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Back pressure?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Dunc03, Feb 13, 2003.

  1. Dunc03

    Dunc03 1/2 ton status

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    How much back pressure should a high performance engine have. I have a 330HP 350 crate from GM and I think thelack of exhaust bak pressure is making it idle rough. I have virtually no back pressure becuse the cat is hollow and the stock muffler is too for the most part. THe cat is completely hollow. Is there such a thing as too little back pressure. The reason I ask this is becuase when you cover the tail pipe with a rag the engine seems to smooth out. IS there anyways to check for back pressure, or are you checking for vacuum level to find the absence of back pressure. I have a vacuum tester, but I don't know how to check for back pressure. Thanks for the help.
     
  2. schmuck

    schmuck 1/2 ton status

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    its too little backpressure when the engine starts poping real bad when you decelerate, for the most part you want the least amount of backpressure as possible, really what you want is a tunned exhaust
     
  3. Judd

    Judd 1/2 ton status

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    I don't think that a lack of backpressure is what is making your engine run rough. You could run open headers and it wouldn't make your engine run rough. You might lose a bit of low end torque if the collectors are not the right length but that's about it. As for the popping sound, it's most likely caused by a air fuel mixture that is too rich. The popping sound is from the too rich mixture not burning completly before it leaves the combustion chamber. My Firebird does the popping thing under deceleration when it's cold {running rich}. Soon as it warms up, it goes away. It has plenty of backpressure, two cats and a muffler. The backpressure thing is a bit overstated in my opinion. JMO
     
  4. mudober

    mudober Registered Member

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    I dont really have enoughf info on your carb & intake, but it sounds like too rich a mixture on your carb. if you gun it & stalls at first, its off.Also check your vacume you should have 15-18 at 600rpm or so.If not did you install the intake, anyway check to see if its had any gaskets for lunch. Dont be concerned about backpressure to much is a bad thing you wont notice to little on a trail rig.
     
  5. sled_dog

    sled_dog 1 ton status

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    Nah, my buddy's Bronco had headers into about a foot of 3" piping then a 3" 18"long glass pack, whats backpressure? For the amount of noise it made it sure didn't go anywhere. but it didn't idle rough. When I get my engine done I am gonna use the same exhaust setup he had, backpressure is a good hting and I should probably have some but there is something special about being the loudest best sounding truck in town /forums/images/graemlins/smile.gif
     
  6. Headbanger

    Headbanger 1/2 ton status

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    What year is the vehicle? Is the system computerized? Does that engine run an EGR valve? If the answer to the last two questions is no then backpressure should not be a problem. If the answers are yes then the system requires a certain amount of backpressure to run properly. Without going into major detail, GM runs backpressure EGR's. No b/p to make it open and close at the right time= rough running, usually but not always at idle. Lack of b/p can also effect the way your oxygen sensor reads your exhaust and sends signals to the computer.FYI /forums/images/graemlins/grin.gif
     
  7. Dunc03

    Dunc03 1/2 ton status

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    The Blazer is and 86 and IT does have an egr valve, but not a computer. It used to have electronic spark control, bt that got ousted with the new engine. The only reason I'm still running hte egr is that it has to be on for the Early fuel evaporation system to work. This system causes the exhaust to be cut off at the manifold and rerouted to the base of the carb. The egr is the only way that I can think of that exhaust gas could be reintroduced into the intake. Should I just chunk the egr and block it off.
     

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