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 GSS (internal name RT75-M) is the hottest Mark II, with a new DOHC 1858cc engine in a body that is basically the same as the conventional 1900 Hardtop SL (RT72-S). It can be said to be a car that continues the glorious tradition of the Toyota 1600GT (RT55/55-M), which dominated touring car races until the first half of 1969.
Before we get to the road test, let’s briefly explain the differences between the 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 of course a new design, a DOHC made of die-cast aluminum alloy that incorporates a great deal of experience gained from the RT55’s 9R engine (which was, of course, a DOHC).
The combustion chamber is fully machined and hemispherical, with the intake and exhaust valves positioned at an angle of 64° to each other (32° to the cylinder axis) to form a cross flow. The valve diameter is large at 45mm for intake and 37mm for exhaust, and stellite steel with excellent heat and wear resistance is welded to the face of the exhaust valve head. The intake valve stem is chrome plated and the exhaust valve is Tufftride treated to improve wear resistance.
The DOHC is equipped with 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 the extent that intake resistance does not increase at high speeds, and the exhaust valve is narrowed to the extent that intake resistnce does not increase at low speeds. Consideration has been given to increasing the flow rate and ensuring good atomization of the mixed fuel. Similarly, the valve timing has been improved without sacrificing torque at low speeds, and the valve overlap is only 36° for a DOHC (the same as the SL, but the cam profile is different).
To improve balancing at high speeds, the weight difference between the pistons is kept within 2g, and the connecting rod is also kept within 3g. The connecting rod is different from the 8R-B, and the pairing cap is not fastened with a bolt and nut, but with a thread cut on the connecting rod side and a reamer bolt. The curved shape of the shaft is also made gentler, and the oil outlet hole to the cylinder is also closed, to avoid stress concentration as much as possible. Two Solex 40PHH-II carburetors are used.
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’s service weight is 170kg for the GSS, and 168kg for the SL. Despite being DOHC, the difference is only 2kg, likely due to the use of a light alloy for the cylinder head.
Taking into consideration heavy-duty use, the clutch is a 224mm diameter diaphragm type (the SL uses a 200mm diameter coil spring type) similar to that used in the 2000GT. The gearbox is based on the 5-speed (fifth gear being an overdrive) used in the RT55 and 2000GT, but the gear ratio has of course been changed to match the power characteristics, and significant improvements have been made, including an increase in the capacity of the first and second gear synchros, tooth surface grinding, and reduced backlash. The differential is the same limited-slip differential 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 different with small caps that make them look capless at first glance.
The suspension is basically the same as the SL, but there are some differences. First of all, 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 reliable 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). The highly fade-resistant M33S mads (the SL has M2200) are standard, with DS-11 pads available as a dealer option for competition use. The rear brakes are of the leading/trailing shoe type with 229mm diameter Alfin drums. Of course, like 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 value, preventing the rear wheels from locking up prematurely.
The 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 the SL’s 9.3kg/ps to 7.5kg/ps. As a comparison with direct rivals, the Skyline 2000GT-R’s power-to-weight ratio is 7.0kg/ps, while the Bellett GTR is 8.1kg/ps.
The performance of the Mark II 1900 Hardtop GSS is best explained by comparing it with the data of the C/G 1900 Hardtop SL, which we purchased exactly one year ago and tested at Yatabe. We were highly impressed to see 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 the speedometer on the test car was unusually lenient, indicating 209km/h when the actual speed was 184km/h. However, the test car was equipped with the optional 165SR-14 tires (radial tires have a slightly smaller rolling radius than bias-ply tires), so some of this may be due to the difference in effective diameter.
The acceleration performance was truly remarkable. With two people and a fair amount of test equipment on board (equivalent to almost four people on board), the 0-400m run was achieved in 16.6 seconds (the same as the catalog value, which is very rare in our long 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, so you can see how much the extra 30ps improves acceleration.
The DOHC engine always maintained good performance throughout the 700km of rigorous testing. A cold start in the morning was a breeze with just two or three strokes of the pedal without using the choke (a Solex strong point), and the warm-up time was very short. However, the idling was rough even by the standards of a hot sports engine, and the needle sometimes swung between 400 and 800rpm, but whenever I stepped on it, it responded well.
The rev counter is redlined from 7000 to 8000rpm. During acceleration tests, I always pulled it up to 7200rpm in first gear, and it was still smooth and the valve response was not lost 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-cylinder, and the vibration period specific to four-cylinder engines was not observed anywhere in 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 even from 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 excellent acceleration of the GSS at high speeds can be clearly seen in the acceleration times above 100km/h compared to those of the SL: 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.
On the other hand, the GSS’s 40 to 80km/h acceleration time is 7.2 seconds in third gear (7.5 seconds for the SL) and 9.4 seconds in fourth gear (11.1 seconds), proving that the GSS’s low-speed range has not been sacrificed for the sake of high-speed performance. The SL is said to be usable at low speeds despite its high performance, but as these figures clearly show, the GSS, an even higher-speed model, is even easier to use at low speeds. For example, the 2000GT-R can only use up to third gear in the city, but the GSS, like a completely normal family sedan, can go into fourth gear at 40-50km/h. So in crowded city streets, you can forget about the existence of fifth gear and use it as easily as a four-speed SL.
The GSS’s 5-speed gearbox is a big improvement over the previous 1600GT and 2000GT. First of all, the gear ratios are well-matched to the power characteristics (the 1600GT’s was originally designed for the 2000GT, so third, fourth, and fifth gears were too close together, and first and second gear 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 handover from gear to gear. Revved up to 7000rpm, the first four gears could reach speeds of 58, 98, 143, and 174km/h. The shift feel was also much lighter and smoother than that of the 1600GT/2000GT. However, some staff members commented that the springs pushing the lever towards the third and fourth gear plane were too strong. Gear noise was generally quieter than in the 2000GTs we remember, but when we shifted from the direct fourth gear to the overdrive fifth, engine noise became lower, but a light gear noise was audible.
Even in the SL, 140km/h is a practical cruising speed that can be maintained without difficulty, but the GSS can cruise safely at 160km/h. Thanks to the overdrive, the revs are only about 5400rpm at this speed, leaving a sufficient safety margin. Even if cruising at 160km/h is “physically” possible, this speed would be unrealistic if the overall noise level and vibration exceeded the limits of human tolerance. However, the GSS is well within this acceptable limit. As mentioned above, the engine is smooth and quiet, but the good balancing of the two-piece propeller shaft is also worth mentioning, and the unparalleled straight-line stability also contributes greatly to easy high-speed driving. At Yatabe, even at speeds of over 180km/h, with just a light touch on the steering wheel, the car went straight as an arrow, and no effort was required to stay on the straight. We maintained top speed and drove for about 15 minutes straight, but the engine conditions did not change at all, the water temperature remained normal, and it idled normally after coming into the pits.
Naturally, fuel economy is slightly worse than the SL, but is rather surprisingly good for a DOHC 1.9-liter engine. Also, the GSS’s overdrive fifth gear was found to be clearly advantageous 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 almost comparable to the SL, and at speeds of 140km/h or more, the GSS actually consumes less fuel. In terms of practical fuel economy, it is 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 braes are basically very good. The front discs are larger than those of the SL, and the pad material is M33S, which has a lower friction coefficient (high wear resistance), so a slightly larger pedal force is required than in the SL. Still, it is not heavy in absolute terms, and it demonstratd 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. Another good thing is that there is very little nose dive. However, even these excellent brakes clearly showed fade in the rigorous 0-100-0 fade test (accelerating from a standstill to 100km/h, braking at the equivalent of 0.5g, accelerating immediately after stopping and braking when the vehicle reaches 100km/h; 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, making the brakes unstable. However, they quickly recovered after a few minutes’ rest. Basically, the brakes are reliable at high speeds.
The suspension is stiffer than the SL, and this feeling is even more pronounced because the test car was equipped with radial tires, and even with the standard air pressure of 1.5kg/cm² at both the front and rear wheels, the ride feels quite rough. The rear suspension in particular gives the impression of being stiff, and at low speeds the car is shaken strongly up and down. The suspension is clearly designed for high speeds, and the ride becomes smoother as the speed increases.
The rear axle of the GSS is equipped with a torque rod. The SL does not have one, and when starting suddenly during acceleration tests, the leaf springs wind up too much, causing the rear axle to swing out of control and making quick starts difficult. With the GSS, the torque rod works in tandem with the limited-slip differential, making it easy to make clean, sudden 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 light understeer up to the limit. The steering is not light, but it is very responsive and accurate. The grip on the road is excellent, and the tires do not easily squeal or slip. Even if it does slip, it can be corrected with firm steering inputs. However, the standard variable ratio steering (19.5:1-21.5:1) is a bit slow and keeps the driver busy, so the optional 17.5:1 steering ratio is more suitable for this car, even if the steering force becomes heavier. This is a “must” for those who participate in gymkhana.
The Mark II suspension is well matched to the radial tires, and even without changing anything else, just adding radial tires improves the handling significantly, and the ride comfort is hardly affected. 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 had increased to 2.5kg/cm².
In terms of the interior, the first thing that is different from the SL is the seats. They are semi-bucket seats with headrests attached, which are extremely comfortable to sit in. The seating position is slightly lower than in the SL, and there is no mechanism to adjust the seat height ike in the SL. The steering wheel is wrapped in black leather, the throttle pedal is elongated to make heel-and-toeing easier, and the standard equipment of the Hardtop SL, such as power windows, rear window defogger, and power antenna, are made optional to reduce weight.
Well, let’s jump straight to the conclusion. The GSS is a high-performance five-seater hardtop that is easier to drive around town than the Skyline 2000GT-R and much roomier than the Isuzu Bellett GTR. It costs about 200,000 yen more than its sister model, the 1900 Hardtop SL, but I can say with confidence that it is well worth it.