strait talk about e -pipe diameter vs. flow...

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  • Jerk
    • Mar 30, 2024

    strait talk about e -pipe diameter vs. flow...

    when I was taking welding classes about 9 years ago I learned something long known in the pipe industry- "double the diameter (I.D.) and you quadruple the flow capability". Now, having brought that to light, I'll enlighten you a bit more, on a NON- mass air flow sensor controled or NON O2 sensored intake/exhaust system, i.e.- a normal carbed engine w/ no electronic fuel dilivery - our jeep/amc engines in other words, YOU CANNOT HURT PERFORMANCE BY PUTTING ON A LARGER I.D. EXHAUST PIPE. the same is not true for the o2 sensored/ mass air flow sensored engine. On OUR engines, If you want to run 8" pipe, it wont hurt a thing. if you run .75" pipe...it will hurt your performance.
    Another point: As far as "fast moving exhaust gasses" goes, it makes no difference how fast the gasses are moving, as long as there is a small or preferably zero level of BACK PRESSURE. back pressure is caused by a restrictive muffler, cat, pipe, manifold, even a high pressure tail pipe exit, as in, the exit end of the exhaust pipe being in a high air flow area of the vehicle, hampering efficient exiting of exhaust gasses. for an extreme example, think about driving 70 mph with your exhaust pipe facing out the front of your wag! dumb example, wouldn't happen in the real world, but its just an example.
    Now bearing in mind that an engine is an air pump, and the freer it can move air in and out of itself the more efficient and thus more powerful it will be, the largest diameter, and shortest running exhaust you can install is undoubtedly the best. of course, there are monetary, legal and spacial concerns as to how big you can go and still be practical.
    With all that being said, I'd go with a good 2.5" or 3" free flowing system, duals or single. the duals and headers really only show a marked improvement at higher rpm, say free way speeds. so find out what you are going to be doing and go from there. as far as my preference sound wise, I'd stay away from glasspacks unless you want LOUD! but that's just my opinion. Matt W.
    A.K.A. - Mr. Tinkerjeep.
  • billyrb
    BJ's Off-Road
    • Aug 15, 2001
    • 10032

    #2
    Got a question, then: if there isn't any back-pressure (zero preferably, as you stated?), won't that eat away at the valve seats?

    ~ryan
    BJ's Off-Road
    [email protected]
    Your source for '63-'91 FSJ Parts

    Comment


    • #3
      no. this is not a factor in exhaust valve seats(I'm assuming you mean just the exhaust valves, since they handle the hot gas) being erroded. If you have a long duration/ high overlap cam and a a super charger, you can get preburned air/fuel mixture drawn through the cylinder and into the exhaust system and it CAN be ignited in the pipe, though! However, I have never heard of a valve seat going bad due to freeflowing exhaust in a street engine. also bear in mind, "zero back pressure" is not a real world achievement on a street driven engine...unless you like to run full drag race prepped engines in your street rig.

      [ March 16, 2002: Message edited by: Matt W ]</p>

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      • Weelspin
        232 I6
        • Sep 07, 2001
        • 38

        #4
        Good information..... however, I just put a 36" glasspack on my FSJ and it really isn't all that loud. Point on this though.... I have a 258 and a cat. It actually purrs quite nicely. I was worried about the back pressure though.... thanks for easing my mind. I had heard the lack of back pressure actually DECREASED my torque.....
        Dave<br />1986 Grand Waggy

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        • ArtsiFrtsi
          304 AMC
          • Jan 05, 2001
          • 2231

          #5
          <blockquote>quote:</font><hr>Originally posted by Matt W:
          YOU CANNOT HURT PERFORMANCE BY PUTTING ON A LARGER I.D. EXHAUST PIPE. <hr></blockquote>

          Right, you can't hurt the performance, but like with heads, or a cam, you will change the effective range. Instead of having X amount of tourque at X RPM's, you'll move those figures around a bit. So instead of peak tourque being at say 2000 RPM's, you might now have peak tourque at 2500 RPM's.

          Comment

          • jeepbob
            Jeep Idiot
            • Jan 16, 2001
            • 2513

            #6
            Arch is right, too big of a pipe and you will lose power on the low end. John Garrety ( I have prolly misspelled his name) wrote a book on R.V. performance and in his book he went to great detail on exhaust systems and thier effect on power and torque. He dyno tested many different styles and sizes of exhaust systems with a chassis dyno and found that you could hurt low end torque and power by going to too big of an exhaust pipe. Also while you could gain upper end power most of us would never see the gains as we do not see more than 4000 rpm on a regular basis. Too big or too small of pipe can and will hurt the power of most of our engines. Even race motors are limited on the size of pipe you should run. If this was not true, you would see 10" pipes (well maybe not that big) on race cars. Exhaust tuning is an scientific art form that takes a lot of dyno time and experience to master.
            The rule of thumb for street motors is no more than 2" to 2.25"dia for a small block motor and 2.25" to 2.5" for big blocks unless these are really built motors.
            BTW too short or too long of a pipe can also hurt performance.
            65 wag. with a bunch of stuff done and more happening.<br /><br />\"If the local junkyard calls you for Jeep parts, you might be a redneck\".... and yes they do!

            Comment

            • Lindel
              Perfesser of Jeepology
              • Jun 15, 2000
              • 9205

              #7
              Too short of a pipe will not only hurt performance, it'll hurt the engine, by allowing the exhaust valves to burn. Too much fresh oxygen, too close at hand.

              Jeepbob, and Arch are 100% correct about the proper diameter for a street/4x4 engine. If you go bigger than 3" max, you'll lose most of your bottom end. Installing an "H" or cross-over pipe will help restore some of it, due to increased scavaging, but not all (technically, it doesn't just go away, but moves to a place in the rpm band, that you aren't likely to get too, more than once).

              For the average 360/401, 2.25" pipes are completely adequate for a dual system, while 2.5" pipe is a good start on a single system. If you do some (or a lot) of engine mods, then you can increase the size of the exhaust, but keep in mind that pipe diameter WILL affect where the torque of the engine comes into play.
              Jeep Grounds
              RRV Homepage
              Texas Full Size Jeep Association
              1987 Grand Wagoneer
              AMC 360/TF727/NP229
              1999 Wrangler Sport
              4.0L/AX-15/NV231


              ?Freedom is never more than one generation away from extinction? by Ronald Reagan.


              Formerly of DFW/Gun Barrel City, TX - eventually to return...

              Comment


              • #8
                The faster you move the exaughst away from the motor the torque # go down. So I have to say bigger is NOT better. If you ever looked at the exhaust on some of the torque monsters the exhaust systems were restrictive. My 70 with the 350 in it I beleive has a 1 7/8" exaughst system. I have no intention of opening it up, I like my LOW RPM torque, you loose that with Headers and Big pipes. However this is a subject that can get kicked around forever. Jeepers choose your PIPES, and go play!!!
                Daily Driver 2008 Wrangler Sahara
                Going, Gone, or Thinking Fishing

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                • Tad
                  • Nov 30, 2001
                  • 17618

                  #9
                  Thank You One & All, I have to rebuild the entire exhaust system on the wag, from the manifolds back. Staying at 2", going to duals, going out the passenger side.
                  2000 Infinity QX4, 3.3L, MPFI, 4 speed auto, 2 speed Nissan tcase, Unibody, IFS front, 4 link rear solid axle with 255-70/16s

                  IFSJA WMS PROJECT
                  EARLY WAG LIFT SEARCH

                  ...Pay no attention to these heathen barbarians with their cutting torches and 8" lift kits!...
                  Self Inflicted Flesh Wound

                  Comment


                  • #10
                    well guys, as usual, some of you are wrong, some right, and some both. low end torque is great for a diesel and 5 gears, not for a 401 and 3 gears, if you have a slightly built 401, or even 360 for that matter, you already have really excellent torque at 1500-2000 rpm. what most people don't realize, and I'm not pointing fingers or calling anyone dumb, is that your hp doesn't come into play untill 3000 rpm on those same built engines, tuning a v8 for a flat power band is essential for a truck and general use engine. this is done by using a decent dual plane intake manifold and streetable cam for low end torque, and tuning exhaust for higher rpm. this not only widens the power band, but makes sure your exhaust is definately not the bottleneck in the airflow into, through, and out of the engine. Edelbrock's book "the great manifold bolt-on" directly relates to this- I reccomend it to everyone interested in volumetric efficiency and power gains! as to the big cu engines of the 70s, the reason no one put big pipes on them or headers, is because they not only didn't need them, (as stated, they never operated above 3200 rpm, not nly because of the lower speed limits, but also a big engine with heavy rotating assembly doesn't last too long at 6000 rpm), also, the intended owners did not like loud pipes in a 4000-5000 lb boat/car. for the boys that did like loud pipes, they made the mustangs/ camaros/ firebirds/ chargers. headers and big pipes don't hurt torque on an engine unless it has little torque to begin with. everyone that grew up in the 50s/60s knows the easiest way to make torque is large displacement- still true today.
                    As far as what racers would be running if it was good for power (the 10" pipe example), I hate to tell you, but top fueler dragsters don't run any pipe but maybe 24" from the exhaust port to the tip, which opens to atmosphere, they also make approximately 7000 hp! any body else who runs pipes in a governed racing event is only allowed to run what the rules say. period. same goes for intake, cam, carb, cubic inches, tires, etc. the list is longer than I can think to write it all down. basically every detail of a governed (like scca, nascar, etc.) car is dictated by a rule book, those rules are not made to allow the best performance possible, only to keep things fair and safe! I don't mean to yell, thats just the way it is. another detail worth mentioning, a jeep, with very few exceptions, is not a nascar racer. the point being, most race cars are very "peaky" in their power bands, and these power bands are very narrow and high up in the engines rpm range. everything is tuned for that range, not so on a street/ off road vehicle. the broader and higher the power(not rpm wise, just higher output) the better, (within reason),in my opinion. - Matt

                    Comment

                    • jeepbob
                      Jeep Idiot
                      • Jan 16, 2001
                      • 2513

                      #11
                      Well Matt,
                      Books do not always translate into the real world. A properly tuned exhaust system can be good for up to 50 hp on a street car or 4x4 depending on what was on it before. Even 401's do not have so much torque that they can afford to lose a whole lot on the bottom end where most of our motors run most of the time. Except for a few classes SCCA does not put a whole lot of restriction on exh systems. Nascar does limit the size of the pipes but only did so after consulting all the enginge builders to find what was the maximum size they would use (source here was Nascar Tech on the tube a few days ago). If you think that top fuel headers are not tuned then you have not talked to the top fuel engine guys. For an idea of some radical tuned headers check out some of the late 60's Indy and road racing cars. They did not call those headers a bundle of snakes without a reason.
                      If you want even more proof come on up here and play is some of our "clay on sand" mud pits or up at the dunes where Torque is king and where high end power won't get you to the top if you can't turn the tires and keep the revs up. Automatics and torque monsters rule the dunes and pits which is why I am dumping AMC motors and going to Pontiac power in my Wag.
                      65 wag. with a bunch of stuff done and more happening.<br /><br />\"If the local junkyard calls you for Jeep parts, you might be a redneck\".... and yes they do!

                      Comment

                      • jeepbob
                        Jeep Idiot
                        • Jan 16, 2001
                        • 2513

                        #12
                        Well Matt,
                        Books do not always translate into the real world. A properly tuned exhaust system can be good for up to 50 hp on a street car or 4x4 depending on what was on it before. Even 401's do not have so much torque that they can afford to lose a whole lot on the bottom end where most of our motors run most of the time. Except for a few classes SCCA does not put a whole lot of restriction on exh systems. Nascar does limit the size of the pipes but only did so after consulting all the enginge builders to find what was the maximum size they would use (source here was Nascar Tech on the tube a few days ago). If you think that top fuel headers are not tuned then you have not talked to the top fuel engine guys. For an idea of some radical tuned headers check out some of the late 60's Indy and road racing cars. They did not call those headers a bundle of snakes without a reason.
                        If you want even more proof come on up here and play is some of our "clay on sand" mud pits or up at the dunes where Torque is king and where high end power won't get you to the top if you can't turn the tires and keep the revs up. Automatics and torque monsters rule the dunes and pits which is why I am dumping AMC motors and going to Pontiac power in my Wag.
                        65 wag. with a bunch of stuff done and more happening.<br /><br />\"If the local junkyard calls you for Jeep parts, you might be a redneck\".... and yes they do!

                        Comment


                        • #13
                          bob, what world are you living on? the books I read are taken from race proven, and dyno proven real world results. as far as me saying that dragsters don't have tuned pipes, I never said that, I just said they only have about 24" of pipe from head flange to end of pipe, all seperate pipes from one another, and they don't seem to have a problem with back pressure or measley torque! I don't care what people drive where you are, or what they are doing, but you should get your info strait when it comes to torque versus horsepower and what a built 401 has for torque numbers. a built 401, with a broad power band makes 400 lb feet at 2000rpm, that goes up to 450 at around 4000 rpm and doesn't drop until after 5000 rpm. as far as hp, it doesn't get into the 300s until around 3000rpm and climbs to 350 at 5000 rpm, roughly. this is my engine to the gnats a$$ on desktop dyno- a paper estimate, but accurate enough for this debate! Now, you put what ever engine you want in your rig, I don't care, stick a 502 crate engine in it if you want, that isn't going to change the FACT that a 401 still has decent torque in the low rpm range. if you want more, build it in! also, your getting a small block engine that can compete with the big 3 big blocks of past years, without the front heavy handling characteristics.
                          As far as high rpm not getting anyone to the top of a hill in sand, it all depends on traction /gearing/ vehicle weight/ wheel base/ power band, not to mention driver skill. Ever seen the Reykevic(sp?) hill climbers? those boys have some nasty rigs and they climb practically strait up flakey, crushed lava hills, let me tell you something bob, it ain't at 1000 rpm that they do it. They use long, light rigs, high hp engines, big paddle tires and sometimes even nitrous. I believe the rigs are 800 hp mills and they are definately 4 wheel drive. the key, actually to driving on sand or mud is momentum, and high speed, ask anyone who runs in the African rallies, or off road rallies...of course those boys know how to drive their rigs too!

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                          • #14
                            Hey JeepBob, I will play with ya in the "clay on sand" mud pits or up at the dunes when I come down this fall. You will have to help me wash my rig when we are done though. Will bring my own torque and empty coffee cup, heard ya have a pot on all the time.
                            Daily Driver 2008 Wrangler Sahara
                            Going, Gone, or Thinking Fishing

                            Comment


                            • #15
                              BTW, bob-
                              I just ran a simulation, note, did not say dyno test in "real world!" and with my exhast system changes from what I have now-small tube headers and mufflers, I get 450 lb/ft at 2000 rpm, this stays in the 400s until 5000rpm and my hp starts at 173 at 2000 which peaks at 360 at 4500-5000 and drops below 300 at 6000rpm. pulling the mufflers off, increases both power and torque across the board, Large tube headers and mufflers increases across the board over that, and finally, large tube headers and open exhaust- 466 torque and 177 hp @2000rpm, 380 hp @ 5000, staying above 300 until 6500-7000rpm, torque peaks to 480 @ 2500-3000 and remains in the 400s until 5000rpm. so what does this indicate? maybe less restrictive exhaust is a good thing for low end torque and high end hp sitting on our Yeep engines. Sorry this is me telling you this based only on a simulation BASED on real world examples and not you seeing it for yourself in your rig, but hey sometimes you cant do it all yourself and a little faith is required. that, common sense, and an analytical, open mind BOB! somethings you haven't been exhibitting here recently. Do you think I'm saying this just to Bull$hit you?, no! I posted the topic because I kept hearing the same "old-wives-tales" concerning exhaust systems and the magical mysteries therin! you know like the one : "if you have an auto tranny that's burning fluid, DON"T CHANGE IT! it'll get worse, it's gotten used to the old fluid!" same deal- "open exhaust will ruin your power and torque, that engine needs the back pressure to make sure it can't breathe too hard and errode the valve seats"-HOGWASH! JOIN THE MODERN AGE PEOPLE! READ A **** BOOK! they aren't just there to fill empty libraries!

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