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Back pressure and valves

 
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edjack



Joined: 04 Feb 2002
Posts: 3887
Location: San Jose, CA

PostPosted: Wed Sep 26, 2007 6:47 pm    Post subject: Back pressure and valves Reply with quote

On another forum, the subject of back pressure came up. I stated that, since the engine is an air pump, one needs to minimize or eliminate back pressure in order to free up power. This was in alignment with everything I've learned about cars.

I was corrected: the poster said that, at very least, one needs back pressure to protect the exhaust valve stems.

Anyone in the racing community ever hear this? And does it apply to street machines as well?

And, do the racers "build in" back pressure to make the engine run well?

And is there any quantitative evidence to back it up?
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'97 540i 6 speed Aspensilber over Aubergine interior.
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Chuck Baader



Joined: 21 Aug 2004
Posts: 95
Location: Birmingham, Alabama

PostPosted: Fri Sep 28, 2007 11:28 am    Post subject: Reply with quote

I think you have found an urban myth. If exhaust and intake ports are designed correctly (as in all BMW motors), any back pressure on the system inhibits the intake process. My race motor, M20B27 has full open exhaust and makes the most power that way. Hope this helps. Chuck
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edjack



Joined: 04 Feb 2002
Posts: 3887
Location: San Jose, CA

PostPosted: Fri Sep 28, 2007 11:47 am    Post subject: Reply with quote

I agree. The myth is hard to kill.
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idanity



Joined: 21 May 2005
Posts: 3097
Location: N.W.U.S.A.

PostPosted: Fri Feb 29, 2008 10:13 am    Post subject: Reply with quote

i was told (when i was 11 or at least before i ever drove) that there is enough back pressure just to have exhaust pipes. and the only time you will NOT have back pressure is if you ever removed your exhaust.


or like some hot-rodders, have tiny exhaust (like 9inches straight through the hood, or out the fender), and they have enough backpressure. but just barely i guess.

i remember now, when i saw the cars for the 1st time, i wondered why have a pipe at all, and someone explained it w/out any pipes, the side of the head may get burnt w/all the open atmosphere and it needs a surrounded bit of exhale on one side to keep the motion stable. *(airflow stable), and not to cool the valves too fast or at least keep them at a steady temp.


its been a long time, and i hope i was able to help w/something.
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