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Rear window and exhaust fumes

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by muddin4fun, Jan 15, 2002.

  1. muddin4fun

    muddin4fun 3/4 ton status

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    What exactly causes that vacum which brings the exhaust fumes inside the truck when you have the rear window down? Like do the vent windows being open cause it etc?

    Reason I ask is I need to get rid of the blazer tailgate. I would be putting on a regular pickup tailgate since I'll have a soft top which means that rear window would be useless. I live in Texas and it doesn't get cold enough to need a hard top during the winter [​IMG]. So...anyone know?

    <font color=blue>Answers: $1, Short: $5, Correct: $25, dumb looks are still free.</font color=blue>

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  2. 74k5inVA

    74k5inVA 1/2 ton status

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    i sure would like to know with my 2 straight pipes coming out the back i suck in mad fumes, but i have a hard top. i think if you run a soft-top you won't have to worry that much because you can leave all the window open. if you have pipes that stick straight out the back, put on some angle tips. should take care of most of the problem there. i haven't done this yet because i like the dual exhaust i have now. maybe one day.

    If it can't be broken then I haven't had it.
    '74K5 NP203/sm465 4"Lift 35"BFGs 350ci
     
  3. denver75k5

    denver75k5 1/2 ton status

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    It's because a blazer is as about areodynamic as a brick! when I had my dual 3" exaust done I had them put is out the rear, MISTAKE, I took A carbon monoxide meter and it read about 440ppm which 200 can kill you, so i took it back and had them put it out the sides which helped quite a bit. With my soft top on the meter read 0 ppm.

    " Wider IS Better!!!"coloradok5.com/gallery/albun51?&page=1
     
  4. muddin4fun

    muddin4fun 3/4 ton status

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    I know, but is there a way to have the rear window down with your hard top on and not suck in fumes....

    <font color=blue>Answers: $1, Short: $5, Correct: $25, dumb looks are still free.</font color=blue>

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  5. denver75k5

    denver75k5 1/2 ton status

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    My best advise would be to check for exaust leaks ( I fixed mine and it made a heck of a differance) also the further back you have the pipes (im assuming you have dual) the less it will come in.
    One other option (for those with hard tops) Is that funny looking spoiler that mounts to the hard top.


    " Wider IS Better!!!"coloradok5.com/gallery/albun51?&page=1
     
  6. four_by_ken

    four_by_ken 1/2 ton status

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    It is not an exhaust leak, it is, as someone else put it, the aerodynamics of the Blazer. Only thing that can really help, is to have the exhaust exit the sides. And even this will only help a tlittle. I hear the little spoiler deal that some have on the top help, but dont know about that one.

    It is the same effect that causes all the light stuff in a pickup bed to accumulate at the front of the bed. The air comes over the top and then down in a swirling effect, and if the window is open, in it comes!

    Ken H.

    I guess if you had an exhasut leak underneat the cab, it would be even worse.


    '86 K5 in parts
    1-ton 454 44 TSLs
     
  7. muddin4fun

    muddin4fun 3/4 ton status

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    It's all brand new exhaust which exits out the sides. I might still have the spoiler I took off the truck.....but I might have tossed it....hmmm....
    It'll be a little while before I get a soft top cause I have to do a vertable chop on it then a rollcage. Hmmm...
    I wish I had those side windows that open. I bet that'd fix it. Ah well...I guess I'll have to put up with that tailgate for a while.

    <font color=blue>Answers: $1, Short: $5, Correct: $25, dumb looks are still free.</font color=blue>

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  8. four_by_ken

    four_by_ken 1/2 ton status

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    The side windows may help a little to lessen the vacuum effect, but dont worry.... I dont think it would have helped very much. Mine has them, still gets the fumes.

    Ken H.


    '86 K5 in parts
    1-ton 454 44 TSLs
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    If you can live with the smell accelerating from a stop, opening the door windows (the main ones) makes it possible to cruise with the rear window down. But every time you accelerate hard, or take off from a light, you WILL smell it, even with the pipes exiting out the side.

    Haven't got around to adding the rear spoiler, but I would imagine it will make a large difference, but again, if there is no airflow, it won't help. Apparently the rear spoilers help keep the window clean, so that indicates that air is forced down, past the rear window.

    Dorian
    My tech/links page: <a target="_blank" href=http://www.dorianyeager.com/index2.html>www.dorianyeager.com/index2.html</a>
    Why insist on counting when the ring gear has the tooth counts stamped in?
     
  10. Calclips

    Calclips 1/2 ton status

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    In a K5, do you smell the fumes with the top off?
     
  11. dyeager535

    dyeager535 1 ton status Premium Member

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    Been so long since we've had a stretch of nice weather long enough to warrant removal, I can't remember! I'd say no though, since if I have the top off, the windows are down.

    Dorian
    My tech/links page: <a target="_blank" href=http://www.dorianyeager.com/index2.html>www.dorianyeager.com/index2.html</a>
    Why insist on counting when the ring gear has the tooth counts stamped in?
     
  12. K5Jimmy

    K5Jimmy 1/2 ton status

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    The little spoiler thingy doesn't help much at all with fumes, but it does somehow help keep the rear window a little less dusty (? go figger)....the side windows that open don't help much either for some reason......seems when I open the wing windows in front, it's even worse than with them closed......with the door windows cranked down, and the tailgate dropped, I'm ok as long as I don't accellerate hard......my advise is to make it a 'vert, lose the tailgate, and wear a raincoat in bad weather......j/k......

    Stuck?...Hell no..I still got some gas left..
     
  13. chevyfumes

    chevyfumes Court jester

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    Watch for the muzzleflash!
    Hey I resemble these remarks...[​IMG][​IMG]

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  14. Confederate9

    Confederate9 1/2 ton status

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    i have dual exhause stright out the back and when i have my top off, i cant smell it at all, but top on, i can

    <font color=red>K5: Because size DOES matter</font color=red>
    85 K5 4IN LIFT 35 GOOD YEAR WRANGLER MT/R, AMERICAN RACING WHEELS
     
  15. txbartman

    txbartman 1/2 ton status

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    It all has to do with aerodynamics. As your "brick" goes through the air, you are forcing the air to split all around you. When the vehicle has passed a spot, there, to an extent, is no air in that space the vehicle just took up. So, the air that was pushed around the vehicle rushes back into that spot. The faster you accelerate, the greater vaccuum is created causing the air to rush in faster. When the rear window is down, the air will get sucked in there by a siphoning effect. If you open a window in the front, that helps air "reclaim" the space take by the vehicle easier by passing through the vehicle. While air is still being forced up and around and still creates a vaccuum, much of the siphoning effect is dissipated by the force of the air moving through the vehicle. During all the filling of the vaccuum and siphoning, much of the air being sucked into this void is the air full of exhaust. If your pipess exit the sides, you have less of a problem. If it exits straight back, you have a major issue. That is why it is illegal in most places to have rear-exiting exhaust on a vehicle with a rolldown rear window that is consistent with the same vertical plane as the exhaust.

    If you have your top off, the air can reclaim its space in stages. One thing this does is to create two vaccuum instead of one big one. And, the air being sucked back into your cab area is not the air filled with exhaust so it has less impact.

    With a soft top, the air is passing over the top with more turbulence as the soft top is not smooth like the hard top. This will cause the vaccuum behind the vehicle to not be as clean. Hence, the vaccuum will last longer in psace, so to speak, and will not be as strongly concentrated in one spot. This, too, will help reduce the exhaust coming in, but may not be noticeable. Opening windows on the side of the vehicle also dosn't always help if they are close to the rear. As air flows in these windows, it will try to reclaim the space closer towards the front. As a result, it can actually slightly increase the siphoning effect.

    This vaccuum, by the way, is part of what causes our rigs to have such poor aerodynamics. If the front were sloped but the rear was still shear, we would still see problems. Only way to completely solve the rear vaccuum issue is to round the rear, like those stupid mini-vans. My convertible Firebird, for example, gets much worse gas mileage with the top down than up. That is because the air moves more smoothly back into shape after I pass through it with the top up versus the sheer drop provided by the edge of the windshield and no top when it is down.

    Finally, as far as the spoiler, this simply gives the air one last thrust up. Most of the dirt and crap on the rear window comes from stuff passing over the vehicle, not as much passing under it. That ends up on the tailgate more. So, by forcing the air up one final thrust, the circulation effect that brings the crap back down and onto the glass moves a little and has less impact on the rear window. However, the air coming from the bottom now must move even more quickly to fill this void as the air from the top of the vehicle is no longer doing it. The effect can be causing even MORE exhaust to come in the rear window with a fin.

    So, no top will solve the issue. Side pipes will help significantly. Open front windows can also be a big help.

    Hope that helps! Next I will explain how a similar vaccuum causes lift under an airplane's wing as the air pressures differ based on the shape of the wing cuasing the plane to rise or fall! But that's next week, same bat time, same bat place...

    Brian
    Check out <a target="_blank" href=http://tx85gmcguy.alloffroad.com>My Jimmy</a> with all her projects!
     

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