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Technical Plenum size for Flathead blower intake

Discussion in 'Traditional Hot Rods' started by revkev6, Dec 7, 2017.

  1. revkev6
    Joined: Jun 13, 2006
    Posts: 3,193

    revkev6
    Member
    from ma

    hey guys, thinking of making my own blower intake for a flathead. I have a few ways that I am thinking of doing this in my head. My blower is an eaton (cheap) that has a farely small mount surface. I Could modify a four barrel manifold to work fairly easily. the single plane unit that speedway made/makes (may be discontinued??) looks like it was made to be a blower manifold, not a flathead four barrel!! they want a lot of money for them IMO..... but notice there is no divider in the middle?? ask anyone how well their flathead four barrel manifold worked when they milled out the divider!
    [​IMG]

    option two is making my own out of steel like below but thats a lot of plenum! [​IMG]

    or something with a much smaller plenum volume like the navarro. the difference being the runners come in to the plenum from the front and back instead of the sides/underneath.
    [​IMG]

    this makes the real question how much does plenum volume matter on a supercharged flathead?? is it better to keep it small?? I know there are calculations that can be done on an NA motor to optimize plenum and runners but I'm not sure that any of this applies when I bolt on a huffer!
     
    Runnin shine and bobo1 like this.
  2. Flame me for not responding about a supercharged flathead, but my friend made a plenum for his FI big block.. and the rule of thumb he used was plenum volume equals engine displacement volume.

    Been running pretty damn good for 20 years that way.
     
    Runnin shine likes this.
  3. CNC-Dude
    Joined: Nov 23, 2007
    Posts: 673

    CNC-Dude
    Member

    1 cubic inch of plenum volume per cubic inch of engine volume is a good rule of thumb. Custom intakes for forced induction applications are often 1-1/2, 2 and 3 cubic inch of plenum volume per engine volume, depending on how much boost and power output is required. For your Flatty, between 1 to 1-1/2 times the engine volume would be a good starting point for your plenum size. I have an EFI intake for the Chevy inlines designed specifically for forced induction in mind and I have it at 2 times the engine displacement and it works great for those wanting higher boost levels than normal.
     
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  4. Bored&Stroked
    Joined: Jan 14, 2005
    Posts: 3,948

    Bored&Stroked
    ALLIANCE MEMBER

    I'm having a hard time imagining a plenum for a 276 cubic inch street flathead Ford (given the port spacing) having say 276 - 414 cubic inches of volume. That would require an extremely tall box. Also, while having a larger plenum makes sense for higher boost full-out racing (as once the plenum is filled under boost pressure, then it has enough capacity to dump into the cylinders), it may not make a lot of sense for lower-boost street applications . . . due to a lag associated with filling the plenum and velocity issues under partial throttle and low boost (which you'll have the majority of time on the street). Given the size of your blower, I'd think that something closer to the Navarro design would work just fine. If you make taller/longer port runners - that should help with low-speed torque (based on better velocity and associated atomization). Just thinking out loud here . . .
     
    Last edited: Dec 7, 2017
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  5. Bored&Stroked
    Joined: Jan 14, 2005
    Posts: 3,948

    Bored&Stroked
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    For a medium to large cube motor and higher boost levels (say 10+ lbs), having a bottom like the one that Tom Roberts designed might work great. I contributed initial funding to help this project get under way - but have yet to put my manifold on an engine. Another 'future project'! Arggggh!

    This is the same manifold bottom that Magnusen then used. BUT - it is going to make an extremely tall package and I believe would work best with a big engine and a big blower (on the street). Notice how the runners for a sort of 'cross ram' effect at the center of the plenum - this makes a lot of sense to me (for port velocity).

    BlowerManifold1.jpg BlowerManifold3.jpg
     
  6. Bored&Stroked
    Joined: Jan 14, 2005
    Posts: 3,948

    Bored&Stroked
    ALLIANCE MEMBER

    Another thing to consider is the type of outlet and where should it be in reference to the plenum. With modern 'delta outlet' blowers, one should set the blower back to move the outlet toward the center of the plenum. While this works great with 14-71 blowers and top-alcohol or top-fuel, it makes for some fun with blower drives. This would probably work the best with the blower you have - but would require a customized drive.
     
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  7. CNC-Dude
    Joined: Nov 23, 2007
    Posts: 673

    CNC-Dude
    Member

    Here is a sample plenum I put together in Solidworks right quick to see what type of volume it requires for the size constraints of the engine bay with LxWxH just to see. The sample is 17" long, 6" wide and 4" tall and is 270 cubic inches in volume. A Flatty probably doesn't have the same air demands as an OHV engine since the airflow thru the ports is often much less in comparison. So a 1 cubic inch of plenum for 1 cubic inch of cylinder displacement is more than likely very sufficient for your needs in a street engine.

    plenum1.jpg plenum2.jpg
     
    Last edited: Dec 7, 2017
  8. Mike VV
    Joined: Sep 28, 2010
    Posts: 907

    Mike VV
    Member

    Using a carburetor...plenum size matters a lot.
    Not so much with a belt driven blower, most any size will work well.
    You need pay more attention to port entrance restriction than anything else.
    If I were building one...it would be the top one in post #5. Longer runners..! Though the second one in post #5 is also plenty good.

    Mike
     
  9. Here are a couple of Flathead blower intakes I make. One to suit a 4-71 blower and the other can have an adaptor plate bolted to the top so you can fit different types / size's of blowers on to it. IMG_0789.JPG IMG_0808.JPG P1070959.JPG P1070960.JPG
     
    Last edited: Dec 8, 2017
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  10. revkev6
    Joined: Jun 13, 2006
    Posts: 3,193

    revkev6
    Member
    from ma

    this is the unit I'm considering, its an M112 and the opening appears to be about 6" square:
    [​IMG]
    [​IMG]
    The motor will be street only. I'm not looking to run hard as the drive line is early ford. 3 speed and a banjo. I'm looking at around 5psi of boost in something around the 270-300 cube range... probably a big stroke with 3 5/16 bore.... 4 and 1/8 or 4 and 1/4 stroke... I want a nice SUUUPPERR flat torque curve that just pulls hard. this will be in a 32 btw.

    One of the reasons I am inquiring about the plenum size is I don't want to put a "race" setup onto a street motor and lose bottom end. I can make the plenum small with short/no runners very easily while keeping my entire assembly low but I think having a longer runner seems to be a good thing for a flathead?? the speedway four barrel looks perfect for long runner small plenum... but it is definitely smaller than even a stock flathead. If consensus is a full 1:1+ ratio of plenum to cubic inch is "good" even on a street motor I can design around that.

    if I get something that just matches up to the outlet size on the supercharger I think I can get a plenum in the 144 cubic inch range. outlet is about 6x6" so figure 4" tall =144. this would have to have the ports coming in from the front and back of the plenum. that's half the size of what your guys are recommending. bigger is much easier.

    Dale, I can see that manifold being ideal, especially in a bigger high rpm motor. larger tapered ports in the manifold. looks nice. If I were to make something like that I would put the ports straight up and down so I could make it lower and narrower without impeding flow. basically it would look like a SBF EFI intake with a box around it.
    like this, mustang on bottom, explorer on the top.
    [​IMG]
     
  11. earlymopar
    Joined: Feb 26, 2007
    Posts: 732

    earlymopar
    Member

    Correct me if I'm wrong but I recall one of the reason Tom's design was tall is that he left room for an intercooler as an option. - EM
     
  12. revkev6
    Joined: Jun 13, 2006
    Posts: 3,193

    revkev6
    Member
    from ma

    I've looked at his page, the inter-cooler is recessed into the top plate, above the lower manifold that Dale posted a picture of.
     
  13. revkev6
    Joined: Jun 13, 2006
    Posts: 3,193

    revkev6
    Member
    from ma

    here is the top plate that bolts on to the lower manifold that would house an intercooler:
     

    Attached Files:

  14. earlymopar
    Joined: Feb 26, 2007
    Posts: 732

    earlymopar
    Member

    Yes, this is how I remembered it. - EM
     
  15. Bored&Stroked
    Joined: Jan 14, 2005
    Posts: 3,948

    Bored&Stroked
    ALLIANCE MEMBER

    Tom hadn't yet designed the intercooler top - which would surely be good if you were planning on any real boost - at 5 lbs, really not necessary. I may have to get in touch with him and see if he still has that top available - would be the right way to go for sure.
     
    paintslinger805 likes this.
  16. Wow that's awesome. Looks like I need one of those. I really like the design of the inside. Big ports at what is that about 45 degree straight to the valves. I think the great Mr. Navarro once said that he got the most power from 45 degree runners. I could be wrong though.
     
  17. sunbeam
    Joined: Oct 22, 2010
    Posts: 3,048

    sunbeam
    Member

    In looking at the supercharger where is the inlet? I always thought a Ford lighting unit would be the easiest to use.
     
    Last edited: Dec 10, 2017
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  18. A quick google search shows its on the back, opposite the drive snout

    Full Kustom, drunk mobile posting
     
  19. revkev6
    Joined: Jun 13, 2006
    Posts: 3,193

    revkev6
    Member
    from ma

    Like stated above it's rear inlet but you can mill put the rear upper portion. I plan to mill the longitudinal webs flatter and tap them to bolt a plate on for 2-3 97's.

    I had originally thought of the.lightning unit as it would be easier. I think the jag unit will look a little better when I finish.

    Sent from my SM-G920V using The H.A.M.B. mobile app
     
  20. did this to my 304 and am happy with it......... used a 4x2 manifold....... easy and quick welded manifold comp 001.jpg blower mani 9 14 001.jpg
    welded manifold comp 001.jpg
     
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  21. revkev6
    Joined: Jun 13, 2006
    Posts: 3,193

    revkev6
    Member
    from ma

    Ive seen 2 3 and fours all converted similar to that. What supercharger are you using?

    Sent from my SM-G920V using The H.A.M.B. mobile app
     
  22. trey32
    Joined: Jul 27, 2014
    Posts: 184

    trey32

    Damn Carl, stack them dimes!!!
     
  23. revkev6
    Joined: Jun 13, 2006
    Posts: 3,193

    revkev6
    Member
    from ma

    OHHH you have it on the grocery getter :D

    badass carl! didn't put two and two together. you ever get a set of leakdown gages for your motor?? they are really a requirement when you build an injected motor. all gages read a bit different and you don't want to be borrowing two different gages and setup your mixture different each time. plus, they are handy to have. I use mine to do leakdown tests on the motor too. been 15+ years since I messed with injection though. my setup was an autocraft VW midget engine running methanol. Had to completely drain and clean the motor out every week so the methanol didn't eat the case :confused:
     
  24. yep gonna order a set for xmas..........:D
     
  25. revkev6
    Joined: Jun 13, 2006
    Posts: 3,193

    revkev6
    Member
    from ma

    ok, I have been doing a bit of reading on intake design and it seems that the common theme is that manifold design really doesn't change much when you add a supercharger. All of the design criteria still applies. runner length and plenum size all work similar but are not as critical as they would be in a standard carbed manifold because the more pressure you have the less the tuned pressure wave can pack in. I like most would run low boost <5psi on a street motor.

    the rough math is below, from the superflow dyno operators manual. the earlier you can start catching the pressure waves the more they can help through the RPM range. the lower the number, the longer the runner has to be to use it. generally the 2nd pressure wave is the lowest practical. each wave is lower strength and shorter than the previous. again the numbers below are the rough and dirty "fudged for general engines" calculations.
    2nd wave = 132,000/RPM = length
    3rd wave = 97,000/RPM = length
    4th wave = 74,000/rpm = length

    so, if you want to setup a flathead that peaks out at 4500rpm
    132,000/4500 = 29 1/3"
    so the runner from the back of the valve to the plenum opening would need to be about 29" which is excessive! trying for the 3rd wave drops us down to 21.5"

    so, on to a discussion of flathead intake design. lets just go to types first.
    stock ford manifold, 1 2bbl, 180 degree dual plane. this intake has longish runners to a small plenum area just large enough to open the ports into the carb.

    four barrel intakes for flatheads, All I have seen are dual plane except the speedway unit. by their nature they have similar runner length to the factory manifold. I have heard of guys milling the divider out of four barrel intakes and they don't work well. I don't think i have ever heard of a four barrel manifold designed from the start as an open plenum single plane.

    2x2 intakes. most dual carb intakes are 180 degree manifolds like the stock unit, but due to the nature of having two carbs the runners are shorter and the plenum area is larger. even a tall manifold like the thickstun or eddie myer which are both dual plane have a plenum below the risers. this plenum wipes out the runner pulse.

    3 and 4 2bbl intakes, I have never actually held one in my hand but I have read that these are mostly an open plenum, single plane design? for the designed purpose of racing and big street engines I don't seem to hear many complaints on performance??

    blower manifolds, most of these have a much larger plenum than any carbed manifold and very short runners. the runners I have seen very from 1-4 inches depending on the manufacturer. oddly enough it looks like the shortest and the longest are both on the "munro" unit. the inner ports are a straight shot from the side/below while the outer ports come in from the ends.

    i plan to try designing something with as close to optimum runner length as possible for the RPM range of the motor.

    is there a flathead intake that has applied runner lengths into the design?? just eyeballing manifolds is really tough to tell. from what I have read you have to isolate the runners individually the entire distance to the plenum or you destroy the pulse. so, having a dual carb manifold will substantially shorten the runner distance of a single. even more so when it's a "super dual". triple and 4 carbs would be very short.

    final question would be... is any of this even worth going after... blown or unblown?? I don't know because I don't think it's been done.
    @Bored&Stroked
     
  26. Vanness
    Joined: Aug 5, 2017
    Posts: 197

    Vanness
    Member

  27. CNC-Dude
    Joined: Nov 23, 2007
    Posts: 673

    CNC-Dude
    Member

    The problem with many of the intakes available is that they are mainly design styles copied from older traditional intakes and don't incorporate modern thinking or design elements in them that we know are important today. The intake that Bored&Stored showed above is a good example of modern thinking and design. Now, saying all of that to say this, they seemed to still work satisfactorily back in the day and in current times, so are those elements really relevant to the design of the intake. So when you compare what we know in today's world with all our technology and formulas we have found to be better in our modern thinking, many aspects of the vintage parts market should need a complete rehab if that is the case. It would be interesting to compare an intake designed with today's newer thinking design elements added to them to see what gains or improvements are experienced over these older designs just for conversations sake. Look forward to seeing what you come up with!
     
    Last edited: Dec 22, 2017
  28. revkev6
    Joined: Jun 13, 2006
    Posts: 3,193

    revkev6
    Member
    from ma

    @CNC-Dude my current thinking for a manifold would be something along the lines of a hilborn injection unit. make a lower port runner for a base and make an upper plenum. use tubing for a riser in between so that the length can be tuned. this would be geared more towards a normally aspirated intake but if designed correctly I think the lower could be the base for a blower unit as well.
     
  29. A low blower intake is fine...but it restricts you to 49-53 water inlet heads. I adapted a 2-two manifold
    to an aluminum top plate "breadbox" ....and stuck the 4-71 on that...simply to retain the center
    outlet early heads.
    I never worried much about what pipe-smokers said about design... and figured tire-smoking was
    more fun than anything. My engine idles at 600 rpm and there is NO lag when I stab it.... so my
    long distance between carb and cylinder is not an issue for me.
     

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