blanking off 1 exhaust

Nobody has replied (yet)...I've read the same thing..possibly even in Roadholder?  But isn't it true that all Norton Singles had exhaust pipes with too large diameter?  One of the things they did to make Dommies a bit quicker was to fit smaller diameter pipes to keep exhaust velocity up.   Any racers out there who can add something?

Three valve twin exhaust  port sloper engines were fashionable at one time. These were usually long stroke slow revving hot running all iron motors . Perhaps the extra ex valve gave a a bit more cooling area to the hard worked ex valves and springs but a blanked off nearside port and one less pipe/silencer  showed  it was really fashion only and made life easier with a sidecar. Rudge showed that 4 valve singles were effective and  an Ulster put 200 miles into 2 hours  in the 1920'.s and  50 odd years ago a 350 Rudge sprinter managed 156 mph !!  and held records , sounds most improbable now. I did give the Ulster its head down the mad mile 10 years back ,and it was bonkers  fast but no brakes made it too exciting. 

The limit on gas flow is more inlet than exhaust.  Inlet has to make its way in with only atmospheric pressure to assist, but exhaust has a lot more escape energy.  If someone has nothing better to do...perhaps the twin exhaust port heads might be worth using in reverse!  Two carbs (at the front) and one exhaust at the rear.  It's been done a few times...there's at least one Manx (2 valve of course) with reversed head.

The two carbs could be arranged to open one after the other...

Those who fancy trying a modern take on this idea should look at the version from Yamaha, used in various bikes including Belgarda's SZR 660 supermono and assorted MZs, and also (apparently) some snowmobiles.

For your money you can experience the joy of setting up a combination of CV and butterfly valve carbs feeding the three inlet valves and exhausting through twin exhaust valves.

With a big-bore kit and hot cams 65 bhp has been reported.

Hello,

Here is my tuppence worth !

Years ago my brother in law blocked one port off on a 1930's twin port 500 that he used as regular transport but it dropped the head off the exhaust valve with dire consequences.  He still has the piston with several holes punched in it and the mangled valve head stuck fast and protruding at 90 degrees to the piston crown !  The evidence (valve colour) suggested that the blocked off port became a hot spot that was trapping heat and overheating the valve.  Mind you he has also had the head drop off in an ES2 head as well, so maybe it is just the way he drives it, he "makes progress" as they say in the police riders manual !

Good luck with whatever decision you take, make sure it is a good quality valve !

Malcolm.

....is complex! 

The down-pipes on my T140 Triumph twin look too small, but actually when you get into it, the pipe diameter needs to be selected to optimise for the best exhaust gas velocity over the running range, and Triumph/Norton did a lot of work on that in the 1970s/80s.

Essentially, a small pipe means higher exhaust gas velocity, which gives exhaust scavenging benefits, as each exhaust 'puff' creates a better negative pressure profile for the next one.

With two-strokes, exhaust design is critical to performance (and even whether the engine will run at all) but few folk realise that this is also true for four-stokes.

When I was a nipper, our neighbour had a Panther 120 with a Busmar on, it was a twin porter but only had a pipe on the offside, the other had a blanking plate in it, not for any performance  reason, just to make it a little quieter  for anybody in the chair.

Panther lore had it that the single porters were a shade faster which, if true,  would have showed the whole twin port thing up to be what it was, just a fad.