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Thread: cat question

  1. #11
    Jr. Engineer
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    I've read that back pressure is just a myth brought from the days before computer controlled cars. You would have to manually tune the engine after putting on free flowing exhaust and if you didn't you would lose power. Also, since the exhaust was so free flowing it would create a vacuum in the carb or something like that but it is not true anymore. Does anybody have true evidence of this? I always hear about back pressure being necessary but why would that ever be true? It does not make any logical sense. I almost think that a lot of people put too big of exhaust pipes on and lose power and blame the lack of backpressure, when it's really that their pipes are too big and exhaust velocity and temperature have decreased. My last two cars did not even have cats on them and they did not lose any power or mpg.
    99 Silverado Z71 4.8
    2" Suspension lift (Add-a-leafs and leveling keys)
    Long tube headers with 3" offroad Y-Pipe
    Volant Cold Air Intake with Ram Air Intake Tube
    Jet Mass Air Flow Sensor
    Diablosport Predator Tuner

    Previous Truck:
    97 Suburban K1500
    3" Body lift
    2" Suspension Lift (Add-a-leafs and cranked up torsion bars)
    35" Yokohama M/T's
    K&N Intake
    Airraid Throttle Body Spacer
    Hypertech Programmer
    Dual Flowmasters

  2. #12
    Legend cascott325's Avatar
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    hmmmm... im gonna have to pass that one to someone who can explain it. i dont know enough about it, just going with what i have always heard.
    2000 silverado with a 5.3l v8, shorty headers, upgraded plugs, 10 mil wires, magnaflow high flow cats, flowmaster 80 series muffler(2 in 2 out) w/ 18 inch stainless silverline tips. i have a k&n CIA. its a 4x4 z71 offroad package. Ls model.
    extended cab 160k miles, truxedo low profile bed cover, diablo sport predator tuner.
    reman tranny @ 130k miles
    Chris

  3. #13

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    cascott, im in louisville. I would have done straight pipes for my silverado had it not been for my previous car. I had a 2000 Impala with straight pipes, no mufflers, and for some reason feel it had something to do with it catching on fire (it got totaled and thats why I have the silverado). I think that not having any mufflers may have caused a cold reversion on the valves and burnt em up over the year and a half I had straight pipes. I AM PROBABLY WRONG, but theres no way to know and I dont wanna take the risk again. The firefighters said it was electrical but they didnt get there until the whole front was on fire. Anyways I've heard so many different sides to the whole backpressure topic that Im not 100% sure what to think. After reading my replies, I probably won't mess with the cat, but im basically stuck between 2 exhaust setups. (willing to take any suggestions):
    --glasspack(s), dualed
    --flowmaster 40(s), dualed
    --or anything else you all might suggest

    thanks, ryan

  4. #14
    Legend cascott325's Avatar
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    louisville. probably gonna hit the swap meet commin up then hit up the pull apart. i have an 80 series flowmaster, and its pretty loud, also, im thinking that if you use smaller diameter pipes, you will have quite a crackle

  5. #15
    Jr. Mechanic thatjerk01's Avatar
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    its not a myth its apart of the egr system. hes an example i have the 5.7L and the 5.3L will beat my truck in a race. well i raced a kid that cut his exhaust off and he has the 5.3L and i smoked him. but legally a shop cant change out your cats unless they r damaged or setting codes which mean there bad.
    Casey Clark
    Currently Arizona
    Soon Idaho
    1996 CHEVY K1500 Z71
    Front end collision
    145000 miles about


  6. #16

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    I'm not looking for too much crackle, so the piping will probably have to be at least 2.5". My neighbor has single in/dual out flowmaster 40 on his 2002 5.3, and it sounds awesome. I also want something that "drones" at cruising speed. Yes you read that right...it doesnt give me headaches or bother me...I just love to hear my car at all times

  7. #17
    Jr. Mechanic thatjerk01's Avatar
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    if u can buy mandral bent tubing doesnt restrict.

  8. #18
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    Quote Originally Posted by thatjerk01 View Post
    its not a myth its apart of the egr system. hes an example i have the 5.7L and the 5.3L will beat my truck in a race. well i raced a kid that cut his exhaust off and he has the 5.3L and i smoked him. but legally a shop cant change out your cats unless they r damaged or setting codes which mean there bad.

    What about part of the EGR system? What does that have to do with backpressure? The EGR gets deactivated when you are at WOT so that has nothing to do with racing somebody. There are many other factors that can cause him to have lost. The 5.7 and the 5.3 are pretty much the same when it comes to power numbers (270 hp 320 tq vs 255 hp 330 tq) maybe he has a taller rear axle ratio than you. That's not really evidence that a decrease in backpressure decreases performance. Also, where was his exhaust cut off at? Right after the manifold or what?

  9. #19

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    Mandrel bent is usually more expensive I think isnt it? And for all you truck guys on here, is my straight pipe theory wrong, as far as burning up the valves or harming anything else?? Just wondering...theres nothin I love more than a straight pipe sound.....thanks for all the help...please reply back with any thoughts/suggestions!!!

  10. #20
    Jr. Mechanic thatjerk01's Avatar
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    egr stand for exhaust gas recirclution. the exhaust pulsates which in turn is when the erg opens. ok and then why is it at the drag strip people with the 5.3 beat my truck. and my truck runs great. yea the gear ratio makes a difference but he would have either taken me off the line or top end and he didnt do either. he cut it off after the cats. plus his truck was 2wheel drive mines 4 wheel drive. and his is just standerd mines extended so i have more wieght then him.

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