I see customised scooters (and less often, motorcycles) with little chrome/stainless secondary exhaust “mufflers” which look to be venting the crankcase to the atmosphere, rather than reburning via the intake manifold, as is the norm.
Whats that about??? I could see blowby gases might reduce the combustability of the charge a bit, (like EGR) but the effect must be tiny unless the engines knackered.
Is it just brainless bling, or am I missing something?
Crankcase breathing is very important, especially on a single-cylinder engine. Remember that the engine is firing once every 720* of crankshaft revolution. The rest of the time the crankshaft is using energy from the flywheel to move gas around and compress it. Hence the term ‘pumping losses’. If the crankcase were sealed the bottom of the piston would be continuously compressing and decompressing that volume of air, which uses power.
Back in the old days of Manx Nortons and BSA Goldstars they found a couple of hp just from freeing up the crankcase breathers or timing them just right. Of course it is one of those ‘last 0.1% of 0.1%’ kind of things you do to extract the last bit of power from a full-on race engine. Modern engines already have very efficient crankcase breathers which exhaust into the intake air to recirculate any oil vapor. So yes, on a scooter it is just bling.
Having now worked on a scooter and seen that its got an EGR system (which I didn’t know), I think the above probably missed the point. I’d bet those “mini-mufflers” are an EGR that doesn’t R, but exhausts directly to atmosphere.
From what I hear, EGR is only really bad news on diesels, but the system on a carburetted scooter probably isn’t very efficient, so you might gain a very little power, as well as keeping your engine cleaner, by disabling it.