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Advantage of having 3" exhaust?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by BLzr4x4ngkid, Jan 19, 2001.

  1. BLzr4x4ngkid

    BLzr4x4ngkid 1/2 ton status

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    Advantage of having 3\" exhaust?

    I am going to put 3" dual exhaust and have it dump right out over the rear axle. I order 3" 40-series(2 chamber) Flowmasters yesterday and a friend from work is going to hook me up on getting the exhaust installed. In the future, I plan on going to a bigger engine and he was saying that 3" would be my best bet. What is the differences between 3" and 2.5", besides .5"?

    "Don't hit me with them negative waves so early in the morning." Odd Ball--Kelly's Hereos
     
  2. Chris Demartini

    Chris Demartini 1/2 ton status

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    Re: Advantage of having 3\" exhaust?

    <font color=blue>Do you have a single or dual exhaust? Unless you have a cam with lots of lift and duration, stick to 2 1/4 for dual and 3" for single. If you have a milder motor and 3" duals, you will have no low-end power. The 3" is more for high-reving motors.</font color=blue>

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  3. SlyDog

    SlyDog 1/2 ton status

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    Re: Advantage of having 3\" exhaust?

    I think he said he had dual exhaust [​IMG]

    [​IMG]
     
  4. Chris Demartini

    Chris Demartini 1/2 ton status

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    Re: Advantage of having 3\" exhaust?

    <font color=blue>Thats just goes to show that I'm about as sharp as a marble!!! [​IMG] A bigger motor wont need bigger pipes unless you get a bigger cam. Milder motors built for torque (what you want in a big heavy K5) will make more power with a slight amount of backpresure, so I would go with the smaller pipes.</font color=blue>

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  5. 6.2Blazer

    6.2Blazer 1/2 ton status

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    Re: Advantage of having 3\" exhaust?

    Too large of an exhaust can actually hurt the performance of the engine. The larger pipe slows down the velocity of the exhaust gases and can actually restrict flow due to the tumbling of the gas. For duals, 2.5" pipes are more than enough for a built small block and there's a bigger difference between 2.5" and 3" pipes than you think. A 3" pipes has an cross-section area of over 7 sqare inches, while the 2.5" pipes have under 5 sq.in. of area, almost a 50% difference in the amount they can flow. For another example, a single 3" pipe will flow more than dual 2" pipes. This is a factor most people overlook, they don't realize that a 1/2" in size makes a huge difference when it comes to pipes.
     
  6. Thunder

    Thunder 3/4 ton status

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    Re: Advantage of having 3" exhaust?

    I have to agree with all of the above posts too. Dual 3" exahust is way too big for a truck unless you have a VERY healthy or super charged motor. It would be great for the drag races in a light rig where you need top end power.
    But K5s and trucks are big and heavy they need low end torque to get them moving ,climb hills, and just creep along on the rocks.
    A single 3" exahust will do you very nicely. Its what I'm running in my K5. With a flowmaster 70 series muff. With the mods i'v done ( MSD, Headers, Modified TBI, HI pref Chip.K&N ect) My K5 runs very well with the single 3" sounds good too.
    Also putting too large of an exauhst may destroy your gas mileage.
    Sorry to be negative but I have made the mistake of putting too big of Exahust(big purple hornies bolted to the headers) on a El Camino with a 327. It ran great at WOT but It was not too good off the line and was kind of boggy. I switched to dual 2" Glass packs dumped out in front of the rear wheels and it ran a lot better for all around driving and I got a lot better fuel milage. I could also hear my stereo.
    If you go with the dual 3" and have performance problems you can always restrict the exahust with smaller Exahust tips. They make many different sizes and it is possible to tune your exahust with them. Or run smaller Diameter tail pipes after the muffs
     
  7. thatK30guy

    thatK30guy 1 ton status Premium Member

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    Re: Advantage of having 3\" exhaust?

    On my two one ton trucks, both with 454's, I have 3" after the headers, before the mufflers, and 2 1/2" after the mufflers for some back pressure. Runs and sounds awesome! If you are running a small block, I would suggest 2 1/2 before muffler and either 2 1/4 or 2 after the muffler all the way back.
     
  8. Goose

    Goose 1/2 ton status

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    Re: Advantage of having 3\" exhaust?

    I had my exhaust end directly in front of the rear axle for a couple years. Just had it finished out to behind the rear tires out the sides this fall. Wow! what a difference, it is much quieter inside, and sounds much better outside. It also doesn't set off my car alarm anymore when I park by my Monte. I have dual 2.5" and the exhaust dude said that was good for up to about 500hp!

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  9. BLzr4x4ngkid

    BLzr4x4ngkid 1/2 ton status

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    Re: Advantage of having 3\" exhaust?

    I ordered 2.5" Flowmasters and when the 3" get here I'm going to send them back. I don't know why, but I feel kind of stupid because I know that 3" exhaust would work for single and not dual. Thanks for all the feed back. It's really sad, but I had 2.5" dual on my '87 Jimmy and I could tell the low-end torque jumped up. I feel kind of dumb. Thanks anyway. HAHAHA

    "Don't hit me with them negative waves so early in the morning." Odd Ball--Kelly's Hereos
     

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