The four-cycle (or 4-stroke, as they are also commonly referred to) engine, in it's basic design, doesn't need exhaust back pressure to operate...
But, because modern engines are required to have very quiet, very flow-restricted mufflers when they are manufactured, they don't run well when a free-flowing exhaust is installed without also altering all the other factors affecting flow rate...
Back pressure in an exhaust system creates back pressure in the combustion chamber...
This decreases the amount of incoming fuel/air mixture the engine is able to draw into the combustion chamber during the 'intake' stroke...
If a free-flowing exhaust is installed to decrease back pressure, more of the inert gases left over in the combustion chamber after firing are able to be pushed out during the 'exhaust' stroke...
This decreases combustion chamber static pressure, and a greater quantity of fuel/air is drawn into the combustion chamber, even without re-jetting the carb...
But, the engine is still 'set up' to combust the old mixture level that contained more inert gases...
This usually results in incomplete combustion of the 'new' fuel/air mixture...
Two of the most important factors in combustion are the ratio of fuel to air in the incoming mixture, and ignition timing...
The fuel/air ratio of the carburetor was factory preset to compensate for the back pressure created by the factory installed exhaust system, and will need to be readjusted if this back pressure is removed...
'Timing an engine' adjusts the time that the spark plug fires to ignite the fuel air mixture...
A greater volume of fuel/air requires more time to completely combust...
The factory preset timing was adjusted for a mixture containing more inert (doesn't contribute any oxygen to combustion) gases, and won't be completely burned at the previous 'timing' setting...
This condition allows combustion to take place inside the exhaust port and pipe area during the exhaust stroke...
The fuel/air mixture is still burning as the exhaust stroke starts, and burning gases exiting the combustion chamber past the stem of the exhaust valve cause damage to the valve stem and seat area, and can also cut right through a new exhaust pipe, similar to the action of a blow torch...
So, changing an exhaust system on today's modern engines is a 'package deal'...
There are other components that need to be changed and adjusted at the same time, or the new exhaust will not have the desired effect of enhanced performance...
But it could have the opposite effect, depending on how restrictive the old exhaust was, and the amount of compensation made in other components...
But, on the plus side, a four-cycle engine, when 'tuned' for a free-flowing exhaust, will run better, due to the greater ratio of active (combustible) components contained in the combustion chamber during firing, and more powerfully than it did with the more restrictive exhaust...
:doah:
