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What causes exhaust rapping?

Discussion in 'The Hokey Ass Message Board' started by Bugman, Jul 6, 2005.

  1. Bugman
    Joined: Nov 17, 2001
    Posts: 3,483

    Bugman
    Member

    I know you get more rap from smaller diameter pipes, but why is that? What is it that causes the rap in the first place? Can you design more or less rapping into a system? The curious want to know...
     
  2. jalopy43
    Joined: Jan 12, 2002
    Posts: 3,085

    jalopy43
    Member Emeritus

    My theroy is that sound waves,are pulses in a closed pipe. The Tighter you pack the pulses the 'rappier' it is. Larger diameter pipes tend to spread the waves out into more of a 'rumble'. I used 1 1/2" straight pipes on my flathead,and it sounds pretty 'snappy':D Sparky
     
  3. tommy
    Joined: Mar 3, 2001
    Posts: 14,757

    tommy
    Member Emeritus

    I like to call it the school bus effect Ever hear a school bus go down a long hill? They rap like a bitch.

    Place the muffler as far forward as possible. A long tail pipe will rap. My 57 wagon has the mufflers under the front seat. Sounds sweet going down hill in second gear.
     
    1971BB427 likes this.
  4. Paul
    Joined: Aug 29, 2002
    Posts: 16,389

    Paul
    Editor

    fuel continuing to burn past the point when the exhaust valve starts to open

    to test remove your exhaust pipes and rev the motor hard and let off sharply
    the rapping will be in the form of loud flaming explosions of fuel burning outside the combustion chamber..
    not to be confused with the external combustion engine;)

    the smaller diameter exhaust pipe compresses the sound waves giving a sharper note and higher velocity exhaust and more distinct seperated pulses,
    the opposite of a large diameter pipe where sound and gasses can move slower giving a more mellow note from sound waves becoming confused and less distinct
     
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  5. prime mover
    Joined: Dec 6, 2002
    Posts: 827

    prime mover
    Member

    small diameter pipe will rap to an extent but the more pipe after the muffler or just straight pipes will get alot more. theres more to it like motor size and compression but I doubt anybody would change those for this reason.
     
  6. Its the difference between a trumpet and a tuba. The smaller diameter pipe makes a sharper sound, its more noticable. If you could make the volume equal in proportion to the smaller size pipe you could get basically the same effect in a lrager diameter exhaust.
    ya wanna loose the rap put a cross pipe in it.

    If you want to be really cool an old school trick was to build brass or copper tail pipes. They really ring when flying down the road, and rap with a real crackle when your backin it down hard.

    There were a lot of brass exhaust systems smuggled out of the Mare Island Naval Ship Yard "back in the day."
     
    Just Gary likes this.
  7. BlackFlag
    Joined: Jan 23, 2005
    Posts: 558

    BlackFlag
    Member

    Mare island as in vallejo mare island? I like it when you can get a good rap after letting off the gas. It just adds the finishing touches to the car
     
  8. Yea that Mare Island. A lot of hot rod parts and hot rodders were born there over the years. Its just a shame they shut it down.

    I got some 3" copper here that I'm going to use for lakes pipes someday when the perfect ride comes along. I made some 3" boom tubes (echo cans?) for a guy a few years back they were copper and sounded really cool.
     
  9. Bugman
    Joined: Nov 17, 2001
    Posts: 3,483

    Bugman
    Member

    How does the copper change the sound? Is it related to the metal's density?
     
  10. Nothing raps better than a pair of the old flattened WELL Pump PIPE...it was small in size-and when installed on a Flathead with just one corner at the motor and straight out with no mufflers....wow!!!THAT'S A sound you'll never forget!;)
     
  11. Mojo
    Joined: Jul 23, 2002
    Posts: 1,872

    Mojo
    Member

    i've read that torque tubes used as pipes would get that sound, something to do with the expanding diameter of the pipe, and the heavy steel it was made of... anyone heard an exhaust made with some?
     
  12. It has to do wqith the composition of the metal I'm sure, but I'm not a metalurgist.
    Copper and brass are very similar metals, horns and bells are made from brass because it resonates so well. I know that on the flatty ('53 Ford truck full house flahead built from the original mill) that we put the copper boom tubes on when it was in resonance it would really sing. Just can't describe the sound you would have to hear it.

    The Ol' Man used to use either torque tubes or old Ford axle housings (from the bell toward the backing plate) when he was building lakster style headers I loved the sound, they just don't sound like a standard open header. it could be the megaphone effect I guess.

    Back in the '70s when side pipes were popular I made several sets of side pipes from old disgarded drive shafts. Straight from the header to the end. They didn't sound too bad, they were just too big around to have a crisp sound to 'em.

    If you want it crisp and you're going to use steel your best bet is a smaller diameter pipe. Actually you gain a little low end torque with a small displacement mill and smaller diameter pipe. I've noticed for instance on a basically stock SBC when you go to say a 3" exhaust it just falls on its face, they just won't rev high enough and don't make enough volumn to take advantage of the larger diameter pipes.

    On a single exhaust its an entirely different story, but that's way off topic for this thread.
     
  13. Fords and MoPars usually rap better under hard decelleration than a GM mainly because they use timed vacuum advance while GM uses direct manifold vacuum.

    When you close the throttle in a Ford or a MoPar, the distributor retards and the rap gets really pissa. In a GM, the distributor advances and the rap is real whimpy.

    My '48 Chrysler with factory Dodge truck (big truck) dual carbs and dual exhaust used to really rap wicked pissa when I backed off the throttle until I put on an Accel 300+ ignition. That extended-duration spark ruined my rap but it sure woke up the engine!
     
  14. Deuces
    Joined: Nov 3, 2009
    Posts: 23,757

    Deuces

    Valve overlap baby!.... :cool:
     
  15. davidvillajr
    Joined: Apr 4, 2005
    Posts: 1,165

    davidvillajr
    Member

    Not trying to drive you away, but *some* folks might take issue with answering a bike question on a hot rod/kustom forum.

    You might get better/more love over here: https://www.jockeyjournal.com

    Good luck, man.
     
    j hansen, Deuces and Blues4U like this.
  16. Yasser
    Joined: Sep 5, 2020
    Posts: 9

    Yasser

    Thank you David. But unfortunately I haven't found a thread like this anywhere. I understand you guys are car lovers, but both engines car's and motorcycle's has the same basics. I am just willing to go deeper into a phenomenon very little talked. Try a and Google exhaust rapping and most post are related to wrapping. I will keep hoping to find more info here.
     
  17. spanners
    Joined: Feb 24, 2009
    Posts: 2,074

    spanners
    Member

    I know this is an old post but what else are you going to read?
    When I was going to Tech. College in the '70s a mate wanted to build his exhaust out of copper pipe. The tech teacher told us it would hamper the performance and rap horribly because the copper pipe would get a halo effect inside the pipe therefore decreasing the inner diameter and restricting the flow. Sounded plausible to a bunch of 15 year olds.
    Personally, I prefer a deep throaty rumble to a rap that sounds like a strangulated fart, but, that's just me.
     
  18. Yasser
    Joined: Sep 5, 2020
    Posts: 9

    Yasser

     
  19. Yasser
    Joined: Sep 5, 2020
    Posts: 9

    Yasser

    Is there a way I could post and mp3 file?
     
  20. Yasser
    Joined: Sep 5, 2020
    Posts: 9

    Yasser

    What about trying about trying different lenghts from the rear axle to the front, in increments of 5 cm, should I get any rapping at the desired RPM?
     
  21. jnaki
    Joined: Jan 1, 2015
    Posts: 9,295

    jnaki

    upload_2020-9-6_7-55-1.png
    Hello,

    The original design of the exhaust pipe on any car, hot rod, or motorcycle was usually designed for performance. Some headers had equal length tubes with no restrictions and no rapping. The smooth one-piece Bonneville TT exhaust pipes on a Triumph gave it more power with less restrictions.

    Your exhaust pipe starts out wide at the port, but because of the stock down pipe and bend to the back, it has some odd connections for a straight through pipe that seems like you are accentuating the rapping idea. Try a one-piece pipe from the exhaust port to behind the rider position to see if that does not help.


    Cars from the factory do not have exhaust rapping because of the design, however restrictive it is. But, when people start changing mufflers and pipes, then the rapping sound comes into play. Even big name companies like Magnaflow, sell straight through mufflers that cause factory stock cars to make the rapping sound. A newish car with loud rapping sounds just does not cut it in performance or quality driving conditions.

    Jnaki

    The T-Bucket roadster had 4 even size pipes of equal distance in length for a mellow sound on each side. But, the CHP needed less sound, so the owner put in baffles to quiet the sound. Going for a ride in the T-Bucket Roadster did not make my ears cringe because of the mellow exhaust sound of the Ford motor.

    The custom Triumph in the colorful print was also fitted with the Bonneville TT pipes and it did not need baffles for smooth sounds while riding around. Remember, the rider knows where that “throttle spot” is to keep the pipes from having the rapping sound or sounds that come out too loud for the local police, despite not having mufflers.

    upload_2020-9-6_7-56-3.png

    NOTE: The old scavenger pipes that were add-ons for most hot rods and cruisers were guilty of rapping sounds. At first everyone thought the straight through pipes from the factory exhaust manifolds would increase horsepower and make a great sound. But, for most, including our 348 Impala motor just sounded like a choking 6 cylinder with a tube muffler.

    That was not what you wanted your drag racing cars to sound like with those attached. Luckily, the tech committees outlawed them as not stock and that was that. They looked good underneath, but the sound was embarrassing to a pure hot rod/drag racing scene. Plus, they added more weight to the already heavy cars.

    There is always the need for smooth flow with no restrictions of any kind for best performance.
     
    Last edited: Sep 6, 2020
  22. lippy
    Joined: Sep 27, 2006
    Posts: 6,825

    lippy
    Member
    from Ks

    I had an O/T roadrunner with a 383 it had stock mufflers blown out I guess, lol and full tailpipes, stock manifolds and the factory chrome tips. It was a 4 spd. If you wound it up on mainstreet and let it back rap, it would darn near knock windows out!! That sucker would report!!!! Lippy
     
  23. Yasser
    Joined: Sep 5, 2020
    Posts: 9

    Yasser

    Great info. I understand that there is more try error on this. Fortunately I have found 40mm copper straight tubes. Once covid allows me I will try and post back.
     
    Jet96 likes this.
  24. Yasser
    Joined: Sep 5, 2020
    Posts: 9

    Yasser

    You are very luck lippy!
     
  25. Mojo
    Joined: Jul 23, 2002
    Posts: 1,872

    Mojo
    Member

    PO was building an old school lowrider with my riviera. 2" off the motor, going to 20" cherry bombs, then into 1 1/2" pipe to the bumper. It raps off like a nascar, way too loud though.
     
  26. 57JoeFoMoPar
    Joined: Sep 14, 2004
    Posts: 6,138

    57JoeFoMoPar
    Member

    I think @Paul nailed it with his answer from 15 years ago. The thread is till on-topic though and a good example of new members using the "search" function properly. Don't want to discourage that.

    Since we're having the discussion, I would add to Paul's excellent post that moving the mufflers forward in the exhaust system design will help keep the RAPPPP sound, whereas mufflers further back in the car will work to stifle it. The closer the muffler is to the engine, the more the pipe will rap, which is especially true with a straight-through muffler design like a Cherry Bomb or steel-pack rather than a chambered design.
     
  27. Yasser
    Joined: Sep 5, 2020
    Posts: 9

    Yasser

     
  28. Yasser
    Joined: Sep 5, 2020
    Posts: 9

    Yasser

    I agree, Paul explanation is brilliant, but in my case shorter pipes won't give bigger rapping. The is some mystic around this sound
     
  29. williebill
    Joined: Mar 1, 2004
    Posts: 3,274

    williebill
    Member

    What causes exhaust rapping?

    Coolness.
     
  30. 57JoeFoMoPar
    Joined: Sep 14, 2004
    Posts: 6,138

    57JoeFoMoPar
    Member

    No shit you're experiencing more rapping with straight pipes. I was simply referring to those of us who actually run mufflers, and on a car, not a motorcycle. Of course straight pipes are going to rap and cackle more than a system with a muffler, regardless of muffler placement. The word "muffler" is not a misnomer. It *muffles* the sound of the engine.

    Your application isn't really relevant to this board but I answered to try to keep the subject on topic.
     

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