Porsche 924 Turbo
With 170 b.h.p. on tap, Porsche's turbocharged version of their 924 is claimed to be the most powerful 2-litre production sports car in the World. It will reach 60 m.p.h. from a standing start in 7.6 sec and has a maximum speed of 140 m.p.h. Maximum power is produced at 5,500 r.p.m. and compares with 125 bhp at 5,800 r.p.m from the standard fuel injected 924.
Torque is increased from in 121.5 lb-ft at 3,500 r.p.m. to 180 lb-ft. at 3,500 r.p.m. Production of left hand drive Turbos has just started, but potential British customers will have to wait for delivery until Autumn next year. Porsche estimate that its price would be about £12,500 if imports started now, placing it nearer to the 911SC than the 924.
This 924 development by the company which must have more petrol engine turbocharging experience than any other is much more than a bolt on conversion. The whole of the running gear has been extensively revised, while ventilation slots in the top of the nose and the front spoiler, a NACA duct in the bonnet and a rubber tail spoiler mounted at the base of the liftup rear window all use air for various beneficial cooling or aerodynamic purposes and make the Turbo instantly distinguishable from the common-or-garden 924.
The 924 Turbo retains the in-line, four-cylinder, belt-driven single-overhead camshaft, eight-valve, Audi-developed engine of 1,984 c.c. The compression ratio is lowered to 7.5:1 and the turbocharger is allied to Bosch K-Jetronic fuel-injection and a transistorised, contact-less ignition system. Maximum boost is 10 lb. sq. in. The aluminium cylinder head has been changed quite radically, even to the extent of moving the platinum tipped spark plugs from the exhaust side to the inlet side. The larger valves are raised into the head to give more combustion chamber volume and the piston crowns are bowl shaped. The camshaft timing is unchanged. The turbocharger, a joint development between Porsche and KKK has the blow-off valve incorporated in the compressor housing, a new development. Positioning of the alternator, air filter, metering unit, headlight servo motor, starter and fuel supply lines have all had to be revised for space considerations. Extra fuel line pressure is developed by a second fuel pump installed inside the tank. A rubber spoiler beneath the engine creates an eddy to aid cooling under the bonnet. An oil cooler is fitted. Engines for the Turbo are built and bench tested at Zuffenhausen along with the flat-six engine.
The entire drive line has been beefed up to cope with the extra torque: the torque tube to the rear transaxle has been increased in diameter by 5 mm, the larger clutch comes from the 911SC, the mainshaft in the 5-speed gearbox, the differential and the half-shafts are all strengthened and first and second gear ratios have been lowered.
Springing and damping has been uprated, hubs-and bearings are borrowed from the 911SC and incorporate a change from four stud to five stud wheel fixing, ventilated disc brakes are fitted to all four wheels and the floating brake calipers are straight from the 928 model. New alloy road wheels of 6J section are increased in diameter to 15 in. Pirelli P75 will be offered as an option next year -- fitted to special 16 in wheels.
The performance claims are by no means exaggerated, as I found on a recent test drive near Stuttgart. Once the engine is turning beyond 3,500 r.p.m. the performance is stupendous for a 2-litre car, yet so deceptive that only the speedometer can tell the real tale. Speeds of 100 to 115 m.p.h. came up with effortless ease on ordinary main roads and the claimed maximum of 140 m.p.h., an indicated 146 m.p.h. could be held without protest on the autobahns. But at exactly 130 m.p.h. indicated, air pressure forced both front door window frames away from their seals to cause deafening wind roar.
The 924 Turbo's handling was beautifully balanced and roadholding first classs, yet at the same time it lacked any sort of lively character -- perhaps too vice-free? Those 928-based brakes had tremendous stopping power, but a hard application from near maximum speed generated dramatic roughness, which took a long time to clear.
I can't understand why Porsche have lowered the two bottom ratios. First is now far too low and the gap between second and third too great. The gate has first on a dog's leg down to the left, a system which Porsche abandoned on the 911 series but have brought back in the five-speed 924 and 928. I'm accustomed to such an arrangement, so this didn't bother me, but I did object to the strong spring bias into the plane, when a central bias would be much more logical. The manual 928 which I have at the moment does not have that right-hand bias.
The uprated suspension has made the ride harsher -- it must be even harder on the P7 option. The steering is lower geared than on the ordinary 924 to compensate for the extra turning forces required on the larger tyre footprint; although a smaller, thick-rimmed 930 Turbo steering wheel has been fitted to put the overall ratio more or less back to normal, the steering felt less pleasant than that of the ordinary 924.
Readers will recall that I complained bitterly about the harshness and coarseness of the early 924 road test car. Many revisions were subsequently carried out which considerably improved the road noise and vibrations, but the 2-litre engine remained coarse. I am sorry to say that turbocharging the engine has not removed that coarseness. The engine of the Saab Turbo, tested elsewhere in this issue, is so much smoother. In spite of the extra torque, the Porsche Turbo engine lacks punch at low revs, and needs all five speeds to be used to the full, but it is flexible if instant acceleration is not demanded from low revs. -- CR