Calculate Required Octane For Compression Ratio

calculate required octane for compression ratio

Discussion in ‘Intake Systems , fuel systems and related‘ started by grumpyvette, Jan 10, 2010.

  1. grumpyvette

    grumpyvette Administrator Staff Member

    What else influences your car’s octane requirements?


    * Temperature: Hotter air and engine coolant increases your engine’s octane requirements
    * Altitude: Higher altitudes decrease your engine’s octane requirements
    * Humidity: Drier air increases your engine’s octane requirements
    * Engine spark timing: If your engine’s spark timing is increased, the octane requirement increases
    * Driving method: Rapid acceleration and heavy loading increase your octane requirement.

    If your thinking of running pump fuel, the simple answer..
    try to keep your dynamic compression ratio at 8:1,or lower, your intake air temp as low as possible,your oil temperature below about 220f and your coolant temp below about 190f and use 92-or higher octane fuel, and use an ignition system with a knock sensor if possible
    BTW E10 fuel, (THATS 90% gas /10% ethanol ) is no longer running correctly at the 14.7:1 fuel air ratio, the best ratios closer to 14.2:1 for E10
    Blending Race Fuel With Pump Gas
    Adding a few gallons of race fuel to varying amounts of pump fuel is a popular octane-boosting method that enthusiasts use. We were curious what a race fuel expert would say about it, so we asked the Rockett Brand team. Here’s what they said:

    “[Mixing race and pump fuels] is an okay thing to do, and much better than ‘octane boosters.’ It does not damage the engine, and improves the quality of the street gas. Octane numbers blend almost linearly, and we actually publish blending charts on our website for those who insist on doing this. For example, if you blend a 92-octane pump gas with a 100-octane race fuel in equal amounts, you will get a 96-octane fuel.

    “But keep in mind, if you don’t want to spend money on straight race fuel, you are watering down the benefits that that straight race fuel has. You may get the octane you need, but you will not get the optimized vapor pressure and blending.

    Read more:…-you-need-to-know-about-octane/#ixzz3lpsdQYn9
    Follow us: @HotRodMagazine on Twitter | HotRodMag on Facebook

    viewtopic.php?f=52&t=727 … lator.html … 70401a.htm … myths.html … power.html … index.html … index.html

    For a typical carburetor equipped 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 … ion-1.html … atings.php … re=related … index.html … ion-1.html



    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 %

    Read more: … z0cEhLg8Gn … index.html

    OK, first fact! the piston can,t compress anything until both valves fully seat, static compression is based on the volume compressed between the piston starting at bottom dead center and compressing everything into the combustion chamber , head gasket quench,volume, that remains when the pistons at TDC
    dynamic compression is the ONLY compression the engine ever sees or deals with, it measure compression from the time both valves seal the chamber,and that is always lower simply because the valves always seat after the piston is already moving upwards on the compression stroke.
    if we look at the crane cam I linked earlier you see the valves seat at about 75 degrees after bottom dead center




    from what Ive seen working on and tuning engines ,those charts are depicting about the ideal maximum compression ratio to run,in your basically stock engine, giving you a bit of a hedge for detonation resistance, and they pretty much assume a 14.7:1 f/a mix ratio to minimize emissions.
    detonation can destroy an engine but its frequently caused by more than just a bit of compression ratio increase in relation to the octane of the fuel alone, get the heat transfer rates out of the combustion chamber and ignition curves and fuel/air ratios correct and you can run a bit higher ratio that the charts depict.
    A great deal of how well your engine runs will be determined by its state of tuning,if you run a non-emission friendly 12.5-13.1:1 fuel/air ratio where you maximize the engines power curve and play with the ignition timing advance curve to get the best torque ,you can frequently boost the effective compression ratio by about .2-.3 and not only get away with it but make noticeably better power.
    now IM not saying you can ignore the graph, but in the real world its not like if the graph says that if your engines compression ratio is at 9:1 your, engine combo instantly self destructs the point you put 89 octane in the tank,or at 9.1:1 compression, if you mis-calculated,or that if the pump says your getting 91 octane, your not occasionally getting 89,90, or 92 octane.
    get the quench down in the .040-.042 range , polish the combustion chambers and keep the coolant temps below about 190F and run a good oil control system with an oil cooler and you would be AMAZED at how far you can push the limits…..push NOT IGNORE!

    viewtopic.php?f=52&t=727 … ns/b/b.htm

    viewtopic.php?f=53&t=726&p=8809&hilit=quench+squish#p8809 Ratios.pdf … atios.html

    viewtopic.php?f=56&t=495 … re=related

    viewtopic.php?f=52&t=1343&p=2942&hilit=+booster#p2942 … index.html

    ETHANOL ALCOHOL CAN BE USED WITH NITROUS to reduce the tendency towards detonation,increase octane and cool the engines exhaust but of course the fuel and injectors or carb must be compatible and locating a nearby source of E85 may not be easy

    Last edited by a moderator: Mar 1, 2016
  2. grumpyvette

    grumpyvette Administrator Staff Member

    A Consumer’s Guide:
    Gasoline Octane for Cars
    from Gasoline Questions & Answers for Your Car
    API Publication 1580, Sixth edition, January 1996
    Q. What is octane?

    A. Octane is a measure of a gasoline’s ability to resist knock or pinging noise from an engine. In older vehicles, knock may be accompanied by engine run-on, or dieseling. Knock is the sharp, metallic-sounding engine noise that results from uncontrolled combustion. Severe knocking over an extended time may damage pistons and other engine parts. If you can hear knocking, you should have your engine checked to make sure it is calibrated correctly and does not have a mechanical or electrical problem, or use a higher octane gasoline.

    In most vehicles no benefit is gained from using gasoline that has a higher octane number than is needed to prevent knock. However, in some vehicles equipped with a knock sensor (an electronic device installed in many modern engines that allows the engine management system to detect and reduce knock), a higher octane gasoline may improve performance slightly.

    Q. What determines my car’s octane requirements?
    A. Your car’s octane requirements are mainly determined by its basic design. In addition, variations in engines due to manufacturing tolerances can cause cars of the same model to require a different octane of several numbers. Also, as a new car is driven, its octane requirement can increase because of the buildup of combustion chamber deposits. This continues until a stable level is reached, typically after about 15,000 miles. The stabilized octane requirement may be 3-6 numbers higher than when the car was new. Premium or midgrade fuel may be advisable to prevent knock.

    Other factors also influence your car’s knocking characteristics:
    Temperature – Generally, the hotter the ambient air and engine coolant, the greater the octane requirement.

    Altitude – The higher the altitude above sea level, the lower the octane requirement. Modern computer-controlled engines adjust spark timing and air-fuel ratio to compensate for changes in barometric pressure, and thus the effect of altitude on octane requirement is smaller in these vehicles.

    Humidity – The drier the air, the greater the octane requirement. The recommendations that vehicle manufacturers give are for normal- to low-humidity levels.

    Your engine’s spark timing – The octane requirement increases as the spark timing is advanced. Both the basic setting of the spark timing and the operation of the automatic spark advance mechanisms are important in controlling knock. In some computer controlled engines, the spark timing can only be changed by replacing modules in the computer. If they are equipped with knock sensors, these computer controlled engines have the ability to retard the ignition temporarily when a sensor detects knock. This temporarily reduces the octane requirement and may also temporarily reduce vehicle performance.

    Method of driving – Rapid acceleration and heavy loading, such as pulling a trailer or climbing a hill, may result in a greater octane requirement. Stop-and-go driving and excessive idling can increase octane requirements by causing the buildup of combustion chamber deposits.

    Malfunctions of emission control systems – An improperly functioning emissions control system can affect the octane requirement by changing the air-fuel mixture or by not providing dilution gases through the exhaust gas recirculation (EGR) system. If a malfunction occurs, your vehicle should be taken to a qualified vehicle service mechanic. Some problems are indicated by warning lights on the driver’s instrument panel.

    Q. How many grades of gasoline are available?
    A. Most places that sell gasoline offer three octane grades of unleaded gasoline–regular at 87 (R+M)/2, midgrade at 89 (R+M)/2, and premium at 93 (R+M)/2. In high-altitude areas such as the Rocky Mountain Region of the U.S., the (R+M)/2 number may be lower by one or two numbers. After January 1, 1996, no leaded gasoline may be sold for highway use.

    Q. Which octane grade should I use in my car?
    A. Use the recommendation in your car owner’s manual as a starting point for selecting the proper gasoline. If you notice engine knock over an extended time and your engine is adjusted correctly, try a higher octane gasoline. Also, higher octane may provide a performance benefit (better acceleration) in cars equipped with knock sensors. Many late model and high-performance (turbo-charged and supercharged) cars fall into this category.

  3. grumpyvette

    grumpyvette Administrator Staff Member

    Last edited by a moderator: Jun 11, 2015
  4. 87vette81big

    87vette81big solid fixture here in the forum

    Grumpy How Relavent in your experience are these Dynamic compression Ratio calculators with Vintage Detroit Iron?
    Cast Iron production heads used.
    Read above yesterday & all links.

    My ’65 Olds 425ci V8.
    Used the old Kieth Black engine calculator, United Machine now.
    Have a tight Quench area stock as is. .032″
    Calculated static 10.4:1
    Math by hand 9.2:1.
    Dynamic with my Isky Cam 6.99:1.

    Many are saying these Dynamic compression calculators have been found useless.
    Pontiac sites included.

    Previous owner said this 425 ran great on 91-93 octane.

    Your chart above says I should be able to run 87-89 octane.
    Be Coll radiator in place. Can easily maintain 180F water temps.

    Last edited by a moderator: Jul 11, 2016
  5. 87vette81big

    87vette81big solid fixture here in the forum

    Building for Torque.
    Redline 5800-6000

    Using stock iron heads. A heads.

  6. grumpyvette

    grumpyvette Administrator Staff Member

    I have tried to maintain an 8:1-8.4:1 dynamic compression ratio and a .040-.042 quench in most of my engines,I try to keep the oil temps at no more than 220F most of the time and ideally under 215F, I try to keep coolant temps under 190F most of the time and try to avoid getting coolant over 215 f, keep in mind that you can,t audibly hear detonation in most engines in the upper rpm range until it becomes rather extreme and potentially rather destructive, and that just because you can,t hear it at lower rpms or when its not happening consistently, its not an indication that the cumulative damage is not occurring over time.
    I lost count of the guys I know who built engines that “for no apparent reason… busted pistons or rings” when those engines get torn down and closely inspected DETONATION is frequently a prime suspect, and todays crappy fuel octane is a prime contributor.
    every combos different and simple things like polishing combustion chambers. retarding a cam a few degrees,using a larger more efficient radiator, and changing the fuel/air ratio and ignition advance curve or adding a highly effective scavenging header on a low restriction exhaust can make or brake a combo as far as its tendency to get into detonation.
    theres several additives that are supposed to make use of ethanol laced fuel far less corrosive,
    if you find a really good additive that works 100% let me know , we have ETHANOL FUEL LACED GAS AND ITS KILLS SMALL ENGINES LIKE LAWN MOWER CARBS AND PRESSURE CLEANER CARBS, in the mean time heres a list of gas vendors that only sell alcohol free fuel

    Given constant pressure (which seems accurate), the temperature of the air is inversely proportional to the number of air molecules
    So colder air means more molecules, and more air molecules means more energy released in each combustion cycle.
    A drop from 30 C to 0 C is roughly a 10% drop measured in Kelvin,
    which suggests 10% more energy for each combustion cycle –> 10% more horsepower!


    for air temperature, colder air entering the engines induction is generally better.
    simply because cold air is more dense, so if you put cold air into a cylinder, you could fit more molecules of oxygen in a given volume of air flow,
    than if you allow the engine to breath in hot air.
    that’s a good reason why when people design an air induction system for a performance application,in their muscle cars,
    they put in cold air intakes, because they allow access too cooler and denser outside air from a cooler area,
    then the engine and exhaust pre-heated air under the hood ,
    a standard engine air cleaner assembly breaths around the engine compartment where the engine sits.
    theres always a point where more is not better, and colder air entering the engine must be above the dew point where water turns to ice, in the air,
    combustion is more efficient at higher temps,



    for the combustion temperature of the actual engine,combustion chamber,
    efficiency is enhanced through higher compression and increased burn temps and combustion speeds,the higher the compression ratio the faster the burn speeds thus the less wasted energy compressing a rapidly expanding mass of burning fuel/air mix BEFORE the piston and rod assembly pass TDC and the greater percentage of the burn energy that can be actually used to increase engine torque driving the piston down the bore on the power stroke AFTER TDC

    thus a good ignition system and thermal reflective coatings on the piston uper surfaces and combustion chamber surfaces that reduce heat loss too the engine cooling system and ceramic coatings on the headers that tend to reduce heat loss help the engine make power. warmer combustion, with cooler denser air containing more oxygen is better for performance. if your car’s cooling system is cooling too much, your engine won’t run as efficiently, so if your car is running too cold, its best to get it set up to operate within a fairly narrow temperature band.
    engine coolant temps in the 160F-200F range with oil temps running in the 180F-215F range have proven to be about ideal in a muscle car.

    and yes increased altitude tends to reduce engine performance meassurably
    ,simply because, a car operating at a higher altitude, performs worse because the air is less dense, and contains less oxygen in the air being compressed so less fuel can be burnt. so less air useful oxygen molecules entering the cylinder.
    a carefully designed exhaust header with the correct dimensions, with a ceramic coating to reduce heat energy losses, pistons, and combustion chamber surfaces, with thermal barrier coating and matching cam timing on a high compression engine can be significantly more efficient with some carefully thought through engineering that enhances its thermal effiency and cylinder scavenging, while most people don,t realize the benefits the addition of carefully thought through and well matched components each STACK or MULTIPLY the effect of the matched changes making each more effective than they would otherwise be!
    these threads may also help`








    Last edited by a moderator: Sep 22, 2015
  7. 87vette81big

    87vette81big solid fixture here in the forum

    Yes I am personally aware of Detonation and Havic Damage it causes.
    Not putting $5 k into this build.
    The “A” Heads on this 425 Olds are supposed to be best ever made stock next to the 1970 W30 455 heads. Some Olds guys say the A heads still better.

    I will start my intial Tuning with 100 LL Aviation gasoline.
    Test WOT @100MPH.
    Tune down for 93 pee water.
    Find octane limits.

  8. philly

    philly solid fixture here in the forum

    Attached Files:

  9. grumpyvette

    grumpyvette Administrator Staff Member

  10. 87vette81big

    87vette81big solid fixture here in the forum

    I am going to print off this entire thread Grumpy.
    Always have the info then.

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