Pipe diameter

I have some nice old megaphone mufflers I would like to put on my Norton. They have 1.5 inlets.
I need new pipes so I was thinking about getting the 1.5 head pipes, I know the stockers are 1 3/8.
What are your thoughts on the bigger pipes on the 850? I don't foresee any real performance issues.
I can tune a carb if need be.

RGM has 1 1/2" pipes and silencers on offer. Suspect that that larger pipes
wont do you much good for a normal street set up. Maybe good for a howling
WFO bike.

Fat pipes used t o be used on older bikes such as t he 1950s Triumph 650s. The faster manx Nortons of the 50s used skinny pipes , however I've heard claiims that the fat pipes were better. In an exhaust pipe you have a resonating column of gas when the motor is running, and what happens between cam timing, and exhaust back pressure and the standing wave is important if you want performance. On a road bike I don't believe you would notice a difference. I always use the skinny pipes on my racer, and always a two into one with the cam timing adjusted to compensate. ( I notice from photos and videos that Kenny Cummings uses the fat pipes, and I believe his motor is tuned for 'top end' power.)

Unless you are going to step the compression ratio up very high, rev it for the extra power and have cam timing and enhanced intake to support all this your bike will perform much better with the 1-3/8 pipes.

I have 1 1/2 pipes on my 750, but it's set up a bit with a cam, head and carbs. They are about 3" longer than stock with adapter plate at the rear. I love them but can tend to promote a lean condition.
As said, bigger is not always better. There are adapter sleeves to attach 1 1/2" silencers to 1 3/8 pipes. I think it is best to go this route until you know how they work for you.

PM sent.

You can get big bore pipes with an inside diameter of 1 3/8.

acotrel said:

The faster manx Nortons of the 50s used skinny pipes , however I've heard claiims that the fat pipes were better.

Click to expand...

Where did this innaccurate snippett of info come from ??
Although we diverge from the topic at hand, as usual....

Manxes have a decidedly fat pipe - 1 & 3/4" - Commando pipes are skinny by comparison.
Restricting the flow in manxes in full cry tends to have the exhaust valve head melt/drop off.
The exhaust valve is sodium cooled, to help prevent this.
(The inside of the valve is hollow, with a little pellet of sodium in there.
It melts, and jumping up and down helps transfer the heat out of the head of the valve, down into the stem.
Same as many piston-engined aircraft and F1 engines use.).

1 1/2 inch pipes will take a chunk of power from the rpm range that is usable on the street. It also makes them harder to tune.

I even ran 1 3/8 inch pipes on the racebike for most short tracks. 1 1/2 pipes would give a little more stuff on the wide open tracks. Jim

Early on Paul Dunstall experimented with exhaust sizes and shapes. He found that Norton twins had far more acceleration to 7,000 rpm when the smaller diameter pipes were fitted. Norton downsized the exhausts on the later models of the 650ss and took his advice to use the same small tubing size for the new Commando model.

Glen