Siamese exhaust vs twin exhausts - performance?

[owain]

Righto, the my reconditioned engine and gearbox are back in it's frame and now I'm thinking of replacing my rusty old exhaust pipes with the siamese type of exhausts (mainly because it means I only have to buy one silencer instead of two). I'm wondering whether the siamese exhausts affects performance compared to the standard twin exhausts?

[duTch]

 Hiya owain, I think there's a number of threads on the subject already, might be worth a look.

  I think the general concensus is   

[JulianS]

A number of different siamese setups have been sold over the years, by BSA and a number of other exhaust manufacturers.

Some of the offerings were small bore and some not so I think it not possible to generalise over whether or not there is a difference in performance.

One recurring issue is the fit of the pipes. The bend of some dont allow the exhaust rose to sit square against the head on the nearside and most are a challenge to fit. Even BSA recommended exhaust paste - see yellow sheet below.

BSA also claimed 3 or 4 BHP power increase when fitting their "power jet" siamese system. Other claims have included improved mid range performance and better fuel economy.

I have used siamese om my A10 for many years and it does not seem to go any better or worse than other similar bikes.

The second photo, from 1962 US East Coast brochure shows small view of simese on Super Rocket in bottom right hand picture.

[muskrat]

Being a bit of a rev head (ex racer) I believe in one carb and one pipe, multiply X 2. In the science of pipes there is a pulse (negative pressure wave) that comes back from the end of the pipe to the valve. That is where we get tuned length of pipes, 1/4" can make a big difference. About 37" for A7's and 31.5" for A10's. If the pulse hits the valve at the wrong time it upsets the scavenging of the pipe, reducing power output. So with a siamese pipe the pulse will be split and not do it's job. Then we also have two cylinders worth of ex gas trying to get out one pipe, so it's actually more restrictive. Some argue that the inertia of the exiting left cylinder ex gas will help "pull" the right ex gas out and vice versa.
The only benefit I can see is the few lb's of weight saved and the cost of one muffler. But I do like the look.
Cheers

[jachenbach]

You got me thinking, Muskie (dangerous). If 1/4" difference in pipe length can make a big difference in performance, then the trimming that (at least in my case) was necessary to fit siamese pipes could mess up even an optimally designed exhaust.

[JulianS]

This is what Eddie Dow wrote in his article "Twin Tips" from his 1967 catalogue.

Does not tell us anything about normal road use.

[Greybeard]

That business of packing the nose of the oil pump is a likely source of trouble. What would happen if you didn't do that?

[mugwump]

I have just re-fitted the oil pump after a re-build and actually made a paper gasket ( more like a washer ) to fit under the front fixing stud ( as described ) for exactly the same reason. Without it the pump would be distorted surely. The same applies to the twin AMC oil pump for those that are familiar with them.

[duTch]

 I thought it was common logic and self evident. .. but let's not get carried away with it in an "Exhaust thread"

[worntorn]

I'm starting to really hate these Armours Siamesed pipes. I glued them into the head with high heat silicone as suggested. That lasted about 150 miles, now the left one has come loose again.
I wish I had stayed with the separate twin pipes. The bike sounded much better on board with twin pipes, had the same amount of power and the pipes stayed put. Too bad they were thrashed.

Glen

[edboy]

i love the siamese system as its so much easier for maintenance and makes no difference to performance. however you cant glue them into the head , they have to be an interferrence fit. no problems with original pipes except rust.

[duTch]

 I've said in another thread somewhere that I had the headers held in with a length of chain around them, and had a padlock for tying to fence posts- didn't look good, but I think it worked... .

 It's also been suggested to hold them in with a short spring to holes in the the head, or between the two.

 I've also said I have a 5" S.S. turnbuckle from the boat shop that I intend to use, when I fix the crappy radius 'bend-straight' of my (non-Armours) header, reckon it shouldn't look too out of place 

[JulianS]

I'm starting to really hate these Armours Siamesed pipes. I glued them into the head with high heat silicone as suggested. That lasted about 150 miles, now the left one has come loose again.
I wish I had stayed with the separate twin pipes. The bike sounded much better on board with twin pipes, had the same amount of power and the pipes stayed put. Too bad they were thrashed.

Glen

I suspect the pipe fit to head needs more than just high temp silicone to take up the clearance.

Try PTFE tap wrapped around the pipe, which should take up the clearance and seal it with some silicone (or exhaust paste) on top.

Tap it into the head with a rubber mallet then trim off the excess which will be pushed out.

This is what I use (on not an Armour system) and it stays in.

The other problem with the Armour pipe is that it does not come with a frame to pipe bracket .

[Greybeard]

..I have a 5" S.S. turnbuckle from the boat shop that I intend to use, when I fix the crappy radius 'bend-straight' of my (non-Armours) header, reckon it shouldn't look too out of place...

[Borderer]

Erm!!
When I bought my bike in 1974 it had Siamese exhaust with an Allen screw tapped into the left hand pipe through the port still solid now,
Tried separate pipes once but it felt flat compared to the Siamese system.