,000 but are reasonably rare - at AUD$55,000 new, Mazda didn’t sell anywhere near as many Series 5 RX-7s as they did earlier models...

Grey import RX-7s are commonly charcoal grey, white, red or custom painted in any number of bright colours. Australian delivered cars were offered in red, white, metallic dark blue and champagne metallic and can be identified by the ‘TURBO II’ stickers and round indicator lenses on the front mudguards. They also have a slightly different taillight design to a grey import S5.

In all, the Series 5 RX-7 offers sports car fans an enjoyable package and good value for money. For rotary enthusiasts, you get more car for the money than an AUD$10 000+ modified RX-3 or early RX-7 and better value compared to a one-year-newer - albeit faster and dynamically superior – Series 6 RX-7 at around AUD$30,000+.

Thanks to Series 5 RX7 owner - and AutoSpeed reader - James Whitbourn for contributing this article.