1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

The great airbox debate...

Discussion in 'Norton Motorcycles (Modern)' started by Fast Eddie, May 15, 2019.

  1. Fast Eddie

    Fast Eddie VIP MEMBER

    Joined:
    Oct 4, 2013
    A topic of conversation at our last meet up was the airbox.

    Wodgedodge has led the way so far with his double ended air filter mod idea and he’s looking to do a mk2 of this on his latest 961.

    I was jus’ wondrin’ if others had approached it differently at all?

    The ‘problems’ with the airbox are:

    1. The airbox size, consensus is it’s just too small for a motor of that size (I say ‘consensus’ cos that’s what folk have said, and that’s what my gut says, but I’ve never calculated properly cos I don’t know how to).

    2. The intake and / or filter is apparently too small and is a bottleneck for the intake (I say ‘apparently’ for the same reasons as above).

    3. This is actually the one that struck me first; actually the intake hole is not the bottleneck because irrespective of the intake size, the intake is sucking all its air from under the seat... and how is that air getting under the seat? From around the side of the airbox, between the seat and frame tubes, etc. In other words, it is a very restrictive and torturous path the air has to take to get under the seat before the supposed restricted intake can suck it in.

    So, has anyone looked into these thoughts at all? Any good ideas in people heads or on the back of a fag packet etc (note to our colonial cousins ‘fag’ means cigarette and has nothing to do with the LGBTQ agenda)?
     
  2. ntst8

    ntst8 VIP MEMBER

    Joined:
    Feb 14, 2004
    Some interesting reading on airbox design, i haven't run through the example at the end with 961 figures but it would be an interesting exercise for a rainy day when there aren't any bikes needing maintenance....
    https://motorcycleinfo.calsci.com/Airboxes.html
     
  3. Fast Eddie

    Fast Eddie VIP MEMBER

    Joined:
    Oct 4, 2013
    Blimey, that’s a bit heavy! But as you say, be good to get ones head around it (I’ve got a long flight coming up soon, maybe then)!

    Until then though, it seems that at 6,000rpm a 961 will require 2,883 litres of air per minute. Or 48 litres per second.

    Now, look again at your airbox guys, or more to the point, where the airbox gets its air from, and visualise 48 litres PER SECOND trying to get in there.

    Not very scientific I know, but I can’t visualise it happening...
     
    richard-7 likes this.
  4. ntst8

    ntst8 VIP MEMBER

    Joined:
    Feb 14, 2004
    I only read the easy parts, need a comfy chair and a whisky for the theory section.
    In the meantime i couldn't resist and at a very rough measure (builders tape measure) i get around 5.5 litres for internal airbox volume. Compared to the 8l for the example in the link which revs much higher, the volume may not be far off.
     
  5. Fast Eddie

    Fast Eddie VIP MEMBER

    Joined:
    Oct 4, 2013
    Maybe. I dunno.

    Personally, my theory of the bottleneck is not airbox volume, it’s feeding air into it...
     
  6. TonyA

    TonyA VIP MEMBER

    Joined:
    Jan 16, 2015
    This may be another Domi advantage with the intake hole at the bottom ? I was hoping that iwilson would update us on his clamp on filters experiment .
     
  7. TonyA

    TonyA VIP MEMBER

    Joined:
    Jan 16, 2015
    What do you think of a K&N type panel filter snapped in at the front of the airbox ? Just below where the brass spigots are , say a 4in x 5in size panel ? There was a member who actually opened the hole straight to the bottom and used a k&N filter , Remember that ? I wonder how that's doing for him ?
     
  8. TonyA

    TonyA VIP MEMBER

    Joined:
    Jan 16, 2015
    Here it is :

    [​IMG]
     
  9. TonyA

    TonyA VIP MEMBER

    Joined:
    Jan 16, 2015
    What I like about this is he retains the oil catching capability of the air box and keeps it looking stock . I'm not too keen on the road spray though. What is missing is the dyno runs to see if any of this helps .
     
  10. Fast Eddie

    Fast Eddie VIP MEMBER

    Joined:
    Oct 4, 2013
    Tony, that is the mod by Wodgedodge as I mentioned earlier. He reports it worked well and intends to do similar on his new 961.

    Hopefully this time he’ll do some before and after Dyno runs, how about it Wodge?
     
  11. Fast Eddie

    Fast Eddie VIP MEMBER

    Joined:
    Oct 4, 2013
    I think the panel filter is an interesting idea Tony...
     
  12. BritTwit

    BritTwit

    Joined:
    Aug 25, 2009
    I’m not a California scientist, but I did sleep in a Holiday Inn last night.:D

    OK, seriously, that is an excellent airbox article, but...

    The article is basically telling the reader that your stock motorcycle was designed by engineers to work very efficiently with the airbox/snorkel/exhaust system that is provided in stock form. That drilling holes in the airbox or otherwise modifying the airbox will be counter-productive more several reasons.

    I AGREE -----> on a completely stock motorcycle or vehicle.

    But, If you are looking for more power, you are now throwing out some basic assumptions and design elements that the original engineers had to work around.

    Now, who modifies only the airbox to generate more power?? Duhhh.

    The airbox is not even the first engine component that normally is modified or swapped out by owner. That would be the exhaust system. This will have limited effect on power since the engine breathing will now be choked off by an airbox designed to flow only the amount of air needed for the original stock engine design requirements. Because, again, that was the design specification given to the engineers to shoot for.

    People looking to generate more power typically seek to increase flow, which usually means improving breathing (intake) and blow (exhaust). So in this case you need to increase the flow into and through the airbox. This is done by:

    1. Removing airbox restrictions like snorkels, restricting baffle plates, etc.
    2. Increase internal plenum (airbox) volume.

    Internal plenum volume is a very important factor because the plenum is a sort of reservoir or buffer of calm air which is available to the intake system. Calm plenum air in the airbox requires less engine vacuum to pull a given mass of air into the combustion chamber than air pulled in through the open environment. So, the engine vacuum is utilized more efficiently to pull air into the chamber – greater volumetric efficiency, more power is the result.

    This is the reason why sportbikes are delivered with large under tank airboxes - to provide a large plenum of clam air for their intake systems.

    I think the standard 961 airbox is probably fine for a stock 80HP machine. That is what the engineers were aiming for. For this purpose, the airbox is not too small or restrictive at all. The under seat airbox intake on a Commando is problematic I think.

    However, If we want to make more power on a 961, then I believe the airbox intake location and plenum volume are definite concerns. Adding a free flowing exhaust will provide a few extra HP but you quickly bang up against the intake flow restrictions of the stock airbox plenum.

    A great deal if airbox info can be had on the Triumphrat forum in the Air Cooled Twin section. Many Hinckley Bonneville’s have had their snorkels/restrictor plates/airboxes modified or removed. The results are undeniable.

    To make lots more power with a Hinckley Bonneville, you gotta chuck the airbox all together, and add individual air filters to the carbs/throttle bodies.
     
    Clive likes this.
  13. TonyA

    TonyA VIP MEMBER

    Joined:
    Jan 16, 2015
    We agree with all that. At this moment we were just discussing some air box mod options.
     
  14. Wodgedodge

    Wodgedodge VIP MEMBER

    Joined:
    Dec 1, 2016
    I’m planning to have a chat with Cain at Wolf Performance who provided the inlet trumpet to see if he can also produce the connection pipe for the inside of the air box. Ideally he could also profile the edge of the trumpet to eliminate the need to dremel a countersink recess into the air box to get the trumpet fitting flush.
    Another benefit of the mod is you get a louder intake snort that covers up some of the motors “tinkling noises”!
    I’ll report back once I’ve spoken to him.
     
  15. Clive

    Clive VIP MEMBER

    Joined:
    Jun 9, 2017
    Reluctantly have to agree With FE - the other day I was looking at the air path round the seat and thinking the same - the only answer is a dyno run in a wind tunnel. The info on the importance of the air box/plenum size is interesting. I still wonder what could be done with the cam profile by someone who knows.

    I then draw parallels with my teenage tinkering on my Honda SS50 and wonder if I'm going to look back and think 'I wish I'd spent more time in the workshop and less time riding'.

    This bike is never going to be a performance bike so I kinda think time would be better spent on styling issues such as a new seat covering or that fairing......
     
    thesixthelephant likes this.
  16. Stephen_Spencer

    Stephen_Spencer VIP MEMBER

    Joined:
    May 31, 2017
    Fairing, did somebody mention fairing! I still imagine a fairing akin to the Thruxton R race fairing. In carbon? Come on you technical titans, let’s get it done!! And while you’re at it, how about that matching carbon fibre single seat unit!!! PS more power would also be welcome!

    Steve
     
    Clive likes this.
  17. Fast Eddie

    Fast Eddie VIP MEMBER

    Joined:
    Oct 4, 2013
    When a get a round tuit, I’ll try and get mine on the dyno, should be easy enough to ascertain if the air starvation to the airbox hypothesis is correct or not by simply doing a few dyno runs with seat fitted followed by a few without.
     
    BritTwit and TonyA like this.
  18. storm42

    storm42 VIP MEMBER

    Joined:
    Jul 20, 2011
    Wouldn't swapping the seat for an arse be worse:D
     
  19. Bernhard

    Bernhard

    Joined:
    Apr 20, 2011
    Kill two birds with one mod-just run a small plastic funnel from the front of the bike via a plastic tube that runs into the air box. Advantages? - It’s called forced induction, and at the usual 60mph you will get a mild power boost, I’ve being doing this for years and years. . . . just do a plug chop at various speeds over 60mph to check gas/air mixture is not weak.
     
  20. TonyA

    TonyA VIP MEMBER

    Joined:
    Jan 16, 2015
    Unfiltered air ? A good idea worth looking into.
     

Share This Page