Two outlet ports on a single?

Itchy · Super Spammer Joined: 07 Apr 2005 Karma :

Anybody know the reason as to why this is so? , in that all the big singles I've seen DR600/650 , XT600/660 MT Armstrongs , Rotrax 650 engines , even the Husquavarna big singles all have two exhaust ports which combine into one pipe just outside the barrel.

But yet on any multi cylinder bike ie my CBR it has one port per cylinder , same with the africa , the NTV and other bikes.

Just curious but why is this?.

Any thoughts?

Ta
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Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Probably gives a slight advantage in exhaust flow, and with a single there is plenty of space to do it. Also possible that they can use different length exhausts from each port to possibly get some scavenging improvements.

All the best

Keith
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truslack · World Chat Champion Joined: 08 Apr 2007 Karma :

thewildblue · Nitrous Nuisance Joined: 17 Jun 2008 Karma :

One for each exhaust valve ?

Pedd · World Chat Champion Joined: 24 Mar 2008 Karma :

yup mines single exhaust out let with twin sparks and twin carbs. Couldn't tell you the technical reasons behind it tho lol.

iirc tho the pipe dia on my single outlet is 45mm but on the rotax engined versions the twin outlets are 38mm each. So more outlets = smaller bore pipe ???
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Kal · World Chat Champion Joined: 02 Jan 2007 Karma :

My GB has twin outputs despie being a single, only 250cc though.
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Mudskipper · World Chat Champion Joined: 22 Aug 2006 Karma :

So did my CB250RS.

https://en.wikipedia.org/wiki/Multi-valve
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Feasty · World Chat Champion Joined: 01 Nov 2006 Karma :

My 650 has 2 outlets on a single, I've always assumed this is just to get the gases away as quickly as possible - I don't know anything about them though!
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loply · World Chat Champion Joined: 24 Mar 2004 Karma :

Seen the size of the valves on a big single? They might as well be two seperate cylinders...
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Robby · Dirty Old Man Joined: 16 May 2002 Karma :

Smaller ports means higher gas velocity. This is apparently a good thing, all of the 4AGE tuning boys (mk1 MR2 engine) tune the small port engine, the big port is harder to get power out of.