Yeh its a pretty common thing these days. Its for balance Jas. and yeh they sound shit without them. The guy who designed the mufflers also gave me dimentions for the merged area in the X pipe and the reducer behind the collectors.
All these dimensions came from some software programme he had after entering some engine specs, ie bore, stroke, comp, estimated peak rpm, cam specs etc. the 2 1/2" reducer behind the collector eliminates the doughy dead spot you sometimes get down low by speading up the gas flow again as it cools resulting in scavenging the dead gas out of the cylinders leaving area for the new air fuel mixture to enter.
Sounds like i know what im talking about hey, but the scavenging thing was explained to me today in a way that i understood. It is also said that the area of the X pipe controlls the torque band. Its all a bit more complicated in a atmo engine thats for sure. But all these things combined are supposed to make for a good efficient system.
Pottsy put a tube in as to measure back pressure, appatently anything over 1.5psi at 5000rpm is getting excessive and can result in power loss.
He also decided that two 3" outlets on each side will look trick so just waiting for some collectors and it will be done.
Fuel rails are well under way also, have managed to find 15mm. Just hope its enough.