Morris Minor Owners Club

1275 marina engine installed with larger bore twin box exhaust. Is the twin box system purely to keep the noise levels down? Or is it part of the old argument about back pressure needed (which I find strange as performance systems are advertised as free flow design). Asking as a new exhaust is on the cards later in the year.

To get a good overall view regarding performance (or not) exhaust systems I would suggest that you read David Vizards book on Tuning the 'A' Series especially the section on exhaust systems. It goes into great depth as regards flow.