compression ratio vs octane

Mopar Performance Issues

  1. blue missile

    blue missile Well-Known Member

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    Ok yall,
    Here goes. This is my first tech thread.
    I must tell you that I have as a passion of mine, since high school, experimental physics. When I sat down to design the new car I approached it with that same passion. While we all have our own opinions, nothing can undo or discount the laws of physics. In each phase of the design process I did research at the above and beyond level. I am not sure if yall want information at this level. If not I will not be offended and will try to keep my posts free of such lengthy answers. In either case I will try to keep my opinion to a minimum, unless asked.

    I have read many threads that try to address the issue of octane requirements vs compression ratios in gasoline powered engines.
    I too became obsessed with this question. Here’s what I found out.
    First the fuel does not care about static compression ratio but only is effected by the actual compression that occurs after the valves are all closed. This is referred to as dynamic compression. While DCR is effected by the factors that you use to find your static ratio the DCR can be manipulated by several factors, more on that later.
    Now I have seen that the guys from Canada say that there numbering system is different. If (R+M)/2 is the method that is used, and it should say so right on the pump, the numbers are the same. If not I have not found an official conversion in the petroleum industry archives, but I will keep looking.
    This first bit is from those archives and is about ten years old if that matters to anyone.


    7.2 What is the effect of Compression ratio?
    (The compression ratio talked about here is actual or as we say dynamic compression ratio. AC)
    Most people know that an increase in Compression Ratio will require an increase in fuel octane for the same engine design. Increasing the compression ratio increases the theoretical thermodynamic efficiency of an engine according to the standard equation Efficiency = 1 - (1/compression ratio)^gamma-1
    where gamma = ratio of specific heats at constant pressure and constant volume of the working fluid ( for most purposes air is the working fluid, and is treated as an ideal gas ). There are indications that thermal efficiency reaches a maximum at a compression ratio of about 17:1 for gasoline fuels in an SI engine [23].
    The efficiency gains are best when the engine is at incipient knock, that's why knock sensors ( actually vibration sensors ) are used. Low compression ratio engines are less efficient because they can not deliver as much of the ideal combustion power to the flywheel. For a typical carburetted engine, without engine management [27,38]:-
    Compression Octane Number Brake Thermal Efficiency
    Ratio Requirement ( Full Throttle )
    5:1 72 -
    6:1 81 25 %
    7:1 87 28 %
    8:1 92 30 %
    9:1 96 32 %
    10:1 100 33 %
    11:1 104 34 %
    12:1 108 35 %

    Modern engines have improved significantly on this, and the changing fuel specifications and engine design should see more improvements, but significant gains may have to await improved engine materials and fuels.

    Based on this information I extrapolated the following expansion of the octane chart

    DCR Octane #
    7.1 87.5
    7.2 88.0
    7.3 88.5
    7.4 89.0
    7.5 89.5
    7.6 90.0
    7.7 90.5
    7.8 91.0
    7.9 91.5
    8.0 92.0
    8.1 92.4
    8.2 92.8
    8.3 93.2


    7.3 What is the effect of changing the air-fuel ratio?
    Traditionally, the greatest tendency to knock was near 13.5:1 air-fuel ratio, but was very engine specific. Modern engines, with engine management systems, now have their maximum octane requirement near to 14.5:1. For a given engine using gasoline, the relationship between thermal efficiency, air-fuel ratio, and power is complex. Stoichiometric combustion
    ( air-fuel ratio = 14.7:1 for a typical non-oxygenated gasoline ) is neither maximum power - which occurs around air-fuel 12-13:1 (Rich), nor maximum thermal efficiency - which occurs around air-fuel 16-18:1 (Lean). The air-fuel ratio is controlled at part throttle by a closed loop system using the oxygen sensor in the exhaust. Conventionally, enrichment for maximum power air-fuel ratio is used during full throttle operation to reduce knocking while providing better driveability [38]. An average increase of 2 (R+M)/2 ON is required for each 1.0 increase (leaning) of the air-fuel ratio [111]. If the mixture is weakened, the flame speed is reduced, consequently less heat is converted to mechanical energy, leaving heat in the cylinder walls and head, potentially inducing knock. It is possible to weaken the mixture sufficiently that the flame is still present when the inlet valve opens again, resulting in backfiring.

    7.4 What is the effect of changing the ignition timing?
    The tendency to knock increases as spark advance is increased. For an engine with recommended 6 degrees BTDC ( Before Top Dead Centre ) timing and 93 octane fuel, retarding the spark 4 degrees lowers the octane requirement to 91, whereas advancing it 8 degrees requires 96 octane fuel [27]. It should be noted this requirement depends on engine design. If you advance the spark, the flame front starts earlier, and the end gases start forming earlier in the cycle, providing more time for the autoigniting species to form before the piston reaches the optimum position for power delivery, as determined by the normal flame front propagation. It becomes a race between the flame front and decomposition of the increasingly-squashed end gases. High octane fuels produce end gases that take longer to autoignite, so the good flame front reaches and consumes them properly.
    The ignition advance map is partly determined by the fuel the engine is intended to use. The timing of the spark is advanced sufficiently to ensure that the fuel-air mixture burns in such a way that maximum pressure of the burning charge is about 15-20 degree after TDC. Knock will occur before this point, usually in the late compression - early power stroke period. The engine management system uses ignition timing as one of the major variables that is adjusted if knock is detected. If very low octane fuels are used ( several octane numbers below the vehicle's requirement at optimal settings ), both performance and fuel economy will decrease.
    The actual Octane Number Requirement depends on the engine design, but for some 1978 vehicles using standard fuels, the following (R+M)/2 Octane Requirements were measured. "Standard" is the recommended ignition timing for the engine, probably a few degrees BTDC [38].
    Basic Ignition Timing
    Vehicle Retarded 5 degrees Standard Advanced 5 degrees
    A 88 91 93
    B 86 90.5 94.5
    C 85.5 88 90
    D 84 87.5 91
    E 82.5 87 90

    The actual ignition timing to achieve the maximum pressure from normal combustion of gasoline will depend mainly on the speed of the engine and the flame propagation rates in the engine. Knock increases the rate of the pressure rise, thus superimposing additional pressure on the normal combustion pressure rise. The knock actually rapidly resonates around the chamber, creating a series of abnormal sharp spikes on the pressure diagram. The normal flame speed is fairly consistent for most gasoline HCs, regardless of octane rating, but the flame speed is affected by stoichiometry. Note that the flame speeds in this FAQ are not the actual engine flame speeds. A 12:1 CR gasoline engine at 1500 rpm would have a flame speed of about 16.5 m/s, and a similar hydrogen engine yields 48.3 m/s, but such engine flame speeds are also very dependent on stoichiometry.

    The web site LS1tech.com has more on doing the math and a gentleman named Pat Kelly has a DCR calculator that I will attempt to give you the link to.
    He also states that keeping the DRC below 8.25 will keep you in the pump gas range.
    All of this discussion does not take into account extra measures such as thermal barrier coatings and the difference between quench and non-quench setups.
    I live in Texas where 93 octane gas is everywhere. I am designing for 92 octane gas because that is the lowest premium gas found in most states. I am into cruising and long road trips, so that’s what I am designing for.
    There is another factor which corresponds to DCR and that is cranking pressure. I have found it to be generally agreed that a cranking pressure under 180psi will keep you on pump gas. There is a very good article in “Hot Rod” Aug 2005 on all of these issues which continues to back up all of the data.
     
  2. blue missile

    blue missile Well-Known Member

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    It seems that I will have to find a different way of getting info onto the page. All of the Paragraph marks were somehow deleted and the document was blended into one paragraph.
    The technical article can be found @ www.mr2.com/TEXT/gasoline_faq.txt
    go to section 7.2-7.4.
    sorry again
    Andrew
    Ps got it mostly fixed with different formatting
     
  3. dgc333

    dgc333 Well-Known Member

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    With modern high swirl closed chamber heads with a tight quench you can easily approach 200 psi or a little above and still run on pump gas if everything else is dialed in. With coatings and polishing of the chambers over 200 psi is not an issue.

    I have a 360 with magnum heads that that has a tight 0.039" quench, has between 180-190 psi cranking pressure and a static compression ratio of 10.6:1. No special coatings on the pistons and no polishing of the chambers and I run the engine on 89 octane and can use 87 in a pinch if I am careful how I drive.

    The particulars of the actual design of the engine has as much to do with octane requirements as anything.
     
  4. moper

    moper FABO Gold Member FABO Gold Member

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    Andrew, cool post... Thanks. There is a dynamic compression ratio calculator at KB-Silvolite.com that i use a bit. In terms of the DCR vs octane, the only time that might be really accurate is if the fuel is.... Pump gas is a bit less precise. I build for use with 89 octane.. Around here that's 10% ethanol too, and it accounts for things like liars and mixups where the high octane is contaminated or simply not what is advertised. You know the factors that effect the DCR. Those are rod length, stroke, intake valve closing event, and mechanical(static) compression ratio. To that end, with the rod ratios and typical stock strokes, I've run 185psi pretty easy on pump 89 with open chamber iron heads using the stock 383 HP camshaft. So I'd say it's more than strictly math. You can "fool" the engine into thinking the fuel is better by using quench, and chamber design, and piston shape, and good porting to enhance wet flow into the chamber. Also the longer strokes available make the pressure rise faster, and allow for much less initial timing.
     
  5. daryl h

    daryl h Well-Known Member

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    Interesting subject. In our racing asocation we are restricted to av gas. Always wondered how high I could go with the compression? Right now running about 14 to 1 with no problems. Many said it would not live. Believe as long as you get the squish right you can run alot of compression an that is horsepower.
     
  6. moper

    moper FABO Gold Member FABO Gold Member

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    It actually depends a lot on rpm of the power peak. That's when burn time is really a big deal. Or perhaps controling the burn time is a big deal is a better way to put it...
     
  7. daryl h

    daryl h Well-Known Member

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    Yes. You have to run alot of fuel pressure an make sure it doesnt bleed off to. Run my moter 7000rpm for about 60 seconds an the first thing was a high flow fuel pump
     
  8. MopaR&D

    MopaR&D Nerd Member

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    This type of stuff fascinates me and is what inspired me to go on to study mechanical engineering in college (which I hope to do after high school). My ultimate plan for my '70 Duster is an aluminum Edelbrock Magnum-headed 360 with around 11.5:1 compression. If dgc333 could use pump gas in his 10.6:1 iron-headed 360 Magnum, then I probably could too with 11.5:1 compression and aluminum heads. I might even try TBC's and will probably work the chambers to round off any sharp edges. Of course, that's all if modern Hemis are still expensive and hard to find by then.
     
  9. Petzschler

    Petzschler →Welcome back←

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    very good quench, rich mixtures (for cooling), and well thought out advance curves allows the use of low octane
    fuels with very high cylinder pressures. i think some of you are not completely comprehending the purpose
    of "swirl" or "heart" shaped combustion chambers are for.
     
  10. moper

    moper FABO Gold Member FABO Gold Member

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    I agree. That's why I mention the chambers and the piston top designs. Edebrocks are not a modern chamber, just a smaller one. The MP chambers are ok, but really need tweaking to be good because the machining and casting is so bad. Indy i think feel has the best design out of the box, then the initial shape of the MP.
     
  11. Petzschler

    Petzschler →Welcome back←

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    yup. those indy heads are very hard to beat OTB, even the other ported head designs dont even come close.
    I'd just wish they'd drop the price somewhat lol.
     
  12. moper

    moper FABO Gold Member FABO Gold Member

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    lol.. I wish they had an employee who could size guides and cut a good valve seat.
     
  13. idrift

    idrift Well-Known Member

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    Does anyone have any additional info they could add to this? I've been reading
    and researching for a week on this now.

    I'm trying to figure out what octane I need in my engine. I don't know the
    compression ratio as I didn't build it, but the pistons are flat tops and I am
    guessing maybe the stock 10.5 and it is bored .060 over. Hot compression
    tests show me about 180-190 psi. The camshaft is a long duro 235 int. and
    245 exhaust which I know helps lower octane requirements. I am also running
    rich on my wideband afr gauge at 13.5. It is really hard to hear anything in
    the engine as the exhaust is kind of loud and I have valve train clatter anyway. Short of putting in a knock sensor gauge, any ideas on how to
    narrow this down? The 110 octane fuel down the street is 6 bucks a gallon!
     
  14. blue missile

    blue missile Well-Known Member

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    idrift,
    I didnt think I would see this post again. I cant tell you if your pressure reading is the same as cranking pressure, but I think it is. If so 190psi is still in the range for 92 octane, but at the very top of that range.

    I would not put in a knock sensor if you have valve train clatter it would be useless.

    If this issue is really critical you need to pull a head and go through all of the measurement steps to calculate your engine's specs. this will also give you the ability to inspect the cylinders for signs of improper ignition.
    hope this helps,
    Andrew
     
  15. moper

    moper FABO Gold Member FABO Gold Member

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    what heads are on it? What does the porcelain on the spark the plugs look like with a magnifying glass? You are just above where I call things safe in terms of cranking compression.
     
  16. idrift

    idrift Well-Known Member

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    Thanks for the reply guys.

    I have iron heads, I think they are off a 360 instead of the original
    70 340 heads and by looking inside the plug holes the pistons are flat
    tops. I dont know if anything was decked or not.

    I was fooling around with curving my distributor and changing and jetting
    carbs so I need to get my plugs cleaned up and then take a look at them.

    It's really hard for me to hear and tell, I need to try to adjust the valves
    again to see if that will quiet them down some (it has crane gold rockers)
    and fix the header leaks, but even then it's hard to tell. The car seems to
    like to run a little richer than lean for sure, there is a big difference
    between a holley 3310 and a street avenger 670 given same jet sizes
    in both.

    I will go ahead and pull a few plugs and look at them to see if I can find
    anything under magnification.

    I was thinking that my valve clatter might show up on a knock sensor, so I
    guess that option is out of the question.

    I have my timing set up now so it's at 16 before at idle and 34 all in at
    2500 rpm but I do not have the vac advance set up yet. Really wanted to
    make sure I have no knock before I hook that up.

    I've been reading the last few days about detonation and I know it's hard to
    come up with any real numbers because there are so many variables so
    anything you guys can share is VERY much appreciated.
     
  17. moper

    moper FABO Gold Member FABO Gold Member

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    What gearing/convertor/clutch does the car have? You dont want to clean the plugs and then look at them. Just pop a few new ones in and drive it as you normally would for a couple miles, then look at them. If you see small blue or black spherical deposits, or any metallic looking material, you're detonating or pinging whether you can hear it or not. I can get 94 Octane super at Sunoco gas stations in CT... You may need that. But you can also tailor the vacuum advance and mechanical advance so it doesnt come in as fast if the convertor is tight and gearing is high. You also may need to carefully tune the carb to work with those curves too. Pinging will almost always be much louder than rockers, detonation you may never hear anyway.
     
  18. idrift

    idrift Well-Known Member

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    I have a 4 speed with 3.23 gearing but shorty tires/wheels at 24" so it runs
    at 2500 rpm at 55 mph and my timing is all in at 2500 @ -34.

    I have a wideband afr in the car and have been tuning 2 different carbs.
    The 3310 750 and a 670 street avenger. The 670 wants to run lean no matter what and the 750 likes to run fat. I've been changing jets and secondary springs to keep wot about 12 on both carbs but the 670 cruises
    at 15.5-16 and the 750 at 13.5

    If I understand detonation somewhat, the fatter mixture helps reduce that
    correct?

    I have my initial timing set down to -16, the car runs better all the way up
    at -20 but I thought I'd keep it at 16 until I can be certain it's not detonating.

    I have a race track about 10 miles away and just outside it a BP station sells 110 octane fuel, but I'd rather not have to run there to mix gas all
    the time. A few local stations sell 93 octane and I'd like to stay with that.
    Octane boosters pretty much do nothing correct?

    Thanks for your help!
     
  19. a-bodytom

    a-bodytom Well-Known Member

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    i got 165 psi im not runin any thin but 92
     
  20. 72swgnr

    72swgnr Well-Known Member

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    Why not run e-85?(If you have it available) and most likely make more power while doing so? Switching over is so easy to do these days with all the major carb manufactures offering all the parts to do so. just my .02 cents
     
  21. idrift

    idrift Well-Known Member

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    I have a friend running e85 at the strip and he is making some impressive
    times with it. Might be a thought....
     
  22. moper

    moper FABO Gold Member FABO Gold Member

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    If you're tuning with the sensor you will notice a few things. The Avenger has a better system of internal passages. IMO the cruise at 15+ is fine. The 750 is oldschool and just flat out doesnt. What you want to be concerned with is the fuel curve. Get a vacuum gage and hook it up with enough hose that you can drive the car with it taped to the window. Watch the gage and where the needle hangs at various throttle openings. You can adjust you power valve rating to get the valve to open exactly when you need it. Most guys just get one that stays closed at idle. That's important too, but by tuning the PV properly you can get great drivability. My guess is the 750 is rich because of that and the 670 stays closed for a longer period. If it's a hydraulic cam, you can go as much as 2/3 turn past zero lash if you have to. It may pick up some idle vacuum when you do this too.
     
  23. idrift

    idrift Well-Known Member

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    I have to agree with you on the differences between a SA and a 3310.
    I did adjust the PV on the 3310 down to a 4.5 but need to do it on the
    SA yet. I think I should be just about golden then. She idles at 14.5-15
    cruises about the same too. I have to get the secondaries finished also.
    Trying a smaller jet in those tonight. Someone put 75's in the back....
    I have 68's front and 70's in the back now. WOT runs about 11 which I
    think 11-12 is best right? The wideband is cool, it let me adjust the
    acc pump and I can watch when the vac secondaries open.
    I think I am getting close, now I just have to see if it won't ping
    on 93 pump gas! I put some 110 in with 92 and she don't ping at all!
    Dont think I can afford that long term though.
     
  24. moper

    moper FABO Gold Member FABO Gold Member

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    You can also slow down your mechanical advance so it doesnt come full in until 3000. If it has an issue, it will be at low rpm in high gear when the rpms are down and the load is high. Once it starts, it's hard to get pinging to stop. So the trick is to keep the initial ping from happening. Solve the issue at 2300 and you'll be fine at 2400 and up. (assuming there is a problem... :) )
     
  25. dustoff440

    dustoff440 Well-Known Member

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    Look at an MSD 8680
     
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