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Thread: JD to almost JDH?

  1. #1
    Join Date
    Dec 2014
    Posts
    14

    Default JD to almost JDH?

    Hello,
    At last the latest magazine arrived into my mailbox as well and what a nice history of the FH motorcycle.
    The twin cam engine looks very attractive and I trust there's extra power compared to standard J model. As we all know the twin cams are very rare and expensive.
    I've been thinking what are the real differences between the engines? It would be nice to upgrade my 1928 JD to the same power level as the JDH.
    If someone knows the cam specs, is there any problem to re-grind the JD cam to JDH specs?
    Also I would need to change the inlet valve springs, right?
    How about the compression ratio?
    Is the carb 1 1/4" or bigger?

    I would like to hear opinions. Have someone else ever been thinking of this upgrade?

    br
    Antti

  2. #2
    Join Date
    Jul 2002
    Location
    North Hills, CA and Pine Grove, CA
    Posts
    5,464

    Default

    Quote Originally Posted by asavolai View Post
    Hello,
    At last the latest magazine arrived into my mailbox as well and what a nice history of the FH motorcycle.
    The twin cam engine looks very attractive and I trust there's extra power compared to standard J model. As we all know the twin cams are very rare and expensive.
    I've been thinking what are the real differences between the engines? It would be nice to upgrade my 1928 JD to the same power level as the JDH.
    If someone knows the cam specs, is there any problem to re-grind the JD cam to JDH specs?
    Also I would need to change the inlet valve springs, right?
    How about the compression ratio?
    Is the carb 1 1/4" or bigger?

    I would like to hear opinions. Have someone else ever been thinking of this upgrade?

    br
    Antti
    If it were only that simple,,,,,,
    Be sure to visit;
    http://www.vintageamericanmotorcycles.com/main.php
    Be sure to register at the site so you can see large images.
    Also be sure to visit http://www.caimag.com/forum/

  3. #3
    Join Date
    Nov 2015
    Posts
    23

    Default

    Yer going down a slippery slope there, but I can't stop ya!

    The JD has a single cam and the JDH has two cams, the two cam can rev up quite a bit more. Below is a link to the Tom Sifton two cam (I've never seen the inside of this bike so who knows if it actually is). Heaps of engineering probably went to that bike... Wonder what the power figures are on it compared to stock!

    http://www.pacificariptide.com/pacif...ydavidson.html

    If you want the two cam experience buy a lower end from Fred Lange slap some JD jugs on it. But I think there's a guy in Germany who has done the two cam route in stock cases?

    Cylinders are the same, just different inlet towers. George Hood made them awhile ago, I hope he does another run...

    I think a better "upgrade" to the J engine is a high lift cam (http://www.leinewebercams.com/ They are expensive....), port/polish the intake and pockets, domed aluminum pistons, flywheel mass reduction, stroker wheels from T&O, flywheel polishing, bearing upgrades, and probably a modern carb... But that's a few grand right there already.. Race bike stuff.
    Last edited by whp; 04-03-2017 at 02:23 PM.

  4. #4
    Join Date
    Sep 2005
    Location
    Sarasota, Florida
    Posts
    4,176

    Default

    The 1930 VL had one more horsepower than a '29JDH; for what that is worth. I can see spending money on a knuck, or pan to get more power for riding, but I think chasing horsepower with a JD is a money pit. I love IOE Harley-Davidsons, but they have limitations, and a wall that can't be penetrated, and all of those limitations are in the top end. Not only is the IOE a pioneer design (based on weak exhaust valves), but the intake pockets are difficult to seal, and keep sealed, even today with all of our miracle sealants. When it comes to a J, or JD, I am most impressed with one that is reliable, easy to start, and fun to ride.
    Eric Smith
    AMCA #886

  5. #5
    Join Date
    Nov 2002
    Location
    France
    Posts
    96

    Default JDH power

    Look at this power curves from Jerry Hatfield book Inside Harley-Davidson :
    http://www.harley-memories.com/jdhpower.jpg

    and also here the result of Muroc race :
    http://www.harley-memories.com/harle...ce-results.htm

  6. #6
    Join Date
    Dec 2014
    Posts
    14

    Default

    Thank you all,
    The reason I'm asking is that I'm totally comfortable riding my several day trips with my ULH, there's enough power to ride constant 100 km/h speed. But with JD it's a bit too much, 90 km/h is cruising speed with my JD.
    It might be wise to let it be so and do longer trips with ULH or knuck.

    Eric, you have interesting data there. There's big difference between JD and JDL peak power, do you know what is the difference in the engine?
    Also, based on your web pages, I assume you are well aware all technical differences between JDH and JD. Could you share the information?
    BR
    Antti

  7. #7
    Join Date
    Nov 2002
    Location
    France
    Posts
    96

    Default JD to almost JDH

    Here are some differences which for me explain the difference between the JD and JDH.
    The main thing is that the 2cam uses 2 camshafts. This allows for better distribution diagrams.
    The pushrods and valve tappets are more direct and lighter.
    The inlet valves are larger.
    The pistons are doomed.

    The JDH has enjoyed all its racing history since the creation of the first 2cam banjo case.


    To my knowledge the difference between the JD and JDL is the presence of doomed pistons


    There are a lot of people who are more specialized than I who can bring other things.

    jdhcamshaft.jpgIMG_1623.jpgIMG_1100.jpg

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