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  1. #11

    Join Date
    Jul 2003
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    1,092
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    South Central NC, USofA
    Tractor
    Iseki TU1700f

    Default Re: Ok, 1 more theoretical ?

    <font color="blue">"What if you replaced the stock exhaust with a less restrictive (louder) exhaust system (Cat back)? Would that cause engine damage?" </font>

    Short answer... no.

    Most aftermarket exhausts have been tested before hitting the market.

  2. #12

    Default Re: Ok, 1 more theoretical ?

    Nope you can't run pump gas and get 5000 hp [img]/forums/images/graemlins/blush.gif[/img].. there actualy around 7000-8200hp now from what some of the teams report on 90% nitro so it's not a gas engine it's a alternative fueled engine so they can run in california [img]/forums/images/graemlins/grin.gif[/img] .

    The fuel rate and the pure pressure of the cylinders and the rpm's of the engine create great pressure far beyond anything a muffler system would produce. assuming 500ci x 8000rpm x 45psi or 3 atmospheres. They also run fuel lines to cool the valves and the rods/pistons are a 1 run ordeal maybe 2 the blocks the only keeper if it's billet around 4-8K if it's cast it's every 15 passes. depending on style type.

    I can tell you the hi-perf. turbo engines are alot less stress on the cranks and rods. The backpressure/intake pressure does load both strokes exhaust and the intake pressure cushions the compression stroke.

  3. #13
    Veteran Member
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    Default Re: Ok, 1 more theoretical ?

    The backpressure creates pulses which help scavenge the exhuast and keep it away from the valves. If the exhaust is not scavenged properly, the exhaust valves will burn (over time). Racing engines have tuned headers which create the small amount of backpressure necessary. Tuning the exhaust helps scavenge the exhaust even better, thus there is better performance. This all takes place very near the engine; running the factory manifold may be good enough for reasonable numbers of hours (perhaps measured in hundreds). On a typical car with a full length exhaust system, running without a muffler will probably do no damage. On a small engine (like a lawn mower engine) where the muffler is the entire exhaust system, running without it will cause damage quickly. Low restriction exhaust systems still provide the amount of backpressure needed, and enhance the performance by providing fewer restrictions to the exhaust flow.

    On piston-ported 2 stroke engines (no valves), running without an exhaust will cause serious damage quickly, possibly even damaging the bearings and crankshaft - this may be the source for the information about the connecting rods, above. The backpressure is absolutely needed to help pull the exhaust from the cylinders through the port; without it, the exhaust gas can build up in the cylinder.

    It may seem counter-intuitive that backpressure (pushing back against the valve or cylinder) actually helps scavenge instead of impeding the flow. But, keep in mind that the backpressure is not constant, it is a momentary pulse that actually creates a void behind it, and nature abhors a vacuum, so the exhaust rushes even more to fill it. Think of it as a puff back and a whoosh out (high tech terms). The shape of the exhaust creates the pulse.

    You might ask why all engines don't run a tuned header; part of the answer is that cast iron manifolds help reduce the noise. Steel tubing headers can be noisy. Also, they don't have as long a potential life. Some cars have beautifully shaped cast iron manifolds (the Fiat 2000 sports car comes to mind), but lack of engine room space makes it hard to get that kind of shape.



  4. #14

    Join Date
    Jul 2001
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    1,806
    Location
    Houston, TX.
    Tractor
    2001 TN65, 1951 8N Ford

    Default Re: Ok, 1 more theoretical ?

    No pipe off of the manifold. When I was a kid I was known to pull the big V-8s out of old luxury cars I found for free or close to it and stick 'em into old pickups that I'd paid the same for. Once I got them running it was time to roll and pipes and mufflers were an afterthought. [img]/forums/images/graemlins/grin.gif[/img] If you try to run 'em with the manifold off you burn the plug wires off before you could get it hot enough to test the theory. Ever look at the exhaust system length of an old Briggs or Tecumseh with a straight through muffler? Not much there to keep the cold air out once you shut it down.

  5. #15
    Epic Contributor MossRoad's Avatar
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    Aug 2001
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    24,933
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    South Bend, Indiana (near)
    Tractor
    Power Trac PT425 2001 Model Year

    Default Re: Ok, 1 more theoretical ?

    <font color="blue"> No pipe off of the manifold. </font>

    I think you'd cook something under the hood. Or at least have a face full of exhaust fumes. [img]/forums/images/graemlins/laugh.gif[/img]

    I had a '78 GMC 3/4 ton with 1 ton springs with a 454 that I ran with just a stub off the mainfolds that ended under the firewall. Ran that way for 5 years. Had to wear ear plugs. The floorboards were rotted out and water would flip the floor mats up and splash in your face if you hit puddles. I'm sure the fumes weren't at an acceptable level in that one either. Paid $1000.00 for it and sold it for $1100.00 when we had children.

  6. #16
    Platinum Member
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    Oct 2002
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    912
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    TR
    Tractor
    MH744

    Default Re: Ok, 1 more theoretical ?

    &lt;font&gt;<font color="blueclass=small">and then I'll quit. Maybe. It is hard on a gasoline engine to run it without a muffler. Why?</font>&lt;/font&gt;

    No one has answered this question by a counter-question like "Isn't it hard on a DIESEL engine with a muffler? Why?"
    The backpressure many have mentioned here is more important in the diesel engines than the gasoline engines. So?

  7. #17

    Join Date
    Aug 2003
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    32
    Location
    CT
    Tractor
    JD 870

    Default Re: Ok, 1 more theoretical ?

    I can't help but chime in here, even though this one's getting beaten pretty good already. This question has been rattling in my brain since about 1982, when a college buddy was hassling a Harley Davidson dealer because he found a leaking exhaust header on his new bike and was convinced that he would suffer a burned exhaust valve. I believe the mechanical strength of the reciprocating parts is far and away sufficient with or without a small cushioning effect from backpressure. That being said, an engine is still a very dynamic beast, and resonance is very important, not just in the mechanical parts, but more importantly in the drawing in and expelling of gases. Maintaining a proper fuel/air ratio can only be accomplished if these resonances are known and factored into the design. If exhaust backpressure is reduced, intentionally or otherwise, the mixture will be affected. If it goes lean, well, burned valves.
    Don

  8. #18
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    Default Re: Ok, 1 more theoretical ?

    <font color="blue"> Isn't it hard on a DIESEL engine with a muffler? Why?" </font>

    In my research on this issue, I chanced on an article by Banks Performance on their diesel brake, intended for motorhomes and such.

    Basically, they said a diesel only compresses air. Air comes into the cylinder, is compressed until it's hot enough to ignite fuel, and the fuel is then injected and ignited by the hot, compressed air. When you close the throttle on a diesel, all you're shutting off is the fuel, not the air. The same amount of air enters the cylinder, is compressed and exhausted, regardless of the throttle setting. Because there is no resistance to the air, there is no engine braking.

    Therefore, to provide engine braking, a diesel brake deliberately shuts off a portion of the exhuast when the throttle is closed, forcing some of the air to stay in the cylinder and provide pressure to (a) keep more air from entering; and (b) force the cylinder to slow down against the resistance of less exhaust. The limit is how much pressure the exhaust manifold and head gasket can stand - if the exhaust was closed fully, the engine would come to a stop, the tires would start screeching, and something would "pop". So, the diesel brake is calibrated to supply just less than harmful backpressure, yet enough to slow down the engine.

    This doesn't hurt the diesel, because it's just air, not an air/fuel mixture, or burned combustion gases.

    When the throttle is open, and there is a fuel mixture being burned, the compression is so high that all the exhaust is expelled efficiently. This isn't the case with a gas engine, especially one with more "overlap" in the cam timing.

    Interesting stuff that I hadn't known before.

  9. #19

    Join Date
    Jul 2001
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    477
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    Fort Kent, Maine
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    B6100D Kubota

    Default Re: Ok, 1 more theoretical ?

    Just happen to have this situation on my Yamaha Virago 750. The stock pipes were replaced with straight pipes. More noise, less back pressure. The only drawback I found was dry fouling on the plugs. Had to go up 2 steps on the heat range to make up for it. A well tuned exhaust system will maintain a plug temp of 850F. Mine was quite a bit cooler.

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