I had a Robin engine in a hovercraft many moons ago ,that had a broken exhaust when I got it . I replaced the the exh. with a piece of pipe ,no baffles. It just would not start! Guy comes along and reckons it may be the exhaust..I bodged it up with baffles ,and hey presto it ran!Was wondering if the Hondas/Lifan /Briggs engines suffered in the same way?I've always made a point of building a baffled exhaust since then ,but was wondering if I could get away with an open pipe? I know the Honda 70/90 bike engines run without an exhaust ,(with a blip ,blip noise!)
Exhaust back pressure?
- Thread starter RED
- Start date
A bad exhaust design sends 'kinetic pulses' of returning exhaust gases back into the cylinder, contaminating the incoming fuel/air mixture...
It is an issue with 4-stroke exhaust design as well as a major factor in how well a 2-stroke engine functions...
Bad exhaust designs can also fail to dissipate enough heat to prevent the exhaust valve from pitting and burning...
An exhaust system is definitely more complicated than one would initially assume...
It is an issue with 4-stroke exhaust design as well as a major factor in how well a 2-stroke engine functions...
Bad exhaust designs can also fail to dissipate enough heat to prevent the exhaust valve from pitting and burning...
An exhaust system is definitely more complicated than one would initially assume...
To expand on what Yellowhand has said, everytime the engine fires, it sends out a pusle of exhaust through the valve. This pusle, (ball), of exhaust travels down the pipe, being pushed by the firing that came behind it. Baffles, or a baffle, create(s) a blockage for the ball to push against. As they force themselves past the blockage, they create backpressue. This, in turn creates a vaccuum that is carried back up to the valve. This suction draws the next pusle out as the exhaust valve opens. The better the vaccuum, the better the scavanging of gases from the cylinder. To a degree, of course. Everthing has a limit. Stock mufflers create a traffic jam and almost negate the scavanging effect.
For what it's worth, no knowledgeable performance oriented Harley rider would run drag pipes on a street bike. No baffles, no backpressure. Each exhaust ball just kind of breaks up and floats out the pipe. This creates a loss of power until the engine reaches the upper regions of the RPM range. Only then can it fill the pipe enough to begin to create the needed vaccuum. But, a simple fender washer, placed in the tail of the pipe, can be tuned to create a very good broadband performance pipe. Just that little bit of backpressure is all it takes.
For what it's worth, no knowledgeable performance oriented Harley rider would run drag pipes on a street bike. No baffles, no backpressure. Each exhaust ball just kind of breaks up and floats out the pipe. This creates a loss of power until the engine reaches the upper regions of the RPM range. Only then can it fill the pipe enough to begin to create the needed vaccuum. But, a simple fender washer, placed in the tail of the pipe, can be tuned to create a very good broadband performance pipe. Just that little bit of backpressure is all it takes.