Why Only 9.6 Compression On The BMW R 18?

[Hugh Janus]

Kevin Cameron (Robert Martin/)

Riders who read the motorcycle press are accustomed to seeing new bikes with 12-to-1 or even 13-to-1 compression. They are surprised to see that BMW’s new 1,802cc (110ci) “Big Boxer” has “only” 9.6 compression.

This big engine peaks at low revs (90 hp at 4,750 rpm) and is aimed at riders who love to use handfuls of low-speed torque. That means big throttle openings at low revs. That in turn means ideal conditions in which detonation (engine knock) develops easily. This happens when temperature-driven reactions in the last parts of the air-fuel mixture chemically alter it into a sensitive explosive. Give those reactions enough time and those bits of mixture auto-ignite and burn at the local speed of sound, generating shock waves that we hear as engine knock, and creating pressure spikes that damage pistons and crankshaft bearings.

Slow engine speeds and big pistons mean longer combustion times; this gives more time for detonation to materialize with higher compression ratios. (BMW /)

The slower an engine turns and the bigger its pistons are, the more time combustion takes, and the easier it is for the conditions that cause detonation to mature. To alter those conditions enough to prevent detonation, compression ratio in large slow-turning engines must be reduced somewhat.

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BMW worked hard to speed combustion in this engine, providing secondary air ports on the inlet side that generate flame-accelerating charge swirl at low throttle. At higher throttle, squish areas between piston and head generate last-moment jets of mixture as combustion progresses.

Peak engine horsepower is at 4,750 rpm for the R 18; this means big throttle openings at low revs, which is a perfect scenario for detonation. (BMW/)

Although it’s a rule that raising compression boosts torque, the Big Boxer has plenty of the latter—116 pound-feet at 3,000 rpm, which corresponds to a highway speed of 70 mph. Twist and go.

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