Air today, gone tomorrow: One last hurrah for Honda CB1100 as air-cooled fours look ready for retirement

Despite the fact that 1970s-styled bikes are in vogue at the moment with manufacturers scrambling to revive machines from their pasts, one of the most convincing of those retro models is on the way out – and taking with it an iconic engine configuration that’s unlikely ever to return.

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Honda have revealed that a ‘Final Edition’ of the CB1100 will debut soon in Japan, and since the machine doesn’t meet Euro5 emissions rules, with no direct replacement on the horizon, it’s also set to disappear from showrooms on this side of the world.

Derogation rules, allowing time to sell leftover non-compliant models, mean Honda can keep selling it until the end of 2022 if there are enough remaining in stock, but after that – or earlier if stocks run out – it will become impossible to buy a brand new air-cooled four-cylinder motorcycle.

It’s amazing that such a machine has managed to cling on this long. With the CB1100, Honda invested in radical, patented air-cooling technology to beat the emissions legislation in place when it was introduced in 2010, when other brands had already abandoned the idea of air-cooled inline fours.

Now even that innovation isn’t enough to remain abreast of pollution rules, and with the planned end for internal combustion engines already appearing on government schedules around the globe, no company will plough the required effort in to revive the air-cooled four configuration.

Given that some air-cooled engines have managed to survive and thrive beyond the date that many believed emissions rules would kill them – BMW’s R1800 range, for instance – why are fours such a problem?

The answer is that while singles, V-twins or boxer twins have their cylinders exposed to the flow of cooling air on all sides, and even the cylinders of parallel twins can have a lot of airflow around them, the middle two cylinders of an inline four (or the centre cylinder of an inline triple) are surrounded by the heat generated by combustion in the other two cylinders.

That means the outer cylinders, with air rushing past them, can be kept cooler than the middle ones, and any imbalance in temperature like that causes headaches when it comes to controlling emissions.

Heat isn’t the problem, but the inability to control it is. Water-cooling, with channels between the cylinders and through the cylinder head, means engines can be warmed up fast – important for emissions tests that include cold start-up measurements – and then kept at a constant temperature thanks to the thermostat controlling water flow through the bike’s radiator.

On an air-cooled engine you can’t do that, and as temperatures change the materials in the cylinder, pistons, piston rings, valves and heads will expand and contract.

Wider tolerances are needed to combat that expansion, making emissions control even harder, and if – and on an inline four – the middle cylinders are hotter than the outer ones, keeping the whole engine in the sweet spot for low emissions becomes even harder.

Could it be done? Almost certainly. But given the lack of obvious benefits apart from the aesthetics, it’s hard to imagine any company making that investment again. It might be on the way out, but here’s how the CB1100 managed to give air-cooled fours a decade of extra life.

Oil cooling

The lubricating oil doubles as a cooling system. When coming out of the oil cooler, at its coldest, the oil pipes go straight to the cylinder head. The CB1100 has an extra oil pump purely for cooling and a thermostat to bypass the system when cold.

Swiss cheese engine

The essence of Honda’s achievement in making the CB1100 was allowing air to flow through the engine as well as around it. Between those fins it’s like four individual cylinders, allowing air to run between them where the highest temperatures arise.

Cylinder head airflow

As well as running between the cylinders, the cooling air is directed up through the cylinder head, through channels around the combustion chambers and spark plugs to further control their temperatures.

Years of development

Although the CB1100 went on sale in 2010, it was shown as a near-production concept back in 2007 (along with the delectable, but never-produced, CB1100R). Even at that stage, Honda’s designers had already put years of work into its 1140cc air-cooled four.

Future compromises

While the air-cooled four is bowing out, firms are getting better at maintaining the ‘air-cooled’ look. Whether it’s via partial water-cooling (water-cooled heads on air-cooled cylinders) or hidden radiators and dummy fins, bikes evoking air-cooled models will be with us for a while.