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Thread: 2 1/2 or 3 inch diameter system?

  1. #1
    Registered Member WagonCrazy's Avatar
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    2 1/2 or 3 inch diameter system?

    Also pondering this question, and wanted some real world experience so went to youtube and found this video that compares 2 systems on an engine via a dyno, tracking and analyzing the results.



    So with my Nomad engine being a bone stock LS1 (about 340 horsepower) and the desire for quieter (3 chamber mufflers), I'm wondering if I need to bother with a 3 inch system?
    Either diameter size would fit (with the thru frame AME cross members I have welded into my frame).

    Let's hear it guys, what's your real world experience with 2 1/2 or 3 inch exhaust?
    1957 Nomad- LS1/T56 on C4 chassis
    1959 Fleetside Apache 1/2 ton, shortbed, big window, 327ci.

  2. #2
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    I built a hybrid 3" to 2-1/2" system. 3" from the collectors to the mufflers, a 3" muffler with 3" to 2-1/2" adapters welded to it, and 2-1/2" beyond that. Flowmaster 2-1/2" mandrel bent tailpipes, modified to suit.

  3. #3
    Registered Member chevynut's Avatar
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    I went with 3.5” pipes from the header collectors to the cutouts and 3” to the x-pipe and beyond that. But that’s with a 502 that pumps a lot more air than an ls1 at any RPM. Unless you’re planning a bigger engine or supercharging I’d stick with the smaller pipes. I think 3” is typically recommended over around 500 hp.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension

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    I have owned numerous street strip cars over the past 40 some years both SBC and BBC and found little if any difference in et open headers or muffled when equipped with a good 2.5 system. Have ran strip only SBC car with exhaust because it really did not hurt performance.

  5. #5
    Registered Member NickP's Avatar
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    Quote Originally Posted by WagonCrazy View Post
    Also pondering this question, and wanted some real world experience so went to youtube and found this video that compares 2 systems on an engine via a dyno, tracking and analyzing the results.



    So with my Nomad engine being a bone stock LS1 (about 340 horsepower) and the desire for quieter (3 chamber mufflers), I'm wondering if I need to bother with a 3 inch system?
    Either diameter size would fit (with the thru frame AME cross members I have welded into my frame).

    Let's hear it guys, what's your real world experience with 2 1/2 or 3 inch exhaust?
    Bone stock, 2-1/2". I see no need for anything larger. If space allows, either an X or crossover should tone it down also. I'm not a fan of the pictured mufflers though but have found Borla to be the quieter of selections out there. Traveling any distance with that constant drone is un-nerving at best.

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    I agree, at your horsepower level, a 2-1/2" system is fine.

  7. #7
    Registered Member enigma57's Avatar
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    Some years ago, David Vizard conducted tests to determine what it would take to build a full length muffled exhaust system for a race car that made the same power throughout the engine's operating range as with open headers.

    He determined that the key to doing this is to build a system that will flow 2.2 cfm per HP generated (measured at flywheel).

    Exhaust sizing......

    Vizard's rules of thumb: A pipe will flow 115cfm per square inch of cross-sectional area (measured as ID). You need 2.2 cfm of flow capacity per horsepower.


    350 HP X 2.2 = 770 cfm


    770 divided by 115 = 6.695 sq. in. CSA = 2.91" ID (round to 3" OD) single pipe or dual pipes 2.06" ID (round to 2-1/4" OD)


    Bear in mind that this is based upon flow through straight pipe. There will be some degree of friction loss at each piping offset or bend. The more offsets or bends, the more friction loss and the tighter the bends, the more friction loss. So use mandrel bends and build your system with as few offsets or bends as possible and make any bends you must use with as long a radius as you can fit in the space available.

    Regarding mufflers, the 2.2 cfm per HP generated rule of thumb applies there, as well. I like the Dynaflow Super Turbo mufflers (quieter and flow better than Flowmaster in tests of same size mufflers). The older listings for the Super Turbo mufflers actually had a HP capacity based upon 2.2 cfm listed beneath the mufflers on their website (may still be there).

    Now that does not mean that a size smaller system will not work with the same displacement and max RPMs at redline (think of the engine as an air pump). It just means that a smaller size system will begin to be restrictive and choke flow (rob a bit of power) before redline RPMs are reached.

    Anyway...... If you are building a system for a 350 HP engine, 2-1/2" OD duals should be all that you need (in theory, 2-1/4" OD would 'just' do it, but in the real world, 2-1/2" should make up for friction losses at offsets and bends). Just make sure your mufflers do not restrict flow volume. One school of thought holds that it would be fine to downsize your tailpipe from muffler exit to point of discharge at rear of vehicle for reasons I won't go into here. On the other hand, I have run larger systems than absolutely necessary on several cars I have built and it has never been a problem. Ran 2-1/4" OD single exhaust system on 1600cc (98 cu. in.) Toyota engine and 2-1/2" OD single exhaust system on 170 cu. in. slant 6 many moons ago.

    Add a full sized crossover (balance) pipe connecting left and right bank and you should be good to go. Either an 'H'-type or 'X'-type balance pipe will do just fine. 'X'-type only has a small advantage over 'H'-type in those instances where center of 'X'-type can be placed at exact point (length) that correctly tuned header collector extensions would terminate if car was running open headers on the track at RPM where max VE is achieved (usually a couple hundred RPMs past peak torque).

    2-1/2" duals with mandrel bends should flow sufficiently to support over 400 HP so long as mufflers also flow 2.2 cfm (or better) per HP at peak.

    Hope this gives you some ideas,

    Harry
    Last edited by enigma57; 12-15-2017 at 07:01 PM.
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  8. #8
    Registered Member WagonCrazy's Avatar
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    I'm going with 2 1/2 diameter. Still pondering the materials though. (Stainless or mild steel). We're still debating pros and cons on that on the other thread I started here in the Exhaust forum.
    Thanks for your advice guys.
    1957 Nomad- LS1/T56 on C4 chassis
    1959 Fleetside Apache 1/2 ton, shortbed, big window, 327ci.

  9. #9
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    Quote Originally Posted by WagonCrazy View Post
    I'm going with 2 1/2 diameter. Still pondering the materials though. (Stainless or mild steel). We're still debating pros and cons on that on the other thread I started here in the Exhaust forum.
    Thanks for your advice guys.
    What kind of mufflers are you going with ?

  10. #10
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    Key question here is does a muffler with 3" inlet and outlet flow better than one with 2-1/2" inlet and outlet? It certainly won't flow less. The muffler is the deciding factor here.

    It's also why I think a Flowmaster at least has the chance of being less restrictive due to its large case. Just a gut feel other than its reputation.

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