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Intake manifold design discussion & facts thread

Discussion in 'The Hokey Ass Message Board' started by Florian, Oct 12, 2009.

  1. Florian
    Joined: Jul 29, 2007
    Posts: 219

    Florian
    Member

    So recently I came up with the idea of building my own intake.

    I found some threads with pictures of homemade intakes and such, even an article out of an old magazine, but no thread that discussed or pointed out the most important facts that you have to consider when trying to build a well working intake.

    I´m not an expert in intake design, not at all...but I would be interested in what some of the more experienced guys say on this topic.

    From what I found out, the most important thing to keep in mind is to always get a nice even flow. But what´s the best way to accomplish this? How does the length of the runners affect performance? What changes with the size of the plenum? What about angles, curves etc.? Are there any rules of the thumb in intake design?

    I´ve got a lot more questions...but I think this is enough for now haha

    We could write this all together, maybe we can get enough good advice together to provide a good source of knowledge for everyone who is interested in making his own intake!

    Thanks,

    Florian
     
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 55,931

    squirrel
    Member

    long, skinny runners and small plenum = low rpm

    short fat runners and big plenum = high rpm

    Did you ever notice that the bottom of most intake plenums are level? and many of them have ribs to help vaporize fuel
     
  3. kwoodyh
    Joined: Apr 11, 2006
    Posts: 641

    kwoodyh
    Member

    I wish it was that simple, How about the cross ram on the 413 not many runners longer than that, not a low rpm motor either?:D
     
  4. kwoodyh
    Joined: Apr 11, 2006
    Posts: 641

    kwoodyh
    Member

    You running an odd ball engine and an intake isn't available? A lot of R&D time and $, $ and time has already been invested even in a stock low rise cast iron intake, you would be hard pressed to match what is already available?
     

  5. squirrel
    Joined: Sep 23, 2004
    Posts: 55,931

    squirrel
    Member

    The special racing version of the intake had the runner divider removed for part of the length....raise the rpm band compared to the stocker, which was tuned for low rpm highway driving, it was a luxury car motor for the most part. gobs of midrange torque
     
  6. budd
    Joined: Oct 31, 2006
    Posts: 3,478

    budd
    Member

    a good example of long skinny runners is a chevy TPI intake, at low RPM's they pull very well, get them up to 4500 and they run out of breath, install a set of aftermarket tubes that are open inbetween and your rpm range goes way up.
     
  7. Florian
    Joined: Jul 29, 2007
    Posts: 219

    Florian
    Member

    Well, not really:rolleyes:

    I just bought a 1955 265cui SBC. It´s not an odd ball engine, at least not intake wise...

    But I still think that it would be cool to build an intake for it. I must admit that one aspect is the look of course, performance being important too, but the thing, I have to say, that I like the most about this idea is that it kind of expresses the whole hot rod spirit. You know, you don´t take an existing part, you build it yourself from cheap parts and try to improve it and learn a lot about it that way. And even if you screw up, you still had a good time and learned something.

    That was also another question I had, why don´t you see homemade intakes on SBCs as often as on Hemis for example? I know it can be done, I have seen some.
     
    LeoH likes this.
  8. 413 is a low RPM motor 6000 to 6100 will et best .
    length of runners from the intake valve to the plenum floor rule of thumb is 90 divided RPM in thousands (IE 6000 =6 , 5000 = 5 tec) .
    Ribs are actually to stop puddling . (so the puddles of accumulated fuel which happens now and then unfortunately don't run down the nearest port smothering that cylinder. )
    Best rule of thumb for runner size is it should be about the same size as the intake valve. I have build a few . Good results. I wouldnt hesitate to build another one. Biggest deal is dont get in a rush and remember the whole thing will shrink when it is all welded so allow a bit extra length to allow for that. Weld only a small amount at a time. Maybe tiny amount is a better way to say it. Dont get in a rush.
    Don
    Here is the last one I did on my Sr Dragster. There is another on my website on a BB Dodge
     

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  9. budd
    Joined: Oct 31, 2006
    Posts: 3,478

    budd
    Member

    to make a homemade intake for a sbc chevy means you have to make the lifter valley cover as part of your intake, yes anything can be done but its more work and there are 10,000,000 intakes for SBC's allready, i have an old single plane SBC intake with a chunk out of it, i have looked at it a few times thinking what can i build out of it.


     
  10. easyrider47
    Joined: May 7, 2004
    Posts: 670

    easyrider47
    Member

    I want to build an intake for my banger to run a Holley 94. When I make the runners, shoud I stagger them so each one is under a bottom port in the carb base or just use an open plate with the runners together? Staggered would seperate the fuel distribution more equally i would think. Any suggestions?
     
  11. fordrat31
    Joined: Oct 3, 2009
    Posts: 380

    fordrat31
    Member
    from Palmer, MA

    If you build an intake keep the carbs low to the engine. Dont make it so the carb is two feet from the motor. The longer the fuel has the travel before it reaches the motor the leaner it will be. I am no expert but this is what I would assume.
     
  12. strombergs97
    Joined: May 22, 2006
    Posts: 1,888

    strombergs97
    Member
    from California

    This intake for a SBC was very complicated to build, it worked great but lots of liquid leaks, no vacumn leaks..This log type intake need to run all the carb at sametime..
    You can build it, have fun doing it..
    Duane.
     

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  13. Very nice, Stromberg
     
  14. squirrel
    Joined: Sep 23, 2004
    Posts: 55,931

    squirrel
    Member

    Think about the normal 4 degree tilt of most engines, too....
     
  15. I'd recommend Pipemax to anyone curious about this stuff. It doesn't explain every detail like a tutorial, rather by changing inputs you can see the effects of harmonics and other issues. It's a fun, inexpensive, and useful toy for a performance junkie.
     
  16. von Dyck
    Joined: Apr 12, 2007
    Posts: 678

    von Dyck
    Member

    "Mixture Formation in Spark-Ignition Engines" by H.P.Lenz available from Society of Automotive Engineers should be required study to fully understand wet air flow dynamics of an engine. It is heavy, in-depth reading and SAE is not cheap. Consider the cost as tuition on your learning curve. (400 pages)
     
  17. RichFox
    Joined: Dec 3, 2006
    Posts: 10,020

    RichFox
    Member Emeritus

    Have you guys seen the later model Hilborn injectors for the SBC? Three piece with a seperate valley cover and thick rubber seals to the intake. Helps out with the large deviation of deck hight on all the aftermarket SBCs these days. Good idea for a carb intake too. Makes it eaiser.
     
  18. Florian
    Joined: Jul 29, 2007
    Posts: 219

    Florian
    Member

    Sounds interesting...have you got pictures?

    Another question to the guys that already "dared" to fabricate an intake for a SBC:

    How did you weld the runners to the plate on the port openings with them being so near together? On a Hemi the port openings are far away from each other so that you can weld around each runner very well, but on a SBC they´re almost directly next to each other....hope you know what I mean...:rolleyes:
     
  19. Florian
    Joined: Jul 29, 2007
    Posts: 219

    Florian
    Member

    According to this formula, I´d get 20.45 inches for my engine witch desired 4400rpm?! 20 inches?! One runner?
     
  20. RichFox
    Joined: Dec 3, 2006
    Posts: 10,020

    RichFox
    Member Emeritus

     
  21. corsair
    Joined: May 16, 2009
    Posts: 287

    corsair
    Member

    Let me preface the following by saying my knowledge is almost all based on EFI, which is mostly dry air manifolds. However, if that formula is what I think it is, it's used to figure out the right length to produce resonance at the desired rpm. If you get it right, you'll get a minor ram effect in a 1000-1500rpm window. 20 inches isn't that otherworldly in EFI land as you can bundle the runners around kind of like a header. Low points don't matter when you are just running air.

    To see a particularly odd example, that mess below is the stock arrangement on a BMW m5. All those tubes are tuned length, and live inside a giant air box. In this case you are essentially eliminating the plenum and using only tuned runners. The runner length includes the throttle body.

    Now with a carb, you could include the carb length on an individual runner setup (like webers), but you'd really muddy the waters with a 2bbl or 4bbl that needs to feed several cylinders.
     

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  22. Sounds about right if you want ram induction that works for that RPM.
     
  23. oj
    Joined: Jul 27, 2008
    Posts: 6,454

    oj
    Member

    Jesus, the BMW mess looked like it was designed by an apprentice ratrodder, on a bad day.
     
  24. Florian
    Joined: Jul 29, 2007
    Posts: 219

    Florian
    Member

    Okay, now that looks more like 20 inchish...:p

    So you say this formula is used for ram-style intakes. If you look at it you can see:

    The longer the runners, the lower the rpm.
    The shorter the runners, the higher the rpm.

    So can this be generally said?
     
  25. Florian
    Joined: Jul 29, 2007
    Posts: 219

    Florian
    Member


    haha now that´s funny.

    I´m from Munich, Germany, the home town of BMW. I know quite some BMW engineers. For what you described as a rat rodder´s bad day, BMW had likely a gazillion of highly-paid engineers working on for a whole year :D

    haha

    I don´t understand those BMW guys either...
     
  26. corsair
    Joined: May 16, 2009
    Posts: 287

    corsair
    Member

    Generally yes, although plenum volume plays a role too. Not to keep throwing o/t pics around, but this illustrates the point. The manifold on the left is a Honda piece from a CRV designed for more low end torque than the Integra piece on the right. The sport models use even shorter runners and a larger plenum designed to zing up to 8000rpm.

    And yes, that bmw system is a bit of a clusterf%&!. That's why they keep it hidden away under a fancy pants air box. Sounds like a million bucks at full wail though. Like the bastard son of a SBC and a motorcycle :D
     

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  27. theHIGHLANDER
    Joined: Jun 3, 2005
    Posts: 10,244

    theHIGHLANDER
    ALLIANCE MEMBER

    Runner length and plenum volume need to work together to achieve the end result. A super long runner (20") will cause a stand off in the flow so the plenum makes up for the add'l volume/length. That's a grossly over-simplified statement before those with more knowledge start throwing stones. The plenum works with all of the runners to balance the intake pulses and such vs having a pipe organ under the hood. Looking at fuel injection stacks could lead one to believe otherwise, but it's injection not an aerosol can (as in a carburetor).
     
  28. corsair
    Joined: May 16, 2009
    Posts: 287

    corsair
    Member

    The interplay between the runner and the plenum is a pretty complex bag of equations. I've taken more than my fair share of fluid dynamics classes, and I still get a headache looking at the formulas people come up with. It's all kind of ballpark anyway, as most tuners I've seen do thier final work by trial and error, playing with different aspects of the design before finishing it up.

    Now that said, typically the plenum isn't what's used to get low end torque without 20" long runners. The sort of exception is instances like the BMW unit above where the runners extend into the plenum and are capped with an airhorn. In those cases, the "plenum" is actually just an airbox designed to serve as a cold air intake rather than part of the resonant structure. I've seen some manifolds that blend the plenum/airbox distinction by extending the runner about an inch into a "tuned" plenum capped with a single t/b. Different tuners have different thoughts on this.

    If you want to gain the low end of a long runner but don't have room under the hood, the usual answer is to taper down the cross sectional size of the runner. That (within reason) will deliver the high velocity flow you want with a shorter length. The trade off is that the motor will be very wheezy on the top end. If you've ever seen the super tall manifolds ford uses for the 351W with efi on the trucks, you'll see OEMs are not afraid of "pipe organs" under the hood ;)
     
  29. '52 F-3
    Joined: Sep 30, 2007
    Posts: 912

    '52 F-3
    Member

    I believe that heating the bottom of the intake can be very important, it'll help keep the fuel atomized keeping the fuel/air more even between cylinders.

    i made this 3 x 1 barrel carb for a ford inline-6, it's not the greatest performance setup, but does idle very smooth and runs good.

    fyi, i heat the bottom with engine coolant.

    [​IMG]
    [​IMG]
     

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