Page 1 of 1

calculate required octane for compression ratio

PostPosted: January 10th, 2010, 2:08 pm
by grumpyvette
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

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 ... 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

Re: calculate required octane for compression ratio

PostPosted: February 8th, 2011, 7:58 am
by grumpyvette
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.

Re: calculate required octane for compression ratio

PostPosted: February 8th, 2011, 8:05 am
by grumpyvette

Re: calculate required octane for compression ratio

PostPosted: August 26th, 2013, 12:28 pm
by 87vette81big
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.

Re: calculate required octane for compression ratio

PostPosted: August 26th, 2013, 12:30 pm
by 87vette81big
Building for Torque.
Redline 5800-6000

Using stock iron heads. A heads.

Re: calculate required octane for compression ratio

PostPosted: August 26th, 2013, 1:02 pm
by grumpyvette
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







Re: calculate required octane for compression ratio

PostPosted: August 26th, 2013, 1:46 pm
by 87vette81big
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.

Re: calculate required octane for compression ratio

PostPosted: March 13th, 2014, 12:54 am
by philly
i use this stuff it works great! ... -of-6html/