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Zed Speed - Part One

We check out Z31 series Zeds and their tuning potential...

By Michael Knowling

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At a glance...

  • First of two-part series
  • A look at the evolution of the Z31 300ZX
  • Tuning potential
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The Nissan Z-car is one of the world’s favourite machines for aftermarket tuning. Why? Well, with rear-wheel-drive, relatively slippery aerodynamics, an easily modified and reliable turbo engine and sophisticated suspension, it’s hard to ignore.

Curiously, despite the number of recent Japanese imports, the 300ZX isn’t as popular in Australia as it is in the US. In this two-part series we’ll look back over the ‘modern era’ Zeds and investigate their hot-up potential.

Nissan Z31 300ZX

The earliest Zed to bring modern turbo technology is the Z31 series, which appeared in 1984. The Z31 was a huge break from previous Zeds which came powered by a straight-six engine. Instead, the Z31 envelopes an all-new 3.0 litre V6 engine, which allows a shorter, lower bonnet and improved weight distribution.

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The 300ZX’s V6 engine was released in two guises – naturally aspirated and turbocharged. Note that the turbocharged variant didn’t appear in Australia until 1986 – and when it did finally appear, it replaced the existing naturally aspirated model.

The naturally aspirated VG30E incorporates SOHC 2-valve-per-cylinder alloys heads, a 9.0:1 compression ratio and multi-point EFI with a hot-wire airflow meter and distributor. Maximum engine output is 124kW at 5200 rpm and 235Nm at 4000 rpm. This compares quite well to other mass produced 3.0 litre engines of the time and represents a significant improvement over the superseded 280ZX’s L28 engine.

But the engine of interest is the turbocharged version.

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The forced induction VG30ET engine employs a T03-type turbocharger, low 7.8:1 static compression ratio (achieved with domed pistons) and generates 155kW at 5200 rpm together with 319Nm at 3600 rpm. Note the low revving nature of the engine – it achieves peak power 400 rpm lower than the comparable RB30ET engine (as fitted to the Holden Commodore VL Turbo). Boost pressure is limited to just 7 psi (largely due to the lack of an intercooler) and a knock sensor helps avoid detonation.

Both engines could be optioned with a 5-speed manual or electronically-controlled 4-speed automatic transmission.

Regardless of transmission choice, the naturally aspirated version of the Z31 300ZX was widely criticised for its lethargic performance – it simply didn’t have enough torque and power to deliver anything like sporty performance. One Australian motoring magazine claimed that the upright-and-proper Nissan 300C (with the same 3.0 litre V6) was quicker over the quarter mile... ouch!

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The turbocharged Z31 is much swifter, but nothing exciting by today’s standards. The auto version is listed with mid 9 second 0 – 100 km/h and mid 16 second quarter mile performance, while 5 speed manual versions hold an advantage of three or fourth tenths in each increment. Top speed of Australian models is limited using a fuel cut-off set to 205 km/h – about 30 km/h shy of its real potential.

Contemporary road testers praised the Turbo’s eagerness to come onto boost at low rpm to provide excellent flexibility and effortlessness. However, at the higher end of the rev rage, the engine felt strangled – there’s no point venturing near the 6000 rpm redline. Interestingly, many testers said that the auto version of the 300ZX Turbo was the ultimate pick – it’s only a shade slower in the important acceleration increments but its everyday kick-down performance and ease of use put it ahead of the long-throw and notchy Borg Warner manual.

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There is no doubt that the turbocharged Z31 took the Zed to new levels of performance but, regardless, it was (and still is) viewed as a sideways step from the original concept. Gone was the fun feel, the affordability, lightweight and raw appeal – instead, the new Zed was built as a go-fast luxury tourer.

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The wedge styling of the Z31 300ZX was quite daring at the time of release but, today, it looks extremely dated. The overall proportions are elegantly balanced, but the overly sharp edges mean there’s no grace. The new Zed also brought semi-retractable headlights to help improve aero efficiency and a Targa roof version (with removable glass panels) was a popular option. The 300ZX Turbo is equipped with the front and rear spoilers (as introduced on 50th Anniversary 300ZX limited edition) and a subtle "Turbo" graphic on the rear – it’s an extremely covert operation.

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Inside, the 300ZX has an oddball clash of themes. Its layout feels sporty and the angled-at-driver gauges are a nostalgic carryover from the original Zed, but the poor quality plastics and overly-decorated Japanese trim kills it. In base-spec, the cabin is fitted with analogue gauges (and a boost gauge on Turbos) while top-line models get a digital dash with G-force indicator, compass, climate control, upgrade sound system and ‘timber-look’ trims. Power windows and cruise control come standard across the Australian range. Note that the 300ZX was sold only as a 2+2 in Australia – a 2-seater version was available in various other market.

Kerb weight was the biggest enemy of the Z31.

The entry-level naturally aspirated version weighs almost the same as a contemporary Ford Falcon family sedan and the fully-equipped Turbo tips the scales at more than 1450kg. Road testers often used its weight as a centrepoint when discussing the Nissan’s handling.

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The 300ZX rides on new MacPherson front struts and a 280ZX-based semi-trailing arm IRS. Drive is, of course, to the rear wheels and an anti-roll bar is fitted at each end. Interestingly, Australian-delivered naturally aspirated Z31s were fitted with a ‘General export’ suspension tune which is way too soft for local conditions. This causes excessive dynamic weight transfer and a poor balance. For the Turbo version, local engineering input saw the release of specially tuned spring and damper rates and altered rear suspension pick-up points. This gives a considerable improvement but the biggest letdown is grip - the standard 215/60 15s simply don’t provide adequate adhesion. The power-assisted rack and pinion is also over-assisted and lacks feel.

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Braking hardware comprises 250mm ventilated discs under the nose and 258mm solid discs at the rear. Early Turbo models come with beefier 274mm front ventilated discs and 290mm solid rear discs – still, the brakes can quite easily be made to fade when exercising all of the kilowatts. All Z31s ride on 5 stud 15 x 6.5 inch alloy wheels.

Marketed against the very capable Toyota Supra Turbo, Nissan updated the Z31 300ZX for 1988.

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Visually, the 1988 300ZX update brings a rounded front-end and rear treatment to enhance appearance and aerodynamics. A third brakelight is also fitted across the top of the rear hatch. But more important are the upsized wheels and tyres – 16 x 7 inch alloys wearing 225/50 16 tyres. These give the Zed much improved grip for a higher level of on-road performance.

Under the skin, the Borg Warner 5-speed manual gearbox fitted to the earlier 300ZX Turbo is replaced by a much slicker-shifting Nissan-based FS 5R30A unit. The front brakes are also enlarged from 274 to a substantial 290mm and the rears are upgraded from solid to 282mm ventilated discs. ABS is also introduced.

The ’88 interior comes with standard beige leather trim with elaborate multi-way electric seat adjustment. These changes bring an associated weight gain – the Turbo now almost touches 1550kg. This puts a slight dampener on straight-line performance.

Now was the obvious time to replace the Z31 with an all-new model – which we’ll cover in the next instalment.

Japanese Market Z31s

The Nissan Z31 was released with a choice of five engines in the Japanese domestic market.

The ‘export spec’ naturally aspirated and turbocharged 3.0 litre V6 (VG30E and VG30ET) 300ZX was sold alongside a turbo 2.0 litre V6 (VG20ET) version that was intended to appeal to budget-conscious buyers. These VG20ET Zeds are known as the 200Z, 200ZS and 200ZG.

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But more appealing are the 200ZR and 300ZR, which were released in late 1985. The 200ZR is powered by a RB20DET straight-six with a top-mount air-to-air intercooler. The ‘red rocker cover’ RB generates 134kW and only j-u-s-t fits under the bonnet. It’s also the final straight-six powered Zed ever manufactured. Equally as interesting is the 300ZR, which is powered by a naturally aspirated quad-cam 3.0 litre V6 (VG30DE). This engine generates 142kW.

Both the 200ZR and 300ZR are quite rare and collectable.

Nissan Z31 300ZX Modifications

In Australia, the Nissan Z31 300ZX is a relatively unusual platform for modifications. For modification know-how we suggest looking to America.

Stateside tuners recommend the Z31’s exhaust and air intake as the starting point. A custom 3 inch turbo-back exhaust is a cost effective enhancement that (with appropriate cat converter and muffler selection), gives scope to generate about 100 percent more power with an acceptable amount of backpressure.

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Many tuners replace the factory airbox with a K&N pod filter mounted in the vehicle’s nosecone. However, AutoSpeed’s flow bench testing reveals that the factory airbox flows well by simply removing the snorkel that feeds the lower section. For an even greater gain you can enlarge the size of the airbox feed. See Box Breaths for details.

With the exhaust and air intake upgraded, it’s wise to fit an intercooler. This improves power (and the potential for further power) as well as minimising the chance of detonation. As far as we’re aware, there are no reasonably priced off-the-shelf intercooler kits to suit the Z31 so you’ll probably need a custom approach. A large second-hand OE intercooler can be nestled into the nosecone or, if you’ve got more money available, a high-quality aftermarket core can be dropped in. In either case you’ll need to fabricate custom pipework to and from the intercooler. This is also the perfect time to integrate a blow-off valve (which was never fitted from factory).

Now it’s time to boost.

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A simple pneumatic bleed can be used to force more psi into the engine but you’ll first need to remove the factory plenum relief valve. The plenum relief valve vents at about 8 psi to protect the engine from over-boosting. A 1 inch NPT fitting can be used as a plug in place of the relief valve.

Adjust boost pressure to provide maximum power, taking care to ensure there’s no detonation and full-load mixtures are suitably rich. It is said that the standard fuel system requires attention when generating more than about 200 – 210kW. At outputs greater than this, the fuel system requires upgrade injectors, pump and/or adjustable fuel pressure regulator to keep pace with engine airflow. The later model Z32 300ZX twin-turbo fuel pump is apparently an easy upgrade.

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The standard turbocharger will be running out of puff near 200kW. There are a variety of later-model Nissan turbos that can be fitted as a cheap upgrade, but be aware that these ceramic turbochargers are notoriously unreliable under duress. A better option is a custom ‘high flow’ turbo or a brand new off-the-shelf turbo with ball bearing technology.

Next, it’s relatively common in America to replace the standard 54mm throttle body for a 60mm unit from the Nissan KA24E engine (as fitted to the 240SX, U12 Pintara and Navara). This is apparently a bolt-on upgrade, but it’s advisable to match-port the upper section of the intake manifold to suit. For those with an adventurous streak, the free-flowing Nissan Pathfinder intake manifold can also be installed ...

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Internally, the engine can be equipped with a choice of camshafts available from America and – if you’re keen – you can splurge on a pair of Nismo cylinder heads. Note that the VG30ET has a sturdy bottom-end that’s apparently reliable at up to around 300kW (just so long as it doesn’t detonate or run lean).

Likewise, there are no problems with the driveline aside from usual wear and tear. The biggest problem is rear-end squat during hard acceleration. This squat induces a huge amount of rear negative camber, which leads to poor traction. Stiffening the rear suspension will reduce this characteristic or you can install a camber adjustment kit to compensate.

Stick around - in the next instalment we’ll be looking at the later Z32 model 300ZX...

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