Twin Carb exhaust size

[oliver-morrisminor]

I saw alanworland's post re back presures. Which has promted me to ask this question.

In all the literature I can find, it mentions that when you fit twin Carbs to a sidevalve, that a slightly "oversize" exhaust with a free flow silencer makes for improved performance.

Am I right in saying the standard exhaust is is 1", as the exhaust ports are approx this size. So would they be speaking of 1 1/8" or 1 1/4" exhaust tube? As they also mention not to go too large as to loose back pressure.

[PSL184]

General statements such as "do not go too large as too loose backpressure" are misleading and unhelpful. You want an exhaust system with ZERO back pressure because the action of the gasses escaping through the system make their own back pressure. So, if you have a system which rstricts and generates 5 psi of backpressure this will increase to 7 - 10 psi when the engine is running at peak power.
Specialist software is available to calculate optimum pipe diameters for any given flow rate but you will need to know the precise outputs from your engine in terms of bhp, revs, peak flow etc. It's not really suitable for ameture use because of this and remember that flow changes with rpm so you will always have a compromise. Basically the smaller the exhaust dia the more torque you have at low rpm but this will decrease with higher rpm so you need to balance the low speed driveability with high speed flow.
HTH

[RobThomas]

Having designed and built a system for an aeroplane (which worked) I'd say that you'd do well to go up 25% oversize where the 4 pipes join into one and keep it that size for a few feet before sleeving down slightly towards the tail end since the gasses shrink slightly as they cool. Keeping 1 inch all the way will create a choke-point where the 4 go into 1. This is a generalisation but the science of exhaust making seems to work out this way. You want to keep as much of a constant flow as possible so the first 4 sections should be exactly the same length and a scientifically derived length as well, although this is rarely possible and will only be 'perfect' for one rpm.
Midget/Sprite 1960-1970 era silencer will work well or even a motorcycle can from the 1950s ought to work.

[alanworland]

Hi oliver-morrisminor, purely out of interest I have tonight removed my stainless silencer which takes a 1.5 in pipe from the manifold and exits to a 1.25 in pipe (both OD) - the box is about 14 in long.
Looking inside the silencer the pipe stubs continue into the body (both ends) with radial holes approx 3mm diameter, making the pipe perforated into the outer case. At a depth of about 3.5 in the perforated tube is blocked off (both ends)
So the exhaust gases enter the front pipe, travel through the perforated tube wall into the main silencer case (I dont know whats in the case but there are some weld marks visible so I reckon there is some sort of internal support) Gases travel along inside the outer case then through the perforations at the other end to exit into the 1.25 in pipe.
Looking into the pipes at either end it is difficult to imagine gases going through very easily as they have to go through these small holes twice! I haven't counted how many holes are in the walls of the pipe, but it doesn't seem enough!
I certainly understand how this contorted route would generate backpressure!

[RobThomas]

Royal Enfield motorbikes arrive in the UK with chrome silencers that ought to be the perfect size and they usually get discarded due to awful corrosion prospects. About £5 from dealers after they get removed upon import. Nice and narrow. Have a visit to a Classic Bike showroom?

[alanworland]

Has anyone used one of these?

[MarkyB]

I have a feeling the reason they replace these silencers is because they are restrictive due to a crude catalytic converter being fitted.
When this gets hot it discolours the chrome.