Porsche 924 Turbo Comparison

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 strongenough 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, exhaustdown, 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 louvredundertray 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 goes up to a magnificent 181Ib ft at 3,500 rpm instead of 122 at the same speed. The car is heavier by 2201b. It will be remembered that the 924 uses 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 carries a small diameter propeller shaft in four sealed-for-life bearings. Such a layout means that a small shaft can 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. n test are always better. 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 explosivecharacter 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 below3,000 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 fast car set out to chase. The 924 Turbo had the edge, but as as slower traffic slowsyou. but accelerating away again means a lower gear before one draws away - while the other driver simply puts his 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 can sound like a flat six, or a V8 or a twelve, and it is not surprising to find that the least exciting thing about the 924 Turbo was its voice, which is really rather ordinary.

That's wasn't surprising remembering its origins; nevertheless we were at first mildly surprised by the small but noticeably rough feel of the engine at times. It is not smooth - but then very few two-litre fours are. On the other hand, it wasn't a noisy engine; intake noise is there, but not much of it, The turbo can be heard only at low speeds; if you try to accelerate in a high gear, you can hear the high musical whine rising, but it isn't noisy, There is some transmission gear whine, again not much, most obvious at around 80 mph on the test car when lightly pulling. The Turbo would not cause offence to the ears of the outside world, but the test car was disappointing in the various sorts of body' noise heard inside.

Wind noise was too high, increasingly dominating all other noises the nearer one got to maximum speed, coming from both Dials in front of gear/ev,er are (from right) volt meter, clock and oil pressure gauge, above cigar lighter, heater controls and radio. (Blanks for electric window switches are behind gear/ever). Pad in steering wheel works horn. Main dials (from right) are revcounter, speedometer and temperature I fuel gauges. Warning lamps for charging, brakes and low fuel, and for oil pressure, indicators and high beam are respectively on left and right of lefthand dial. Switches, below main instruments, (from right) are for hazard and rear window heater and for dashboard light dimming and lamps. Right stalk deals with wiper I wash and leftone with signalling A and B posts - possibly better door sealing would avoid this.

And like its big tront-engined,brother the 928, it suffers badly from bump-thump; coarse motorway concrete sets up a loud drumming even at 60 mph. W'e would not expect any improvement here if the 70-profi le test car's tyres were exchanged for the more expensive 16in. P7s. A small point common to all turbocharged cars, with their more elaborate exhaust plumbing, applies to the blown 924, Stop after driving it fast for any time, and listen to the crackling and clicking of hot metal contracting as it cools - all cars do it a bit, but not as much as the turbo ones.

Economy Potentially Remarkable

With this sort of power unit, you have in effect a choice of two engines. There's'a 2-litre unblown one for pottering about town or touring gently with which will easily return around 25 mpg - and the at-least 3-litre equivalent employed when you use the performance for high speed cruising or very fast driving, which understandably increases the thirst a lot - to around 20 mpg. The worst we saw was just over 16 mpg, but that was duringvirtually flat-out driving for considerable periods, The tank is ten per cent bigger than the original 924's at 15 gallons, which gives a range of at least 280 miles and up to 375, helping make the car a genuin long-distance cruiser, Filling up to maximum is not made difficult bytoo much blowing back over the last half gallon. Although judgifl9 from the rush of in-going air as one opened the filler, the test car may have had a blocked breather, we noticed no effect on performance, Its fuel level warning lamp came on relatively early, flashing . intermittently when there turned out I to be at least three gal!ons still on board.

Road Behaviour

The steering' is accurate and surprisingly light; on first acquaintance it feels a little deadbut after a while you learn that there is just the right amount of kickback and that although a little low geared at 3.8 turns lock to lock for a quite handy 33ft turning circle, it is just about "quick" enough in its gearing forthe job it has to do. That number ohurns incidentally compares with 3.0 for the normal 924 which has 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. For the 924 Turbo remains a car with nearly neutral normal handling; apart from a mild and unimportant tendency towards steering fight on long fast bends, it is beautifullystable at all speeds -.J the 142 mph maximum speed runs were amongst the easiest we have ever undertaken, with no suggestion of lift or anyother sort of instability - yet it takes only a small movement of the wheel to start it turning.

It has that sort of response without being twitchy; change your mind in mid turn, and the car straightens without any twitch. Sidewind stability is excellent too. Ultimately, its cornering limit is very high. Keep the foot down and it will eventually understeer, but not as much as most front-engined cars do. Lift off sharply in mid corner and-the small camber change produced inthe semi-trailing-arm-Iocated rear wheels produced some breakaway, but by no means abruptly or unpleasantly as in th classic (rear-engined) Porsches, you can catch it easily. The result is an enormously secure-feeling car that inspires deservedly greatconfidence. There are detail criticisms; a suspicion of over-centre lightness is present towards extremes of lock, and to our taste a round steering wheel would be preferable to the ellipitcal one fitted to the test car, which interferes a little with the car's xcelfent self-centring when you tryto let it slip through the hands.

The only other point about stability oncerns the odd-feeling slight ndering you get on braking - "asthe front end is walking about" is' 0':'" one tester put it - a curiousity hich occurs also in the other Ride is firm. You get jarred noticeably but not unacceptably over sharp bumps. The car doesn't seem, however, to be disturbed seriously if it hits such a bump in a bend. It does roll, but not much, so that combined with the handling and grip, it's a car which invites fast driving at anytime. BrakesAdequate but not perfectLight pedal efforts are all that is needed for any stop; even the maximum retardation available required only 801b. 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 we were 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. Our fade test, which is deliberatelyperformance-related (the faster the car, the higher the speed from which it has to stoprepeatedly), showed that the improvements to the car's brakes are just adequate. Pedal effort had risen from 25-301b for the first V2g stop to 40-45 for the last three, which is still within bounds. 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 thepedal. 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. The handbrake on the test carsuggested that, as usual, the change over to rear discs does not help in this respect; it returned only 0.1 5g.

Behind The Wheel

Seats, rather garishly upholstered in pvc with panels of pseudo tartan fabric, are superbly comfortable, supporting the back well and giving tolerable sideways support. Passengers found them very easy to sleep in on a long drive. The car is far easier to drive neatly and precisely than the 928, in spite of the way even a tall driver feels somewhat sunk in it, because it is not too wide, and is blessed with quite good all-round vision except at the back, whose heiqht makes one suspicious when reversing that something vital is hidden. Also the of the glass gives one a distorted view of cars behind, rendering them lower and fatter than they really are.

The wiper blade in front of the driver has a heavy aerofoil meant to keep it on the screen. It works well up to 110 mph, which it starts lifting - but since, even in a car as 'safe as this, one doesn't want to chance such highspeeds in the wet, this is not a criticism. Mirrors are up to the performance; the outside electric one judders only a little from 130 mph onwards, so that it is still usable.Driving position is typically German - excellent, with no real awkwardness. Stalk switches are conventional, and easily worked, and for once on a German car there is a good set of instruments - 260. kph (160 mph) speedometer with press-to-zero trip, 6,600 rpm red-Iined revcounter, oil pressure, wat~r temperature, fuel and battery/volts gauges, plus a clock. Their conical glasses do not however, altogether avoid unwanted reflections.The heater be?,aves like a tolerably refined water-valve one, giving adequate temperature centre which responds only slowly to any desired change. Its controls are lit a night. Ventilation is good. The horn is worked ideally, from the large pal in the centre of the steering wheel.

Living with the 924 Turbo

By sports-car standards, this is 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, although we were surprised to find that it could only be opened with th: key on such an expensive model.One could do with more odd men room. The lockable glove compartment is quite usefully sized, and it is automatically lit on opening Underneath there's a quite long-reach but shallow open shelf. The door pockets are of limited use because their openings are obstructed by the armrests; maps have to be bent to put them in, and feeling if something small has been left in them is awkward.

There are handy hidden wells each side of the Spacesaver spare wheel under the carpet.To hinge the back seats forward, you must release them by pulling a plastic strap. To do so needs such effort that the strap can be felt stretching - a cheap arrangement unworthy of the car. On the other hand, one admires the usual magnificent tool kit which includes a tyre pressure gauge (dial type) and that jewel of an electric air pump; necessary for the Spacesaver; the same goes for the luggage strap arrangement, which when unused winds back on to its spring-loaded reel. At the front, the bonnet release is easily found, but the bonnet. which is quite heavy, has no self-propping stay.

The Porsche 924' range At present, 'you can't buy a Turbo in Britain; we gather that right-hand-drive cars will begin production in September, for introduction in October. It should be remembered when looking at the Dutch test car that the British market cars will come in with an appreciably higher standard of equipment - P7 tyres, electric windows, electric door mirrors and so on. This will put the 924 Turbo where it belongs, well on the way to the bottom of the 911 range (the 911 se which at present starts at £14,549), are obviously very much on top of the 924 pile. The least expensive 924 available is the four-speed model at £8,549 or £8,998 in semi-automatic form. The five-speed version starts at £8,940. The better-equipped 924 Lux four-speed is £8,998 or £9,448 semi-automatic, while the five-speed version is £9,389.