too little backpressure?
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Joined: Jun 2003
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Let me be clear; backpressure is evil. The goal should be as little as possible, while maintaining velocity, in any high performance oriented application. The stock 300 has none of the qualities that qualify as high performance. The stock exhaust is restrictive and creates a lot of backpressure. However, some of this evil backpressure compensates for flaws in the exhaust system at the head. It's as simple as that. Streak, I have an exhaust system that I just dying to install on my '92 w/ a 4.9L. It's the stock system from a powerstroke F350; 3-1/4" stainless steel, mandrel bent, enormous muffler, and the factory hangers are in the exact same locations. This will be the simplest, least expensive(free) exhaust system I have ever done. I know better than to install it now. It will flow TOO well for my stock motor. I'm in the process of porting a spare EFI head. Only when the ported head is ready to install with a long tube header will I install the exhaust. I'll be documenting the whole thing with dyno numbers and now it looks like I may have to make a run after I hack the stock system off just to back up what I've been stating throughout this thread. 
Postmaster
Joined: May 2002
Posts: 3,241
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From: Broken Arrow, OK
IIRC the outlets from the EFI manifolds are about 1.875 or 2". A pair of 2" pipes into a single 2.5" would probably work well for you, but 1.875 might be a little better if you can get it and make it seal to the manifolds well.
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Joined: Jun 2003
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Hey Streak, if it's not backpressure awareness week; why don't we make it so? How appropriate being April Fool's day. The Reynolds equations does not apply here. We're not dealing with steady state flow first of all, and secondly to divide fluid denstities and fluid velocities by fluid viscosities which are virually non-existent in exhaust gas leaves you with, well, basically nothing. The math is not that involved, however many variables must be known. How rough are the exhaust ports? Are they carboned up? How much misalignment exists between the head and the manifolds? How much flash is hanging into the exhaust stream inside those manifolds? Some say you just have to "Uncork the exhaust, it's not rocket science". Well actually it is rocket science. To allow a high energy wave front to meet and equalize with atmospheric pressure in an uncontrolled and highly distored fashion is a waste of that energy. Sometimes, as in the case of the stock 300 six, limited control of a grotesquely distorted wave front can be achieved with backpressure. I apologize to Turdbird for hijacking this thread and I promise I will post nothing futher on the subject. 
Postmaster
Joined: Mar 2002
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From: Coal Region
i have something to say here...
unless ur running open head with nothing else there will always be some backpressure, right?
anyway, i would like both of ya'll to continue this friendly debate...perhaps you two can come to a happy medium and get one answer for all of us to use?
unless ur running open head with nothing else there will always be some backpressure, right?
anyway, i would like both of ya'll to continue this friendly debate...perhaps you two can come to a happy medium and get one answer for all of us to use?
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