Does an increase in airflow (intake or exhaust runner) move peak torque up or down the RPM band?

Small Block Mopar Engine

  1. greymouser7

    greymouser7 Vagrant Vagabond “Veni Vidi Vici” FABO Gold Member

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    Or is the torque band strictly dictated by engine displacement and camshaft?

    Just curious,

    Thank you
     
  2. Oldiron440

    Oldiron440 Well-Known Member

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    The tq and hp curves are defined by the cam but I have seen the heads vastly improve them in there entirety or at one end or the other but generally overall.
     
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    • AJ/FormS

      AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s FABO Gold Member

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      Also unless the engine is airflow choked early, there is a fairly stable relationship between the peaks of about 1600 to 1500rpm,depending on the LSA. So if the powerpeak occurs at 5500, the torque peak is likely to occur at around 4000. Whatever transmission you use, it sorta needs to work in that range, plus say 300rpm at the top; so 1800rpm.
      This is why 4 gears used to be faster,cuz the powerband requirement was just about 1600/1700 when shifting at 5800. Whereas the autos drop 2300/2400rpm on the 1-2, and back up to 1800 on the 2-3. So for a streeter, that never/rarely gets into third gear, they sometimes/usually need a different cam LSa.
      Why so vague? Well a hi-compression 360/727, with a 2800tc,and 3.91s is likely gonna burn all the way thru first and a good way thru second so LSA is not that important; whereas a stock smogger-teen with a 904 and 2.76s will probably not even chirp the tires and gets a full 2000rpm penalty shifting at 5000. That 318engine is gonna want a wide LSA to cover both ends of the working rpm band.
       
      Last edited: Oct 12, 2019
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      • RustyRatRod

        RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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        I'm not sure it would effect top end or bottom end any more than the other. You're asking about increasing air FLOW, which is velocity. More velocity will equate to more power everywhere. I believe it will be left up to the camshaft mainly to determine WHERE peak power is developed.
         
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        • TrailBeast

          TrailBeast Slightly Twisted Member

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          Since plenum and runner volume have all kinds of RPM range applications we have to realize that the intake affects everything to do with HP, TQ as well as the range in RPM's that they produce them.
          So it's not just the cam that determines.

          The actual answer to your specific question the way you asked is both HP and TQ are increased with more flow.
           
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          • RustyRatRod

            RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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            Maybe I should have said what I was thinkin for your benefit. I was gonna throw in there somewhere "all else equal", but I didn't figure I had to. I shouldda thought about you. LOL Ain't seen you post a lot lately. You doin ok?
             
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            • TrailBeast

              TrailBeast Slightly Twisted Member

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              I added to my post a little just so you know.:D
              I'm doing fine, just busy on the cars, house, yard and everything else.
              Sometimes it feels like the Wife is trying to get everything she can out of me before...
              I'll leave it at that.:D
               
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              • rumblefish360

                rumblefish360 So close, yet so far away

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                Let’s start by keeping it simple. An apples to apples comparison. We will take the engine, any engine, in dead stock form, dyno it, get the graph then run it again with headers as the only change. For simplicity, we will say both engines have a 2-1/2 H piped exhaust system properly connected to the exhaust manifolds & headers like you would in a car. Each engine tuned for each system tested.

                The headers will increase power on both the Tq. & HP due to the improved breathing. With the efficiency going up, from start to finish, the light will shine best at the top side of the run. Headers alone will increase the power and add a few RPM to the curve due to the increased efficiency of breathing.

                Adding an upgraded intake will also increase power all over for the same reasons as I said above. If the air pump can breath in easier, then the power and rpm band will increase slightly.

                The camshaft dictates the rpm band in which the engine operates in. Again, starting at the top like we did with the headers first and intake second, the only change is the new cam is installed first for testing, the cam will changed the operating rpm band but also show a power gain because (most of the world) goes with a bigger cam. Adding the exhaust and intake will only up the anti.

                If you have a dead stock engine and your looking for mileage gains, then of course the bigger cam is skipped but the intake and exhaust can still be upgraded will good results.

                Retuning the carb & ignition for best results along with a multi spark box will show good improvements.
                Don't believe the need for back pressure myth. Size the exhaust tube for intended power output.
                 
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                • RustyRatRod

                  RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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                  That's what I had said. lol
                   
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                  • PRH

                    PRH Well-Known Member

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                    The only real answer to the original question, as asked, is........ it depends.
                     
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                    • Wyrmrider

                      Wyrmrider Well-Known Member

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                      you may or may not get more velocity (till higher RPMS)
                      you may just get more volume
                      which was not good for Boss Fords
                      There was a good point about sizing headers- toobig can give more volume but also hurt (till high rpm)
                       
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                      • rumblefish360

                        rumblefish360 So close, yet so far away

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                        Some people here are assuming to much of the basic question. I’ll assume it will sooner or later apply to your specific future build? Have you thought much about it since last we spoke?
                         
                      • Garrett Ellison

                        Garrett Ellison Amateur driver on public roadway, do not imitate.. FABO Gold Member

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                        Agree completely with the back pressure myth. In my own experience, I have yet to see a full exhaust system that doesn't sacrifice power at all points in the operating range in comparison to uncorking the headers and letting it scavenge properly.
                         
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                        • greymouser7

                          greymouser7 Vagrant Vagabond “Veni Vidi Vici” FABO Gold Member

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                          Was thinking about the talk of port velocity and what you refuted on smaller exhaust @rumblefish360 , am still up in the air on intake manifold and can hear Rusty telling me just to get it done.

                          I was thinking, after all the advice here, that I’ll build the 360, the car, and eventually the 360 again-probably wanting more power the second time-

                          -So I am going to go with 2 1/2” exhaust with a crossover. Should be plenty and not restrictive. Versus the 2 1/4” I was originally considering(@rumblefish360 ).

                          If I remember right though, the f bodies usually had a single exhaust and wonder if it will hang low or be very tight around the axle.

                          I have two pieces of exhaust pipe/w/muffler welded-up that dump before the axle for the initial driving.

                          Have been digging through the garage gathering engine parts.

                          Am using the .030” 1970 360 block, rods, pistons you got me @rumblefish360 with J heads I have assembled (&ready to go 901 springs, 2.02 & 1.60” installed but no hardened valve seats ???)
                          I picked up a 1406 Edelbrock to start with

                          Am debating over used 273 adjustable rockers or Chinese stainless 1.6 roller rockers

                          I think I need calipers, and am not sure what other tools I need for assembly. Am going look at the manual tonight.

                          Thanks everyone!
                           
                        • Oldiron440

                          Oldiron440 Well-Known Member

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                          It's funny that the Boss 302 heads were brought up because I happen to own a pair. I was looking for a pair of 69 - 70 351W heads and a guy said he had a pair for a $100 but they tuned out to be Boss heads with new SS valves. Any how the runners and valves are huge, I could see them going on a 400 M motor easly.
                           
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                          • MOPAROFFICIAL

                            MOPAROFFICIAL Well-Known Member

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                            @greymouser7
                            To the title , mostly...yes.

                            I would add heads to the CID & cam.
                            What would you be using the response here to decide on?
                             
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                            • greymouser7

                              greymouser7 Vagrant Vagabond “Veni Vidi Vici” FABO Gold Member

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                              intake manifold and exhaust pipe size (with an x pipe crossover)
                              People talk about port velocity with the 302 heads allot, so I was concerned (opposite direction, losing velocity) with a small cam engine using the chinese aluminum heads because they supposedly flow 239 cfm intake vice the typical 180-210 cfm often witnessed by you guys with stock heads. RAMM and others talked me out of using 302 heads for my goal-mopar daily driver
                              I have 3 or 4 360's to build with 90% of parts purchased
                              block, rods, & pistons: .030" unknown pistons & .040" h116 speed pro pistons, & .060" KB107 pistons & block <-all stock stroke
                              j head set 2.02/1.6", chinese head set 2.02/1.6"<-both assembled ready to go, then w2 econo and chinese aluminum bare head set <- not assembled, missing 1 valve on w2-no valve seats, no springs
                              headers and dual plane, air gap intakes at least for all
                              cams:
                              crane 693901, 693511, 693801 and a comp cams XE 262, XE 274 <-all are small, probably skip out on the 693901 because it is so small, the 801 is similar to a XE268
                              not sure which parts to match to what, i have some ideas...the .060" block IS thick enough on the thrust sides to do anything street after getting it sonic checked and asking you guys for tolerances-moper helped with that

                              i have a set of indybrocks I want to eventually send to you @MOPAROFFICIAL (probably tax season) that have ?500 miles on them and sat in my garage. Rustyratrod helped me get that engine running after sitting, but it had a minor valve train failure with the jesel rockers (i kept the motor floored where ever i drove it on the freeway)
                               
                              Last edited: Oct 13, 2019
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                              • justinp61

                                justinp61 Well-Known Member

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                                Why waste a good set of heads? 351M and 400M engines are junk.
                                 
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                                • Oldiron440

                                  Oldiron440 Well-Known Member

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                                  I will never use them unless I trip over a pile of cash and a Boss block.
                                   
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                                  • Garrett Ellison

                                    Garrett Ellison Amateur driver on public roadway, do not imitate.. FABO Gold Member

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                                    Good heads for a 427 Clevor build, talk about fun in a 67 truck... Fly F250, Fly!
                                     
                                  • MOPAROFFICIAL

                                    MOPAROFFICIAL Well-Known Member

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                                    Spend a little outside the box you're in if need be. You have a few combo in that which will be fun to drive. I wouldn't take any block and just bore it .060 unless that's all that was left in it or it was a class limit. I'd shelve those or could sell with the smaller cam and get another set of kb's ...maybe quenchies for the j heads and one of those cams. Post the specs again, int close etc. 107's and the AL heads would typical no brainer. Then what is the power goal
                                     
                                    Last edited: Oct 13, 2019
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                                    • Wyrmrider

                                      Wyrmrider Well-Known Member

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                                      400 M did great at enginemasters
                                      boss heads will appreciate the additional displacement
                                      either 400 or stroker
                                       
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                                      • oldkimmer

                                        oldkimmer FABO Gold Member FABO Gold Member

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                                        The 470 in my race car picked up a little on the low end when I put on header extensions. Kim
                                         
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                                        • yellow rose

                                          yellow rose Doctor of Thinkology.

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                                          I don't doubt that. Most collectors are way too short. That's why some guys add mufflers and go quicker. They needed the collector length. And some go quicker with exhaust because they have traction issues and the loss of HP from mufflers gets the car to hook.
                                           
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                                          • justinp61

                                            justinp61 Well-Known Member

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                                            Then I'd sell them to a Ford nut and buy a pile of Mopar parts. Those heads are pretty desirable.
                                             
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