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Old 03-12-2007, 03:41 PM   #7
Unit 5302
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Join Date: May 1999
Posts: 5,246
Default Re: Not enough back pressure?

Absolutely. There is more to your problem than a rich/lean fuel ratio. Lean engines actually develop more horsepower to a point. When you have a higher flowing exhaust system paired up to a 5.0 requiring much less flow, you can create too much scavenging as you're indicating.

Since exhaust is like any other gas, it will flow from areas of high pressure to areas of low pressure. Obviously, the upstroke of the piston helps force exhaust out of the cylinder; however, a large diameter, straight through exhaust system will actually create a sort of vacuum for gases inside the cylinder at the top or near the top of the stroke. The reason for this is two fold. Heat creates pressure and so does confined spaces. Having a large exhaust space opening up from the exhaust port creates an area of low pressure the exhaust gas will accelerate into. In addition, the temperature further down the exhaust is cooler, which will tend to draw the gases out of the cylinder.

You’ll see large chambers on 2 stroke engine exhaust systems in order to dramatically increase scavenging and get the burned exhaust mixture out of the engine quicker, thereby increasing the clean air for the next combustion cycle. At the same time, the exhaust is shaped to create back pressure waves as the chamber is suddenly filled with high pressure exhaust because a two stroke engine does not generally have any sort of valve to close off the flow of air through the engine and into the exhaust. If there were no constrictions, the engine would run very inefficiently because air and fuel would be blown directly through the engine into the exhaust without stopping for compression. As it is, the 2 stroke engine is inherently inefficient because a backpressure wave is not nearly as efficient as a valve sealed cylinder like 4 stroke engines have.

Onto your 4 stroke engine. Scavenging works the same way on 4 stroke engines. It gets the old burned exhaust that will decrease burn performance out of the engine while simultaneously promoting a negative pressure environment to pull new fuel air mixture inside the cylinder on the intake stroke. The problem on a 4 stroke engine is mainly overlap. During overlap, both the exhaust and intake valves of the engine are open so an exhaust system designed to flow far more than the engine needs can create a vacuum effect inside the cylinder and pull unburned, fresh intake air/fuel out of the cylinder before combustion takes place. If this happens, cylinder pressure on combustion will be reduced thereby directly decreasing the torque generated, especially at lower rpms. At higher rpms, the effect is somewhat nullified or reversed because the scavenging improves the efficiency of the engine regarding the removal of waste gases and pulling fresh air/fuel into the engine becomes more important than the amount of cylinder pressure lost in the thousandths of seconds the overlap is occurring.

In summary, exhaust, just like everything else on your engine is best matched to your combination for maximum performance.
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