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Thread: High altitude and octane

  1. #1
    Registered User bikegeezer's Avatar
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    Originally posted by The Veg
    100LL in Denver sounds like a real misstake! You may recall that the pumps only go to 91 there, rather than the 93 we lowlanders get. There is a reason for that: the higher the octane, the denser the fuel. In thinner air a less-dense fuel works best.
    It's a real mistake to use it in Denver or anywhere else. I know you have no reason to believe me, but here's a quick & dirty explanation for the lower octane fuels at high altitides. High octane fuels aren't denser than lower octane fuels. They burn slower than low octane fuels. They also have more resistance to detonation caused by extreme combustion chamber temperature and pressure.

    Thinner high altitude air reduces the compression and combustion pressures in your engine's cylinders. An engine with say, 200 psi cranking compression at sea level might have perhaps 185 psi at 5,000' elevation. This pressure reduction has the effect of slowing the combustion process, causing the fuel burn to end too late on the power stroke for optimum performance. The fuel grade recommended by a vehicle manufacturer is designed to provide best power and economy by keeping pressures and temps just below the point at which knock will occur (plus a safety buffer for unusual conditions). This is why many of us find that using slower burning high octane fuels at lower elevations, in an engine designed to run on regular, actually lowers fuel mileage. Increase the altitude, and the situation worsens.

    You could get around this problem by advancing the spark timing at higher altitudes to just before the point that knock begins, or the same effect can be had more conveniently by using a faster burning fuel...lower octane. The whole idea is to control combustion pressure and temperature by keeping the fuel burn end point from retarding at higher elevations. I hope I haven't been too technical. You can probably find this info and maybe some pictures on Howstuffworks.com.

    As for 100 octane Low Lead aviation fuel, that stuff has five times more lead in it than any automotive fuel ever had. So it has a tendency to coat everything in the combustion chamber, including the spark plug electrodes, with lead deposits. A bike or car engine doesn't need that stuff, at least not initially. But use it long enough, and the increased compression due to the deposits will create such high pressures and temperatures that 100LL is all the engine will run on without knocking. Kind of a self-fulfilling prophecy. Also, avgas contains compounds that tend to destroy the rubber components commonly used in automotive fuel systems.
    Last edited by bikegeezer; 02-20-2004 at 02:02 PM.

  2. #2
    What's that noise...? basketcase's Avatar
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    Yikes!

    Stu ... I think I understand what I heard you say, but what I am not sure of is if what I heard is what you meant!

    Actually, the explanation was very helpful. But what about those of us with the computerized ignition systems?

    I had altitude plugs for the K100 and K75, (for riding above 5,000 feet) but I (1) never used them, and (2) let them go when I sold the bikes. And the highest altitude I've ridden at to date is crossing the mountain on the Cherohala. (The brevity of the moment means it is not worth the hassle of putting in the plug).

    That said, is there still a place to get a plug for a K model? And, could you speak to how the gadget helps the engine?

    Thanks,
    Rick
    '98 BMW Z3 Roadster, '00 R1100RT

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  3. #3
    Registered User bikegeezer's Avatar
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    Rick,
    Are you referring to high altitude spark plugs or a ECM plug for high altitudes?

  4. #4
    What's that noise...? basketcase's Avatar
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    The ECM plug -- about 2" long, and the thickness of a 1st grader's fat style beginner pencil. Snugs into a plug hidden away under the seat of the K models.
    '98 BMW Z3 Roadster, '00 R1100RT

    If you insist on exercising a right to burn our flag, first be so kind as to wrap yourself in it and then douse yourself with gasoline just before you strike the match.

  5. #5
    Registered User bikegeezer's Avatar
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    That's what I thought you meant. Those plugs are only necessary when riding continuously above 5000', like if you moved somewhere like Denver, but it might also be helpful if you planned to spend a week or longer riding at those elevations. If you're just passing through once in a while, don't bother. As for availability, I couldn't say.

    Hopefully, these systems will move out of the Pleistocene Era by incorporating a way to monitor barometric pressure and manifold pressure (or mass air flow) as most fuel injection systems do. Then, there'll be no need for "altitude chips." Maybe that's already happened; I'm not up to speed on BMW's latest systems, but the ones I've looked at calculate air flow and density from intake air temp, throttle position, and engine rpm. That becomes inaccurate above a certain altitude without pressure info.

    Maybe we should move this out of the Airheads area, eh?

  6. #6
    Focused kbasa's Avatar
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    Originally posted by Chickenman_26
    That's what I thought you meant. Those plugs are only necessary when riding continuously above 5000', like if you moved somewhere like Denver, but it might also be helpful if you planned to spend a week or longer riding at those elevations. If you're just passing through once in a while, don't bother. As for availability, I couldn't say.

    Hopefully, these systems will move out of the Pleistocene Era by incorporating a way to monitor barometric pressure and manifold pressure (or mass air flow) as most fuel injection systems do. Then, there'll be no need for "altitude chips." Maybe that's already happened; I'm not up to speed on BMW's latest systems, but the ones I've looked at calculate air flow and density from intake air temp, throttle position, and engine rpm. That becomes inaccurate above a certain altitude without pressure info.

    Maybe we should move this out of the Airheads area, eh?
    The new Motronic systems have an 02 sensor and I believe they have a closed feedback loop that will monitor CO production in the exhaust. This helps the FI system tune the fuel input to the right ratio.

    The older FI systems, like the ones on early Kbikes, don't have any feedback at all and are really not much more advanced than the even earlier mechanical systems on cars.
    Dave Swider
    Marin County, CA

    Some bikes. Some with motors, some without.

  7. #7
    Registered User TRJeff's Avatar
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    Thumbs down fuel

    Remember as you ride around the states alot of areas are what is called nonatainment areas by the E.P.A., so the fuel is reformulated to polute less ???????? your bike may not like the stuff they call gasoline.
    where I live on the lakeshore of Wisconsin we have reformulated fuel, 20 miles inland you get a diffrent blend, I have noticed my bikes don't like the fuel I can buy in town, but run much better if I buy away from the nonatainment area. I think much of the fuel you will purchase at altitude will be a reformulated blend and MAY cause some running problems. My 2cents worth.

  8. #8
    Registered User bikegeezer's Avatar
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    Kbasa,

    Just this one more post, then I promise I'll shut up before you guys hang yourselves...or me. Anyway, you're correct about the feedback loop. The 02 sensor allows the ECM to fine tune things during cruise, but it's not intended as an altitude compensator. It's a limited-authority sensor, the main purpose of which is to provide the catalyst with the correct oxygen content in the exhaust gases to perform its oxidation and reduction functions. The ECM uses 02 info to constantly adjust the cruise air/fuel mixture slightly above, then slightly below 14.7/1. There's a lot more to this subject, but I have the feeling I've already overstayed my welcome in this thread.

    Those early BMW FI systems...were those the Larrytronic and Curlytronic? ;>)

  9. #9
    Focused kbasa's Avatar
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    Not at all, man. You've got some great information.



    What else do you know about this stuff?
    Dave Swider
    Marin County, CA

    Some bikes. Some with motors, some without.

  10. #10
    Registered User bikegeezer's Avatar
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    Originally posted by KBasa What else do you know about this stuff?
    A lot more than I should post here if I know what's good for me. I haven't seen a lot of it on this site, but when you jump into threads and start busting myths, you often get flamed. I don't want to be seen as a smartass, but I'll try to help if someone asks.

    BTW Kbasa, how do you post pics here? I tried to post a 300 X210 pixel JPEG, and the message said it was too big a file.

    Stu O

  11. #11
    Focused kbasa's Avatar
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    Originally posted by Chickenman_26
    BTW Kbasa, how do you post pics here? I tried to post a 300 X210 pixel JPEG, and the message said it was too big a file.

    Stu O
    Hey Stu, I finally got off my ass and wrote a post that should help. It's right here. Hope it helps!

    Dave
    Dave Swider
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    Some bikes. Some with motors, some without.

  12. #12
    Registered User bikegeezer's Avatar
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    I think I got it. Thanks.
    Attached Images Attached Images

  13. #13
    Jim Bud
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    Effect of Techusion R259??

    Stu O, what do you think of the design/operation of the Techlusion R259 on an oilhead? Yes, I have one, and I'm trying to learn more about how best to tune it?? It runs well and I like it, but there is always more to learn...

    http://www.techlusion.us/r259.pdf

    Thanks, Jim Bud

    http://www.techlusion.us/r259.pdf
    Jim Bud...

  14. #14
    Registered User bikegeezer's Avatar
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    Hi Jim,
    Techlusion operates by altering the closed-loop 02 signal voltage seen by the bike's computer and altering the on-time command to the injectors. This essentially defeats the machine's emission control system. The Techlusion can also apparently add acceleration fuel, top end fuel, and open-loop cruise fuel. That last one's a mystery to me; unless you disconnect the 02 sensor, the bike won't be cruising in open loop except maybe for the first 2 minutes after a cold start.

    The Techlusion literature suggests tuning the four potentometers using a seat-of-the-pants technique. But aside from removing the surging, you'll never really know what else you've accomplished unless you tune it on a dyno. If I had purchased a Techlusion R259 and didn't have access to a good dyno shop, I'd adjust only the pot controlling closed-loop cruise and leave the other three at base setting. Techlusion's instructions for that pot are very good. Then again, one might be able to accomplish the same thing without an R259 simply by disconnecting the 02 sensor wires.

    Stu

  15. #15
    Jim Bud
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    R-259 tuning

    Stu, your comments fit with my observations fairly well. While I added the unit only after many careful tune up's ( TB balance, valve adjustment, etc) and attempts at TPS adjustment; to correct the mild surging and engine roughness around 4,000 rpm; and while these things did improve the engine operation, they did not give me the ongoing operational consistency I thought they should. After initially trying the R259, set up as recommended, I found it to be too "abrupt" in it's approach to throttle management, with "around town" gas mileage greatly affected. ( maybe this was significantly influenced by the relative ease of hot-rod starts and aggressive acceleration from the standard tune recommendation) However, upon further investigation of the "tunability" of the unit; I discovered that I could bring each of the four operational Pots down considerability in their effect(using careful DVM adjustments) and produce a very pleasant perormance level with significant improvement in local fuel milage. And, effectively, I have ended up very close to your recommendation as a very pleasant solution to making the beast a great riding machine. I have not tried to just disconnect the oxygen sensor; and it was my understanding that the electrical signal from the O sensor was also modified by the unit; thereby allowing for adjustments that wern't defeated by the normal O sensor singal?? Not sure on this though.

    thanks for your suggestions.

    JIm Bud
    Jim Bud...

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