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Technical 2" tail pipes on a 2 1/2" duals system, big power killer?

Discussion in 'The Hokey Ass Message Board' started by drptop70ss, Jul 24, 2021.

  1. drptop70ss
    Joined: May 31, 2010
    Posts: 1,185

    drptop70ss
    Member
    from NY

    Tail pipe question!
    Building a 55 Buick that I put a truck arm rear suspension in, and clearance around the axle /shocks / panhard bar is tight. 300HP engine so not big power. I have mid length headers to 2 1/2" pipes to cherry bomb turbos, but the only way to continue the system with 2 1/2" pipes would be to exit before the rear wheel on both sides, and I would really rather get pipes to the rear bumper.
    I think 2 1/4" tails would not be a big deal, but not even sure I can make that work. Would 2" tails be a huge hit on HP, or does it not really matter past the mufflers?
     
  2. I highly doubt you would ever feel a difference.
     
  3. squirrel
    Joined: Sep 23, 2004
    Posts: 50,270

    squirrel
    Member

    The bonus of having tailpipes, should outweigh the slight performance loss.

    btw I have a full 2 1/2" exhaust system on my 700ish hp blown 427....I take the pipes off for racing, but in street driving, it has plenty of power. Since you're starting with less than half the power, you should be able to get away with a smaller tail pipe, eh?
     
  4. oldiron 440
    Joined: Dec 12, 2018
    Posts: 2,544

    oldiron 440
    Member

    As exhaust cools it takes less physical volume so tailpipes can be a smaller diameter than before the mufflers with no power loss. Now how much smaller?
     
    loudbang, onetrickpony, X38 and 2 others like this.

  5. Unless they've been enlarged, that 1955 Buick has like 1 3/8" exhaust valves. I doubt that you would ever notice the difference.
     
    VANDENPLAS and lothiandon1940 like this.
  6. catdad49
    Joined: Sep 25, 2005
    Posts: 5,317

    catdad49
    ALLIANCE MEMBER

    I did basically the same thing on the A to get around the ladder bars, runs good to me!
     
    lothiandon1940, -Brent- and 42merc like this.
  7. drptop70ss
    Joined: May 31, 2010
    Posts: 1,185

    drptop70ss
    Member
    from NY

    Nailhead was long gone when I got the car, but will make some 2" tail pipes and see how they fit.
     
  8. As the others have said, but also, at what RPM?
    WOT or driving around town RPM?
    Just do it, you'll be fine.
     
    lothiandon1940 likes this.
  9. BJR
    Joined: Mar 11, 2005
    Posts: 7,244

    BJR
    Member

    I have 2" pipes on a 472 Cadillac in my 49 Buick. Lots of power, and good sound.
     
  10. oldsman41
    Joined: Jun 25, 2010
    Posts: 1,556

    oldsman41
    Member

    My avatar has 2 1/2 to mufflers than 2 inch up and over the rear axle,seems ok to me
     
    lothiandon1940 likes this.
  11. jaracer
    Joined: Oct 4, 2008
    Posts: 1,313

    jaracer
    Member

    A little back pressure helps low end torque.
     
    lothiandon1940 likes this.
  12. 1946caddy
    Joined: Dec 18, 2013
    Posts: 1,780

    1946caddy
    Member
    from washington

  13. anthony myrick
    Joined: Sep 4, 2009
    Posts: 8,955

    anthony myrick
    Member

    2 1/4 or 2 inch after the muffler ain’t gonna hurt nuthin
     
    lothiandon1940 likes this.
  14. FrozenMerc
    Joined: Sep 4, 2009
    Posts: 2,776

    FrozenMerc
    Member

    These guys are right, you would never know the difference. I would even be willing to bet a box of doughnuts that it would be difficult to see a noticeable difference produced on a chassis dyno given back to back runs, nothing outside of the range of acceptable measurement error anyways.

    That said, the engineer in me likes to think about an actual answer to this sort of question. So for arguments sake, lets say that the 2" pipes creates an extra 1 psi of back pressure as seen at the piston over the duration of the exhaust stroke. Being a V8, that means 2 cylinders are always seeing this increase in pressure at any given moment. 1 psi on a 4" bore = 12.5 lbf of additional force the engine must overcome per cylinder to push the gases out. On a 3.5" stroke, that will require 3.6 ft-lbs of torque to overcome. Multiply that by the 2 cylinders always undergoing the exhaust stroke, and you have 7.2 ft-lbs of additional parasitic drag. At 5250 RPMS, = 7.2 hp lost, at 2500 rpms, 3.4 hp.

    I doubt you would see a full 1 psi increase in back pressure, but it sure would be fun to measure it and find out.
     
    Moselli likes this.
  15. choptop40
    Joined: Dec 23, 2009
    Posts: 4,090

    choptop40
    ALLIANCE MEMBER

    i was gonna run my 2 1/4 inch exhaust without mufflers and end it with 1 3/4 turn outs in front of the rear wheels..AKA Mopar sneaker exhaust....reason...back pressure ...low end torque...
     
  16. el Scotto
    Joined: Mar 3, 2004
    Posts: 4,490

    el Scotto
    Member
    from Tracy, CA

    Hahaha!! I was just pondering this the other day, I have 2.5" Corvette manifolds on a mildly built tripower 283 but have the 2" exhaust off another heap and was considering just using it since I already have it. Eventually maybe I'll redo it if I stuff the 12:1 327 with the ported 492s in it...

    I had read this as well and it makes lots of sense.
     
  17. jimmy six
    Joined: Mar 21, 2006
    Posts: 10,277

    jimmy six
    ALLIANCE MEMBER

    If you want a “soundalike” go by a big block Chevelle which I don’t think you are looking for with a 55 Buick with any engine transplant. I gave up on noisey street cars log ago and could not be happier..That chart fits my 225 horse engine perfect.
     
  18. anthony myrick
    Joined: Sep 4, 2009
    Posts: 8,955

    anthony myrick
    Member

    An old round track guy I knew said after a fresh build, they would spray paint the exhaust.
    Then run it on the track.
    Cut the exhaust just past where the paint burnt.
    He said anything after that was waste.
    I don’t know. Cool story anyway.
     
    jimmy six likes this.
  19. Atwater Mike
    Joined: May 31, 2002
    Posts: 10,728

    Atwater Mike
    ALLIANCE MEMBER

    More than a couple of dyno runs before-and-after large dents were hammered into header tubes (preceding the collectors!) closing 3/8 of the tubes' diameters (2 tubes, left side for clearance) NO H.P. was lost.
    I was surprised...but saw it done in yet another instance, both were Chevy engines, one a 402" SBC and the other a 472" BBC.
    So...2" tailpipes aren't going to 'neck down' any notable H.P.!
     
  20. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,608

    Rusty O'Toole
    Member

    I doubt the stock Buick SINGLE exhaust was that big. Don't worry it will be plenty big enough.
     
  21. 56MercMan
    Joined: May 22, 2008
    Posts: 114

    56MercMan
    Member

    I went down to 2.25 on an OT carbureted vehicle and it actually increased my torque. I could chirp the tires in 2nd gear after doing the little smaller pipes which I never could do with the larger pipes. Same muffler, header and everything on the motor. Only change was the smaller diameter pipes after the muffler.
     
  22. oldiron 440
    Joined: Dec 12, 2018
    Posts: 2,544

    oldiron 440
    Member

    That's exactly what I have done on my OT Plymouth, I use collector extensions and cut them using that technique.
    It tunes the header to the motor output.
     
    jimmy six and anthony myrick like this.
  23. anthony myrick
    Joined: Sep 4, 2009
    Posts: 8,955

    anthony myrick
    Member

    My 500 caddy came with 2.5.
    runs great.
    About to build a stainless y pipe and run the single 2,5 out before the rear tire.(need the y pipe to run on the opposite side to clear the tank)
    Why the single?
    The 500 will never run in the rpm range where a dual exhaust is needed.
     
  24. seb fontana
    Joined: Sep 1, 2005
    Posts: 7,213

    seb fontana
    Member
    from ct

    There are places that sell oval exhaust tubes and bends for clearance solving. Next can argue if traditional..
     
  25. drptop70ss
    Joined: May 31, 2010
    Posts: 1,185

    drptop70ss
    Member
    from NY

    I built these two cars with identical tuned engine, transmission, and rear axle. Both with cast manifolds, one with a full 2 1/2" system and one with a 2 1/2" Y into 2 1/2" single. Both ran great but the dual exhaust car was amazingly quicker from a standing start as well as from a roll. I dont think there was enough of a weight difference to allow for the seat of the pants difference. Myself I would not put a single exhaust on a cad 500, but that is your call.

    228.JPG
     
  26. anthony myrick
    Joined: Sep 4, 2009
    Posts: 8,955

    anthony myrick
    Member

    It weighs 7500 pounds.
    I’m not drag racing anyone.
     
    Last edited: Jul 26, 2021

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