Toyota Corona Mark II GSS (1970)
Publication: Car Graphic
Format: Road Test
Date: January 1970
Author: “C/G Test Group” (uncredited)
Summary: 140ps twin-cam engine, smooth and relatively quiet up to 7000+rpm, extremely high cruising speed, good flexibility at low rpm, sturdy 5-speed gearbox that is comfortable to use even in the city, radial tires give great cornering power, comfortable bucket seats, slightly rough ride from the rear suspension.
Road testing the Toyota Corona Mark II GSS
The twin-cam–engined version of the Corona Mark II, which has long awaited by many, has now made its debut under the name 1900 Hardtop GSS (Grand Super Sports), and went on sale on October 15 at a price of 1,055,000 yen (Tokyo). C/G promptly undertook a test covering approximately 700km of driving, including accurate top-speed measurements at the Yatabe Test Course as well as handling tests at the Japan Auto Sports Center. The following is our report.
The GSS (internal designation RT75-M) combines a body essentially identical to that of the existing 1900 Hardtop SL (RT72-S) with a new DOHC 1858cc engine, making it the hottest Mark II yet. It can fairly be described as carrying on the revered lineage of the Toyota 1600GT (RT55/55-M), which dominated touring-car racing through the first half of 1969.
Before turning to the road test, let us begin by outlining the differences between the Mark II GSS and the conventional SL. The new 10R engine uses the same 1858cc cylinder block as the 1900SL’s 8R-B, with almost no modifications (four new 5mm water holes were added to improve cooling around the exhaust valves in the cylinder head, and the oil level gauge was moved to the other side as it interfered with the twin Solex carburetors). The cylinder head is an all-new DOHC design made of die-cast aluminum alloy that incorporates a great deal of experience gained from the RT55’s 9R engine (which was, of course, also DOHC).
The combustion chamber is fully machined and hemispherical, with the intake and exhaust valves opposed at a total angle of 64° (32° to the cylinder axis) to form a cross-flow layout. The valve diameter is large at 45mm for intake and 37mm for exhaust, and Stellite steel is welded to the exhaust valve faces for superior heat and wear resistance. The intake valve stems are chrome plated, while the exhaust valves are Tufftride treated to improve wear resistance.
The double overhead camshafts are driven by a two-stage double roller chain, and since this engine is designed with particular emphasis on practical use around town, the intake port is narrowed to an extent that does not increase intake resistance at high speeds, in order to raise intake gas velocity at low speeds and promote better atomization of the fuel mixture. Similarly, the valve timing has been chosen to improve high-speed characteristics without sacrificing torque at low speeds, and the valve overlap is only 36°, small for a DOHC (the same as the SL, though the cam profiles are different).
To improve balancing at high speeds, the weight difference between the pistons is kept within 2g, and the connecting rods are kept within 3g. The connecting rod design is different from the 8R-B, and the bearing cap is not fastened with a bolt and nut, but with a thread cut on the connecting rod side and a reamer bolt. The curvature of the shaft is also made gentler, and the oil outlet holes to the cylinder are also shaped to avoid stress concentration as much as possible. Carburetion is by two Solex 40PHH-II units.
The result of all these changes are that, while the SL’s 8R-B type engine generates an output of 110ps/4000rpm and a maximum torque of 15.5kgm/4000rpm, the GSS’s 10R generates 140ps/6400rpm and 17.0kgm/5200rpm. Equally impressive is that the engine service weight is 170kg for the GSS, and 168kg for the SL. This difference of only 2kg, despite the adoption of DOHC, is largely due to the use of light alloy in the cylinder head.
The clutch, intended to withstand heavy-duty use, is a 224mm diameter diaphragm type (the SL uses a 200mm diameter coil spring type) identical to that used in the 2000GT. The gearbox is based on the 5-speed unit (fifth being an overdrive) used in the RT55 and 2000GT, but the ratios have of course been changed to match the power characteristics, and significant improvements have been made, including increased synchro capacity for first and second gears, tooth surface grinding, and reduced backlash. The limited-slip differential is the same unit used in the RT55. The standard final drive ratio is 4.38, with 4.63 and 4.88 available as dealer options. The wheels are 5J x 14, which are larger and wider than the SL’s 4J x 13. The standard tires are 6.45S-14-4PR, with 6.45H-14-4PR and 165SR-14 available as manufacturer options. The wheels themselves are also model-specific, with small caps that make them look capless at first glance.
The suspension is basically the same as that of the SL, but there are some key differences. In the front suspension, the pins on the upper arms are oil-lubricated metal threaded bushes (the SL has oil-free rubber bushes), aiming for more precise steering. The number of rear leaf springs has been increased to five, raising the spring constant to 1.95kg/mm (the SL has four, with a spring constant of 1.7kg/mm). The vehicle height is 10mm lower than the SL. Also, to deal with sudden torque fluctuations, a torque rod has been added to the rear axle.
The brakes are also stronger than those of the SL. The front brakes are Girling S16-type discs with an increased diameter of 258mm (the SL’s have a diameter of 244mm), with highly fade-resistant M33S pads as standard (M2200 on the SL), and DS-11 pads available as a dealer option for competition use. The rear brakes are the leading/trailing shoe type with 229mm diameter Alfin drums. As with the SL, they are equipped with a vacuum servo, and the rear wheel circuit has a proportioning control valve that reduces the hydraulic pressure to the rear wheel circuit when it rises above a certain level, preventing premature rear-wheel lockup.
Vehicle weight is 1050kg, 30kg heavier than the SL, but since the power output has been increased by 30ps, the power-to-weight ratio has improved significantly, from 9.3kg/ps for the SL to 7.5kg/ps for the GSS. As a comparison with direct rivals, the Skyline 2000GT-R’s power-to-weight ratio is 7.0kg/ps, while the Bellett 1600GTR’s is 8.1kg/ps.
The performance of the Mark II Hardtop GSS is best expressed by directly comparing it with that of the C/G 1900 Hardtop SL, which we purchased and tested at Yatabe last year. We were highly impressed that the GSS’s high-speed performance (which is undoubtedly world-class for a practical, full five-seater vehicle) was achieved without sacrificing any low-speed flexibility. Starting with the top speed, we averaged 184.7km/h around the 5.5km circuit at Yatabe (unusually, the speed for the 1km straight section was lower at only 183.7km/h, which was probably due to the wind; the speed for the C/G SL was 170.1km/h). The engine speed was 6050rpm in fifth gear, nearly 1000rpm below the red zone. The C/G test group uses an accurate electronic fifth wheel speedometer to measure speed, but it should be noted that the test car’s own speedometer was unusually lenient, indicating 209km/h when the actual speed was 184km/h. Part of the discrepancy may be due to the optional 165SR-14 radial tires, which generally have a slightly smaller effective rolling radius than bias-plies.
The acceleration performance was equally remarkable. With two people and a fair amount of test equipment on board (equivalent to a load of four people), the 0-400m run was achieved in 16.6 seconds, matching the catalog value (which is very rare in our experience), and 0-1000m was achieved in 29.9 seconds. The C/G SL’s times were 17.9 seconds and 34.0 seconds, respectively, clearly showing the effect of the extra 30ps on acceleration.
The DOHC engine maintained consistently good performance throughout the 700km of rigorous testing. Cold starts in the morning required only two or three dips of the pedal, without using the choke (a Solex strong point), and the warm-up time was very short. However, the idle was rough, even by the standards of a hot sports engine, with the needle sometimes fluctuating by 400-800rpm. Still, whenever we pressed the throttle, it responded well.
The rev counter is redlined from 7000 to 8000rpm. During acceleration tests, we always pulled it up to 7200rpm in the lower gears, and it remained smooth, with no loss of valve response at all. However, the power dropped off rapidly over 7000rpm, so there was no point in revving it any higher. This engine is exceptionally well-balanced for a large inline four, and we felt none of the vibration period characteristic of four-cylinder engines throughout the wide rev range up to 7000rpm. Mechanical noise is also exceptionally low for a DOHC, at least in the cabin.
While the GSS’s engine is designed for high speeds, response holds up well even at low speeds. In the direct-drive fourth gear, the GSS can accelerate smoothly from as low as 40km/h (about 1500rpm). However, the power starts to come on at 2500rpm and above, and the engine starts to come “on the cam” only after exceeding 3500rpm. The GSS’s excellent high-speed acceleration can be clearly seen in the acceleration times above 100km/h. Compared to the SL, the GSS accelerates from 100 to 120km/h in 4.0 seconds (5.3 seconds for the SL), 120 to 140km/h in 5.9 seconds (8.6 seconds), and 140 to 160km/h in 9.5 seconds (18.9 seconds), making the GSS overwhelmingly faster.
At the same time, the GSS’s low-speed range has not been sacrificed for the sake of high-speed performance. In third gear, acceleration from 40 to 80km/h takes 7.2 seconds (7.5 seconds for the SL), and in fourth gear, the time is 9.4 seconds (11.1 for the SL). The SL is known for being flexible at low speeds despite its high performance, but the GSS, despite being an even higher-speed model, is even easier to use at low speeds. For comparison, while the Skyline 2000GT-R can only use up to third gear in city driving, the GSS can be shifted into fourth from 40-50km/h, just like a normal family sedan. So in crowded city streets, you can forget about the existence of fifth gear and just drive the GSS as if it were a four-speed SL.
The GSS’s 5-speed gearbox has been significantly improved over that of the previous 1600GT and 2000GT. Most notably, the gear ratios are well-matched to the power characteristics (the 1600GT’s, which was originally designed for the 2000GT, had third, fourth, and fifth gears too close together, while first and second were too far apart). The gear ratios are 3.074 (3.143 in the 1600GT/2000GT), 1.838 (1.636), 1.256 (1.179), 1.000 (same), and 0.856 (0.844). This results in a much more efficient hand-off from gear to gear. Revved up to 7000rpm, the GSS reaches 58, 98, 143, and 174km/h in the first four gears. The shift feel is also much lighter and smoother than that of the 1600GT/2000GT, though some staff members felt that the spring pushing the lever towards the third and fourth gear plane was too strong. Gear noise is generally quieter than we recall from the 2000GT, but when we shifted from direct fourth to overdrive fifth, the reduction in engine noise was partially offset by a noticeable light gear whine.
Even in the tamer SL, 140km/h is a practical cruising speed that can be maintained without difficulty, but the GSS cruises comfortably at 160km/h. Thanks to the overdrive, the engine is only turning about 5400rpm at this speed, leaving a sufficient safety margin. In the case of some cars, even though cruising at 160km/h is technically possible, actually doing so is unrealistic due to the noise and vibration levels exceeding the limits of human tolerance. However, in the GSS, these are well within the acceptable range. As mentioned above, the engine is smooth and quiet, and the good balancing of the two-piece propeller shaft is also worth mentioning. The exceptional straight-line stability also contributes greatly to relaxed high-speed driving. At Yatabe, even at speeds of over 180km/h, the car ran as straight as an arrow with just a light touch on the steering wheel, staying on course with no effort required. We drove at top speed continuously for about 15 minutes, and the engine’s condition did not change at all. The water temperature remained normal, and it idled normally after coming into the pits.
Fuel economy is, as expected, slightly worse than the SL, but for a DOHC 1.9-liter engine it is in fact surprisingly good. In this regard, the GSS’s overdrive fifth gear gave it a clear advantage on highways. That is, fuel economy in fourth gear is generally about 15% worse than the SL’s top (fourth) gear, but in fifth gear it is roughly comparable to the SL, and at speeds of 140km/h or more, the GSS actually consumes less fuel. In terms of practical fuel economy, we observed 5.5-6.0km/l in urban areas, and 7-8.5km/l on national highways. The total average for the test distance was 7.2km/l.
The brakes earn high marks overall. The front discs are larger than those of the SL, and with their M33S pads, which have a lower friction coefficient (higher wear resistance), the brakes require slightly more pedal effort than in the SL. Still, the effort not heavy in absolute terms, and it demonstrated a deceleration of 0.95g at 40kg. The PCV in the rear wheel circuit is effective, and the braking force of all four wheels is well balanced even when braking suddenly. It is also commendable that there is very little nose dive. However, even these excellent brakes showed clear signs of fade in the rigorous 0-100-0 fade test (accelerating from a standstill to 100km/h, braking at the equivalent of 0.5g, and accelerating immediately after stopping; this is repeated ten times to see if fade occurs). The initial pedal pressure of 18kg began to fade rapidly around the seventh stop, and from the eighth stop more than double the pressure was required, and braking effectiveness felt unstable. However, they quickly recovered after a few minutes’ rest, and in practical use, the brakes are reliable at high speeds.
The suspension is stiffer than that of the SL, and this feeling was even more pronounced because the test car was equipped with radial tires. Even with the standard air pressure of 1.5kg/cm² at both the front and rear, the ride feels quite firm with these tires. The rear suspension in particular gives the impression of being overly stiff, and occupants feel strong vertical motions at low speeds. The suspension settings are clearly tuned for high speeds, and the ride becomes smoother as the speed increases. In this respect, it has something in common with cars from Alfa Romeo. That said, it is by no means intolerable in everyday use.
The rear axle of the GSS is equipped with a torque rod. The SL does not have one, and in sudden starts during acceleration tests, the leaf springs tend to wind up severely, causing the rear axle to leap wildly and making quick starts difficult. With the GSS, the combined effect of the torque rod and the limited-slip differential make it easy to make clean, fast starts with minimal wheelspin.
The handling is surprisingly good for a car with a rigid rear axle. This is probably more due to the 165SR-14 Bridgestone radial tires on the test car than the reinforced suspension, but it maintains just the right amount of mild understeer up to the limit. The steering is not light, but it is very responsive and accurate. The tires’ grip is excellent, and they do not squeal or slide easily. Even when they do, corrections can be made confidently thanks to the accurate steering. However, the standard variable ratio steering (19.5:1-21.5:1) is on the slow side and feels a bit busy; the optional 17.5:1 steering ratio, though heavier in effort, is better suited to the car. In fact, this option should be considered a “must” for those planning to participate in gymkhana.
The Mark II suspension is well matched to the radial tires, and even without changing anything else, simply fitting radials improves the handling significantly, while hardly affecting ride comfort. We started our testing with the tire pressure at 2kg/cm², and when we measured the temperature after driving at high speed for about three hours at Yatabe, the temperature had risen only slightly and the pressure increased to 2.5kg/cm².
In terms of the interior, the main difference from the SL is the seats, which are extremely comfortable semi-buckets that appear to be based on the 1600GT’s, with headrests added. The seat is positioned slightly lower than in the SL, and there is no mechanism to adjust the seat height as in the SL. The steering wheel is wrapped in black leather, the throttle pedal has been elongated to make heel-and-toeing easier, and some of the Hardtop SL’s standard accessories, such as power windows, rear window defogger, and power antenna, are made optional to reduce weight.
In conclusion, the GSS is a high-performance five-seater hardtop that is easier to drive around town than the Skyline 2000GT-R, much roomier than the Isuzu Bellett GTR, and is slightly less expensive than either. The price is about 200,000 yen higher than its sister model, the 1900 Hardtop SL, but we can say with confidence that is well worth it.
Postscript: Story Photos
