back pressure

[tylersmd1]

umm i was told that if i run a straight pipe that i wll have little back pressure and that it is bad. can sum1 help? or inform me?

[Jblankster]

bad? other way around. theres nothing bad about it, if anything less =better performance but yea. today i jus ran a 5 horse flat head with propane, had no carb or muffler. ran fine.

[tylersmd1]

so its alrite..cuz they said that it will ruin the rings??or sumthin like that..cuz i really want to run a sraight pipe.

[Mud Pup]

There is one universal truth about engines and open exhaust. It will sound cool.

In terms of performance, there are many contributing factors, and not all engines respond the same way. Some will have improved performance and others will have decreased performance.

You will be decreasing the backpressure, and some of raw fuel (and fresh air) will be "scavenged" into the exhaust system. This reduces the amount of air and fuel in the cylinder at the time of combustion and causes the fuel to burn in the exhaust system instead of the combustion chamber.

You will lose some cylinder pressure and this will decrease your available torque, although your rpms may increase.

Your air/fuel mixtures will be leaner and you may burn your exhaust valve(s).


There will be a tendency of back-fire (or popping) during deceleration of the engine.

I know all the above to be true, but I still run an open header on my grandson's minichopper, 'cause it looks and sounds cool.

[Jblankster]

^ there you go. he hit the nail on the head

and yes they do sound cool. in my auto shop at school we rus ran a 5 hp flathead on propane with NO exhaust at all. that thing was insane.

[Russ2251]

Most 4 cycle engines depend on a certain amount of back pressure to keep unburned gasses in the combustion chamber (purposely) to be remixed with the next intake cycle and actually re-burned. This is carefully calibrated by engine manufacturers. This is part of EPA and CARB regulations/mandates regarding emissions. You may realize a small increase in torque but, any increase would not be noticed by just anybody and would be very difficult to prove by ordinary means (dynamometer?). Also, combustion chamber temperatures would be compromised and may yield some scavenging problems. This can lead to a lean condition which can burn valves. Also, the neighbors will appreciate a quiet engine.

[tylersmd1]

thanks guys for your information..you guys are really helpful

[2or3wheels]

ive ran my 5 hp flathead briggs with no muffler and i didnt see any difference in performance, all be it that the muffler on it one of the studs for the exhause it broke and leaksso i dont know.