Results 1 to 10 of 10

Thread: fuel octane - 2.4 motor

  1. #1

    fuel octane - 2.4 motor

    Just been reading through the Porsche Mechanical Fuel Injection Check Measure Adjust booklet.

    Noticed a note regarding YM72 onwards 2.4 motors & the use of high octane fuels :

    'From Model '72 on, all engines use 91 octane fuel.

    If these engines are run on higher-octane petrol, the fuel will not be properly utilized.

    Reason : the higher the anti-knock characteristic of a fuel the less easy it is to ignite.

    Thus if a 2.4 liter engine is operated on 98 octane fuel, the CO contant in the exhaust gas will be high and very difficult to adjust properly.

    When difficulties arise adjusting the CO content, ask the customer what kind of gasoline he is using.'

    Just thought it was interesting
    1973 2.4S
    1993 964 C2
    987 Boxster Spyder


  2. #2
    I've read that to in the Bosch manual. I agree to a point because if a car sits alot (like mine) the fuel we get today breaks down rather quickly and after a month or so is pretty much stale.
    I keep racing fuel in the car during the winter and have been doing it in my antique bikes for years. Never have a problem with stuck gummed up carbs or a no-run situation when spring arrives.

    Early S Registry #235
    rgruppe #111

  3. #3
    Senior Member
    Join Date
    Jan 2003

    I've tried both standard and super unleaded in mine down south (so 95 and 97/8ROn) and honestly the car runs better over all on the lower octane fuel.

    It has more top nd pull on the higher octane stuff but does not idle as cleanly or pull as strongly a low revs.

    Autofarm, who lokk after it have said that its fine to run it on the low octane, as it was originally recommended to run it on 2-star fuel admittedly that was leaded.

    I have not noticed any problems after not using it for a few weeks. ok I own up to letting her sit unused for more than a week or so, but then we do not get the evapouration issues over here that may be a problem in California.

  4. #4
    I usually run the car on optimax. When the summer months come around & I start putting some miles on the car I think I will do some back to back test with optimax & standard unleaded to see if there is any performance difference
    1973 2.4S
    1993 964 C2
    987 Boxster Spyder


  5. #5
    91 Octane is all we get here in CA, unless you drive up to Sear's point and fill up at the Union76 station inside the Paddock
    Chris Purpura @civilizedmisfit
    Member #479
    Current Cars:
    1972 911T aka The "Civilized Misfit" Build
    Miss February - EarlySRegistry 2023 Calendar
    1968 911S Ossi Blau/Beige Corduroy

    Past Cars:
    2019 911 Carrera GTS (sold, no regrets)
    73S - #1100 (restored and now somewhere in Europe)
    1997 993 Carrera 4S Black on Black (sold)

  6. #6

    100 Octane Unleaded....

    Hey Chris, here in LA, there are many (well, not many, but a few...) 76 stations where you can fill up (for $4.99/gal or so) on 100 Octane Unleaded. Some have higer octane unleaded AND leaded racing gas...

    San Vicente and Bundy - (about 2 minutes from my house)
    Entrada - (SM CYN) - 10 minutes
    Union 76 in Palos Verdes - 50 minutes...

    so if one was so IS available...albiet...$$$$$
    Bahia Red '72 911S
    Meerblau PTS 2019 Speedster
    GP Silver, 2018 GT2RS WP....the BEAST
    Daytona Gray 2021 RS6 Avant....BEAST #2...Best daily EVER

    ES #333

    GONE...MANY, many great ones....

  7. #7
    So, then, my 10.5:1 2.4S motor shouldn't be run on 100 octane?!?!

    Sometimes I question the factory's sense ...
    SReg. #778 OGrp: #8 RGrp: #---
    TLG Auto: Website
    Searching for engine #907495 and gearbox 902/1 #229687

  8. #8


    Just this last weekend I saw 2ea 5gal drums of Union 76 110 leaded race gas on the shelf of my local Monument Auto Parts store. I did not see a price, but if you need it you could buy something like this and blend it into our crappy super unleaded we have available here in CA.

    As an added bonus I know its an old racers trick to blend leaded and unleaded fuels for a little free octane boost due to a chemical reaction between the two fuels.

    Doug Roberts
    68 912 Slate Gray
    64 356C Ivory

  9. #9
    Originally posted by Mr9146
    So, then, my 10.5:1 2.4S motor shouldn't be run on 100 octane?!?! Sometimes I question the factory's sense ...
    I realize you're teasing, but I read the same thing. Seems that STOCK motors had lower compression starting with the 2.4 and should run better on lower octane. My '71 S motor has 9.8:1, I believe. So, I need the good stuff or better (read blend of small amounts of race gas). So, in terms of today's gas and ratings, what is the best gas for the 8.5:1 2.4?

  10. #10

    California Gas

    These cars'll run all day long on California's 89 ... they just won't make any power. Most will agree that 87 is looking for trouble.

    91 is, and will always be, the way to go for a stock 911. I've said it before and I'll say it again, I run my high compression 2.4 on 91 all the time with no problem (except for excrutiatingly hot Valley days when she wants to ping).

    But, ya know, when I put 100 in at the track the other day, my car ran better than it ever has before. It had more oomph from a stand-still and would just sing in the upper revs ... smoother, better power everywhere. But 91 still does the trick ...
    SReg. #778 OGrp: #8 RGrp: #---
    TLG Auto: Website
    Searching for engine #907495 and gearbox 902/1 #229687

Similar Threads

  1. Caffene and Octane
    By BBausser in forum Georgia
    Replies: 5
    Last Post: 03-06-2012, 12:10 PM
  2. What fuel / octane to use?
    By 72911E in forum Technical Info
    Replies: 15
    Last Post: 04-23-2007, 04:28 AM
  3. From Octane mag
    By Chuck Miller in forum For Sale/Wanted: Early 911 Cars, 1965 - 1973
    Replies: 2
    Last Post: 10-13-2005, 06:09 PM
  4. 1972 Porsche Motor 2.4L Manual Fuel Injection
    By 72targa in forum For Sale: 911 Parts
    Replies: 0
    Last Post: 09-15-2005, 06:47 AM
  5. Motor Fuel Injection decal
    By ml911rs in forum General Info
    Replies: 8
    Last Post: 03-02-2004, 10:15 PM

Posting Permissions

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

Message Board Disclaimer and Terms of Use
This is a public forum. Messages posted here can be viewed by the public. The Early 911S Registry is not responsible for messages posted in its online forums, and any message will express the views of the author and not the Early 911S Registry. Use of online forums shall constitute the agreement of the user not to post anything of religious or political content, false and defamatory, inaccurate, abusive, vulgar, hateful, harassing, obscene, profane, sexually oriented, threatening, invasive of a person's privacy, or otherwise to violate the law and the further agreement of the user to be solely responsible for and hold the Early 911S Registry harmless in the event of any claim based on their message. Any viewer who finds a message objectionable should contact us immediately by email. The Early 911S Registry has the ability to remove objectionable messages and we will make every effort to do so, within a reasonable time frame, if we determine that removal is necessary.