I disagree that "back pressure" increases performance:
Tuned exhaust
A
tuned exhaust system is an exhaust system for an internal combustion engine which improves its efficiency by using precise geometry to reflect the pressure waves from the exhaust valve or port back to the valve or port at a particular time in the cycle.
Tuned exhaust systems are common on motor racing engines, light aircraft engines,[1]model engines and two-stroke motorcycle engines, and are not restricted to these applications. They are of two main types:
- In a four-stroke engine, tuned extractor manifolds are used to promote scavenging of the exhaust gases by delivering a pulse of negative pressure just before the exhaust valve closes.
In either case, the efficiency of the system is greatest at a particular engine speed, which is determined by the exhaust system geometry. Tuning the system for best effect is typically done both by calculation during the design of the exhaust system, and by trial and error during engine development.
An important recent development is variable geometry exhaust systems, both for two-stroke and four-stroke engines, which allow the tuning to be effective over a wider range of engine speeds.
Exhaust Scavenging
In automotive usage,
scavenging is the process of pushing exhausted gas-charge out of the cylinder and drawing in a fresh draught of air or fuel/air mixture for the next cycle.
This process is essential in having a smooth-running internal combustion engine. If scavenging is incomplete, the following stroke will begin with a mix of exhaust fumes rather than clean air. This may be inadequate for proper combustion, leading to poor running conditions such as four-stroking.
Scavenging is equally important for both two- and four-stroke engines. However it is more difficult to achieve in two-stroke engines, owing to the proximity, or even overlap, of their induction and exhaust strokes. Scavenging is also equally important to both petrol and diesel engines.
Additional Notes
By removing the mufflers, you have changed the geometry of the tuned exhaust. It will not be as efficient at expelling exhaust gasses and will cause lazy exhaust to "hang out" in the exhaust system until it is pushed out. A tuned exhaust system "pulls" exhaust out of the combustion chamber when the exhaust valve opens, by using of the force of the previous exhaust pulse. An exhaust system with too large a diameter for the application will have the same effect.
