Porsche 924 Turbo

Introduction

THE 924 HAD A HUMBLE BEGINNING when you consider the donk was derived from the 2 litre unit used in a Volkswagen van. Of course the engineers went to work on that engine so that, by the time it was installed in the engine bay of the 924, there was little resemblance. But the 924's chassis was always cring out for more power - so how did the 2 litre cope with 10 psi of turbocharger boost?

Although the 1984 c.c. iron-block engine's capacity remained the same and its bottom end was deemed strong enough to be left unaltered, there were considerable changes elsewhere. The cylinder head was entirely new, aluminium alloy as before but with different combustion chambers, recessed valves, 3mm larger exhaust vaIves, sparking plugs with platinum silver electrodes moved across to intake side, and the usual lower compression ratio necessary for any supercharging - this was partly done with different pistons - 7.5 instead the 924's 9.3-to-1; the effective maximum compression at 0.7 bar (10.15 psi) supercharging pressure was 10.8-to-1.

The German-made KKK exhaust turbocharger sat low down near the front under the lee of the heavily inclined engine - it lay, exhaust -down, at 50 deg to the horizontal - taking air from the Bosch K-Jetronic flap valve box and air filter assembly which was mounted transversely across the front of the engine. It blew through a cast pipe up and over the head via the throttle chest horizontally into the intake manifold, which turned the flow down and back again into the ports on the upper side of the engine.

As on the 911 Turbo, an exhaust waste gate was used to limit the output of the exhaust turbine; in addition, to improve throttle response, a Porsche 930 blow-off valve between the pressure and inlet sides of the inlet system relieved back pressure which otherwise slowed down the turbine undesirably when the throttle was closed momentarily. The Bosch K-Jetronic injection system was larger and re-calibrated to suit the higher output. To deal with the severe cooling problems imposed by the change, the 924 Turbo had an oil cooler, extra front air entry openings, a NACA-type exit vent on top of the bonnet and a louvred undertray below.

The result was a very worthwhile 36 per cent more maximum power - 170 bhp (DIN) instead of 125 - at a slightly lower speed - 5,500 rpm instead of 5,800, and an even more useful 48 per cent more torque, which went up to a magnificent 181Ib. ft at 3,500 rpm instead of 122 at the same speed. The car was heavier by 220lb. The 924 used an interesting transmission layout - effectively an Audi front wheel drive transaxle mounted at the back, Alfetta-style, connected to the clutch housing on the engine by a large diameter transmission tube which carried a small diameter propeller shaft in four sealed-for-life bearings. Such a layout meant that a small shaft could be used without it whipping, thereby reducing the rotary inertia loads imposed on the synchromesh.

For the 924 Turbo, the shaft sizes went up from 20 to 25mm (0.787 to near-enough 1 in.) which altered the whipping frequency and nodal points so that one less support bearing was needed. Half shafts for the semi-trailing arm rear were stronger too and the gearbox ratios were changed slightly. Larger tyres were fitted - 185/70 VR 15in. Pirelli Cinturato CN 36 SM on 6in.-rimmed cast alloy spoked wheels, but to be the same maker's more expensive (and even more effective) P7 205/55 VR 16in. for some markets. With the CN36s, overall gearing worked out at 25.44 mph per 1,000 rpm. Springs, dampers and anti-roll bars were stiffer and the brakes had a larger servo and were considerably bigger - ventilated disc at both ends (where the unblown car had rear drums) using an amalgam of other Porsche parts from the 911SC and 928. The steering was lower geared (by 75 per cent) and the new wheels had slightly more positive offset. Wheel bearings were stronger and there were five instead of four wheel bolts per wheel. Apart from the extra air inlets and vent, the car stood out from its less powerful elder brother with the addition of a small polyurethane rear spoiler added under round the hatch.

A 924 That Really Did Offer Performance

Porsche performance claims were nearly always honest to the point of conservatism, so that the performance figures correlated to test figures quoted by various motoring authorities at the release of the 924 Turbo show that when the journalist was behind the wheel, they could nearly always turn in better times. For the 924 Porsche claimed 140 mph and 0-60 in 7.8sec. But even cars with only a few clicks on the odo would soon better that speed, with 145 more the norm. 0-60 mph would take only 6.9sec, 0-100 in19.7sec and a standing quarter-mile in 15.0sec. It was a remarkable machine, quicker than a manual 928 (4 1/2-litres) and nearly as quick as the 3-litre 911 SC. The figures told much of the story.

The 924 Turbo had superb traction which was shown by the fact of that extraordinary 2.2sec 0-30 time, compared to the 911 Turbos 2.8 sec. The traction of the 911 Turbo 3-litre was undoubtedly even better, but because turbocharger design of the era meant that full power didn't come until just after the engine was under full load, the 911's traction would slow the engine as you dropped the clutch, waiting for the turbo unit to catch up - hence that 2.8sec. The 924 was not heavily rear-biased in weight, so you could get past that first gap by letting the clutch in abruptly at 5,000 rpm to produce wheelspin which produced by far the best results. The turbo had an appreciably lower first gearbox ratio and a not-so-much-Iower second than the five-speed unblown 924; overall gearing in both cases was still slightly higher than for the plain 924, the ratios being lowered to help offset boost lag at the start, but this meant that they become rather wide.

The 924 Through The Gears

Most road testers found it best to change up at 6,000 rpm, 500 beyond the power peak, for the first two changes, in the first place to bridge the gap and in the second to make sure of a good 0-60 time; thereafter it was best to change at round 5,500, since power dropped off from 5,700 rpm onwards. Changing like this, at 34 mph (revs dropping on the change up from 6,000 to 3,350), 61 (6,000 4,800) and 106 (5,500 to 4,150) was best, but it showed up the gap between first and second - where the revs dropped to below the maximum torque rpm (3,500) - and the slightly bigger gap between fourth and fifth than between third and fourth.

The gearchange itself was not ideally suited for the fastest-possible acceleration runs because it had the usual Porsche racing-style gate, with first on the dog-leg opposite reverse instead of opposite second as on the Alfa Romeo gate - but no doubt the more professional drivers will tell us we are misguided - and we invite those who find this setup better to place their comments below. There were none of the signs of synchromesh problems due to the bigger prop shaft expected by some on first inspecting the design. The somewhat explosive character of the power curve of any turbocharged car - and especially this one - made it a matter of some practice before you could accelerate smoothly during normal fast driving. The engine - or rather the turbo - took charge but made it difficult to use the normal method of smoothing out a change by lifting off the accelerator as you approached the change point.

Nothing much happened below 3,000 rpm, so that it seemed almost to hang fire, although there was no actual flat spot in the normal sense; just a subdued crackle from somewhere at 2,700 to 2,900 rpm which was the only herald of what was to come, Then, suddenly yet smoothly, that wonderful pressure on the back would come in, rushing you forward tremendously. We loved the feeling in our all-to-short experience in a stock 924 - and the Turbo version was something else. Obviously the extra herbs were exciting; providing a special sort of sensation that you would never tire of. There were debits of course. Porsche had once again succeeded in cutting down lag (the lapse in performance while the supercharger turbine accelerated back to its working speed - up to 100,000 rpm in this case - after the throttle had momentarily shut for any reason) to the one-second maximum expected from any good setup.

Weissach Design

Even that little second was all it would take to make a superbly quick car seem less than ideally responsive - but it was after all a very small delay, and the return to full song more than made up for it both psychologically and practically, in the way you left behind that much bigger fast car that had caught up the momentarily baulked Porsche - or the other one that laughed at you for driving a 924 instead of a 911. There was another, more notable effect when you used a turbocharger to boost a 2-litre car's performance to that of a 3-litre - even though Weissach (Porsches design and development centre) obviously sized the turbo unit to give good mid-range performance, the turbo principle did not allow as much power to be produced evenly throughout the range as the mechanically-coupled, positive-displacement supercharger did - or, more to-the point, as the equivalent 3-litre unblown engine.

The turbo engine worked best towards its top end, meaning you had to use the gearbox more often than in the unblown car, changing down when, say, baulked briefly on a fast road, to maintain performance, You noticed this particularly (or so we have read) on Germany's autobahn's when some larger and faster cars would set out to chase. The 924 Turbo had the edge, but as slower traffic inevitably made you ease off, accelerating away again meant a lower gear before you could draw away - while the other driver simply put their foot down in the same gear. The 924 Turbo was still quicker, more than convincingly in most cases, but it did have to work harder. No four-cylinder in-line engine could hope to sound like a flat six, or a V8 or a twelve, and it was not surprising to find that the least exciting thing about the 924 Turbo was its voice, which is best described as ordinary. That's not surprising when you remember its origins; nevertheless more than the odd driver would be surprised by the small but noticeably rough feel of the engine at times. It was not smooth - but then very few two-litre fours were. On the other hand, it wasn't a noisy engine; intake noise was there, but not much of it - the turbo could be heard only at low speeds; if you tried to accelerate in a high gear, you could hear the high musical whine rising, but it wasn't noisy.

There was some transmission gear whine, again not much, most obvious at around 80 mph or when lightly pulling. The Turbo would not cause offence to the ears of the outside world, but unfortunately wind noise was too high, increasingly dominating all other noises the nearer you got to maximum speed, coming from what we assume to be poor door sealing. And like its big front-engined brother, the 928, it suffered badly from bump-thump; coarse freeway surfaces would set up a loud drumming even at 60 mph.

Economy Potentially Remarkable

With this sort of power unit, you had in effect a choice of two engines. There was a 2-litre unblown one for pottering about town or touring gently which would easily return around 25 mpg - and the at-least 3-litre equivalent employed when you used the performance for high speed cruising or very fast driving, which understandably increased the thirst a lot - to around 20 mpg. The worst figures we have found quoted by any motoring journalists who put the 924 through its paces was just over 16 mpg, but that was during virtually flat-out driving for considerable periods. The tank was ten per cent bigger than the original 924's at 15 gallons, which gave a range of at least 280 miles and up to 375, helping make the car a genuine long-distance cruiser. The 924's fuel level warning lamp would come on relatively early, flashing intermittently when there turned out to be at least three gal!ons still on board.

The 924's Road Behaviour

The steering was accurate and surprisingly light; on first acquaintance it felt a little dead but after a while you learnt that there was just the right amount of kick-back and that although a little low geared at 3.8 turns lock to lock for a quite handy 33ft turning circle, it was just about "quick" enough in its gearing for the job it had to do. That number compares with 3.0 for the normal 924 which had the same lock. We would say that some sporting owners would in fact prefer the original higher gearing, at the cost of greater parking effort, in order to improve further the already good response of the steering. The 924 Turbo remained a car with nearly neutral normal handling; apart from a mild and unimportant tendency towards steering fight on long fast bends, it was beautifully stable at all speeds. At its 142 mph maximum speed road testers all claimed there was no suggestion of lift or anyother sort of instability - yet it took only a small movement of the wheel to start it turning.

The 924 had the sort of response without being twitchy. Sidewind stability was excellent too. Ultimately, its cornering limit was very high. Keep the foot down and it would eventually understeer, but not as much as most front-engined cars of the era did. Lift off sharply in mid corner and the small camber change produced in the semi-trailing-arm-Iocated rear wheels produced some breakaway, but by no means abruptly or unpleasantly as in the classic (rear-engined) Porsches, meaning you could catch it easily. The result was an enormously secure-feeling car that inspired deservedly great confidence. There were detail criticisms; a suspicion of over-centre lightness was present towards extremes of lock, and some road testers would have preferred a round steering wheel to the ellipitcal one fitted, which they felt interfered a little with the car's excelfent self-centring. The only other point about stability concern was the feeling of slight wandering you would get on braking. Even though you were jarred noticeably over sharp bumps, the car didn't seem to be disturbed seriously if it hit such a bump on a bend.

The 924 did roll, but not much, so that combined with the handling and grip, it was a car which invited fast driving at anytime.

The brakes were adequate but not perfect. Light pedal efforts were all that were needed for any stop; even the maximum retardation available required only 80lb. Unlike previous 924s, we found that the balancewas not right, allowing both front wheels to lock for only 0.85g some time before the backs were anywhere near locking. One must, however, remember that wewere not testing on a known surface as at MIRA, and so although it looked good enough, this relatively low figure may be as much a consequence of the road coating as anything.

Fade tests conducted by some motoring authorities showed that the improvements to the car's brakes were just adequate over the unblown iteration. Pedal effort rose from 25-30lb for the first stop to 40-45 required after 3 hard stops. Front disc temperatures were obviously very high and while the pads coped, there was some temporary evidence of hydraulic fluid vaporising which disappeared on pumping the pedal. Interestingly the front brakes rumbled too easily if worked at all hard during ordinary driving, but did not make themselves heard at all after the fade test.

Behind the Wheel Of The 924

The 924's seats were rather garishly upholstered in PVC with panels of pseudo tartan fabric, although they were superbly comfortable, supporting the back well and giving tolerable sideways support. The car was far easier to drive neatly and precisely than the 928, in spite of the way even a tall driver would somewhat sink in it, because it was not too wide, and it was blessed with quite good all-round vision except at the back, where the height hid anything low down. Also the top of the glass would create a distorted view of cars behind, rendering them lower and fatter than they really were.

The wiper blade in front of the driver had a heavy aerofoil to keep it on the screen. It worked well up to 110 mph, after which it starts lifting. The driving position was typically German - excellent, with no real awkwardness. Stalk switches were conventional, and easily worked, and for once on a German car there was a good set of instruments - 260kph (160 mph) speedometer with press-to-zero trip, 6,600 rpm red-Iined revcounter, oil pressure, water temperature, fuel and battery/volts gauges, plus a clock. Their conical glasses did not however, altogether avoid unwanted reflections.The heater was a tolerably refined water-valve one, giving adequate temperature control which responded only slowly to any desired change. Its controls were lit a night. Ventilation was good. The horn was worked ideally, from the large pal in the centre of the steering wheel.

Living with the 924 Turbo

By sports-car standards, the 924 was a very practical car, with its tolerable rear accommodation for the occasional extra passenger, the hatch at the back giving easy access to a good amount of luggage space. The lockable gloveboxwas usefully sized, and was automatically lit on opening. Underneath there was a quite long-reach but shallow open shelf. The door pockets were of limited use because their openings were obstructed by the armrests. There were handy hidden wells each side of the Spacesaver spare wheel under the carpet. To hinge the back seats forward, you had to release them by pulling a plastic strap, which required considerable effort - it was a cheap arrangement unworthy of the car. On the other hand, there was the usual magnificent Porsche tool kit which included a tyre pressure gauge (dial type), an electric air pump; necessary for the Spacesaver; the same went for the luggage strap arrangement, which when unused wound back on to its spring-loaded reel.

At the front, the bonnet release was easily found, but the bonnet, which is quite heavy, had no self-propping stay. The Porsche 924 range came in a rather austere version for continental Europe, with cars destined for the US and UK coming with an appreciably higher standard of equipment - P7 tyres, electric windows, electric door mirrors etc. The least expensive 924 available was the four-speed model, followed by the semi-automatic, then the five-speed version. Some web sites falsely claim the Turbo came standard with a five-speed transmission - but this was only the case in certain markets. But no matter what version you opted for, it remained well below the price and performance of the flagship 911. But we hate the idea of calling it a poor mans Porsche - it was much better than that. If you have driven one, you would know that Porsche simply demonstrated that brilliance could be packaged at a cheaper price. If you can find a good example, it will provide more driving enjoyment than you, or your friends, would ever expect.

924 Carrera GT

The production version 924 Carrera GT was presented for the first time, in 1980, at the Weissach Development Centre, and differed from the 924 Turbo largely through increased performance and more-sporting layout. With compression ratio raised from 7.5 : 1 to 8.5 : 1, a charge-air intercooler and entirely new digital control of ignition timing, the engine of the Porsche 924 Carrera GT produced 154 kW (210 hp) at 6000 rpm. This took the car to a top speed or 240 km/h - 150 mph. The 924 Carrera GT accelerated from 0 to 100 km/h (0-62 mph) in 6.9 s. (924 Turbo, 7.8 s.). The increased geometric compression and completely new ignition system with digital control were not used solely to achieve such a notable increase in road performance, however. They served sensationally low consumption at the same time.

Despite its high-performance characteristics, the 924 Carrera GT scarcely used more fuel than the normal induction-engined Porsche 924. Despite wider track and wider tyres, enlarged air intakes and nose spoiler, Porsche managed to maintain the same, extremely favourable drag coefficient of 0.34 as that of the 924 Turbo for their 924 Carrera GT. It was the result of minute detail work. The fact that the vehicle sat 10 mm (0.4") lower in front and 15 m (0.6") lower in the rear, while its windshield was bonded to the body, contributed to this achievement. Furthermore, Porsche was not only offering something entirely new for passenger cars in the ignition system for the 924 Carrera GT, but showing a new bodywork feature as well: for the first time the guards were constructed of fibre-reinforced polyurethane. These were flexible, like the nose and tail pieces of the Porsche 928, so that light blows would leave no trace. In addition, they resisted corrosion and stone damage. Porsche offered the 924 Carrera GT for a price of around A$30,000 in Germany.