Not enough back pressure?

[USMC302]

I picked up a tenth and .5mph with dumps.....

[Unit 5302]

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.

[Lethal5Oh]

Dang unit 5302! That explains everything. Guess ill have to trade in the offroad h pipe for the UPR catted x pipe. The recyclers should give me one heck of a good price for the stock cats. Thanks again for the post, YOUR THE MAN UNIT 5302.

[Unit 5302]

I'm not necessarily telling you to swap the setup out. You'll need a dyno to tell you which system is actually delivering better performance throughout your powerband. The "seat of the pants" meter is unlikely to be able to accurately analyze your cars true performance between the two.

Of course, if you're happier with one than the other, that's the ultimate test, isn't it?

[warlock 92]

I did the same changed out the factory cats for the offroad h-pipe mated to 2.5 flowmaster. THe car runs like a champ but is running real rich from inside the car I can smell gas fumes. Do I need make adjustments to my fuel regulator( AmFlo ).

Has anybody heard of AmFlo fuel regulators?

[Lethal5Oh]

You would have to have a air fuel gauge to tell that. My car smelled rich too, your smelling the unburned gas thats being sucked out of the cylinder.

[warlock 92]

Originally Posted by Lethal5Oh

You would have to have a air fuel gauge to tell that. My car smelled rich too, your smelling the unburned gas thats being sucked out of the cylinder.

have a Accel guage coming in the mail. Sounds like both our cars have about the same mods what PSI should I be running?