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With Volvo's torque converter we romped up Scottish hills

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7th November 1969

Page 55

Page 60

Page 55, 7th November 1969 — With Volvo's torque converter we romped up Scottish hills

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by the Technical Editor

• At the Scottish Motor Show, .opening today, 44-ton tractive units make news. One model in this category is the Volvo F88. It has been shown before and, in fact, one of these models started all the speculation about gross weights perhaps going one day above 40 tons; Ailsa Trucks, concessionnaires for Volvo commercials, used one in 1967 with a Hands semi-trailer to show that 42 tons was feasible without exceeding present axle weight limits.

The latest development on the F88 has been in transmissions. As already reported, two new units were introduced in September, both based on the existing F160 eight-speed synchromesh gearbox used in the F88. One adds a torque converter in front of the box, the other splitter gearing to make 16 forward gears. The first F88 fitted with one of these units arrived in Britain last week-end—it had the torque-converter transmission—and I was able to make an appraisal of the design on Monday. The tests were in the area around Ailsa headquarters at Barrhead, near Glasgow, and confirmed Volvo claims of improved gradient ability. But I found that an even more valuable feature was the very much easier driving, particularly in heavy traffic, which the torque converter permits.

If European legislators (including ours) follow those in Germany, there could be a general requirement in all countries for a power-to-weight ratio of 8bhp per ton (Germany goes to this in 1972). At 40/44 tons this means 320/344 bhp engines. Volvo says that these power units will be big, heavy and thirsty for fuel. Like many others, the firm's engineers feel that the relationship between engine output and gross weight is not the only way to ensure that goods vehicles have an adequate performance. In effect they say: "Let's make the best of what

we have—retain existing engines and make more use of the power available"—Volvo is not behind on power, and its 9.6-litre turbocharged diesel in the F88 gives 270 bhp gross.

The two new versions of the R60 gearbox are the outcome of this thinking. The effect of doubling the basic eight ratios is obvious—the driver is able to pick exactly the right ratio to keep the engine at maximum output. But the addition of the torque converter is even more interesting, and has the effect of boosting the torque supplied to the main part of the box. In the Volvo application the torque multiplication is relatively low-1.9 to 1—but this almost doubles the tractive effort, which is particularly valuable in giving improved gradient ability—both restarting and climbing. It would have been perfect to have been able to check the performance of the MR 61 in one of the 40/44 ton outfits, but this was not possible. The vehicle brought over for demonstration is a four-wheeler and no suitable trailer could be obtained to allow the tests to be made at a high gross weight. The actual weight of the F88 turned out to be 18.25 tons, and at this weight there was obviously no shortage of power—over 14 bhp per ton. It was difficult to find hills to tax the vehicle and permit practical measurements to compare performance with and without the torque converter in use.

There were, however, three fairly severe hills in a 15-mile test circuit selected for comparative runs. They would have brought the speed of a heavy artic right down, and while the F88 romped over them on both runs, on the journey with the torque converter in use one less downward change was needed in each case as compared with the run when the converter was "locked out" all the time.

On the run employing the torque converter I used it only when I felt it desirable. The main purpose was to assess the advantage in obtaining extra torque and this was clearly demonstrated a number of times on hills, where flicking the switch to "unlock" the converter saved the change down. But the ease of starting and the reduced driving effort in towns was the most notable improvement. At the weight tested there would be no need to use the "low" range in the gearbox except on very steep hills.

Restarts on the flat were easy in fifth (bottom in "high" range} and possible in the same ratio on upward gradients of up to about 1 in 30. Starting from rest was like driving with a fullyor semi-automatic transmission: engaging the clutch to select gear, then letting it in before releasing the brakes and depressing the accelerator to move off. I tried various driving techniques and found it best to change from fifth to sixth at about 16 mph and then lock up the converter at about 20 mph, changing to seventh and top normally.

A major advantage was that a quick flick of the dash-mounted switch to unlock the converter saved changing down when meeting a hazard such as a sharp bend. This enabled the drive to be maintained and adequate concentration given to negotiating the hazard without having to make quick changes-down to suit the reduced road speed after braking. Again, when going through a town, top, gear could be held permanently unless one had to stop, the torque converter taking care of wide variations in road speed; the F88 would pick up quite smoothly and give adequate acceleration in these conditions from speeds as low as 10 mph.

There was no noticeable difference in acceleration performance with and without the converter and this bears out test results provided by Volvo. Figures published by the manufacturer also support my findings on hill climbing and show that at 16 tons a 1 in 5.26 gradient could be climbed in fourth, while at 42 tons the same hill was possible in second when using the converter; with a fixed drive the 42-ton outfit needed first to climb a one in 10 gradient.

Mention of fluid transmission couplings raises fuel consumption questions. I was not able to check this point, but Volvo reports that the fuel usage in town traffic at 16 tons was 12 per cent worse with the converter, but that figures up to 8 per cent latter resulted in "highway" and "mountain" running. This is feasible, the extra fuel used because of slip in the converter being outweighed by the ability to .surmount gradients in higher gears.

After my tests I am convinced that the MR 61 gearbox can offer major benefits. The F88 is a most pleasant vehicle to drive in its standard form. It is very quiet, comfortable(except for rather bouncy front suspension), light to handle and has excellent brakes—there was no feeling that we were overloaded. With the torque converter transmission, I rated the F88 four-wheeler one of the best vehicles I have ever driven.