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Technical Ideal Flathead Combustion Chamber?

Discussion in 'Traditional Hot Rods' started by HuskerNation, Jul 27, 2021.

  1. HuskerNation
    Joined: Dec 28, 2010
    Posts: 337

    HuskerNation

    As I think about pulling the head on my 1937 L head straight 6 engine to swap in a performance cam & larger intake valves, I come back to wondering what is the ideal flathead chamber shape?

    I have been told by a number of reliable sources that getting the correct “squish or quench” height is of great importance to an L Head Flathead, if not the most important thing along with a compression increase. Unfortunately there are a number of things one can & can not improve upon too much with a stock cast iron head. So what are the things one should focus on & try to tweak with a Flathead combustion chamber? Is there really an ideal shape to the chamber or areas of the chamber that are really the critical areas?


    A little background: Around 2 years ago on this engine I did some things to try to bump performance around the combustion chamber side of things. 1) I found an oem cast iron replacement head which bumped compression from 6.25 to 8.5:1, 2) I polished the chamber, 3) unshroud the spark plug, 4) took off areas of the head that were overhanging the head gasket & could create hot spots, 5) to my best guess I tried to open up the transition area, or what I thought was the transition area. 6) I had a solid copper head gasket made to achieve the ideal squish/quench height but I couldn’t get it to seal & a bunch of water got into the cylinders. Thus I abandoned that head gasket & used an old NOS FelPro as there are no modern gaskets made.
     
  2. 6sally6
    Joined: Feb 16, 2014
    Posts: 2,459

    6sally6
    Member

    Since I have NEVER even seen the inside of an L-head 37 you should take this with a grain of salt but.....
    All the things you did were pretty standard hot rodding tricks.
    It also lowered your 8.5:1 CR a dab because of the metal you removed polishing/unshrouding/removing the overhang/etc.
    Why not cc the head and see what your CR is now after these mods you performed!
    A bigger cam and intake valves (along with the above mods) should move the HP curve up noticeably. Bigger carb and header with a hotter ignition should make it a screamer! (in 1937 era terms)
    How about a lightened flywheel?!
    6sally6
     
  3. FrozenMerc
    Joined: Sep 4, 2009
    Posts: 3,092

    FrozenMerc
    Member

    I think the Harley-Davidson KR is about the pinnacle of Flathead combustion chamber technology. HD exploited a loop hole in the flat track rule book that allowed flathead motors to run much higher displacements. HD was getting 50 to 57 hp (as much as 65 hp if you believe some of the rumors) out of a 45 ci V-Twin at 6:1 compression - naturally aspirated. Scale that power output ratio up to a 239 Ford V8 and you would get 290 Hp. Obviously an air cooled V-Twin had advantages for port configuration that would be difficult to replicate in a liquid cooled V8 or even inline 6 or 8 and create similar air flow protentional, but it is still pretty incredible to think about that kind of power to displacement ratio in a flathead.

    [​IMG]

    1958 HD KR
    [​IMG]

    [​IMG]
     
    Last edited: Jul 27, 2021
  4. Fordors
    Joined: Sep 22, 2016
    Posts: 5,372

    Fordors
    Member

    Do you know if the manufacturer of the copper gasket had annealed it prior to shipping? It not and you still have it you might try that, it can be done at home by heating it, even using a gas barbecue grill. Look for info online about this, and if you do anneal it be sure to scrub off the residue on it with solvent and a mild Scotch Brite pad.
     

  5. hotcoupe
    Joined: Oct 3, 2007
    Posts: 588

    hotcoupe
    Member

    Frozen Merc, thanx for posting the images of the N. O. S. (?) Harley Davidson KR cylinder heads,I have never seen one before. This subject is of interest to me, i'm currently re-building my '48 Indian (stroked to 80" w/pop-up pistons) and i've thought about incorporating some of the features of the KR cylinder head. Does the trough run from the intake valve to the plug, or the exhaust valve to the plug?
    Thanx,
    Tom
    Mods, if motorcycle talk is inappropriate, please delete.
     
  6. HuskerNation
    Joined: Dec 28, 2010
    Posts: 337

    HuskerNation

    24C2F297-30A2-489C-BBF8-9F0F8B04EE91.jpeg 455A450D-7353-4E3F-87B9-7429D5C91963.jpeg C42B82B2-D087-4486-87A4-5242EA36EDF3.jpeg 6Sally6, The compression ratio of 8.5:1 is an approximation after all the work that was done to the head. I did CC all the chambers & balanced their volumes. I had a friend use this information in his program to determine Compression Ratio. I agree this stock cam needs to be modified & Isky has a couple grinds they can do to it. Currently both intake & exhaust valves are 1&3/8” but I had determined a 1.5-1.6” looked like it would fit. In the end I did find a very nice Manley valve with all the goodies & a little less weight than the ones in the engine.

    FrozenMerc, I have heard of the HD head you mentioned & seen a few pictures as well. It’s pretty amazingly stuff they were doing & the power is really incredible! I’m hindered with completely vertical valves but I had thought about doing some kind of a cutback at the edge of the cylinder to possibly help with flow into the cylinder. Naturally it’s a balance with loosing compression as I take away more material. As I studied the KR head I’m assuming the recessed groove from the valve to the cylinder is the intake? If so it looks like they may have tried (or succeeded in creating a swirl type of a fill to the cylinder? Although that could also be wishful thinking….

    Fordors, yes I still have the gasket & it was supposed to have come annealed. However I would like to find a way to make it work or find another source that could make something that would work. I’ll be doing more research on that too.

    I’ll attach a couple pictures of the head as it was finished.
     
  7. HuskerNation
    Joined: Dec 28, 2010
    Posts: 337

    HuskerNation

    4B367010-88F1-45D2-8F21-848997B67A26.jpeg AA05B2F3-0661-499D-AE9E-0EE16E87CFF7.jpeg
    Here is the original head chamber vs the one I used, before I worked it over.
     
  8. @FrozenMerc is correct, the KR info is some of the best out there. One of my flathead books even has a full chapter on them
     
  9. Marty Strode
    Joined: Apr 28, 2011
    Posts: 8,796

    Marty Strode
    Member

    Some friends of mine built flathead 1/4 Midget engines, and got 2 hp per inch, by having a nob on one side of the piston top, and a dome in the head to match, and grinding a radius in the top of the cylinder near the valves.
     
    HuskerNation and Tman like this.
  10. nrgwizard
    Joined: Aug 18, 2006
    Posts: 2,513

    nrgwizard
    Member
    from Minn. uSA

    Hey, Marty;
    That's the 1st time I've ever heard of the cyl being rolled. Never got that far into building a flattie, but I've wondered for ~40+yrs why it wasn't done. I'd think the sharp cyl edge would cause flow & fuel separation issues, & tumbling in the chamber(actually can't see that it doesn't hurt flow). For a one-off, it could be easily cut w/a router & a decent bit. Just have use a file to touch up the outside edges near where the throat area - whether relieved or not - gets close to the gasket.
    Navarro had some pop-up pistons for the flattie, but iirc, they went into the head dome, not w/a 2-stroke-type pop-up that helped fill the throat. What comp did your friends' mill run? Any pics?
    TIA.
    Marcus...
     
  11. Ned Ludd
    Joined: May 15, 2009
    Posts: 5,025

    Ned Ludd
    Member

    The great disadvantage of a flathead engine is that flow and compression tend to militate against each other.
    The great advantage of a flathead engine is that it is relatively easy to combine tight quench with a low static CR, which cries out for forced induction.

    Are the plugs over the intakes or the exhausts? Later thinking was to burn the end gas in the coolest part of the combustion chamber, which argues for a plug near the hot exhaust valve, but that HD head seems to be the other way around. It is perhaps moot with a low CR.
     
  12. 31 Coupe
    Joined: Feb 25, 2008
    Posts: 383

    31 Coupe
    Member

    Hey Marty and Marcus,
    It's refreshing to hear someone else make that comment about a radiused and blended bore entry.
    I'd been thinking that it SHOULD improve the air flow from the intake valve seat into the bore and be a better option than a conventional relieved deck. Imagine a crowned profile between the valve seat and the radius into the bore. The exhaust flow is usually pushing it's way to the valve and typically takes the chamber roof pathway.
    I'm building a 276" motor so compression shouldn't be compromised too much.
    Would love to hear opinions from the plethora of Flathead experts on here.
     

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