Aftermarket BMW F800ST exhaust fuelling problem

Q. I have recently bought a 2009 BMW F800ST and changed the can to a Remus exhaust, which has improved the torque and general performance.

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I have had the bike on the dyno to check it out and the power at the rear wheel is up by 3.5bhp. However the fuel/air ratio is reading out at 17.5, which, as you will know is very lean.

I asked the supplying BMW dealer what it should be, but they had no idea. After some pressure they asked BMW UK without success.

Not only am I very disappointed with this lack of information, I do realise that if I am running the engine much leaner than its design envelope I risk burning the valves/seats, etc.

Trevor Harvey, Norfolk.

A. I’ve made some enquiries and part of the problem is that the Remus isn’t supported by BMW, who sell the Akrapovic pipe, so they don’t have the data for your exhaust.

Be that as it may, we can’t work out how the dyno company is getting a ‘lean value’ or 17.5 as this reading (if measurable) should be variable due to the oxygen sensor (lambda probe).

As you know Lambda is one lump of fuel to about 14 lumps of air. This is the perfect burn ration, but as you cannot get an engine that conforms to Euro 3 test cycle requirements you have to use a program that causes the mixture control to vary the mixture on a sinusoidal wave form (ie, rich – lean – rich – lean etc)

In the lean section the value would go to one lump of petrol to 14 lumps of air, in the rich part of the cycle it would be one lump of petrol to 10 lumps of air.

If you have left the oxygen sensor disconnected the missing sensor reading would cause the engine management system to flat line, thereby working the fuel injection value as a fixed constant.

If you leave it like this you may have pre-ignition trouble or burn your valves out as you suggest.

That won’t be covered by the warranty so I suggest you go back to that dyno house or get a second opinion as soon as possible.