Double, double boil and trouble - Mark's airbox mod . . .

Discussion in 'Dyna Models' started by markfsanderson, Sep 8, 2012.

  1. markfsanderson

    markfsanderson Active Member

    This is something that I decided to try out, but certainly don't recommend anyone do. This modification will permanently modify a part of the motorcycle that might be under warrantee . . blah blah blah and so on. That being said, this modification did work for me, and replacing this airbox for resale or whatever should be pretty cheap.

    Here is a link to the photo gallery that I posted some pictures and comments in . . . or just down below and you'll see the important stuff.

    Here's a picture of the tool that I used to open up some holes on my airbox. Its use prior to this was opening up holes for various reasons on lexan bodies on my model R/C racking trucks. Its available at hobby stores and etc - be careful - it is VERY sharp.


    Since just about day one, I've had a K&N stock replacement air filter for my 2011 FXDC. I was tired of the sometimes anemic performance on very hot days, stuck in traffic and etc. Deciding to take matters in to my own hands, I noted that the air cleaner got dirty on on the bottom. I used the pictured lexan auger to open holes along the perimeter of the airbox starting from the top of the airbox all the away around to the rear. If I had it to do all over again, I'd use my drillpress to start the holes and finish off with the auger. The advantage the auger has over the drill is that you'll get a slightly beveled inlet that should minimize/eliminate any whistle. So far I haven't heard a peep from it. I believe that I'm getting a bit better throttle response and that the engine does also reduce RPM quicker as well. I also noticed that instead of the normal 42-44 MPG, I'm now getting nearly 51 MPG (95/1.88 = 50.53!). Those results don't really make any sense to me now . . . but someone else might have a better idea why I'm getting better mileage . . . less restriction - less work? Not sure.

    Thanks - Mark!
  2. Hoople

    Hoople Account Removed

    Mark, Nice job but double check those MPG numbers with a few more tanks of fuel. If it turns out to be true, it means your running terribly lean. The air portion of the A/F ratio has increased. The fuel portion of the A/F ratio is remaining the same as before. If it was a carb bike and not FI, it would be different.

    If you push more air into the engine, you must tell the injectors to do the same or else the ratio will change..
    1 person likes this.
  3. markfsanderson

    markfsanderson Active Member

    The TFI is going in tonight and tomorrow morning - I'm taking video of these (mis)adventures . . . already forgot to disarm the Smart Alarm 2 . . . darn things are very loud!
    Hmm . . . so you say that (assuming the mileage was an accurate read) for some reason the sensor is now reading less oxygen/air than before? That would be the only reason to reduce the fuel flow - right? We did do lots of twisties . . . except on the way there and back on straight roads never got much above 60 MPH . . . It didn't 'feel' hotter . . . but I don't know for sure though . . . Let's see how the TFI does with my 'new and improved' H-D A/C box . . .
    Good news is the Dewey got my check, and my new A/C cover and cleaner is on the way . . . if I'm lucky, the H-D chrome back plate will be not so shabby looking as before - I had to reject one for bad chrome on Friday.

    Thanks for your help Hoople!

    Mark - Thanks!:newsmile026:
  4. fin_676

    fin_676 Experienced Member Staff Member Moderator Contributor

    Nice job and worth doing in my opinion
    I have done similar modifications in the past to 2 bikes although i used a hole saw to cut neat holes the first time i did it was on a 94 sportster along with a larger slow and main jets to allow more fuel to compliment the air flow i also replaced the mufflers at the same time and it really woke the bike up
    I also used the same procedure on my softail to allow sufficient air to feed the mikuni hsr i had just fitted
    I found on the evo bikes the air inlet into the air box was the most restrictive part of the air flow and opening up the air box did really help with those 2 bikes

  5. Breeze3at

    Breeze3at Well-Known Member


    Don't be surprised if the next backing plate is also disappointing. I have never seen one that really looked nice. It seems they do no polishing of the metal before plating. :(
  6. markfsanderson

    markfsanderson Active Member

    Ouch - any suggestions on where I can get a backing plate to fit a round SE/Bigsucker - pretty much the same fitment as the SE back plate? Paying more than 50.00 for bad workmanship doesn't really appeal to me . . .

    Mark Thanks!:newsmile026:
  7. Jack Klarich

    Jack Klarich Guest

    Last edited by a moderator: Sep 9, 2012
  8. markfsanderson

    markfsanderson Active Member

    Hmm, (and big sigh!) my heart is set on a nice piece of chrome there . . . the style of A/C that I'll get will show that area I think that chrome would look better against my engine . . . I certainly won't convert my bike to an all-chrome machine, but there it seem to work the best for me. I'll try the H-D one again . . . and reject for a black one if need be. I'm on the look out for a nice chrome piece . . . doesn't need to be triple-plated show chrome - but just nice. I'll keep look until I find what I want . . . I suppose I could buy two and one off to be re-chromed . . . thats a thought . . . it would probably cost an arm and leg though . . .

    Brian, yup I'm doing the TFI thing today . . . hopefully I'll be done with it in time to do a medium length ride. I'm also going straight to the gas station to re-verify the mileage I was getting with a stock FI set up (no TFI).
    Around here in San Antonio; I was noticing that recently (very hot!) at traffic lights, stop signs and traffic my FXDC was not a happy girl at all. After opening up some holes, she seemed to breathe just a little better. No startling performance increase; just what seemed like 'normal operations'. The throttle seemed a little snappier, engine rpm increased and decreased a bit more rapidly . . . acting like it 'should' if you will.
    Hopefully, the TFI and the SuperTrapp will add a little strut to her step . . . we'll see!

    Thanks - Mark!:newsmile026:
    Last edited by a moderator: Sep 10, 2012