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Mopar flathead six on the dyno PLEASE HELP!

Discussion in 'The Hokey Ass Message Board' started by moparsled, Sep 30, 2008.

  1. man my flat head revs to 4500 then starts to stumble also thats on the free way going about 100 mph it acts kinda like its floating the valves that could be what is happening to yours remeber that ting stock red lines at about 3800-4000 rpm
     
  2. Shifty Shifterton
    Joined: Oct 1, 2006
    Posts: 4,964

    Shifty Shifterton
    Member

    Wow that thing is serious for a mopar flattie. Saw the thread title and thought "where'd he find a dyno that read in inch pounds" but you've really stepped on it.
    Good luck chasing the rev problem and low ET, you've added another follower to the progress thread
     
  3. CNC-Dude
    Joined: Nov 23, 2007
    Posts: 1,031

    CNC-Dude
    Member

    Ouch, that was cold! Im sure you were kiddin'...
     

  4. Shifty Shifterton
    Joined: Oct 1, 2006
    Posts: 4,964

    Shifty Shifterton
    Member

    Totally kidding.

    Seriously though, you don't often hear mopar flathead six, and dyno, in the same sentence. Caught me off guard.
     
  5. CJ Steak
    Joined: Sep 23, 2008
    Posts: 1,377

    CJ Steak
    Member
    from Texas


    WOW. I would LOVE to do something similar (probably not as radical) with the 226 Super Hurricane in my Willys pickup.

    Your miss @ 4750 is screaming ignition to me... but the other thread pretty much covered everything. I would say have someone else go through the ignition while you step outta the room. Sounds stupid... but i'll check something over and over and OVER... then call my dad... he comes over and notices something simple (or sometimes really obscure) and it's back running again.

    Other than ignition... maybe it's a valve train issue? 4750 is pretty low to be floating a valve... but at that RPM maybe one isn't adjusted right, or you've got a weak spring. I've had that happen to me in my built to the hilt Triumph TR6. I would hate to see you tear down that engine again though!

    Thanks for beefing up your engine. It really motivates us flatty 6 guys to follow suit. I know i've learned a lot from your posts. Your engine have 7 mains, or 4? :)
     
  6. RichFox
    Joined: Dec 3, 2006
    Posts: 10,020

    RichFox
    Member Emeritus

    I replied to the other thread. Just from running my Plymouth flathead four, I would back the timing down some and see what happens. If it's good, back it some more. If it's bad put it back where it was.
     
  7. thanks for the props guys. This thing is getting REALLY frustrating.

    "Inch Pounds" that's funny a hell!!! Almost lost my coffee on that one!

    Four main bearings.
     
  8. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    Re: "We will find that a fundamental limitation on the maximum gas flow is the occurrence of sonic flow at the valve aperture, and we will be able to relate this to the average piston speed"

    This is based largely on an OHV chamber, where maximum valve size is always a function of bore diameter. In a sidevalve engine this doesn't apply.

    I agree - more point spring tension.
    Also: longer primaries, much longer collector.
    Also: I can't see how 2 1 bbl. are going to make it. You need about 2 × 1.50" venturis for 5,000 RPM and 70% VE @ 1.5 Hg" vacuum (how big are they?), plus enough manifold volume to buffer the intake vacuum cycles. Try big spacers under both carbs.

    Future: bigger intake valves (1-5/8" now?), minimal stem-to-head radius like 3/8", 5/16" stem in the bowl, 30° seat. An 80" H-D engine (same cylinder size) can use 1-15/16" valves. I don't know how big you can go before too close to the exhaust seat, but easily 1-13/16". A very useful cheap valve is the V2275 replacement for the Chevy 235, .340" stem, 1-7/8", good head shape, 30° seat.
     
  9. I posted to the other thread, but I'll post here too, that the engine isn't running out of steam at 4750. It develops a miss, missfire, stutter, whatever you want to call it. We are the ones backing out of the throttle when the miss occurs. The engine wants to keep going.

    we are going to try a 4bbl tomorrow, on an adapter hat. If the ignition doesn't get it, and the carb doesn't get it, I have to think there is a mechanical issue.
     
  10. TraderJack
    Joined: Apr 10, 2008
    Posts: 330

    TraderJack
    Member

    You might try colder plugs in it!

    Traderjack
     
  11. moose
    Joined: Jan 11, 2005
    Posts: 353

    moose
    Member

    It would seem to me that the flathead would be more limited by this sonic flow condition that I made note of on the other page. In an overhead valve chamber, the whole circumference of the valve can be effectively used, whereas on a flathead the half of the valve that is pocketed by the head, would in effect restrict the flow introducing a sonic flow condition sooner.

    I agree with moparsled, something else is happening at 4750 though. You are right Panic, probably weak springs somewhere. Good info on the bigger valves too. I think there's the need for more carburetion too. Longer runners are usually better for low end torque, but a quick look at the manifold tuning chapter says that around 18" of 1.5" intake runner would be tuned for 6000RPM.

    moparsled: do you have any manifold pressure readings at those RPM's? I'd really like to try some more of the formulas in my old textbook.

     
  12. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    Yes, extending the manifold away from the head (4 huge spacers) would have to be impractically long to actually tune, but it would also add volume and inertia fill, which typically flathead cams don't do very well due to conservative (early) IVC to conserve cylinder pressure.
    IMHO that's nowhere enough cam, but probably not the time to add new stuff. A H-D engine (yes, different bore/stroke but similar size per cylinder, lower compression) would be using 280-290° nominal duration but at least 250° # .050". My mild 1937 82" (3-7/16" × 4-3/8") engine has 294° @ .010", 270° # .050" .377" intake and 278° @ .010", 256° # .050" .407" exhaust. Springs: 100 lbs. closed, 210 lbs. full lift. More notes on flathead cam selection from my site: http://victorylibrary.com/kns/camnotes.htm

    I hate to say cut up your exhaust, but since your primaries are parallel you could use male/female stubs and add a big piece (like 12") to all primaries and just move the collector back. Yes, there might be a space problem; hint: the collector doesn't have to go straight back, a big radius turn-down works fine.

    My guess at required CFM is based on the minimum vacuum level used for 4 bbl. carbs. If the carbs are smaller they don't stop flowing, they just use more power pumping the air in. You could use even bigger carbs, about 1.61" venturi, and get perhaps 4-5 hp by going down to perhaps 1.0 Hg" but your response may suffer.
    You can also add some CFM to the existing carbs with 2 radiused velocity stacks inside a big air box, but the right shape is crucial. The old-school scoops, etc. are worse than useless, science has progressed a lot on this. Important features:
    1. the stack ID ends smoothly into the air-horn surface (no steps)
    2. very big radius at the top, at least 3/8" minimum
    3. height doesn't matter
    4. the radius has to extend beyond the carbs and "turn over":
     

    Attached Files:

  13. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    Ideal intake valve shape (made to my drawing by Donovan Engineering - excellent but $$):
     

    Attached Files:

  14. moose
    Joined: Jan 11, 2005
    Posts: 353

    moose
    Member

    I had a chance to do some calculations: if you can keep the same bmep(cylinder pressure) to 6000 RPM this thing would be making 210HP.

    Since you hadn't done much tuning yet, I think you should be able to bring the bmep up enough to make 200HP by the time you get it to 52-5500 RPM range.

    Stay with it!
     
  15. GOT IT!

    changed to racing plugs-Autolite AR73-and another (third) set of plug wires, no more miss.

    real, honest to gosh tuning starts tomorrow.

    on the subject of the carbs- These are two 1948 Plymouth carbs. There are B&B's with bigger dia. venturis and throttle bores, and they are certainly on my list. I spoke with George Asche about this, he uses "truck carbs", says they flow more than 97's.






    HUGE thanks to all, for the advice, the props, everything. The HAMB truly is the best.
     
    Last edited: Oct 2, 2008
  16. BarryA
    Joined: Apr 22, 2007
    Posts: 643

    BarryA
    Member

    Great News!! I've been watching this for a while (with nothing to contribute)
    Really looking forward to seeing what you can get out of it!
    Barry
     
  17. TraderJack
    Joined: Apr 10, 2008
    Posts: 330

    TraderJack
    Member

    As an old Autolie Field Rep, I approve.

    Traderjack
     
  18. Don Coatney
    Joined: Aug 11, 2002
    Posts: 41

    Don Coatney
    Member

    George Asche is the B&B man. I took these pictures on my last visit to his shop.

    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
     
  19. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    Those appear to be Carter BBR-2 "Special Heavy Duty" Series used 1939–60, with a letter/number combination stamped on side of airhorn vane.
    If yes, the throttle is 1-11/16" (1.688"), the venturi is 1-11/32" (1.344"), flow is about 150 CFM each.
    This is going to restrict peak power, not sure how much. Unfortunately, these early carbs were made to SAE standard sizes, and the next size larger carb very likely has a different bolt pattern.
    What's the bolt pattern between centers?
     
  20. Hudsonator
    Joined: Jun 19, 2005
    Posts: 335

    Hudsonator
    Member
    from Tennessee

    Obviously, this is the same Panic of Victory Library fame.

    Lots of respect for this guy. I didn't know he kept B&B info in his catalog of knowledge.

    How much does the 1.5" throttle bore carbs flow? I too would be interested in manifold pressure numbers, I hope Mr Mopar gets around to that whilst tuning. We're hoping the best for him this weekend, no doubt he's tuning his posterior off.

    Hud
     
  21. posted to my build thread, but, here it is.

    we had it close- kinda ok, maybe a bit better than ok.-- pulled the exhaust pipe off, and the thing went to SHIT. way too rich all over again.

    For the B&B curious, here were my #'s, measured in drill sizes, that were "ok" before removing the exhaust pipe.
    main jet #54 (.0550") main air bleed #61 (.0390")
    idle emulsion tube #71 idle air bleed #54 (.0550")
    step up jet #65 air horn bleed # (.0670")
    two coils cut from the step up springs.

    Monday we'll try it again without the step up springs. Right now, this thing needs either more airflow in, or less out.
     
  22. Keep plugggin, Dont get frustrated. As far Mopar sixes. Chrysler Cwrons with dual carbs have been doing a wonderful job powering wooden launches for years. Have built some for customers a few years ago. Crane even ground me a hot cam once.
    Removing the pipe should lean it out not richen it in most cases.
     
  23. 4woody
    Joined: Sep 4, 2002
    Posts: 2,110

    4woody
    Member

    I just found this thread.
    Anything new happen since the 4th?
     
  24. rustydeuce
    Joined: May 26, 2006
    Posts: 855

    rustydeuce
    Member

    I have a buddy looking for a twin/triple carb intake manifold for a Mopar Flattie six if anyone knows of or has one for sale. :)
    Cheers
    Dale
     
  25. moose
    Joined: Jan 11, 2005
    Posts: 353

    moose
    Member

    I might be able to help him out...
     
  26. rustydeuce
    Joined: May 26, 2006
    Posts: 855

    rustydeuce
    Member

    Will email you via your website Moose.
    Cheers
    Dale
     
  27. no bux rod
    Joined: Mar 26, 2007
    Posts: 123

    no bux rod
    Member

    Moparsled, Carburetor tuning makes tuning a modern fuel injection system via computer look easy.

    Keep pushing, we want to see that 200 HP number soon.

    N B R
     
  28. Hudsonator
    Joined: Jun 19, 2005
    Posts: 335

    Hudsonator
    Member
    from Tennessee

    Has a Year gone by already? Sheesh.

    How are things going Moparsled? Any updates lately.

    Hud
     
  29. a quick summary of the last year-

    The engine came off the dyno last october, but I didn't finish paying the bill, and bring the thing home, until March.

    From March until July I focused on getting the car built, didn't mess with the engine at all.

    In June or so, I sent my carbs out, they returned with fully tuneable circuits all the way through, (special thanks to a HAMBer who shall remain nameless unless he gives himself up) I'm pretty excited to run them, but haven't yet, as I try to work out other issues with the car. The fast idle cams on these carbs ride on a little post cast into the carb base, and with them removed, the carbs would over center and stick at WOT. The simple solution was to drill out two 1/2" nuts and locktite them to the posts, with the WOT stop pressed up against the flat of the nut. This allows the throttle blade to stand up perfectly straight, but not over center. Next thing I ran into was that the linkage I had made was pulling wrong- it would rotate over and then try to pull horizontally rather than in an arc, so the carbs would never reach full throttle when hooked to the linkage. I redesigned the linkage, started to make it, and this is where I am stopped.

    I need to finish the linkage, mount the gas tank, finish the fuel line, and rewire the distributor for it to fire again.

    here's a link to the chassis build:
    http://www.jalopyjournal.com/forum/showthread.php?t=346803
     

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