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Head flow test results

Discussion in 'Rootes Racing' started by jdoclogan, Nov 2, 2022.

  1. jdoclogan

    jdoclogan Platinum Level Sponsor

    Andrew Gibson at Gibson performance completed the flow test for an Alpine head with various valve configurations.

    Here are the results:

    And the winners are:

    Flow test #3 INTAKE uses a Datsun diesel valve - LD28 Provides 21.5 to 22.5 % increase vs stock at .400" to .450" lift
    Flow test #4 EXHAUST uses a Datsun 240 Z valve - L24 Provides 31.5 to 34.1% increase vs stock at .400 to .450" lift

    The chambers (except stock) were all given a Vizard modification and mild bowl work (smoothing).
    Flow test #2 shows the Vizard modification has a slight improvement on air flow vs stock.
    The valve modification results in significant improvement for air flow vs stock .
    Gibson recognized that the valve stem length of both LD28 and L24 were slightly shorter than stock. This would change the rocker arm's geometry. He will look to find a perfect matching stem to retain the original rocker geometry.[/ATTACH]

    Attached Files:

    Last edited: Nov 8, 2022
    alpine_64 likes this.
  2. bernd_st

    bernd_st Bronze Level Sponsor

    Jerry, amazing findings. You are really making good progress. 22.5% on the inlet & 34% on outlet are rather significant flow improvements. Interesting to me is that further increase in lift reduces the gain...
  3. Five Reviver

    Five Reviver Silver Level Sponsor

    Jerry, so generous of you to share this. It removes the blindfold.
    I would love to hear if your man Andrew comes up with those valve sizes with perfect matching stems or longer ones which can be shortened.
    In my current build I was leaning towards sticking with my current standard valves and Vizard head, to go with Dan's new school pistons and long rods but now I'm thinking a bit more adventurously.
  4. spmdr

    spmdr Donation Time

    For comparison, some old info. Of unknown origin.

    Seems to corroborate the info quite well.


    Last edited: Nov 2, 2022
  5. alpine_64

    alpine_64 Donation Time

    @Five Reviver


    The vizard mod will increase flow bit loose compression... Also depends how skimmed your head is ... And what compression you get via flat tops, deck height and bore.... But carefully deshrouding the spark plug and cleaning ports with bigger valves is also good.

    What compression are you aiming to run?

    This was the head on my racer with larger datsun valves:

  6. spmdr

    spmdr Donation Time

    I have been using Chevy V6 valves because they have slightly bigger stems (8mm?), perfect for reaming old Alpine guides to.

    And they have close enough stem lengths.

    In the future, I plan to bite the bullet and find valves with smaller stem diameters, 6 or 7 mm.

    Good timing, I have a connection to a new head Guy. Planning to talk to him soon.

  7. spmdr

    spmdr Donation Time

    One thing of note, port flow STILL dies (VERY diminished returns) at the upper valve lifts, of a VERY old school of around .400 or more.

    These days, .500+ is STREET stuff and .600 is more common on RACE engines.

    More work needs to be done on RACE Alpine heads, Me thinks.

  8. alpine_64

    alpine_64 Donation Time

    How do we overcome the limitation of the Rooted exhaust ports o_O
  9. Five Reviver

    Five Reviver Silver Level Sponsor

    Michael, 650547F6-34A6-40AD-A162-B4D4E5C02EC9.jpeg Here’s a combustion chamber, cc’d at 38. Compression last assembly was 10.3 as far as we could work out. Above all I want this engine to be durable.
  10. jdoclogan

    jdoclogan Platinum Level Sponsor

    Dan, interesting chart. Clearly the valve selections I have made are a significant improvement over the Holbay configuration. My intake and exhaust valves provide a 26% and 51% improvement over the Holbay head.
    Now the task is to align the cam shaft with these flow rate outcomes.
  11. Barry

    Barry Platinum Level Sponsor

    The data in the original post indicates that a suitably modified 1725 head can flow about 165-170 CFM on the intake side and about 125 CFM on the exhaust side.

    Current wisdom is that a "good" naturally aspirated 4-cylinder gasoline engine should be able to make about 1 HP for every CFM of intake port flow. Part of "good" is that the exhaust port flow should be at least 75-80% of the intake port flow, that the compression ratio needs to be about 10:1 and that peak power needs to be 6,500+ RPM. I suspect that something appreciably higher than 91 octane would be required to avoid detonation around peak torque.

    A little more exhaust flow would probably help , but 165-170 CFM intake port flow and 120-125 CFM exhaust port flow should produce something like 160 HP on a good day. The bigger challenge may be not letting the smoke out.
    windlessdog likes this.
  12. spmdr

    spmdr Donation Time

    Yes, CW HAS been, put a LARGE % of your effort

    and Real Estate on the Intake side.

    The problems with the Alpine head may suggest

    A rethink on the CW numbers, Ex to In.

    Clearly, a bigger Ex valve is working on the flow bench.

    Now, How big can we make those PORTS?

  13. jdoclogan

    jdoclogan Platinum Level Sponsor

    That is where my 4 into 1 exhaust system will come in. It should provide significant exhaust scavenging allowing for more flow. We shall see after dyno testing.
  14. spmdr

    spmdr Donation Time

    I finally bit the bullet and cut up a head. It's a 1966 SV head casting 1980987.

    This head was deemed the least likely to be reused. It was surfaced severely on an angle,

    front to back and getting into the side cover bolt holes.


    My immediate thinking is to move the valves closer to the manifold side to open up the short turn area.

    This potential change has several benefits.

    Last edited: Nov 17, 2022
    alpine_64 and jdoclogan like this.
  15. spmdr

    spmdr Donation Time

    And a look at the original IRON head suggests not much change in the port shape casting cores.

    So my guess is that all the spark plug tube type aluminum heads have a very similar port wall thickness/casting core shape?

    Last edited: Nov 17, 2022

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