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Exhaust size?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by txbartman, Dec 18, 2001.

  1. txbartman

    txbartman 1/2 ton status

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    I am looking at converting to true duals with dual cats and flowmaster 40s. What size pipe should I use? I have heard go with 3" if single exhaust and 2.5" if dual. Does this sound correct? I just need to order the right cats/mufflers.

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

    shawnboy 1/2 ton status

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    I've also heard that going with too large of an exhaust reduces back pressure to the point where on a stock engine it can actually decrease performance. I have my theories why but honestly have never researched the matter. You might want to.
    Shawnboy.

    <font color=red>If you are having too much fun it's probably illegal.</font color=red>
     
  3. Burt4x4

    Burt4x4 3/4 ton status

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    Yup that is what I have read. I wouldn't go any bigger that 2 1/2" pipe with duals some have even ran 2" to try and get more torque...I don't know the detail but I am running 2 1/2" and I'm happy[​IMG]

    72K5[​IMG]Led Zeppelin[​IMG]Rock ON![​IMG]
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  4. jekbrown

    jekbrown I am CK5 Premium Member GMOTM Winner Author

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    I am running a single 3" and dig it. Maybe its just me, but paying for 2 cats and 2 muffs? bah. Not worth it IMO. For high HP racing wanna-be rigs or something it might be cool, but 3" is plenty of room for my 350 to breathe through.

    Jason


    See the on-going build up of my '85 Jimmy! http://community.webshots.com/user/jekbrown
     
  5. mudhog

    mudhog THEGAME Staff Member Super Moderator

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  6. muddin4fun

    muddin4fun 3/4 ton status

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    For what motor? Your tired one or the new one you're thinking of?
    On my 305, I had single 2 1/2" pipe. If you're gonna drop a 350 in, you really should decide how much HP and Torque you think you'll wind up running before you put several hundred into an exhaust system. Those flows will sound sweet on anything though [​IMG]

    <font color=blue>What part of my brilliance do you not understand?</font color=blue>

    <a target="_blank" href=http://www.bluefox.tv/~lara>My Wonderful Site</a>
     
  7. txbartman

    txbartman 1/2 ton status

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    This is for my new 350. I am expecting somewhere between 260-285 HP. I expect that I will either go 2 1/2" for duals or 3" for single.

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

    shupach 1/2 ton status

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    psht exhaust, i just cut the mufflers off(no cats cause its a 76, exempt) and took some flex tubing and bolted it on and pointed it down. now thats wat i call an exhuast system.

    phill
     
  9. 84_Chevy_K10

    84_Chevy_K10 Banned

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    I run a 2 1/4" Dynomax header dual kit. It's CHEAP and you can get it from Jeg's High Performance.

    2 1/2" sounds better but if you're under 300 HP and 5,000 RPM or so, 2 1/4" is good enough.

    Tim
    '84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
     
  10. Slayer

    Slayer 1/2 ton status

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    <font color=green>2.5 inch shorty duals with Flowmaster 50's and crossover. It was highly recommended, and it really works...............................</font color=green>

    <font color=blue>I don't know everything, I just think that I do.............</font color=blue>[​IMG]
     
  11. muddysub

    muddysub 1 ton suburban status Staff Member Moderator GMOTM Winner

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    i'm trying to decide what i want too. i have the stock single exhaust and manifolds. i'm getting a crate motor next year but i want headers before then. i've been thinking of what type and brand to get. but i'm still not sure as to what i want.

    people that get good mileage aren't having any fun!
    scott,
    1989 GMC suburban 3/4t 4" lift rollin on 36" swampers, tbi350, th350, np241
     
  12. barbastard

    barbastard 1/2 ton status

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    I've got a 350 with "true" dual exhaust, 2 1/2'' pipes, no catalytic converters (smog exempt), and dual flow master 40's and it sounds AND runs SWEET!!! The key to the whole system is you gotta have a cross over pipe linking the two pipes together to equalize backpressure of both of them. Trust me....this works!!!
     
  13. JIM88K5

    JIM88K5 1/2 ton status

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    Jekbrown is right. Why have two exhaust systems under your rig. JBA shortys, custom Y pipe, Flow master collector,
    Hi-flow cat &amp; Flowmaster muffler works great and sounds nice. My 3/4 ton camper special 454 had 2.5 duals &amp; Flowmasters. Two and a half inch duals on a stockish 350 is
    more than enough system. My 68 mustang has tri-y's 2-1/4
    duals w/Flowmasters &amp; that works good.-Jim

    88 5.7 5in lift 35"procomp at's
    700-208-456-3in exh w/flowlowder 10 bolts & some ORD stuff & CV driveshaft
     
  14. jaybird

    jaybird Registered Member

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    Brian,
    I run 2.5" on my Jimmy, If you run to large of exhaust you reduce port velocity to the point of not getting all of the spent gases out of the chamber. The smaller pipes will maintain heat in the exhaust gases and port velocity. go with 2.25 or 2.5 duals with a crossover pipe as close to the engine as possable.

    Jay

    Jay VT
    79 Jimmy 454, 700R4, NP208, D44, 14 bolt, 4.56:1, 38's
     
  15. BlazinRojo

    BlazinRojo 1/2 ton status

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    the balls of exhaust gas going through the pipe actually creates a vacuum behind it whic helps draw the next ball of gas along...too big a pipe reduces this effect ("scavenging" i think)

    Shutup Brain or I'll stab you with a Q-Tip - Homer J. Simpson
     
  16. Twiz

    Twiz 1/2 ton status

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    Take into account, majority of the published reading is based on "Street performance/Hot-rods". That means it's high RPM use (3-4 grand and up) and therefore, passes more air threw the engine. So, lager exhaust works. But.. for us... the RPM range is different.

    In theory, it's a game of vilosity(S/P?), not "flow".
    Atomispheric preassure pushes back on the exhaust pulse, so it works against the engine in pushing the exhaust out. But, if the exhaust pulses are correctly timed, and the diameter of the pipe is keept to a minimum (for the given RPM range), each "pulse" will pull the next. (get it?)

    What I'm tring to say is that: Operating RPM range should be considered, and for us, thats from idle to 4500, so 2.5" exhaust is to large (IMO), 2.25" or 2" would be better, to keep the air pressure at bay and out of the pipe at all RPMs.

    Back pressure costs.
    If the exhaust sys. is restrictive, the engine has to work harder to push the exhaust out. Any-back preassure will cost H/P. Anyone that says different, hasn''t correctly calibreated the Air/Fuel mix as needed after the exhaust work was completed. If the exhaust sys has a restrictive miffler or is bent exsessively, the diameater of the pipe should be increased as needed to compensat.

    In a nut shell, 2" or with high-out-put 350 (or larger cid), 2.25" dual with good flowing mufflers, crossovers equilize the exhaust pulse into a uniform flow, also help compensate for restrictions in the sys. (I think, if a cross-over is used, the next step smaller pipe could be used)

    In theroy according to Twiz.


    <font color=blue>Wow factors
    A SBC (4" X 3.48") with 2" intake-valve at 6,000 RPMs:
    1- The piston reaches a maximum speed of 60 MPH
     

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