Airhead salt racer

Discussion in 'Airheads' started by adrenal, Jan 30, 2014.

  1. ME 109

    ME 109 Long timer

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    It's the dead centre of town on the outskirts of town, and it's mostly a lawn cemetery with lots of nice trees etc.
    Very quiet, mostly. 'Cept for the bloody bagpipes every so often.
    Or a 9" or 5" grinder at my place when there are no funerals underway. :D
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  2. Kai Ju

    Kai Ju Long timer

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    Re the dyno numbers. As far as I'm concerned the real value in a dyno is to evaluate changes made. Before and after. I would almost advocate that whatever number you achieve on your first runs, be arbitrarily labeled 100. Subsequent runs are either above or below, depending on how successful your mods were.
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  3. pommie john

    pommie john Long timer

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    Have you not touched the ports yet? That's where the horses reside. Standard inlet ports are shockingly bad!
  4. adrenal

    adrenal skrunkwerks.com

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    The ports had a going over by a local wiz, but he doesn't know Airheads so he gingerly applied some general porting principles. My flow bench says he didn't do a bad job but, depending on how much more I can tune the runners and exhaust, will need a few more CFM (while keeping port velocity up) if I'm to reach the target 90 DJBSRWHP!
    Actually, the post 1988 ports are not a bad starting point but yes, I wouldn't be getting the 82 DJBSRWHP with the ports in stock condition..
  5. pommie john

    pommie john Long timer

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    My recollection is that you're correct about later heads being better. There is more meat on the floor of the port which helps . If I recall correctly a decent bowl above the inlet valve makes a good improvement.
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  6. DanMay

    DanMay Adventurer

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    Very Funny, "DJBSRWHP" ! From my experience, on my Dyno using ISO 1585 correction for air and temp conditions, the BMW airheads produce about 18-20% less at the rear wheel than the published BMW crankshaft specs. So a R75/5 which has a published 50 crank HP from BMW, puts out right around 40 RWHP on my Dyno. Moorespeed also states he gets about an 18% loss from published numbers. I have run an R50/5, R60/5, R75/5, and a R90S with similar results. My dyno has a RWHP to Crank correction factor, but I never use it. Like you, I am really looking for A-B-A, before and after testing (you would be surprised how much more power you make with hot vs cold oil). More important to me, is Average HP and Average Torque over the shift power band.

    Dan
  7. DanMay

    DanMay Adventurer

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    I was digging through some boxes in the garage, and I found a set of the CC Products High Ratio Rocker Arms. I thought you might like to see some pictures of them. Of course, one had a catastrophic failure.

    IMG_5193.jpg IMG_5194.jpg IMG_5195.jpg
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  8. Jim K in PA

    Jim K in PA Long timer

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    Thanks for sharing those pictures. I've never seen them before, and I am amazed that they ALL didn't fail at that adjuster bore. There's nothing left after they bored it for the jam nut!!!!

    Did CC products also sell stiffer push rods? The stock ones seem to deflect pretty easily.
  9. DanMay

    DanMay Adventurer

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    Yes, here is a picture of the push rods. IMG_5196.jpg
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  10. adrenal

    adrenal skrunkwerks.com

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    Hey Dan. It's hilarious that the world of RWHP has been pushed up by the magic power making properties of the DynoJet. It's more hilarious when you consider that we can thank a 1980's Yamaha V-Max for the powerboost....or so the story goes. Apparently, in Dynojet fledgling days, they added a factor to square their machine's numbers with the Yamaha published HP which was no doubt at the crank. Now everyone's machine conveniently benefits from the exact same drive train loss! Very obliging of Dynojet but, of course, a complete nonsense.

    Richard Moore uses a Dyna Pro dyno. If you look at Richard's dyno charts on his web page, you'll see the words 'DJHP Corrected power' in the lower RH corner. It's unfortunate that, due to Dynojet's influence in the dyno world, Dyna Pro have felt obliged to add their own (optional) nonsense factor. They make their reason clear in their manual which uses thinly veiled disparaging language without mentioning names. But 'DJHP' is the give-away..

    Re: 18% airhead drive train losses. Another perplexing number! 18% is a high number by any standard including drive train losses in cars. BMW most likely used a steady-state engine dyno for their specifications because it is the only way to get true HP. Having your own inertia(?) dyno, you probably have twigged to this as well Dan, but I think the number is high because we are perhaps comparing the factory steady-state number with numbers produced by inertia dyno's which are the more common type.

    For those that may not be familiar with the nature of inertia dyno's: The thing about inertia dyno's is that roller acceleration is not the only mass-acceleration being measured. An inertia dyno effectively measures the acceleration of all the rotating masses from the crank, through the gearbox, shaft and bevel drive to the back wheel and the dyno roller. Change any of these and you'll get a different RWHP number. The airhead will always read lower than exactly the same engine with a chain drive, not only because of the additional friction in the drive shaft/bevel gear but mainly due to the work required in spinning up these additional masses. Change a worn tyre to a new one with a different profile and you'll get another RW number. This is why it is a complete nonsense to blindly compare RWHP figures (even on the same dyno) between two different bikes using an inertia dyno. A steady state dyno effectively dials out these differences.

    To confuse the situation even more, what if you are using a Dynojet in pure inertia mode? Then presumably 10 - 15% of the 18% is already accounted for by the V-Max factor?? i.e. the difference with BMW factory numbers is reduced to 3-8% !!! Spare me!!

    The upshot of all this: forget HP comparisons! Unless HEAVILY specified and qualified. And even then..
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  11. DanMay

    DanMay Adventurer

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    It is crazy, and fortunately I do not perform Dyno runs for a living, just for personal and friends use. Like any good tool, once you understand how to use it and get predictable results you will also realize its limitations. I am very hesitant to quote HP numbers outside the shop. It just does not do any good comparing apple to oranges. What I will share is before and after HP and Torque curves, which is all that really matters. The most important part of the Dyno is dialing in jetting with the air fuel meter. That is where I have learned the most.

    It is also a great tool to break-in new engines, especially in sub zero winters...
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  12. adrenal

    adrenal skrunkwerks.com

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    Thanks for posting the pics Dan.
    It's interesting that CC didn't quote the actual rocker ratio??
    Having gone thru the process myself, I'd be surprised if they were much more than 1.5:1 which is only one pip above stock. But I may be mistaken. It looks like the axle is in the same position across the head but the posts are probably higher. I'm curious: Would you mind measuring the height from the head mounting surface to the axle centerline?
  13. DanMay

    DanMay Adventurer

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    The rocker blocks are stock BMW, I measured against another set and they were the same. Actually, the needle bearings and shafts are stock BMW also. So, the centerline is at the same deck height on the CC Products as the stock BMW. I tried to measure the distance between the centerline of the pivot point to the centerline of the roller tip and the centerline of the pushrod to the centerline of the pivot point to calculate the ratio, but it seems to tough to get an accurate measurement with the caliper I have. I came out to about 1.6 but I would like to figure out how to get a better measurement. (Perhaps it would be easier to measure from the photo ?).
    IMG_5199.jpg IMG_5198.jpg IMG_5197.jpg
  14. adrenal

    adrenal skrunkwerks.com

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    Yeah, its a difficult thing to measure. Thanks for trying.
    Mine are 1.62 aided by longer valves that are needed for the extra lift on the 340 lobe and higher deck height for the shaft. Also had to move the shaft towards the pushrod so that the tapered rods would clear their tubes. This also helped the ratio.
    Can't see how CC would have achieved 1.6 unless I'm missing something..
    Not good that the thrust bearings were acting directly on the aluminium rocker. There should be a hardened washer between them.
  15. DanMay

    DanMay Adventurer

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    I tried to measure again, but I cannot get consistent measurements. See the attached photos (I used an IPhone ruler app), and even these numbers are difficult. I wanted to add that I never actually used these. They came with a box of spares along with a race bike I purchased. The bearing wear marks in the side of the rocker may be from improper installation. There were plenty of shims with the rockers, and I suspect the installer could have sandwiched the bearing between them. If you like, PM me your mailing address and I will send you the broken one. You can keep it for your R&D.

    FYI, I use the stock +1985 rockers. They perform the best and move around the least. I get all my lift and duration from a custom IMG_5200.jpg IMG_5201.JPG cam using stock lifters.

    Dan
  16. adrenal

    adrenal skrunkwerks.com

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    Thanks for having another go and the offer but I think the pic is enough to tell the story..

    The pic of the stock rockers next to the CC's says a lot. BMW did a pretty good job of optimising strength/weight and they hold up in race conditions. Though the stock rocker geometry is not so flash - particularly on the valve side. I think it may have been Richard who mentioned that the rockers with the feet per your pic don't hold up as well as the other types. Manufacturers of high performance rockers have moved away from aluminium in preference to HT steel or Stainless Steel. My rockers look very different as they were designed with a different manufacturing process in mind. (Ssssshh, don't tell anybody, but prototypes are currently being made in Titanium using EBM additive manufacturing. Haven't put it out there yet as it may all go horribly wrong! Will need to develope a custom cam and porting for this arrangement to fully exploit the target lift - around 14.5mm at the valve).

    I had to compromise on the geometry to keep the rocker gear under the stock rocker covers. The shaft would have to be raised a full 11mm to optimise the goemetry - you'd end up with a wider bike.
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  17. nk14zp

    nk14zp Been here awhile

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    Then throw port velocity into the mix. The most flow doesn't always make the most power.
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  18. adrenal

    adrenal skrunkwerks.com

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    You betcha! Some say velocity is king. And is one of the main reasons a flow bench can seriously throw you off the scent only to go backwards! I'm just about to use mine for the first time in anger. have a velocity probe - wish me luck!!
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  19. mach1mustang351

    mach1mustang351 Long timer

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    See!!! You guys get it!!
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  20. Kai Ju

    Kai Ju Long timer

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    Unless you care about your fellow BMW road racer, engine width should be of no concern on the straight and narrow, should it now ?
    Regarding the valve not fitting, I can't see how you could not fashion a spacer ring between the head and the valve cover. Or better yet, weld in a speed hump where the rocker would interfere. Another option might be to take 4 valve covers and cut them in such a fashion that when you weld them back together you end up with the additional height needed.
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