Results 1 to 2 of 2

Thread: Blowers, altitude, and octane

  1. #1
    I'm wondering if the octane and elevation relationship (not sure I know how to state this) is as relevant with the supercharger. Our Mini Cooper takes premium because it has a blower, and it doesn't seem to care what the altitude is, although, admittedly, the Grapevine and Tehachapi (~4,000 ft) are about as elevated as we get. I remeber reading somewhere that the P-51 Mustangs were fitted with superchargers so they could get more altitude than the Messershmitts (sp?). They also had nitrous oxide and water injection systems in various models.
    Has anyone run a supercharged boat motor at Powell on the lower octane fuel?
    My Merc 600 SCi runs great on the Mercruiser recommended 91 octane pump gas at Havasu. When I go to Powell this year, for example, or Cour d'alene or Lake Roosevelt next year, which are all higher than the ~500 foot altitude of Havasu, will I need octane booster, can I run lower spec 87 octane fuel, or something in between? A high compression naturally aspirated motor I can see would be affected by higher elevations, since the air is no longer being pushed into the engine by 14.7 PSI nominal seal level air pressure. A blower, however, takes whatever is there, and shoves in more until it reaches the preset boost level (at least the modern systems) no matter if at sea level or not. At least, that's what I'm thinking.
    What this all goes to is that my motor is brand new, and under warranty. I don't want to void the warranty, and I don't want to break or damage it even if it is under warranty, because I'm taking it on the boat vacation that I've been trying to get for about five years now. I also don't want to pay extra for fuel specifications that I don't need. Thanks!

  2. #2
    ULTRA26 # 1
    Not an expert here but have spoken to a few guys about this issue. Most recently, I spoke with Mike {clownroyal)
    (DCB F32) when we were on Coeur d Alene over the 4th of July. His twin Merc 700's which use almost identical computer controls. He runs the same fuel on CDA as he does on Havi, and is getting near the same MPH. As I recall 136 on Havi and 130 on CDA. The ECU will adjust the air fuel ratio and timing for the altitude, so no need to worry about octane boosters. Also, I have seen dyno sheets on a Merc 496HO and Merc is running at 13:1 or below throughout the power band. You will be fine.

Similar Threads

  1. Re-jetting for altitude
    By franky in forum Gear Heads
    Replies: 9
    Last Post: 07-15-2006, 06:52 AM
  2. Low Altitude...
    By Jbb in forum Sandbar
    Replies: 3
    Last Post: 03-06-2006, 09:46 AM
  3. Altitude????
    By ELIMINAT THIS in forum Gear Heads
    Replies: 11
    Last Post: 06-02-2005, 04:15 PM
  4. Altitude adjustments
    By lovemyultra in forum Gear Heads
    Replies: 4
    Last Post: 04-08-2003, 07:39 PM
  5. 460 Altitude Adjustment
    By 77_Tahiti in forum Bench Racers
    Replies: 7
    Last Post: 07-19-2001, 10:35 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts