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2x2 or 3x2 for flathead

Discussion in 'The Hokey Ass Message Board' started by gear27, Mar 12, 2009.

  1. There is no simple answer to this question -- the "it all depends" is the only correct answer. For a small cubed motor like yours (239 plus an overbore), the 2x2 will be a better choice for all around usage. When you get to larger stroker motors, with port work done, big cams, etc -- the 3x2 will be much better in general.

    Most guys with smaller motors will be one heck of a lot better off with a simple 2x2 setup. They are FAR easier to tune and run than a progressive 3x2 setup. I used to run a 3x2 progressive on a 284 cube street motor -- was a pain in the butt keeping things setup correctly. Also, there was really no easy way to know that all three were in sync at any throttle position - as you couldn't run it like that in your driveway and measure anything.
  2. carbking
    Joined: Dec 20, 2008
    Posts: 2,537


    This post should automatically be inserted in every thread where some newcomer to our hobby asks about "what carburetor should I choose".

    Excellent advice.

  3. Flat Ernie
    Joined: Jun 5, 2002
    Posts: 8,410

    Flat Ernie
    Tech Editor

    In general, power is power. The question is really, where is the power band and is it somewhere where my engine usually lives. The advantage of the flathead is the long, flat torque curve. A typically flathead torque curve is virtuallyall in by 1500 rpm and doesn't begin to drop off until 4000 (stock) - the cam will move this, of course, but the low end rarely moves significantly. The bottom line is engines with a large flat torque curve can generally "compromise" some bottom end in exchange for top end w/o the driver noticing significantly (obviously, there are extremes to this).

    No doubt. These were all identical 97 carbs built by the same big name, well-respected carb builder.

    Again, we're dealing with flatheads and very restricted breathing. So any increase in intake flow is well rewarded. The intakes tested were the Thickstun 2x2 (least power), Offy Super (middle power), & Offy Triple (best power & straight linkage) - all on a dyno mule engine (basically stock 239). Are there tuning efficiencies to be realized? You bet. But the basic challenge with the flatty is airflow. It should be noted that there was not a HUGE difference in power, we're talking maybe 15HP from worst to best if I recall correctly...all at top end. I don't have the dyno charts handy.

    Flow tests are different than dyno tests which are different from dragstrip testing. Here again, the flathead would've been helped by any increase in volume of flow.

    The thing that gets my head going round and round is why 4 carbs works even better on a flatty. It's because with most of the manifolds, you're now into IR-type configurations and throttle response is incredible, as is power, and you generally need much larger carbs! 4x48s for all my friends! ;) :D

    Yes, yes, yes. Especially last sentence. However, as mentioned above, the flathead has loads of torque, all in from not too far off idle, so you are forgiven many sins a modern engine wouldn't...

    So true!

    Well, the best you can do is measure your throttle positions & ensure the idle is synched by flow. After that, you've got to trust your carb builder did everything the same on the carbs and they're as close as a production model can be to another.

    I run a 2x2 on my '40 right now. Easier to set up (straight linkage), but my choice was mainly due to generator placement - on the majority of my street engines, I run Edmunds or Thickstun 2x2 so I don't have to move my generator - I'm lazy & they run well enough. ;) :D
  4. gashog
    Joined: Dec 9, 2005
    Posts: 936


    Old dyno test from California Bill Fisher's Ford Speed Manual, 1952 Edition. You can read the full article at

    <DT>A 89HP at 3600RPM - Stock 100HP Ford flathead V8 (corrected) <DT>B 114HP at 3800RPM - Same engine with dual intake manifold. <DT>C 124HP at 4000RPM - Same as B with 8.5:1 compression heads. <DT>D 140HP at 4400RPM - Same as C with <SUP>3</SUP>/<SUB>4</SUB> race camshaft. <DT>E 145HP at 4500RPM - Same as D with special exhausts. <DT>F 177HP at 4500RPM - Same as B, bored <SUP>1</SUP>/<SUB>8</SUB> inch, stroked <SUP>1</SUP>/<SUB>8</SUB> inch, ported, relieved, 9:1 heads. <DT>G 197HP at 4700RPM - Same as E but with super track cam and methanol fuel. </DT>

    Attached Files:

    Last edited: Mar 14, 2009
  5. banjorear
    Joined: Jul 30, 2004
    Posts: 3,769


    "I don't doubt the more power of a 3X2, but I do have a question. Does more power on a dyno directly correlate to more usable power on the street?"

    Man, I really need to re-read my post before I press the enter key. Above is what I meant to say. I don't doubt the potential power of 3X2 is what I meant.
  6. Flat Ernie
    Joined: Jun 5, 2002
    Posts: 8,410

    Flat Ernie
    Tech Editor

    I've got that book & have read it a few times - neat stuff. I love those old dyno tests...wish they'd show the torque curves with the HP curves...
  7. To add to this, there must be a lot of turbulence when a Tardel "open" type riser is used, as the charge is going to hit the divider in the manifold. That part of the charge that hits the divider is effectively going to split and go sideways into the part of the charge that's going straight through, thus causing the whole lot to slow down and start tumbling........ I think.
  8. The 2X2 will generally have a better response for street driving but a lot will depend on your linkage . If the outboard carbs are set to open fairly late you'll retain sufficient vacuum and good low end

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