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Why Exhaust Shops LIE about Back pressure

This is a discussion on Why Exhaust Shops LIE about Back pressure within the External Engine forums, part of the LSx Technical Help Section category; I have heard this constantly talked about. Stuff like no cats will warp your valves or your valves need back ...

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    Awaiting Activation jsremmos's Avatar
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    Cool Why Exhaust Shops LIE about Back pressure

    I have heard this constantly talked about. Stuff like no cats will warp your valves or your valves need back pressure to open and close properly and all the other crazy wacked-out redneck fabrications about off road equitment!

    Ok here is the long story short, Exhaust shops have to tell people something when they are asked to make or install off road parts and for some reason they choose to lie instead of telling truth.... which is exhaust shops are not aloud to install off road parts LOL. The only way they can is if the car is noticably an off road car like if it was trailered in and if it is not road registerd. The funny thing is I was just having this conversation whith the owner of performance exhaust in kilgore texas as I was delivering a couple sets of my headers to him. We were talking about the cutouts I sell to him and the best placment for them and we got onto the topic because I told him to put them befor the cats if possible. He told me that that is against the law because of all the restictions on exhaust shops and he told me all the restrictions. Anyways they can get hit with a 10K fine for not following regulations and this is the obvious reason why they are yanking your chain when you ask them to. They dont want you to think it is more performance friendly to have a exhaust with little back pressure. Why because then you wouldnt buy theirs LOL. Trust me I have heard alot of BS from alot of dif shops and the reason is always the same they want your money spent there not on off road parts that they cant supply or install.... dick heads and look at all the crap its caused. The truth is open your exhaust up if you want to it will not damage anything internally and I know this is controversial but at low RPM a SMALL amount of back pressure is desirable to help make low end torque SMALL but the bends in your exhaust tubing, the X,h,y pippe and the length of your exhaust is enough to give you a sufficient amount of back pressure at low RPM and at high RPM the less the better. You just dont want to have no back pressure at low RPM which isnt possible even dumped straight out the headers you have a 6 to 10 inch collector providing a small amount of BP. What everyone should worry about on these motors is maxamizing the scavenging effect and that is a hole different more fun ball of wax!! I hope this helps with any future questions or concerns

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    Member 408WS6's Avatar
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    There is nothing wrong with open headers (other than it is noisy). You are right that the headers will provide the small amount of back pressure that the engine needs, but if your O2 sensors are enabled I've heard that open headers are bad because the cool air can rush in and throw off the sensors. If your O2 sensors are disabled then it doesn't matter. I have bullet race mufflers just after the header collector. Runs like a mofo now. I don't want to put my y-pipe back on...

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    Awaiting Activation jsremmos's Avatar
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    Yup i think open headers are sic and sound even better and it wont harm the engine like the people that have heard it can warp your valves... Well at least not with long tubes. I have heard that about the o2 also but one of the products I have recently developed had something to do with that and we found if you had atleast 3 inches of collector behind the o2 location that you are just fine. Though I would have to believe on headers that are tuned for high scavenging this would cause a problem as it does create a good % of vacuum. Most people are not running such headers.

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    $.02 Queen OneSlowL36's Avatar
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    Quote Originally Posted by jsremmos View Post
    The truth is open your exhaust up if you want to it will not damage anything internally and I know this is controversial but at low RPM a SMALL amount of back pressure is desirable to help make low end torque
    Simple question... how in the world can you effectively scavenge your cylinders with any amount of back pressure?


    Quote Originally Posted by 408WS6 View Post
    There is nothing wrong with open headers (other than it is noisy). You are right that the headers will provide the small amount of back pressure that the engine needs, but if your O2 sensors are enabled I've heard that open headers are bad because the cool air can rush in and throw off the sensors. If your O2 sensors are disabled then it doesn't matter.
    You also need to be careful on open stock headers that cool air doesn't revert back up the manifold; if you cool off the exhaust valves to quick you can damage them. And also, how would you run fuel injection without any 02 sensors?
    Last edited by OneSlowL36; 12-19-2009 at 02:08 PM.

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    Member c5z28's Avatar
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    I don't think you need to tell anyone here, maybe the honda guys need it.

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    Awaiting Activation jsremmos's Avatar
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    Quote Originally Posted by OneSlowL36 View Post
    Simple question... how in the world can you effectively scavenge your cylinders with any amount of back pressure?




    You also need to be careful on open stock headers that cool air doesn't revert back up the manifold; if you cool off the exhaust valves to quick you can damage them. And also, how would you run fuel injection without any 02 sensors?
    I said a small amount of back pressure meaning almost none. if you are running lt headers you will achieve the small amount of pressure that is forced down a primary and when it crosses into the collector the pressure flowing past the other empty primaries will effectively cause a vacuum in the other three primaries. Which inturn will "suck" the exhaust gasses from the other combustion chambers (savenging) Its alot more complex than this but its a good example.


    And you said stock headers which I have never heard of. And I have also never heard of someone running around with open manifolds no catts LOL. I agree in that case the 10 or so inches between the exit of the manifolds and the valve is not at all enough to effectively keep all of the fresh cool air from being pulled back into the head. Me and the other guy were talking about long tube headers which will completly protect your valves frome cool air. OK try this take a 12 inch by 1.5 inch tube and take a torch and heat the tube up untill it is as hot as a normal engine temp (165-210) and then blow as hard as you can through the tube with your hand on the other side and ill bet a chunk of change the air that will exit will be pretty damn hot. It is impossible for the radiant energy to not heat up the flowing air substantally. On another note I have been working with stainless steel professionaly for about 15 years and I have to tell you blowing air at hot metal to cool it is like puting a bandaid on cancer LOL. Metal has to be rapidly and unevenly cooled to warp from a temp change like dipping in water or oil. But even then if the metal was evenly heated and cooled it will not warp this is called TEMPering.

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    jebus... you had better tell the race car drivers that run completely open exhaust that they need back pressure and that the cool air that gets sucked in will destroy their engine...

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    Slow'er'Ass Mr. Luos's Avatar
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    The problem with the O2's isn't while it is running.
    It is right after you shut off the car...they will start to go to crap after repeated 'hot to cold' right away occurances.

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    Member 408WS6's Avatar
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    Quote Originally Posted by OneSlowL36 View Post
    And also, how would you run fuel injection without any 02 sensors?
    Easy, you PCM simply runs off the VE tables that are setup. You have to have a tuner that knows how to do this right, its called speed density tune. As far as the O2's you simply turn them off with the software by elevating the engine temperature at which the sensors are set to come on to a temperature that the engine will never reach (like 300 deg). I have had mine turned off for about 3 years now, runs like a bat out of hell.

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    Quote Originally Posted by 408WS6 View Post
    Easy, you PCM simply runs off the VE tables that are setup. You have to have a tuner that knows how to do this right, its called speed density tune. As far as the O2's you simply turn them off with the software by elevating the engine temperature at which the sensors are set to come on to a temperature that the engine will never reach (like 300 deg). I have had mine turned off for about 3 years now, runs like a bat out of hell.
    speed density is without the MAF...open loop is without the O2's.

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    Member 408WS6's Avatar
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    Quote Originally Posted by jsremmos View Post
    And I have also never heard of someone running around with open manifolds no catts LOL.


    Neither have I!

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    Member 408WS6's Avatar
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    Quote Originally Posted by 0rion View Post
    speed density is without the MAF...open loop is without the O2's.
    oops, my bad. Anyhow, I've got 'em both gone. No maf, no O2's. I wouldn't have it any other way.

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    SD tunes use your MAP readings for fueling.....

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    Quote Originally Posted by SOMbitch View Post
    SD tunes use your MAP readings for fueling.....
    that's basically what he was saying....the VE table is just your map vs rpm

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