[quote]From what has been said on exhaust ports I am not sure larger diameter pipes, beyond say 1 1/2 ID are going to make a lot of difference, particularly for road use, but whatever diameter remember that it is pipe volume that is important...larger dia pipes need to be shorter, and you may then have problems with mounting silencers if using conventional routing.......[quote]

My first race bike had a Steve Maney bottom end, crank, ported & lightened head, 4s cam, Maney pushrods, twin 32mm Mikunis going into a 2 into 1 exhaust with 1 3/8" head pipes. It put out 52 hp when we put it on the dyno, right before we did some work on it. Herb made a 1 3/4" head pipes 2 into 2 exhaust and put twin 38mm Mikunis with long inlet tracks. The only engine work we did was to put a 7s cam in. Back on the dyno it now put out 67 hp. I would say big bore pipes would help you but mostly if you've got more air going in too. I have that dyno sheet print out with both graphs on it.
I've got a set of 810 pistons stamped Dunstall on the bottom that are out of street engine I just had taken apart. The guy had bored stock barrels until they were paper thin at the bottom to make them work- the bike ran fine but that paper thin bore at the bottom was not going to last. I had the motor rebuilt as a 750.

This is a subject of great interest to me as Im about to make up some pipes for my 1360 Vincent. Ive always thought (wrongly perhaps) that people tend to go too large on pipe diameter and end up losing exhaust velocity and some performance.

I recall reading that Paul Dunstall tried all different sizes of pipe on his racing Dominators and finally decided that the smaller 13/8" size gave those bikes the most power and greatest top speed.
. Later on, Dunstall was a consultant to Norton for the development of the first Commandos. One piece of advice he gave was to use 1 3/8 pipes on the Commandos, so that is what Norton did.
On the subject of long inlets, I have always thought (again, perhaps wrongly) that stock Norton twins have terrific throttle response due to the short inlet that engine type has. A Vincent has a longer inlet trac and it seems to be a little slower engine to rev. Both engines have heavy flywheels in stock form.
One well known Vincent is in a Seeley frame. The act of shoehorning the Vincent into the Seeley was documented in a British cycle mag about twenty years ago when this was done. The article was entitled "big job" which it was.
Due to the positon of the frame tubes a very long inlet stub was required for one pot, so the owner/builder, made the other stub of equal length.
That bike is one of the slowest accelerating Vincents Ive encountered. It did eventually show some power from 5 to 6 k (all done at 6k) but by the time it got up there the rest of us were already in the next town!

Btw Doug, I have the 650ss that Herb rescued from a junkyard in Ontario a few years back.

Glen

im not trying to make my 810 a track bike
i just wanted to try and improve on the weaker parts so i have a 810 dunstall
that hopefully will be reliable and not damage bottom ends by the sleeve breaking
and hopefully the inserts in the barrell wont work loose and cause problems
but still have a fairly fast norton that will the do business

thans val

Val, if your main goal is simply to end up with an 810 Dunstall, then I think you are going the right direction in mods to improve its reliability. Back in the '70s I built a race bike with a set of Dunstall 810 cylinders that I sleeved down to 750. Because Dunstall made the cylinders in both 810 and 750 bore sizes, LA Sleeve stocked replacement sleeves in both sizes. It was pretty simple to just replace the 810 liner with the 750 liner. It worked fine for a couple years, with the only problem being the need to regularly tighten the base nuts, because they would sink down into the soft alloy Dunstall used. Eventually the alloy cylinders cracked between two of the fins, and I had to replace them with iron cylinders, but the engine had suffered a lot of abuse on the race track before it finally gave up. Good luck with the project.

Ken

[quote="Doug MacRae"][quote]From what has been said on exhaust ports I am not sure larger diameter pipes, beyond say 1 1/2 ID are going to make a lot of difference, particularly for road use, but whatever diameter remember that it is pipe volume that is important...larger dia pipes need to be shorter, and you may then have problems with mounting silencers if using conventional routing.......[quote]

I just had a good test of that theory. I had two similar fresh builds. One was 850 ccs with 1 3/8 inch pipes and normal peashooters. The other was 880 ccs with 1 3/4 inch pipes and hollow peashooters. [ no center tube-very loud]

Both bikes had the same cam [street performance] ,compression ratio and cylinder head mods. They were both fuel injected.

Both engines hit the torque peak almost identical. Above the torque peak the big pipe 880 went downhill. At 6500 RPM the 850 with small pipes and mufflers with a center tube made a good 15 horse more than the big pipe 880 with open mufflers. The difference was dramatic. Jim

I have a friend that has three dunstall 810 cylinders he said he would get rid of, if anyone is interested Pm me a phone # and I will give it to him so he can call you.

comnoz wrote:
Both engines hit the torque peak almost identical. Above the torque peak the big pipe 880 went downhill. At 6500 RPM the 850 with small pipes and mufflers with a center tube made a good 15 horse more than the big pipe 880 with open mufflers. The difference was dramatic. Jim

So change the exhaust...or...tear down the motor and get a wilder cam in there...and a few more head mods....to work with the trick looking pipes....but don't go shopping on it....

Actually what would be interesting is if shorter big bore pipes would work with it, on the dyno, but I guess that it would need some form of high level exhaust to feed into those peashooters to actually be able to ride it....

Or, try it with 1 1/2 pipes, first and then centre tubed mufflers....as a workable compromise?

Back in the Day, I bought a new 1971 Commando. After finding that it would not catch the Kowa Tripples I started to Dunstallize it.
First I bought the 810 Kit & PD3 cam, & ported the head as per Paul D's instructions.
It was faster but not a rocket ship.
I bought one of the Dunstall 2 into 1 into 2 exhaust systems & the decibel silencers and the bike got faster.
A couple of years later I was traded a Paul Dunstall cylinder head with the re angled and larger valves.
This turned the bike into an 11.7 second bike. I never had any problems with the 810 kit, or lay shaft bearing. I did re do the mains to the superblends a some time . I drove it into the mid 1980's when I had to sell it to make a morgage payment.