View single post by chris
 Posted: Fri Jan 11th, 2019 02:28 pm
PM Quote Reply Full Topic

Wrench spinner

Joined: Thu Oct 5th, 2017
Posts: 266
Reputation Points: 982
The concept of engines needing "back pressure" in the exhaust is erroneous - they will perform better with low restrictions in the exhaust system.

Basically the silencer is only there to quieten the engine to acceptable levels - straight through silencers present less resistance in the system than baffled silencers and are to be preferred.

System design is complex - the best systems use reflected waves of lower pressure at critical rpms to enhance cylinder scavenging during the exhaust cycle - these waves are generated wherever the exhaust path increases in volume - so a silencer / cat / end of pipe will generate a wave which returns to the ports in the head - ideally the wave(s) from one cylinder arrives back just as another exhaust valve is opening.

Motor manufacturers tend to get this right for their average customer but this may not suit tuned engines or those run at other than normal rpms.  If you change the exhaust system or state of tune of the engine you may well upset the power delivery.

Silencers without cats don't have to be noisy at all - I run a single large straight through silencer at the very back of my MGB V8 and even with 3.6 litres & 200 bhp it has a quiet burble which is very pleasant!

MG power