Can someone support or disspell the backpressure myth
Record Breaker
Joined: Feb 2006
Posts: 1,567
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From: Richmond, Va
RE: Can someone support or disspell the backpressure myth
ORIGINAL: MEINEKEMAN1
I won't be able to explain it very well. As I mentioned I only have a limited understanding on this subject. If you search this forum, and the internet, you'll be getting much better answers than what I will be able to give.
I'll do my best, and await the criticism from those who know that will no doubt follow...
If there isn't enough exhaust velocity, then the exiting exhaust gases don't create enough momentum to pull allong (syphon) the rest of the exhaust gases behind it. Engines use an overlap between closing the exhaust and opening the intake valves to benefit from the vacuum of the exhaust gases exiting the combustion chamber. When the exhaust gas velocity is down, there isn't enough vacuum created in the combustion chamber to adequately pull out all of the exhaust gases and benefit from the overlap between the intake and exhaust vales pulling fresh fuel/air mixture for the next compression cycle. When there is enough velocity / vacuum in the expelling exhaust gases, this will help pull all of the spent exhaust gas and pull a little more fresh fuel/air into the cylinder before the exhaust valve closes. This gives more of a bang in the combustion cycle, thus more power. Now, if you're running higher rpms with an open exhaust, this will increase the exhaust gas velocity and begin creating the vacuum during valve overlap as I mentioned. The more the exhaust system is open, the harder it is to create this vacuum, so the engine has to increase its rpms to make power. If you plan on using power at HIGH rpms only and built you engine accordingly, then this would be a good thing. Since your engine isn't built to run at the high enough rpms to benefit from open exahust pipe, it will feel sluggish because it is not getting that desired pull on the exhaust gases exiting the combustion chamber.
If you've ever noticed when looking at camshaft profiles, they always talk about lift, and duration, and valve overlap, etc. These things are built into the lobe shapes to make power in specific rpm ranges, and they are based to a degree on this overlapping of the valves.
For not being an expert as you said,you damn well explained it like one.
The only thing I can do on your post is to bump it!!
Great explanation and understanding of this.
I won't be able to explain it very well. As I mentioned I only have a limited understanding on this subject. If you search this forum, and the internet, you'll be getting much better answers than what I will be able to give.
I'll do my best, and await the criticism from those who know that will no doubt follow...
If there isn't enough exhaust velocity, then the exiting exhaust gases don't create enough momentum to pull allong (syphon) the rest of the exhaust gases behind it. Engines use an overlap between closing the exhaust and opening the intake valves to benefit from the vacuum of the exhaust gases exiting the combustion chamber. When the exhaust gas velocity is down, there isn't enough vacuum created in the combustion chamber to adequately pull out all of the exhaust gases and benefit from the overlap between the intake and exhaust vales pulling fresh fuel/air mixture for the next compression cycle. When there is enough velocity / vacuum in the expelling exhaust gases, this will help pull all of the spent exhaust gas and pull a little more fresh fuel/air into the cylinder before the exhaust valve closes. This gives more of a bang in the combustion cycle, thus more power. Now, if you're running higher rpms with an open exhaust, this will increase the exhaust gas velocity and begin creating the vacuum during valve overlap as I mentioned. The more the exhaust system is open, the harder it is to create this vacuum, so the engine has to increase its rpms to make power. If you plan on using power at HIGH rpms only and built you engine accordingly, then this would be a good thing. Since your engine isn't built to run at the high enough rpms to benefit from open exahust pipe, it will feel sluggish because it is not getting that desired pull on the exhaust gases exiting the combustion chamber.
If you've ever noticed when looking at camshaft profiles, they always talk about lift, and duration, and valve overlap, etc. These things are built into the lobe shapes to make power in specific rpm ranges, and they are based to a degree on this overlapping of the valves.
For not being an expert as you said,you damn well explained it like one.
The only thing I can do on your post is to bump it!!
Great explanation and understanding of this.