I currently have an 850 fitted which is still running the exhaust setup from previous 1293, medium bore LCB + 2 inch single box stainless system.
I'm looking to gain a little more power by fitting a small bore 3 branch manifold and 1.75 inch mild steel system leading to either a Sprite Mk1 box or an RC40 can.
Crikey, the exhaust is so big i`m surprised all the gases don`t crawl back into the engine and choke it to death
On a serious note the cast iron manifold separated from the inlet and that`s only to get rid of the hot spot is all you need on an 850 and most 1000`s to be honest.
You are losing power with a 2" system as you really need to keep the gas speed up to slow reverse flow on valve overlap or half your cylinders will be filled with exhaust gas instead of fresh fuel.
I have had some " quick " and brilliant fun 850`s that would " pull " 73 mph in third gear, that`s 6.5 - 7000 revs with nothing more than a bit of porting and deshrouding of the valves, 10:1 CR, air box ( big plastic ) and needle change and the washer being knocked out of the middle box of the standard exhaust system.
An 850 needs gas speed going in and coming out so the standard inlet with a bit of cleaning up is as good as you will get, separating the exhaust manifold will help torque and allows a higher compression before detonation and an 850 needs more compression.
I believe somewhere in the Vizard bible it states that the standard manifold is actually quite good so unless you are going to be pushing out 50+ bhp I don`t believe there will be any gains with a 3 into 1. An LCB is not the thing to use on any small bore other than full race, that`s not to say it won`t work but a 3 into 1 is better.