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BackPressure....

Discussion in '1982-Present GM Diesel' started by FastFob, Oct 1, 2002.

  1. FastFob

    FastFob Registered Member

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    Do diesels need/like backpressure?
    Would opening up the exhaust on a 6.2L help or hurt?
     
  2. BigBluOx

    BigBluOx 1/2 ton status

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    I am by no means an expert on the subject, but I have heard nothing but good things about opening up the exhaust on diesels, especially on the turbo models (much like turbo/gas cars). Anyone with real experience/knowledge wanna chime in and set us straight? /forums/images/icons/grin.gif

    Jon
     
  3. imiceman44

    imiceman44 1 ton status

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    You got it guys, a diesel is basically an air pump, you need to help it get more in and help it get it out fast. That's how you build more power.
    That's why all the kits first put a bigger exhaust, and then for more power they add a turbo which stuffs more air.
    IceMan
     
  4. DieselDan

    DieselDan 1/2 ton status

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    6.2s also like a crossover pipe. Becomes real noticable if you run a "J" manifold without the interconnection for an EGR.
     
  5. imiceman44

    imiceman44 1 ton status

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    OK so that's what the J manifold is. It's the one from a one ton or a HD 3/4 ton right?
    IceMan
     
  6. calcide

    calcide 1/2 ton status

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    So, a diesel doesn't scavenge the exhaust like a gas motor does, even though they are both 4 stroke and evacuate the cylinder the same way? If you put too big of an exhaust system on a gas motor, you can hurt the low end performance. Since diesels are all low end, I would think it would matter quite a bit.

    If I'm not right, please tell me how gas and diesel exhaust dynamics are different.
     
  7. 84CUCV

    84CUCV 3/4 ton status Premium Member

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    Is there a way to put 3-4" pipe for exhaust on? I know someone did this before. mike
     
  8. DieselDan

    DieselDan 1/2 ton status

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    Somebody correct me if I'm way off-base here. When we talk scavenging we're talking about resonate scavanging with headers. Without equal length/correctly sized primaries, low/no restriction exhaust, scavenging doesn't really exist; right? /forums/images/graemlins/confused.gif

    Now a lower restriction exhaust is a properly sized exhaust pipe without being so large as to cause poor flow at lower RPMs, and large enough not to restrict the top end. As far as a comparison with a gas engine: I would think it would be similar but I would like to compare the actual CFM numbers. Diesel's lower RPM range but no throttle plate restricting flow would be more CFM YES/NO?

    And finally "J" manifold has no EGR valve and no crossover exhaust passage.
     
  9. Muley

    Muley 1/2 ton status

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    I would think that he exhaust pipe would need to be twice the size as it would be on a gas motor. Mainly due to the higher compression of th diesel(20:1). Higher compression means it is going to be forcing more air past the exhaust vavles quicker, I guess that would be a higher number of CFM's. So you need larger pipes to get the exhaust away from the cylinders.

    Just look at Dodge trucks with the Cummins. They come out of the factory with a 4" pipe.
     
  10. calcide

    calcide 1/2 ton status

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    You are right. The factory manifolds do not promote scavenging, they are not properly configured
    to form an effective resonant cavity.

    The orignal question asked if opening the exhaust would help or hurt. Everyone ALWAYS says something like "hell, yeah, a bigger exhaust is always better!", which is not entirely accurate. A system can be too big.

    At least nobody brought up glasspacks.
     
  11. calcide

    calcide 1/2 ton status

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    </font><blockquote><font class="small">In reply to:</font><hr />
    I would think that he exhaust pipe would need to be twice the size as it would be on a gas motor. Mainly due to the higher compression of th diesel(20:1). Higher compression means it is going to be forcing more air past the exhaust vavles quicker, I guess that would be a higher number of CFM's. So you need larger pipes to get the exhaust away from the cylinders.


    [/ QUOTE ]

    I'm pretty sure that CFM is determined by engine displacement, RPM and volumetric effiency. Do diesels have a better VE than gas motors? Is it twice as good? What about the higher RPM range of gas motors? Does that even out the CFM difference?

    </font><blockquote><font class="small">In reply to:</font><hr />
    Just look at Dodge trucks with the Cummins. They come out of the factory with a 4" pipe.

    [/ QUOTE ]

    You are comparing apples to oranges here. 6.2s are NA.
     
  12. arveetek

    arveetek 1/2 ton status

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    This topic has been hashed and re-hashed many times over on the Diesel Page. There's been a lot of scientific jargon brought up that goes way over my head. Some people feel that the scavenging effect is very important, and that a single exhaust is much more useful than a dual system. Others feel that a dual is much better. Some feel the smaller pipes produce more torque. Some have used these long, dragged-out scientific equations with words I don't understand to help prove their point.

    Here's my opinion:

    The diesel and gasoline exhaust systems are totally different and really shouldn't be compared. The main reason: the diesel doesn't have any throttle valves or plates restricting the intake like a gasoline, so a diesel sucks in and expels a ton more air and combustion gases than a gasser.

    Here's my highly scientific approach: place your hand over the intake opening of a gasoline engine at idle. You feel a very slight tug. Now do the same on the diesel. Your hand nearly gets sucked in! And this is at idle! The amount of air being moved through that thing is unreal. Place your hand over the exhaust of a gasser. You don't feel much but a slight breeze. Now place your hand over the tailpipe of your diesel. Feel the massive wind? I think someone referred to these diesels as air pumps, and I think that's a good analogy. I think unrestricted air in, and unrestricted air out is the way to think. I don't think you can go too big on the exhaust.

    I do agree that the n/a engines don't need a 4" or 5" exhaust like some of the new trucks have, but still, a single 3" to 4" or dual 3" is probably the best way to go. My '81 will eventually have a turbocharger with 2.5" crossover pipe, 3" downpipe, 3.5" transition pipe, 4" muffler, and 4" tailpipe, possibly with a 5" chrome tip. I have most of the parts, just waiting for time to install.

    Some of the Duramax guys are running 5" systems from the turbo back. I mean, that's honking big, now!

    To sum up, bigger is better on the diesels, whether n/a or turbocharged. Smaller pipes will only choke the motor.

    That's my opinion, anyway! /forums/images/graemlins/crazy.gif

    Casey
     
  13. DieselBurb

    DieselBurb 1/2 ton status

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    I don't remember where I read it, but they said that for naturally aspired engines, it is possible to over-scavange the exhaust, but with turbo engines, there is no limitations on exhaust size. /forums/images/graemlins/burb.gif
     
  14. calcide

    calcide 1/2 ton status

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    I personally would put a larger exhaust system on a NA 6.2 than the factory had. Like DieselDan pointed out, the factory manifolds kill whatever scavenging you get so running a larger pipe shouldn't hurt. There is still a proper size for the pipe to get max torque, but I think the gains would be minimal. Certainly not worth running the risk of having the pipe too small.

    Does anybody know what the diameter is of the flange of the exhaust manifold?

    </font><blockquote><font class="small">In reply to:</font><hr />
    The diesel and gasoline exhaust systems are totally different and really shouldn't be compared. The main reason: the diesel doesn't have any throttle valves or plates restricting the intake like a gasoline, so a diesel sucks in and expels a ton more air and combustion gases than a gasser

    [/ QUOTE ]

    The exhaust system on a 6.2 and a gas 5.7 are almost identical, as far as the way they operate. A piston pushes the gasses out through a valve and into the exhaust port of the head, etc.

    Your test of feeling the air coming out of the pipe at idle is interesting, but idle isn't where the engines are doing their work. A gas engine under load moves a ton more air than it does at idle, I wonder how they (gas and diesel) compare under load. I would assume the diesel moves more air, due to its more open intake design, but a gas motor revs to a higher RPM. More RPM, more air.


    &lt;thinking out loud here&gt;
    But then, diesels have bigger turbos, so maybe they do move more air. Or maybe just that a diesel can run at higher boost pressures and won't automatically self-destruct if you overboost (like a gas engine).
    &lt;/thinking out loud&gt;
     
  15. BlueBlazer

    BlueBlazer 1/2 ton status

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    </font><blockquote><font class="small">In reply to:</font><hr />
    I would assume the diesel moves more air, due to its more open intake design, but a gas motor revs to a higher RPM. More RPM, more air.

    [/ QUOTE ]

    Diesels have much better volumetric efficiency, thus they flow more air, mostly the reason for the lower RPM maximum speeds is due to the heavier parts and that diesel make more power at low end, and running slower helps them live longer.
    </font><blockquote><font class="small">In reply to:</font><hr />
    But then, diesels have bigger turbos, so maybe they do move more air. Or maybe just that a diesel can run at higher boost pressures and won't automatically self-destruct if you overboost (like a gas engine).


    [/ QUOTE ]

    Exactly, there is no chance of running "lean" with a diesel, thus no chance of detonation, but an overboost condition with more than enough fuel to match the boost can easily melt pistons.
     

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