Airhead Cams - A Comparative Religious Study

Discussion in 'Airheads' started by adrenal, Sep 15, 2021.

  1. bmwrench

    bmwrench Long timer Supporter

    Joined:
    Nov 10, 2006
    Oddometer:
    9,152
    More valve clearance.
    #21
  2. Jim K in PA

    Jim K in PA Long timer Supporter

    Joined:
    Mar 15, 2011
    Oddometer:
    5,747
    Location:
    Pocono Mountains, PA
    Hmm. And probably assymetrical clearance, no? ( more so than stock).
    #22
  3. adrenal

    adrenal skrunkwerks.com

    Joined:
    Jun 12, 2008
    Oddometer:
    1,171
    Location:
    Melbourne, Australia
    @Arktasian after reading this:
    https://www.motortrend.com/how-to/hrdp-1011-cams-for-turbocharged-engines

    Particularly the bit about off-boost lower RPM performance in making a good broad, streetable machine. It occured to me that the Schleicher 296 Enduro might be an interesting candidate. It has the smallest overlap area of them all and less duration but higher lift than the 308.. It's an interesting cam. One of the most aggressive in the table with a Major Intensity of 24!. All good for off-boost low down torque/responsiveness then boost takes care of the mid to upper end. Has considerably higher opening/closing acceleration and nose de-acceleration than even the Schleicher 340 racing cam so you'd have to be careful with spring force etc if you want to push the red line. But I can help you with that.

    Only way to find out mate - a dyno shootout between the 308 and 296. Get onto it!
    #23
    Arktasian, bmwrench and Jim K in PA like this.
  4. adrenal

    adrenal skrunkwerks.com

    Joined:
    Jun 12, 2008
    Oddometer:
    1,171
    Location:
    Melbourne, Australia
    The table is up to Revision 6 : have added both versions of the Stock 308: The earlier with no advance and the later (/7 models on?) with 6 deg advance.
    Same with the BMW 336 'Sports': Earlier with no adv and later (Post 1978?) with 6 deg adv.
    #24
  5. RGregor

    RGregor Been here awhile

    Joined:
    Oct 31, 2007
    Oddometer:
    838
    Location:
    Bavaria
    If you mean Allan Millyard, he also builds his cams that way.
    I'm not sure this would work with our engines as you would need the weld right in the center of the cam, where flex would be highest ....
    #25
    adrenal likes this.
  6. Arktasian

    Arktasian Feelin the BMEP

    Joined:
    May 23, 2014
    Oddometer:
    2,278
    Location:
    BC
    Thanks for the article. In particular, I hadn't seen the reference between excessively high exhaust back pressure being a large influence towards less overlap before - happily I have a pretty good match and my pressure ratio remains close to equal intake to exhaust throughout range I use.
    Meanwhile, I'm thinking to ignore your bad influence in providing a test candidate (as I initiate a search on the Schleicher 296 :hmmmmm)
    #26
    adrenal likes this.
  7. Arktasian

    Arktasian Feelin the BMEP

    Joined:
    May 23, 2014
    Oddometer:
    2,278
    Location:
    BC
    Bud - are you saying - increasing valve lash/ clearance to reduce overlap? (just a lowly "northfounded, igloo bound S.A.)
    #27
  8. adrenal

    adrenal skrunkwerks.com

    Joined:
    Jun 12, 2008
    Oddometer:
    1,171
    Location:
    Melbourne, Australia
    Millyard! Like Bert Munro, a soft spoken genius. I find his vids mesmerising.
    #28
    DonniBoi and RGregor like this.
  9. bmwrench

    bmwrench Long timer Supporter

    Joined:
    Nov 10, 2006
    Oddometer:
    9,152
    Yup. Apples and arnges, but I discovered (by accident, of course) that increasing valve clearances by .002" gave my Triumph Trident a nice boost in torque. Todd Schuster once told me that he ran his RS valve clearances at .010", but he may have been "Jerseying" me.
    #29
  10. Arktasian

    Arktasian Feelin the BMEP

    Joined:
    May 23, 2014
    Oddometer:
    2,278
    Location:
    BC
    ^ "Perfectly Superlative" - slight reduction in overlap benefits towards less fuel charge being wasted (via boost), increased heat rejection via longer valve closed time, better safety factor between valve adjustments, cost is a wash - back to Andrian's excellent theme here.
    #30
    adrenal likes this.
  11. Arktasian

    Arktasian Feelin the BMEP

    Joined:
    May 23, 2014
    Oddometer:
    2,278
    Location:
    BC
    Let's all pause for a muffin or two from the kitchen, prior to watching the "how to build airhead cams" episode.
    #31
    Doppio and adrenal like this.
  12. Arktasian

    Arktasian Feelin the BMEP

    Joined:
    May 23, 2014
    Oddometer:
    2,278
    Location:
    BC
    For that, we have wool - its colder up North.
    Any comments on running increased valve clearances? Pro's and cons but if it doesn't harm the cam lobes or lifter faces then can be an ingredient in valve timing.
    #32
  13. adrenal

    adrenal skrunkwerks.com

    Joined:
    Jun 12, 2008
    Oddometer:
    1,171
    Location:
    Melbourne, Australia
    Ja, can be used as a tuning tool in changing the seat to seat duration. Our donks are rather sensitive to too much low lift flow particularly with large overlap cams. Increasing lash can be helpful to reduce this flow in older cams with fat Jurassic clearance ramps. 'Retail' cams like most of the ones we talk about will have conservative clearance ramps to deal with vagaries in usage/maintenance - best to leave lash settings stock for that reason.

    Custom racing cams different story: in order to open valves as quickly as possible, tolerances are tightened based on carefully measured parameters and a particular lash setting with less wriggle room that is continuously monitored.
    #33
  14. adrenal

    adrenal skrunkwerks.com

    Joined:
    Jun 12, 2008
    Oddometer:
    1,171
    Location:
    Melbourne, Australia
    Quick thanks to Rudi ( @RGregor ) and Michel Schenk for getting together and scrounging some cams from among their friends. Winging their way down-under are three cams: The Schleicher 332, 344 and the 336 BMW Sports Cam (aka Holy Ghost). Stay tuned for the big Airhead reveal of the century: Are the Schleicher 332 and the BMW Sports cam in fact the same cam??!!

    A reminder to anyone who may have a cam not in the table (including those listed with a #) and don't mind loaning it for me to measure. Postage on me.

    http://skrunkwerks.com/skrunk/airhead-cams-a-comparitive-religious-study/
    #34
    Arktasian, Jim K in PA and DanMay like this.
  15. RGregor

    RGregor Been here awhile

    Joined:
    Oct 31, 2007
    Oddometer:
    838
    Location:
    Bavaria
    Adrian, it's even better. We're shipping five cams! OK, two of them should be the same ....
    1 * Schleicher Ro340.10 (330°)
    2 * BMW 336° sports cam. Michel thought it would be a good idea to measure both and compare the results
    1 * Schleicher Ro214.100 (344°)
    1 * Schleicher KA508 (332°)

    More flesh for the comparative study ...
    #35
    adrenal likes this.
  16. adrenal

    adrenal skrunkwerks.com

    Joined:
    Jun 12, 2008
    Oddometer:
    1,171
    Location:
    Melbourne, Australia
    Brilliant! Should have picked that up from the shipping weight.. The 330 is one that I'm eager to measure given its reputation of chewing out lifters and its own lobes. It would provide a boundary condition. From the data you have already provided, I suspect the problem is due to the combo of overly small nose radius, extra spring force required (more than even the Schleicher 340 for a given rpm) and the small lifter crown radius. Discontinued as a result. TBC...
    #36