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?

    “your fuels Octane” has little at all to do with potential engines power it is simply a way to label or list the fuels measure, of detonation resistance to self ignition, due to heat and compression. ”.

    Octane is only a unit of measurement of knock resistance and does not in any way describe any fuel’s burning characteristics, flame speed or anything at all to do with combustion …… except knock resistance.


    * 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:

    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



    Last edited by a moderator: A moment ago



    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: Sep 27, 2020
  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

  4. 87vette81big

    87vette81big Guest

    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 Guest

    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!




    if your experiencing detonation issues that are cured by swapping to higher octane rated fuel, and you would prefer to use the lower octane , less expensive fuel, you should adjust your cars ignition advance combination , so that its advance curve has either less initial timing, or delaying the mechanical advance vs. rpm with some stiffer springs, or a combination of both might reduce the pinging under load at 2500-3500 rpm where its most commonly seen,. Does this detonation or pinging, only occur at WOT? If not, limiting the vacuum advance with a stop, or using an adjustable vacuum advance unit and raising the amount of vacuum required vs. the amount of vacuum advance might be warranted also and installing a lower temp rated t-stat and adjusting the engine fuel/air ratio a bit richer may also help..


    the guys have a point, best power and in many cases durability, is generally found at nearer 12.7:1 f/a ratio, I don,t ever remember your engine getting near that lean while we discussed getting it tuned up?
    link too bore vs stroke info on hundreds of engines

    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: Jul 16, 2020
  7. 87vette81big

    87vette81big Guest

    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 Guest

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

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    theres always the question raised or the assumption made...

    that swapping too a higher octane fuel will result in a higher power out-put.

    an engines power production is effected by several factors
    the fuels octane is simply one of many factors,
    keep in mind your fuels octane rating has little to do with the power potential, its a measure of the fuels resistance to spontaneously ignite due too heat and compression, rather than the engines ignition system, the higher the octane rating the more heat and compression the fuel will tolerate, without entering the tendency to self detonate/ignite due to that level of heat and compression.
    in theory,and within reasonable limits for the fuel used, the higher the engines compression level, the more power per cubic inch of displacement its likely to produce.
    but to operate correctly an engine must run fuel of a high enough octane to prevent entering detonation.
    first Id state that on average a boost in an engines static COMPRESSION of one point IR from lets say 9:1 to 10:1 generally on average results in about a 3%-4% boost in TORQUE.
    (provided the fuel octane is high enough to eliminate any tendency to detonation.)
    while changes in ignition advance curves under higher loads does tend to show a slight improvement if the octane allows higher loads without reaching the detonation thresh hold.
    be aware that, the fuel octane, being used, alone is not the only factor here!
    iron heads hold heat and transfer heat to coolant at a lower rate!
    running a 180f -190f t-stat and use of a 7-8 quart oil pan, an oil cooler will tend too lower the effective operating temp.
    both the coolant and oil temps, and ignition advance curve, will effect the range where detonation will occur,richer fuel/air mixtures (12.5:1-13.5:1) tend to burn a bit cooler than lean mix ratios,(13.5:1-15.5:1)
    aluminum heads, transfer heat much faster, and benefit from, some, simple mods , like keeping the quench in the .040-.042 range, polishing the piston deck surface and combustion chamber and rounding the edges on the combustion chamber and piston valve notches, will reduce the tendency to get into detonation
    most cars are set up to run a bit lean to maximize mileage ,reduce exhaust emissions,and reduce the potential for spark plug fouling, at near a 14.7:1 ratio
    max power is generally found at closer to a 12.7:1 fuel air ratio,
    but that results in higher emissions and minimally more engine wear,
    as a slightly higher F/A ratio can reduce piston ring lubrication.
    power tends to increase as the compression ratio increases at a fairly consistent relationship up too about 13.6:1 where the RATE of increase , in power to compression increase tends to slow, when were limited to more common "GASOLINE" wither its common pump or race octane fuel
    as long as your fuel octane is a decent match to the engines dynamic compression , boosting the octane will have only a minimal improved result if any.
    in every test Ive seen the result of upgrading the fuels octane tends to show a minimal or no power increase UNLESS the the engines compression ration and the fuel used are close too the detonation limits , where the octane increase will help minimally.
    the chart pictured below is important but a bit mis-leading,
    because it shows only about a 15% gain in power if you went from a 8:1 compression ratio to a 14:1 compression ratio,
    in reality , you could more likely see an 15%-18% gain,
    with some changes in cam timing and exhaust tuning alone,
    and with other changes, 20% gains would be reasonable with that increase in compression.
    so to put that in perspective, if you had a 9.5:1 compression 454 chevy,
    that made lets assume 425hp, and swapped a cam and pistons,
    and increased it to 12.5:1 compression .
    it would be rather reasonable to expect, a gain in the range,
    close to 10%, or a boost to near 470 hp once the correct octane fuel was used.


    yes its been tested several times on dynos and in magazine articles
    result of a similar test
    Last edited: Jan 26, 2021
  12. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    if you think about it, the one area that seems to have been rather over-looked in race car engine performance is the development of what many people would regard as exotic fuels.
    normal "GAS" mixed with outside air that contains about 21% oxygen content, burns and produces pressure in the cylinder above the piston, rather slowly,when compared with the time available during the compression and power stroke, at low speeds and with low compression it can take .0070 of a second to burn and produce pressure , at higher compression and faster engine speeds this is reduced to .0045-.0050 but that still requires ignition advance to light the fuel/air mix before the piston reaches TDC to maximize pressure over the piston ATDC and the burn cycle rarely lasts past 20-24 degrees past TDC. the result is that during the 180 degrees the crank rotates through the power stroke, more than 155 degrees the pistons is pushed down the bore by significantly less than the full pressure the combustion cycle produced.
    if you've ever wondered why nitro-methane in some race engines produces much greater power its because nitro-methane contains and releases a significant amount of oxygen during combustion thus it can burn at much richer mixture, ratios and much greater quantities of fuel can be burnt, this allows significant increases in engine power, than the 12.5 too one fuel/air ratio that gas finds ideal.
    nitro-methane produces both much higher cylinder pressure and a much longer and more efficient pressure curve in the engines cylinder

    if you super charge or use turbos read this

    I'm certainly not suggesting we all swap to nitro-methane, as its very expensive, but certainly there.s room for tweaking and improving what is currently available in race engine fuels, for example, E85 (ETHANOL has some potential ) and some serious problems



    remember theres almost no effective pressure forcing the piston down on the power stroke after about 35 degrees past TDC , so your power is derived during less than 5% of the engines rotation,(remember theres 720 degrees in a cycle )



    well I'll assume you previously read the links on engine building,and quench, and you have at least tried to build a well balanced combo with reasonable quench,and tried to match the cam duration and lsa to the engines compression and intended power range, and you selected a fairly well matched cam timing and reasonable compression, but at this point in the tuning ,your still having indications your getting into detonation.
    keep in mind that keeping reasonably consistent and as low as practicable , combustion chamber temps are a huge factor in avoiding detonation issues, having an auxiliary oil cooler and a trans fluid cooler with a powered fan, and the proper fuel/air ratio and ignition advance curve along with matching your cars engine dynamic compression ratio to the available fuel octane can go a long way toward avoiding detonation issu

    [​IMG] Fuel Characteristics.pdf

    Last edited: Jun 17, 2020
  13. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    I spent several hours a couple days ago helping a friend get his 1965 corvette started, the engines not original, its a 10.7:1 compression 377 SBC engine I built for him about 30 years ago, (thats a 400 block and a 350 crank, with a crane flat tappet solid lifter cam, (crane 11o921 crane roller rockers and an edelbrock dual plane intake with a 750 holley, , and a 7 quart fabricated oil pan, and vertex magneto, the engines still running fine , but he forgot to use the fuel stabilizer last year when he put the car in storage)
    to be fair, the corvette gets driven maybe 300-400 miles a year at most , so I doubt it has over 20K moiles in 30 plus years, its a toy for special occasions, the vettes got a muncie trans and 4.11:1 gears.
    its the first time he forgot to use only ethanol free fuel. and the first time he ever had any issues. once we changed out the fuel filter , and air filter and spark plugs, and checked the ignition timing.
    and used some fresh fuel and a bit of starting ether the car started up, Failue.pdf

    most reasonable lower compression car/truck engines , built in the last 30 years, can run the crappy low octane 10% ethanol laced pump swill thats sold at your local gas station.
    if you have an older muscle car that had an 11:1 or higher compression engine or your running an engine like a chain saw or lawn mower that may be sitting for months between uses that ethanol laced crap is very detrimental, as it sits without any movement it tends over extended time, frames, to separated out, ethanol is hygroscopic
    (of a substance) tending to absorb moisture from the air.
    relating to humidity or its measurement.
    since water is heavier than gas, it settles to the bottom of the tank and in direct contact with the metal surface it can and frequently does cause corrosion issues
    adding 6 oz of each of these 3 products to your fresh fuel , assuming the common, (16-20 gallon tank capacity) before you put your muscle car in storage is a huge help in preventing problems from developing over a winters storage
    heres a web site that lists stations that carry reasonably priced real gas with zero ethanol

    E85 gas station locations ignition) Master_Low Res.pdf
    Last edited: Jul 6, 2020
  14. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    We test a 105-octane fuel that costs less than 87 pump gas and makes the same power as race gas
    Air/Fuel Ratio
    Stoichiometric 14.7:1........ 9.7:1
    Max power (rich) 12.5:1.... 6.9:1
    Max power (lean) 13.2:1..... 8.4:1
    14.7:1-15:1 static compression and 12.5:1 dynamic compression,
    is generally considered about ideal for ethanol powered e85 engines
    Compression makes horsepower on any fuel.

    Methanol makes more power by providing more heat.
    Gasoline at 20,943 BTU’s/lb at 14.7:1 air/fuel ratio by mass equals 0.068 lbs of fuel/lb of air or 1415 BTU’s/lb of air.
    Methanol at 9,770 BTU’s/lb at 6.4 air/fuel ratio by mass equals 1527 BTU’s/lb of air. About 22% more bang.
    Nitromethane at 5,160 BTU’s/lb at 1.7:1 air/fuel ratio by mass equals 3035 BTU’s/lb of air.

    Ask Away! with Jeff Smith: What is Safe Effective Compression Ratio for E85?
    Posted by Jeff Smith on November 20, 2015 at 4:16 pm
    What is a general safe effective compression ratio on E85? The internet results in varying opinions from 13:1 up to an extreme 20:1.

    Jeff Smith: This is an interesting question that came out of last month’s discussion about E85. To quickly refresh, E85 is 85 percent ethanol (grain alcohol) and 15 percent gasoline. It has a commonly accepted octane rating of 105 although that may vary depending upon the quality of the 15 percent gasoline. According to Tim Wusz of Rockett Racing Brand fuels, the quality of that blended gas may not necessarily be as good as 87 octane. So with that as a variable, we might safely assume the octane to be closer to around 100. Rockett sells its own brand of race E85 listed as 112 octane. All gasolines use an anti-knock index (AKI) for the octane number created by averaging the fuel’s detonation-suppressing capabilities based on two very different tests. The first is a calculated number called Research octane and the second is Motor octane generated by actually testing the fuel in a variable compression ratio single cylinder engine.

    In the case of Rockett Brand’s E85, the Research number is 116 and the Motor number is 108. Adding the two and dividing by two ((R+M) / 2) produces an AKI of 112. Evaluating these numbers, the “real-world” Motor number is always lower, but the goal is to have the two numbers be relatively close. Wusz says that of the two numbers, greater emphasis should always be placed on the Motor octane number.

    Before I get too deep into this, let’s try to quickly answer your question with some actual dyno testing as opposed to just theory. As I mentioned last month, we tested a small block Chevy with a static 10:1 compression ratio with 9 psi of boost using E85. This was run on normal E85 and we checked to make sure the fuel was actually 85 percent ethanol and according to our test it was very close. The engine ran perfectly and we experienced no detonation problems. If you look at our Effective Compression Ratio Chart (we’ll explain what that is in a moment) this would be like running a normally aspirated small-block with a 15:1 static compression ratio. The engine made 600 flywheel horsepower using a Magnuson supercharger.

    One way to estimate the actual static compression ratio is by using a simple formula that produces a result referred to as Effective Compression Ratio (ECR). This simple formula converts the amount of boost to additional compression ratio as if the engine were normally aspirated. This formula does not take into account the increase in inlet air temperature that occurs when you compress air – we’ll get to that later. Given that warning, the formula looks like this:

    Effective Compression Ratio (ECR) = [(Boost / 14.7) +1] x Static Compression Ratio

    Let’s plug in our 10:1 compression engine with a psi of boost into this equation:

    ECR = [(8 psi / 14.7) +1] x 10

    ECR = 15.4:1

    This reveals our engine was running an effective compression ratio of over 15:1. That means that you could reasonably build a 14:1 compression ratio engine and run it on E85 and not expect problems with detonation. This also reinforces Rockett’s listing their E85 fuel as compatible with a normally-aspirated engine with up to 16:1 compression ratio.

    To answer your question directly, it appears both from our ECR chart and our own testing that you could run E85 on a normally aspirated, static compression ratio of 15:1. The issue becomes the ability to generate that much compression. My guess is that a huge domed piston would hurt combustion efficiency more than it would help. We did a quick scan of the JE piston catalog and the highest compression off-the-shelf piston for a big-block Chevy, for example, was 14:1 with a 112cc chamber.

    According to Wusz, even given the cooling effect of E85, his recommendation would be to limit the static compression ratio to 13:1 on a 4.00-inch bore small block and slightly lower in a larger bore engine like a big block Chevy. His recommendations will be conservative since we were talking about pump E85 as opposed to his company’s version of E85 which has a significantly higher octane rating than normal pump E85. There’s still plenty of advantages to building a street engine with high compression that will take maximum advantage of the fuel’s reduced energy content.

    Remember that cylinder pressure is directly affected by cam timing. This is because compression cannot begin until the intake valve closes. This means that a late closing intake point (with a long duration cam) reduces the engine’s dynamic compression ratio. If you put a mild cam in a 13:1 compression engine, the dynamic cylinder pressure might be equivalent to 15:1 with a larger cam. This is certainly a bit of a no-man’s land since there probably aren’t too many people with practical experience along these lines, so you should perhaps be conservative and keep the compression in a range where everything is manageable. Then there are other considerations such as how difficult it will be for the starter motor to crank over a hot, 15:1 compression engine.

    The big advantage that E85 has over gasoline is alcohol’s excellent latent heat of vaporization. This is a fancy term for alcohol’s ability to absorb heat when it evaporates. If you’ve ever swabbed your skin with rubbing alcohol, it evaporates quickly, leaving your skin feeling very cool. The alcohol in E85 does exactly the same thing in the intake manifold, cooling the air as it evaporates. This reduces the inlet air temperature. This is important because cooler air is more dense, which means it packs more oxygen into the cylinder for the same volume of air.

    Next, because of the cooling effect, we can safely assume that the inlet air temperature for our normally-aspirated engine will be dramatically cooler using E85 fuel. This number is a little harder to predict, but I think it would be safe to estimate the inlet air temperature entering the cylinders could conservatively be 25 to 30 degrees cooler than ambient air temperature. This is important because Wusz says that based on OE testing, for every 25-degree reduction of inlet air temperature, the engine’s octane requirement will be reduced by one full octane point. So if our 14:1 engine really was at the knock limit of the fuel with an inlet air temperature of 80 degrees, reducing this by 30 degrees would mean the engine is now “safe” by roughly one full point of octane – which is a significant drop. Cooler air is denser, which should produce higher cylinder pressures and nudge the engine towards detonation. But as we’ve mentioned, cooler air also reduces the tendency for an engine to detonate. So colder air is always better and E85 is very good at cooling the air entering the cylinders.

    We’ve included a couple of charts that may help make all this a bit easier to understand. As you can see, there are a ton of variables, but the bottom line is that I think a 13.0:1 big block to perhaps 13.5:1 small block engine would be very responsive when running on E85 and that extra compression will return some of the lost mileage from E85’s reduced BTU heat output compared to gasoline. Think about that – a 13:1 compression street motor on E85 that makes excellent power that could also deliver acceptable mileage. Very few people believe that’s possible – but I think it is.

    Anti-Knock Index Chart
    The AKI number is calculated by adding the Research and Motor numbers and dividing by 2.

    AKI = (R + M) / 2


    Effective Compression Ratio (ECR) Chart
    This chart is a just a guideline, but if we assume we can safely run an ECR of 16:1, then a static 10:1 compression with 9 psi of boost is an ECR of 16.1:1 and that’s exactly the combination we safely ran on the dyno.

    Last edited: Jul 28, 2020
  15. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    Nice New chart Grumpy.
    Gas Station E85 is Real cost race fuel.

    The VP E85 Race fuel sold in 5 gallon cans is expensive.
    It's as much as 110 Race Gas.
    No cost savings.

    Methanol alcohol is the Only low cost race fuel sold by Vp and others in 5 gallon cans and 55 gallon drums.
    After each use of methanol the entire fuel system has to be flushed out with mineral spirits or gasoline.
  16. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    If you have a good wide band 02 you can compensate for different batches of pump gas station E85.
    They change the blends 2 times a year here in the midwest.
    When it is cold its close to 50/50 blend.
    Corn alcohol and gasoline.
  17. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    Methanol is spark ignited with a Magneto.
    Nothing else will work reliable.
    Takes high amps Joules of current to light it off at the spark plug gap.

    E85 hard to light off at times too.
  18. Loves302Chevy

    Loves302Chevy "One test is worth a thousand expert opinions."

    We’ve included a couple of charts that may help make all this a bit easier to understand. As you can see, there are a ton of variables, but the bottom line is that I think a 13.0:1 big block to perhaps 13.5:1 small block engine would be very responsive when running on E85 and that extra compression will return some of the lost mileage from E85’s reduced BTU heat output compared to gasoline. Think about that – a 13:1 compression street motor on E85 that makes excellent power that could also deliver acceptable mileage. Very few people believe that’s possible – but I think it is.

    Cool. If E85 ever comes to my area, maybe I can actually get some use out of my 12.8:1 static CR 350 SB Chevy.
    Jorandom likes this.
  19. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

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