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Thread: Altitude Limits on Speed

  1. #1
    Z8 Novice
    Join Date
    Aug 2004
    Posts
    20

    Altitude Limits on Speed

    I recently found myself in a position to make some all out runs at altitudes between 6700 and 7000 feet.

    My S2 Z8 would not make more than about 5000-5200 rpms suggesting by math (and the onboard speedometer) a final speed of about 150mph.

    Does anyone have any data with respect to altitude and its relationship to maximum rpms (and therefore mph)?

  2. #2

    I think this may be indicative of something else....

    as the altitude shouldn't effect the engines ability to rev thru the gears, though it may possibly limit the top speed in top gear. I often run up to 8,000ft here in the Angeles Crest, and never noticed any restricted breathing, but I'm mostly in 3rd & 4th on those roads. GM lives at 6,000ft in Montana, and has an S2, so he would be the best sounding board on this.
    Andrew Macpherson

    Expert Z8 Inspections, with full support for both Z8 sale and purchases.

  3. #3
    Grease Monkey
    Guest

    Z8 at altitude

    If you were able to hit redline in 1st through 5th gears but ran out of steam in 6th, you simply ran out of power. Your S2 modified Z8 is rated at 461 HP at sea level. At 7000 feet that same engine is producing roughly 364 HP, which is less than a stock Z8 at sea level. That should go a long way towards explaining the brick wall you ran into at 5000 RPMs, which with stock tire diameter and gearing, would equate to 142 MPH in 6th.
    On the other hand, I suspect there may have been something else involved here since I believe your Z8 should have been able to go faster, even at 7000 feet. If the ambient temperature was high, your ECU may have dialed back power production to protect the engine from potential damage (this is normal) resulting in an even greater power loss. There is also the possibility that your ECU did not properly compensate for the increased richness of the fuel/air mixture which results when operating at high altitudes thus preventing your engine from making its full power potential.
    One thing you might consider is changing your rear axle ratio to 3.64:1 which will not only provide faster acceleration in all gears but allow you to reach redline in 6th under most circumstances. The increase in torque available due to the gear change should provide a higher top speed under the circumstances you described.
    Hope this helps!

  4. #4
    Z8 Novice
    Join Date
    Aug 2004
    Posts
    20
    Pondering your answer which is much appreciated.

    I anticipated your comments re the differential. Installation of a Quaife 3.64:1 LSD is underway in the shop as I write this.