To see if the latest high-powered four-cylinder tractors can compete with their six-cylinder counterparts, we pitted equivalent powered MF tractors against each other. James Rickard and Richard Bradley report. Pictures by Marcello Garbagnoli.
Aside from front axle and abundance of the six-cylinder's throaty tones, it is difficult to tell the difference between the 6718S and 7718.
Ever since diesel power took over from steam, there has been a long debate as to whether a four-cylinder tractor could ever match up to an equivalent-powered six-cylinder.
It is an argument which is more prevalent than ever, as manufacturers continue to push the power envelope with smaller engines.
One manufacturer which is pushing the limits of four-cylinder output is Massey Ferguson, with the recent launch of its 6718 S model, a 200hp four-cylinder tractor. To compare its performance, we put it up against its equivalent six-cylinder stablemate, a MF 7718.
Both machines swapped between heavy draft on the plough, and worked the combi-drill for some more agile duties.
Apart from engine and front axle, the specification of both tractors is identical. In addition, to help with traction and balance for all tasks, each tractor was equipped with an 850kg front weight block.
To find out what the real differences are between the two tractors in real-life situations, we carried out various tasks, which took into account fuel use, engine characteristics, stability, manoeuvrability and traction.
For simplicity, throughout the test we shall refer to the four-cylinder as the 67 and the six-cylinder as the 77.
To get an idea of the engine performance of each tractor, both were subject to a dynometer test.
Before even getting to the field, the dyno results showed there is very little difference in engine performance, both in terms of power and torque.
Weighed with and without the plough, the aim here was to see just how much of a weight difference there is between the two tractors, particularly around the front axle.
Both tractors were weighed with full tanks of fuel and 850kg front weight blocks. The plough weighs 1,900kg.
Fuel use was measured using a metered pump for total fuel used during muck spreading duties and fuel observations were made, via the tractor’s own telemetry, for ploughing and drilling activities.
Working together, each tractor completed 30 loads from farm to field, with a 1.5-mile round trip following a twisty farm track.
Tyre pressures were set at 20psi to handle the weight of the spreader and to be able to cope with high transport speeds, but still forgiving enough to provide decent traction in the field.
However, the four-cylinder is a little more responsive to throttle input, but the biggest difference comes from engine noise; the six is a lot smoother and sounds sweet right through the rev range, while the four has more of a metallic rattle when it gets to high revs above 1,650rpm.
Ploughing at a depth of 250mm and a furrow width of 350mm, soil types ranged from medium loams to heavy and sticky clays. For traction, tyre pressures were reduced from 20psi to 15psi.
However, as its front-end could slip when trying to quickly turn around on tacky headlands, this made it less manoeuvrable than the 77 in some cases.
Only when met with heavy clay and a downpour did much difference begin to show. In this case, the 77 was still able to achieve a decent bite on the ground, while the 67 relied more on Massey’s famous draft control to keep moving.
When it comes to engine performance, you can hardly put a feeler gauge between the two, with both the 67 and 77 having similar characteristics and fuel use.
The differences mainly come from the physical size of the tractors, which has a noticeable effect on stability, manoeuvrability and traction.
As highlighted by our tasks, when it comes to tight manoeuvres, it is the shorter wheelbase of the 67 which gives it the edge every time. Conversely, it is the 77’s longer wheelbase and extra front-end weight which offers up extra traction and surefootedness.
Therefore, when choosing between the two tractors, the size of the engine is not the issue in this case, it is more of a question as to what physical size of tractor you need for a particular job.