An interesting article on vintagenorton.com about getting performance out of a 16H. Nothing new in the ideas but interesting to see all applied.
The 2000 mile trip he mentions was in the company of a CS1. The 16H is nothing flash to look at until you try to keep up with it.

Thank you for the link Iain - it is an interesting read. One thing that did jump out at me was the comment that the exhaust pipe diameter on the 16H is too big (and presumably does not assist gas flow). Does the same thing apply to the M20?

Exhaust pipe diameter and length is a primary tuning tool.

A smaller diameter gives an increased gas speed resulting in higher gas inertia and greater pulse effect, resulting in a lower gas pressure at the piston and improved cylinder filling.

Recent data shows that even on a standard road engine the exhaust 'extraction' is actually r fagreater than that caused by the piston moving down the cylinder.

All powerful stuff, well known and exploited for years (just spent the weekend racing my Kawasaki 2 stroke - complete with tuned exhaust and variable exhaust timing).

Against this science is - HOW IT LOOKS - for years big exhausts = powerful engines.
Such that even the japs got in on the job - but using twin walled exhausts with nice scientific small bore inner pipe and a BIG shiny outer pipe + various box's and resonance-chambers hidden under the engine and side panels.

So where does it leave your question -
Unless there has been a very significant change in engines spec (cam, capacity, etc.) a 1.5" i.D pipe of something like the correct length, exhausting into a good silencer is as good as it gets without messing about.
The guys who designed these things where not muppets and certainly knew about exhaust tuning.

If you have a highly tuned engine - you can use all the theory you want, but the only way is to get it on the DYNO and start hacking/welding the exhaust.

I have a genuine '75 CCM (probably ex works - but ar'nt they all?) - it has a massive 1.75" I.D exhaust - way too big for the exhaust port and spec;
BUT - it is necessarily very short as there is no room.
They have increased the volume by using a non 'tuned' diameter, to achieve tuned system within the space available.
It works - very well and is widely copied.

So L.T.F.A and if want to go faster, buy something else.

Unless of course you want to make a sidevalve go faster...

Here's a picture of just such a project..'BADGER'...A 750cc J.A.P. V twin in an Ariel frame and turbo charged for a bit of extra OOMPH!...

It's taken about 10 years in total from acquiring the engine, through building it and getting it to run well...Definitely a long term project..Ian

email (option): ian@wright52.plus.com

Well maybe not muppets but I seem to remember massive megaphone exhausts in the 50's and 60's.
I recall several well respected motorcycle engineers branding Eric Walsh's 80MPH 125 cc D! Bantam, everything from a lap timming error, to a down right lie, till he arrived in the UK and blitzed the Junior classes against all the works competition bikes tuned by those "non-Muppets".

It is hard to go against conventional wisedom.
Even worse in the pre calculator, slide rule days you had to do a lot of work just to get consistant results from the math then get some one to draw it and finally make it before you could put in on an engine to test it.
This costs money and money was always tight.

Now days we can sit with our smart phones do computer simulations on the effect of changes in the angle and widths of the exhaust seats, let alone pipe diameter, pipe length and oft overlooked pipe wall thickness.

It was not till the work that Rex Bunn did in 2002 that the "coventional wisdom" on single cylinder ( included parallel twins ) crankcase breathing was found to be wrong.
So assuming "they" had it right is some what of a long bow.
Then lets not forget management prejudices. The engineering staff at BSA wanted the A 50/65 engines to be split horizontally which would have saved BSA millions and maby even the entire company but the board would have none of it because that was not how we English do things chaps now run along and make one like the old one.

OTOH I see nothing particular edificaing about taking a 40 year old bike then using technology and materials not available at the time of its manufacture making it go faster than it was designed to go particularly considering the limitations of the breaks & structural integrity of the frames.

If you want some thing that goes 100 mph go buy a 10 year old Honda 350 or H1 Kawaka. And sell your Notrun to some one who will appreciat it for what it is.

email (option): bsansw1@tpg.com.au

['OTOH I see nothing particular edificaing about taking a 40 year old bike then using technology and materials not available at the time of its manufacture making it go faster than it was designed to go particularly considering the limitations of the breaks & structural integrity of the frames...']

It's plainly not just about going fast...Take another look at the 'BADGER' and consider why my friend decided to do it...

It's about having a liking for something old, mechanical, being interested in how an engine works and setting yourself a bit of a challenge...however 'illogical' that may be..

For anyone who doesn't want any of those things and only wants to go fast then a cheque book and a Japanese sports bike is probably the answer..

It will certainly go faster but doesn't require a lot of thought and won't really make you feel you've achieved or learnt anything personally will it?...Ian

email (option): ian@wright52.plus.com