I was reading a review of a new single cylinder motorcycle (BMW G 310 R) which mentioned it has the intake in the front of the engine and the exhaust in the rear - the opposite of the normal arrangement (intake in the rear, exhaust in the front) - which made me wonder, why the latter is the standard arrangement? Does not the former seem to intuitively make more sense?
In the days of air-cooling, having the exhaust in the front kept it cooler than if it was blocked from airflow.
In the case of the BMW
It’s water-cooled so placement isn’t the issue it once was.
If my memory serves, Harleys had the rear cylinder slightly offset so it got some airflow as the rear exhaust was at the rear of the “V”.
The exhaust valve and port are the single hottest part of the cylinder head, and as running coach said it was beneficial to have it up front to receive direct cool air. plus, when you have multi-cylinder engines like the UJMs with four exhaust header pipes, you can easily route them down from the front of the engine under the bike towards the rear. if the exhaust ports were at the rear of the engine they’d be right under the gas tank and seat.
nope; Harley V-twins use fork-and-blade connecting rods so the cylinders are in line with each other. Back before the big baggers (the Touring range) went to water-cooled cylinder heads, they had a mode called EITMS which would kill the rear cylinder if the cylinder head temp got too hot at idle/low speed.
The other point is that a rear exhaust is under the rider’s butt, rather than out away from the rider in the air flow. Depending on routing, you need a setup that avoids accidental touching and helps keep things cool while idling in traffic. Plus, the intake is attached to a carburetor and traditionally that takes up a decent amount of space. (was attached? Fuel injection?) Normally the air filter and carburetor(s) sit under the seat. If the intake is out front, either the carb is hanging out there too or there’s a long-ish air pipe from under the seat around.
The traditional design worked best for the traditional configuration of engine. Perhaps BMW had a better idea…