Which petrol

Discussion in 'Small Block Tech' started by [JP], Apr 26, 2018.

  1. [JP]

    [JP] Well-Known Member

    Hi all,

    somewhere on my build thread of my 350 I remember someone saying "use the higher octane petrol when you get it running again"

    and I didn't think any of it, thought petrol will be the same.
    anyway, the other day I read on here, on some thread, someone saying that their car ran really well on the 87.

    As I had never heard this number I researched a bit. fuel is different in the US than to here in the UK.

    So apparently you guys have 85 AKI (90 RON) and 91 AKI (95 RON)

    Over here we have 95RON, 97RON and 99RON. - A few years ago there was even 102RON available.

    So... out of all of these which one would be best to run? This question arised when I read about the 87 someone using and the fuels being different, so maybe my higher octane fuel is to high?
    or should I just go for the higher octane rating as to avoid the early detonation? or is it a case that a 97 or even a 95 will be just fine?


    thanks!
    JP
     
  2. TrunkMonkey

    TrunkMonkey Well-Known Member

    The higher octane rating the better the anti-knock factor.
    You should run the lowest octane numbered fuel you can without knock. ( A properly tuned engine is key)
    Light pinging under hard acceleration/load is OK.
    Running a higher octane rated fuel is mostly tossing money out the tailpipe.

    Do you know what the compression ratio is for your engine?

    I found your earlier thread about the engine, so at 10.25, you will be fine with the 95 RON.

    I have a nailhead 10.25 and am running 87 non-ethanol, equal to 91-92 RON, with no issues.
     
    Last edited: Apr 26, 2018
  3. alec296

    alec296 i need another buick

    I would suggest using the mid grade minimum. The lower grade is fine for the later low compression engines. You have more compression. So I would tune on that. Then try the lower grade and see if it affects how it runs or if you get spark knock.
     
  4. DEADMANSCURVE

    DEADMANSCURVE my first word : truck

    light pinging is not ok .
    use normal higher octane gas . i don't even use #87 in my lawn mower . saw a good demonstartion a long time ago comparing low and hi octane fuel . low = fast burn rate , hi = slow burn rate . translation is that fast burn is like hitting the top of your piston with a hammer ( thousands of times ) , slow burn is more of a "push" .
    i've been driving a long time - never had to repair anything related to rings , ring lands , valves etc . maybe a clean up from carbon but nothin' busted .
     
  5. Schurkey

    Schurkey Silver Level contributor

    ^^^Bold is false.

    ^^^Bold is false.

    Light ping under LIGHT LOAD is acceptable, but not preferable. GM used to call it "The Sound of Economy" to get out of "fixing" it under warranty. Any detonation under heavy load is very likely to cause damage.

    Octane rating by itself does not change the burn rate. Burn rate can vary by fuel composition, but it's not an "octane rating" thing. "Gasoline" can be made from a mix of about a thousand hydrocarbon chemicals, and different mixes can have the same octane rating...but not the same performance otherwise. Burn rate also varies by mixture strength (rich/lean) and by in-cylinder turbulence. There are probably other factors.

    What is true: Higher octane rating than actually needed to prevent "ping" or "spark knock" is wasted. If in doubt, start with the higher-octane fuel and blend downward until you have spark knock...then increase octane until the knock goes away.
     
  6. DEADMANSCURVE

    DEADMANSCURVE my first word : truck

    here is the demonstration I saw put on by a champion spark plug tech . he was actually showing us examples of what effects dif heat range plugs have on cylinder temp using a single cylinder motor outfitted with various temp sensors . FYI 2 heat ranges hotter is a problem area concerning metal fatigue .
    he then did a little thing with a long glass tube sealed on one end and a ping pong ball size thing inside . put in a few drops of regular gas , held his hand over the end , rocked the tube back and forth a few times ( air-fuel mix ) , uncovered the end and held up a bic lighter to the open end = sounded like a small gun blast . scared the crap outa the girl holding the lighter . same process using premium fuel , uncover end and lit it off , made a longer duration loud "whoosh" sound . made sense to me and he had no reason to trick us or bend the obvious . didn't have anything to do with selling spark plugs .
    not that it makes any dif but it was mid 70's gas he was using .
     
    300sbb_overkill likes this.
  7. TrunkMonkey

    TrunkMonkey Well-Known Member

    What I said is correct.

    Light ping under heavy load, for example, you are in a higher gear at a lower RPM, a place you would not normally be indicates the threshold of ping or detonation and typically not a concern as no one operates in that realm for more than a second or two at any time. Similarly, one tunes an engine by vacuum and or RPM, or "smoothness", when you get to the threshold, you back off, and are for the most part, "optimal", but you do not operate at that threshold.

    Having ping under light throttle condition is more likely to be a "bad sign" and under heavier acceleration would result in detonation or hard knock.

    And as far as the whole argument of "octane" and fuel blending, it is a bit deeper than what most people deal with in their real world lives, they understand what the meanings attributed to the words used. I know fuels, and such discussion is far beyond what the OP asked. One is not likely to be obtaining more than a few variations in blended fuel at the gas pumps. It will pretty much be the same thing, within a margin undetectable variable from one fillup to the next, at any given gas station. And the swings in for summer/winter conditions to meet evaporation (phase threshold) requirements.

    If we all start breaking out hard science and engineering, the only people remaining in the room will be Rocket Surgeons, and all the pretty girls will have left with the guys in the cool cars.

    Remember, this is supposed to be fun.
     
  8. Test and tune carefully. I feel it's best for the OP to note that no audible detonation, over any prolonged time, should be acceptable, especially on an engine built for performance.

    Also, an engine experiencing detonation under light load, does NOT mean that it will ping under WOT. While part throttle cruising, with vacuum advance, the engine is advancing timing for a more economical burn. The carb could be tuned much richer at WOT therefore eliminating detonation during heavy load/acceleration.

    I feel that the extra cost of a higher octane fuel is a better option than potentially burning holes in your pistons.
     
    300sbb_overkill likes this.
  9. MrSony

    MrSony Well-Known Member

    That was probably my post you saw. My 7.8:1 engine liked 87 octane, which is the lowest octane you can pretty much find in the US. I run 91 octane as much as I can in my current '70 SP engine (originally 10ish:1, with thicker gaskets now about 9.3-9.5:1). Gets the best mileage with 91. 89 is so so, 87 you can almost see the needle going down. Essentially, to take all the guess work out of it, run the highest grade gasoline/petrol you can afford and tune accordingly.
     
    8ad-f85 likes this.
  10. [JP]

    [JP] Well-Known Member

    Oh wow!! what a great discussion! wasn't expecting this much! thank you all very much!

    I did have a read about the octanes, basically what' I read is that the lower octane doesn't cope very well with the higher compression, meaning air pockets can start exploding before the actual explosion created by the spark plug.
    the higher octane is more resistant to the compression, meaning explosion occurs when the spark plug ignites, and therefore preventing the pinging...

    So, looks like I'll be playing around with the petrol we have here. Now.... just so that you can all feel sorry for me, I'll put the average prices, converted to US gallons, in my area. Bear in mind I live in the country side, prices in London are way higher!

    95 RON - $6,5 per US gallon
    98 RON - $7 per US gallon

    can't find the 99RON price... but you get the idea, it's not cheap!
     
  11. Fox's Den

    Fox's Den 355Xrs

    Well, I read somewhere that higher octane has a slower burn rate than the lower octane. Isn't that why you can up the timing with higher octane gas? Put in some 110 race gas and watch the black smoke come out the tailpipe and the car will run slower. I tried this when I had the lower compression engine in and it did not work.

    What I would do for over in the UK is run the middle grade and see how it runs and check the plugs to see how it is running. Go to the track and play with the timing then play with the jetting, use MPH as your guide.
     
    TrunkMonkey likes this.
  12. [JP]

    [JP] Well-Known Member

    just out of curiosity, I know petrol is cheap in the US, but... how cheap? what do you guys pay there for the gallon?
     
  13. TrunkMonkey

    TrunkMonkey Well-Known Member

    Around $2.75-$3.30 gallon.

    It varies in different areas, and summer the cost goes up.
     
  14. MrSony

    MrSony Well-Known Member

    Freakin $3.04 a gallon here. Takes $48 to fill up my 16 gallon tank.
     
  15. MrSony

    MrSony Well-Known Member

    I remember the last two times my car was down for the count, gas was well under $2. -__-
     
  16. Brad W

    Brad W Miles from nowhere...

    $3.71 here SO. Cal. for 91 octane... You can find cheaper if you hunt around...
     
  17. JoeBlog

    JoeBlog Platinum Level Contributor

    Here in middle America (Kansas), it’s 87 - 89 - 91 octane at $2.38 - $2.48 - $2.58 respectively, with no-ethanol 88 octane coming in at $2.68. That’s about the way it is for the mile or two area near my house.
     
  18. Fox's Den

    Fox's Den 355Xrs

    We pay from 8-12$ a gallon for 110 cam 2 I always put in 5 gal with each tank fill up.

    Right now BP premium is 3.60 a gallon. I am sure Sunnoco and Shell are about the same.

    NE Ohio
     
  19. [JP]

    [JP] Well-Known Member

    damn!!! I knew it was cheap... didn't realise that cheap!
     

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