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Backpressure debate

Discussion in 'The Garage' started by Leadfoot, Sep 22, 2004.

  1. Leadfoot

    Leadfoot 1/2 ton status Premium Member

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    After the discussion in another post (Flowmaster post) I decided to do some research. Tim was correct is saying backpressure is bad (for all RPM's). No backpressure is ever good (for HP or Torque). I may have mislead some by saying a little backpressure at lower RPM's is good....it is NOT and I was wrong.

    That being said, running open headers or "open exhaust" on the street WILL hurt low end performance but it has nothing to do with backpressure, and won't necessarily cause burnt valves (although it can). Again as Tim stated richening your mixture when running open exhaust is needed to keep valves from burning as running open exhaust causes the mixture in the combustion chamber to be lean.

    It is based on pressure differentials, volumetric efficiency, and scavenging (pressure waves). Searching the web will provide hundreds of articles discussing the issue (some factual, some not), but after reading and looking at formulas it does make sense. At high rpms the gasses exiting the combustion chamber are moving at a much higher velocity than at lower rpm's, valve timing is reduced (open for the same amount of crankshaft rotation, but since the rotation is faster the actual time in Milliseconds is reduced), and the physics of fluid (gas is a "fluid") combine to make exhaust size/shape irrelavant as long as it is not restrictive. At lower rpm's it becomes important to maintain volumetric efficiency (VE). Lower VE for a given rpm = less power and higher VE for a given rpm = more power. Running an open exhaust or very big chambered exhaust (size if most important at and near the head) lowers VE at low rpms and will cause a reduction in power (torque/hp). Sizing the pipes for low rpm so that they "scavenge" and create a high VE without restricting the flow at high rpms is a delicate balance. Ideally a variable exhaust along with variable valving would net the best power over a large rpm range, but until that becomes a mass market reality, we have to live with a comprimise. For any street driven vehicle, running an open exhaust is not an optimum setup. Even those running exhaust "cutouts" for occasional drag racing are not an optimum setup as they are either running lean on drags, rich on the street, or a little of both (unless they know how to tune for each occasion AND do so).

    Stock motors (especially older carbureted ones) were built to work in a variety of settings (both ideal and unideal). Many performance enthusiests refer to this around here as a sloppy motor. They are great for the street as they will work well in varying enviroments and don't need as much tweeking to keep running right, but not great in a specific enviroment (i.e. racing or all out performance). A "tight" motor will work well in a specific environment but is easily thrown off by changes in the environment and is very sensitive to even minor tuning changes. Because of this, changing a component on an engine (such as manifolds to headers) will not net a large amount of power as the rest of the motor is not "setup" to notice the gains.

    In looking at some articles of manifolds vs headers, it looks like headers win out in the performance range from anything at or just above idle rpms (assuming stock street motor). The reason many manufactures used (and continue to use) manifolds is that they provide a common chamber close to the head (great for very low idle efficiency) is that they performed well for idle smog checks, sealed better than headers, reduced flex, and lasted longer (at least longer than the warranty period). For a rig built for longevity, ease of maintenance, and not built to put out every ounce of power it can, manifolds are an excellent choice. Headers in and of themselves don't provide a huge power gain unless complimented with other items (mainly the rest of the exhaust). For example, put headers on a BBC that has a Y pipe and single 2.5 inch exhaust most likely will not see much if any performance gains over the stock manifolds. Many people around here still use stock manifolds (ram horns if they can find them) with custom duals. I went with ceramic headers wanting the most power I could get for pulling, but even they have suffered heavily from the salt/sand of New England. The 78 K10 already had headers installed, but when they rust out, I am planning on putting manifolds back on for ease of maintenance as it is a D/D plow truck that rarely sees high RPM use.

    To each his own (as that's what makes the world go round), just wanted to let everyone know I was incorrect with what I said before (not a first and won't be the last /forums/images/graemlins/blush.gif)
     
  2. TSGB

    TSGB 1 ton status

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    So, since I live under a rock;

    backpressure is how much exhaust hangs in a tube, preventing gas from nearer the engine moving easier?

    /forums/images/graemlins/dunno.gif
     
  3. hawkeye649

    hawkeye649 1/2 ton status

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    Need someone to go off road with? All my friends are trading in their trucks for cars.
    Manchvegas area here....
    /forums/images/graemlins/k5.gif /forums/images/graemlins/peace.gif
     
  4. 84_Chevy_K10

    84_Chevy_K10 Banned

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    [ QUOTE ]
    So, since I live under a rock;

    backpressure is how much exhaust hangs in a tube, preventing gas from nearer the engine moving easier?

    /forums/images/graemlins/dunno.gif

    [/ QUOTE ]

    No. Backpressure is the force pushing the gasses back towards the engine. Thus why it is called backpressure.

    You're on the right track though. That lingering gas (the reason it lingers and lose velocity is that it loses heat and volume) is what creates backpressure mostly.

    IMO the majority of so called, "Performance" exhaust systems that people recommend for trucks are too restrictive and too small. People are so afraid that they'll lose, "backpressure" and thus lose low end torque that the myth hold strong enough that they undersize their exhaust systems.

    IMO, freeing up more restriction via good mufflers and mandrel bent (or going a little larger on pipe if not mandrel bent) goes a long way.

    That said, I gave up on exhaust systems built for power a long time ago. I go 100% for sound now. /forums/images/graemlins/rotfl.gif I understand the theory though, which is the beginnings of building something that truely increases performance.
     
  5. 91GMCSuburban

    91GMCSuburban 1/2 ton status

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    Nice post.
     
  6. SF87K5

    SF87K5 1/2 ton status

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    So people who are running a TBI were the computer controls the mixure we need a new chip to run open exhaust right?

    I am running manifolds with straight pipes and 4" tips on my DD. The noise is very loud and I have poor low end power but great top end power. I was going to put headers and a single muffler exhaust on my K5 to try to get my low end back.
     
  7. RustBuket

    RustBuket 1/2 ton status

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    The computer is capable of adjusting mixture on the go with readings from the O2 sensor and such but only to point I believe. If the change were major enough, you'd be forced to get a chip burnt I would think.
     
  8. SF87K5

    SF87K5 1/2 ton status

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    The reason why I ask is when I let off the gas my exhaust pops. I now its not my timing cuz I had it checked and re-timed. I have been told by 4 different mechanics and 3 different exhaust shops that my popping is from my open exhaust.
     
  9. 84_Chevy_K10

    84_Chevy_K10 Banned

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    [ QUOTE ]
    The reason why I ask is when I let off the gas my exhaust pops. I now its not my timing cuz I had it checked and re-timed. I have been told by 4 different mechanics and 3 different exhaust shops that my popping is from my open exhaust.

    [/ QUOTE ]

    That is normal.
     
  10. RustBuket

    RustBuket 1/2 ton status

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    I beleive the popping is a result of the motor running rich (which was a symptom of open exhaust) and the excess fuel building up in the exhaust and then where there is enough, it ignites.
     
  11. 84_Chevy_K10

    84_Chevy_K10 Banned

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    [ QUOTE ]
    I beleive the popping is a result of the motor running rich (which was a symptom of open exhaust) and the excess fuel building up in the exhaust and then where there is enough, it ignites.

    [/ QUOTE ]

    You would be correct.
     
  12. RustBuket

    RustBuket 1/2 ton status

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    Good to know I'm not completely out to lunch! Thx.
     
  13. CyberSniper

    CyberSniper 1/2 ton status Premium Member

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    [ QUOTE ]
    The reason why I ask is when I let off the gas my exhaust pops. I now its not my timing cuz I had it checked and re-timed. I have been told by 4 different mechanics and 3 different exhaust shops that my popping is from my open exhaust.

    [/ QUOTE ]

    Look for exhaust leaks... when you let off the throttle you end up with a vacuum in the exhaust system so then it sucks in the atmosphere through the leak and ignites the excess raw fuel in the mixture.
     
  14. CyberSniper

    CyberSniper 1/2 ton status Premium Member

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    Backpressure was used by a lot of manufacturers for tuning purposes. It's a redneck way to try to alleviate the problem of metering fuel twice in a carbureted engine.

    Backpressure no longer serves any real purposes besides emissions. It is used much the same way an expansion chamber is used on a 2 cycle motor.

    Power and volumetric efficiency is all about flow... and backpressure doesn't help.

    Any more than 1psi of pressure at the collector is hurting performance.

    A prime example is the 03+ Cobra which has a catalytic converter and exhaust system that generate ~4psi at redline.

    All of this applies to backpressure only... not to design or scavenging or anything else.
     
  15. gravdigr

    gravdigr 1/2 ton status

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    I understand some of this. My recent experience with an exhaust was with my 86 blazer when I got it. The po installed a 2 1/2" dual exhaust with flowmaster 40 series mufflers on a 305ci motor. It sounded awesome at idle and howled on the highway once the RPMs went up but it could barely get out of it's own way in town. I would have to rev it high and pull out in sm465 low so as not to burn the clutch or stall. Since getting visual emissions inspection added to our safety inspection I had To get a Y pipe and cat. The difference in low end torque was amazing and the only change I made that would have affected it was the exhaust which is now a single 2 1/4" pipe. It quieted it down a lot and it doesn't sound mean but you feel it in the seat of the pants when you pull out now.

    Now my trail rig on the oter hand will have a 350HP 350sbc a friend of mine built, hooker headers dumping into dual 2 1/2" pipes and those flowmasters I saved. It will sound mean /forums/images/graemlins/thumb.gif
     
  16. Thunder

    Thunder 3/4 ton status

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    The reason you loose low end with large diamater pipe is flow velocity. The gasses dont flow thru large pipe at lower rpm as well as they do at higher rpm because:
    1. There is a larger area of atmospheric pressure to overcome. This slows down velocity
    2. Gasses flow thru a pipe in a circular motion. The faster the spin the better it flows. A larger diamater pipe has more surface area so it also slows down the gasses velocity at lower RPM.
    3.Each time a power stroke occurs and an exhaust valve opens, a positive pressure occurs in the exhaust manifold. A negative pressure occurs in the exhaust manifolds between the positive pressure pulses, especially at lower engine speeds. Having oversized pipe can cause the exhaust pressure pulses interfere with each other at LOW RPM. The exhaust flow is slowed, causing a decrease in volumetric efficiency and thus decrease in horsepower at LOW RPM

    There are other factors but the end result is your motor has to work harder to push(pump) the gasses out at LOW rpm with an over sized exhaust.

    Back pressure isn't the issue. Its where you dont have backpressure is what matters.
    For a DD you want the best flow at low/mid RPM. So you use smaller tube.
    For performance your engine should be built to overcome any low end backpressure/velocity issues.. Use as big of tube as you can. So you have no restriction at the top.
     
  17. Leadfoot

    Leadfoot 1/2 ton status Premium Member

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    Good point, I guess i didn't explain it as well. What you are talking about is in regards to flow with little or no backpressure (i.e. ideal size pipe or oversized pipe), I was more concerned with undersized pipes causing backpressure which they will if small enough and will affect VE as you can only get in the motor what you can get out (obviously more of a problem at higher rpms). Good points though /forums/images/graemlins/peace.gif
     
  18. Thunder

    Thunder 3/4 ton status

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    For those of you who think a little backpressure is good.
    I have the Ideal product for you.
    [​IMG]
    "O" Pipe"
    You've heard of the Y pipe.. the X pipe.. the H pipe.. nothing can touch the O pipe!!! It provides 100% backpressure feedback on ALL engines!! Torque increase and kuneutson valve float brings in much lower revs so you don't hurt your motor! Pipe shown is powdercoat black for car with DUAL EXHAUST. Not CARB Legal. Please specify Stainless or Powdercoat and also if you are using dual or single exhaust. 3" diameter.
    Only $200
     
  19. fortcollinsram

    fortcollinsram 1/2 ton status

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    Here's my $.02.
    Unless you are running a forced induction motor (this is especially true if you run a turbo), some back pressure is a good thing. On my 454, I run Thorley headers with true dual 3" pipes and Flowmaster 50's that dump just in front of the rear axle with 3" turn-downs. When I first put my 454 in I had my old, true dual 2-1/4" with cats. The low end performance was definitely better with the smaller exhaust but another thing I noted was the "gurgling" my exhaust made then I was off the throttle slowing down in gear. It barely "popped" "gurgled" with the small exhaust but it does it quite a bit with the 3". I believe that my small oval port heads and mild cam just don't flow enough air to create any back pressure in the exhaust. I think a dual 3" exhaust is better suited for big rect. port heads and a cam with at least .550" of lift and 220* durationI am actually going to be swapping out my Flowmaster for some 2-1/2" DynoMax UltraFlo mufflers to try to regain some bottom end torque and to try to get rid of some of the popping. Check out some of the LT1 and LS1 sites! There was a write up a guy did about backpressure and he actually measured the backpressure and changed exhaust configurations and did dyno testing to determine what the optimal backpressure was.

    I think on my current setup, one reason it pops off the throttle is b/c the lack of backpressure in the exhaust allows the exhaust gasses to "pulse" more than they would if there was some backpressure to aid in the scavenging effect that backpressure creates...

    Chris /forums/images/graemlins/thinking.gif /forums/images/graemlins/thinking.gif
     
  20. 84_Chevy_K10

    84_Chevy_K10 Banned

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    [ QUOTE ]
    Here's my $.02.
    Unless you are running a forced induction motor (this is especially true if you run a turbo), some back pressure is a good thing.

    [/ QUOTE ]

    Nope, wrong.


    [ QUOTE ]
    On my 454, I run Thorley headers with true dual 3" pipes and Flowmaster 50's that dump just in front of the rear axle with 3" turn-downs. When I first put my 454 in I had my old, true dual 2-1/4" with cats. The low end performance was definitely better with the smaller exhaust but another thing I noted was the "gurgling" my exhaust made then I was off the throttle slowing down in gear. It barely "popped" "gurgled" with the small exhaust but it does it quite a bit with the 3".

    [/ QUOTE ]

    It is painfully obvious that the only reason you can't fully take advantage of your more efficient exhaust is that your engine is not tuned correctly.


    [ QUOTE ]
    I believe that my small oval port heads and mild cam just don't flow enough air to create any back pressure in the exhaust.

    [/ QUOTE ]

    Even without any exhaust system bolted to the heads at all, there is still SOME backpressure. Eliminating backpressure completely is absolutely impossible.


    [ QUOTE ]
    I think a dual 3" exhaust is better suited for big rect. port heads and a cam with at least .550" of lift and 220* durationI am actually going to be swapping out my Flowmaster for some 2-1/2" DynoMax UltraFlo mufflers to try to regain some bottom end torque and to try to get rid of some of the popping. Check out some of the LT1 and LS1 sites!

    [/ QUOTE ]

    There is absolutely no question in my mind that if you do this, you will lose power. IMO, any big block, in a truck, car, or otherwise, should have 3" head pipes back into the mufflers, and at LEAST 2 1/2" tail pipes. Flowmasters flow like crap though, so you may gain power simply from freeing up restriction.

    [ QUOTE ]
    There was a write up a guy did about backpressure and he actually measured the backpressure and changed exhaust configurations and did dyno testing to determine what the optimal backpressure was.

    [/ QUOTE ]

    Optimal backpressure is zero. This is practiaclly impossible so optimal backpressure for us is to eliminate as much backpressure as possible. If the hyno gained power by adding backpressure, the engine was not tuned correctly.

    [ QUOTE ]
    I think on my current setup, one reason it pops off the throttle is b/c the lack of backpressure in the exhaust allows the exhaust gasses to "pulse" more than they would if there was some backpressure to aid in the scavenging effect that backpressure creates...

    [/ QUOTE ]

    Wrong again. Backpressure does nothing but hinder the scavenging effect. Either you don't understand the terminolgy or you're completely confused about the theory, I can't tell, but I can tell you that your ideas are WAY off in the quest for more power.
     

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