When Ford launched its six-cylinder 7810, it was roughly the same horsepower as its 7610 at around 110hp.

The pros and cons argued in the 1970s are such the same as what they are today. The biggest difference now is that manufacturers can get far more power out of a four-pot engine than they could then.

Today, 160hp is normal for a four-cylinder pocket-rocket, with some examples already getting close to the 200hp mark. There are obvious benefits to building a four-cylinder tractor that can produce a lot of horsepower. For medium-sized farming operations, in particular mixed operations, the benefits are many. A short wheelbase four-cylinder tractor can operate as an effective loader tractor during the winter. Developing 160hp means it can operate as a prime tillage tractor with a plough or power-harrow/drill combination.

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Can a 160hp four-cylinder tractor perform as well as a 160hp six cylinder tractor across all operations and rpm ranges? In theory, yes, helped by modern engine design and an electronic engine control map, which dictates how and when fuel is injected into the cylinder.

Table 1 shows a comparison of engine characteristics from Deutz-Fahr’s 6-series tractor. Some extra data was available from the engine manufacturer Deutz AG.

All power, torque and rated speed figures are quoted according to tractor manufacturer’s listings. These are identical for both four and six-cylinder tractors. This effectively gives potential customers looking for a 156hp tractor the option of purchasing it in four or six-cylinder build.

What’s the difference between the four and six-cylinder Deutz-Fahr 6160.4 and 6160? In length, there is a difference of 228mm (9in). It doesn’t sound much, but it has considerable effect on manoeuvrability and frontal stability.

It is likely that someone looking at tillage and haulage operation would go for the six-cylinder version. Everything in front of the rear axle acts like a lever or seesaw over the back axle.

In physics, this is called moments and it is relatively easy to work out for different weight and dimension. A long lever needs less weight than a short lever to apply the same force over the rear axle. This contributes greatly to stability.

The practical application of a four-cylinder tractor is that it would likely be a better choice as an all-rounder with a loader on-board. For heavy three-point linkage equipment and draught work, extra ballast would be needed up front to provide the same stability and traction as the longer six-cylinder tractor.

Grunt

We have tested quite a few tractors here in the Irish Farmers Journal, six-cylinder tractors and a few pocket-rockets among them.

Is there a difference? Yes, there is. Working at rated speed, there is little or no noticeable difference between four and six-cylinder tractors of the same rated power. The engine is running at its designed maximum and as engine rpm drops off under load, torque increases to help pull the engine’s rpm back up.

This is the key characteristic of a diesel engine and enables it to maintain its rated engine rpm and power under constant load or return to it after peak loading. Peak loading could be a hill on the road or a tough spot in the field when power-harrowing.

Running at lower engine rpm or taking off from idle, we have found a noticeable difference between four and six-cylinder tractors of the same rated power. There appears to be less grunt, for the want of a better term, at lower engine rpm in the four cylinder variants. This is particularly noticeable when taking off from a standstill with a loaded trailer behind the tractor or starting uphill and accelerating from a low rpm upwards.

Why does this happen? Well, the most likely reason is the difference in the number of cylinders. Tractor diesel engines are tuned to perform to their peak in the upper one third or quarter of the engine’s rpm range. Not so much at lower engine rpm.

Now, take a high-idle engine speed of 1,200rpm, for example. At 1,200rpm in a four-stroke engine, there are 600 power strokes per piston per minute. In a four-cylinder engine that equates to 2,400 power strokes per minute or 40 per second.

Add two more cylinders and at 1,200rpm, you now have 3,600 power strokes per minute or 60 per second. Yes, the power may be similar or the same at low rpm if measured on a Dyno. How the power is delivered is different though it’s smoother because there are more pistons on a longer crankshaft making that delivery.

It’s a numbers game really. At 1,200rpm, a six-cylinder engine has 50% more power strokes per second than a four-cylinder engine. And that, in my opinion, equates to 50% more grunt.