[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]

Re: newbie questions ... (snow tires, and octane number)

          If you notice a difference or not between 87 and 91 will
          be a function of your engine and setup, compression,
          timing advance, boost level and if you have/don't have
          dynamic knock-sensing ignition control. If your setup
          does not experience detonation with a given fuel
          octane level, then running higher octane fuel quality
          above that level will not help or be noticable.
          If you are getting detonation with a given fuel quality,
          and/or if you have knock-sensing ignition that can
          advance the timing more with higher quality fuel, then
          you should notice an improvement in performance and/or
          reduced or no detonation.

          If you want to test this, for example, with an older
          non-knock-sensing 4000Q, run some rot-gut 86 octane fuel.
          Advance the timing gradually untill you get detonation
          under heavy load, accelerating in a high gear and/or
          uphill works well. Then switch to a higher-octane fuel,
          like 93 or 94. You should now notice that the detonation
          is gone and your vehicle responds better and feels faster.
          This is the "hot setup". You want to run as much advance
          and compression as possible without detonation. The ideal
          setup is just on the edge of detonation. Only time and
          testing can achieve this high level of tuning with the
          ol' "manual adjust" ignition timing systems.

          If you have a knock-sensing dynamic system you could try
          running timing tests at a local drag strip with rot-gut
          and with 93/94 Ultra and compare the difference in ETs.
          This will be difficult because of other factors beyond
          your direct control such as atmospheric pressure, ambient
          temp, himidity etc. It might take several attempts under
          similar conditions and/or averaging the results of many
          runs to observe the difference. Then again, you might be
          able to tell right off.

          The bottom line is, with "manual" ignition systems, if you
          are willing to take the time to get the timing adjusted
          right on the edge of detonation you should be able to
          get better performance and MPG using the highest quality
          fuel available. If you just want to set the timing at stock
          and get average performance, then good ol' 87 should be
          just fine. If you get detonation, back it off a bit more.
          It's time/performance/fuel-cost tradeoff.