Compression Ratio Fuel Octane Chart: (All You Need To Know)

Every engine has a specific compression ratio (C.R) that calls for a specific fuel octane rating. Getting the right fuel octane for the right C.R for your engine helps prevent detonation in the cylinder.

The compression ratio represents the volume of the air-fuel mixture in the cylinder when the piston is at TDC (Top Dead Center) compared to the volume when the piston is at BDC (Bottom Dead Center). An engine with a low C.R will require fuel with a low-octane rating and vice versa.

This article gives more information on the compression ratio fuel octane chart and different ways of determining the right fuel for your vehicle.

Compression Ratio Fuel Octane Chart

Below is a summarized chart for compression ratio fuel octane requirements for your engine from the department of energy, USA. The manufacturer’s manual uses the gas with the recommended octane ratings.

Compression RatioMinimum Octane No.Maximum Octane No.
8.18789
9.18992
10.19296
11.196102
12.1100108

What Is Compression Ratio?

Compression ratio means the amount of fuel and air mixture the pistons compress. Each engine has its compression ratio. In a four-cylinder engine with a 2-liter fuel capacity, every cylinder has a 500 cc capacity.

During the movement of one piston, it draws in 500 ccs of the fuel-air mixture. Once the 500-cc limit is achieved, the inlet valve closes, and the piston moves up to the Top Dead Center.

The upward movement compresses the air-fuel mixture. The compression ratio is the ratio of the uncompressed air-fuel mixture to the compressed mixture. For example, if the 500-cc mixture is compressed to 50 ccs, the compression ratio is 10:1.

Interaction Between Compression Ratio and Octane Ratings

After the successful compression, a spark is introduced under normal conditions to ignite the air-fuel mixture for combustion. However, this is not always the case.

At times, the compressed air-fuel mixture self-ignites and explodes in the chamber. Such an occurrence is known as detonation or pinging, commonly known as knocking.

This brings us to the octane ratings of fuel.

What are octane ratings?

Octane ratings or octane number is obtained as an average of two different rating methods. The two methods include motor octane rating and research octane rating.

In other words, octane rating measures a gas’s ability to resist knock pinging or detonation. One needs to use the right fuel octane number for their engine with a certain compression ratio.

Engines with a high compression ratio require a high-octane fuel rating to prevent detonation that can damage the engine. An engine with a low compression ratio requires fuel with a low-octane number.

Types Of Gas Based On Octane Ratings

In the United States of America, there are three types of gasses based on the compression ratio fuel octane ratings.

Regular gasoline

This fuel has the lowest octane rating. They are also known as unleaded fuel.

It is preferred for engines with a very low compression ratio. Using regular gasoline helps save money as it is cheaper. The rating number ranges between 85 and 87.

Midgrade gasoline

As the name suggests, this fuel is rated midway between gasoline and premium fuel. It portrays characteristics of a mixture of gasoline and premium fuels.

The ratings are from 89 to 90.

Premium gasoline

This is the fuel with the highest-octane rating, also known as super. That means it has the highest ability to resist detonation. It is more expensive than the other gasses; Premium gasoline ratings start from 91 to 94.

There are other fuel octane ratings for special use, namely race gas and 85 octanes.

  • Race gas: Race gasses are high-octane fuels containing additives to boost the octane level. Boosters include lead, methanol, and ethanol. Adding these additives raises the octane number by more than ten numbers and can comfortably work under high pressure and temperature.
  • 85 octane gas: This is a special gas for carbureted machines in high-elevation regions. Consult with the engine’s manufacturer to ascertain if it’s right to use 85-octane fuel on your block.

How To Measure Compression Ratio

There are two ways of determining the compression ratio of your engine. You can either measure the compression ratio manually through calculation computations or use a manometer to measure the compression ratio.

1. Calculate The Compression Ratio Practically.

Manual measurements are better suited for a new engine or while constructing a new engine. Having the right tools and accuracy would be best to avoid error accumulation.

Necessary tools:

  • Bore gauge
  • Paper, pen, and calculator
  • Micrometer
  • Tape measure
  • Degreaser or a cleaning cloth

Step 1- Cleaning the engine

For accuracy and neat work, wipe the cylinders and the piston clean. Use a degreaser/detergent and a cloth.

Step 2 – Take the necessary measurements

Use a bore gauge to determine the cylinder’s diameter. Take several measurements and then get their average. Divide the diameter by two to get the radius (r) of the engine cylinder.

Use a tape measure or a ruler to take the height/depth of the engine cylinder (h). use the values to compute the volume (v) of the engine cylinder. Use the formula of the volume of a cylinder.

Step 3 – Establish the ignition chamber’s volume

Check the combustion volume from the car manual in cubic centimeters. You can also contact the manufacturer.

Step 4 – Take the piston measurements

Use the manual to find the piston compression height between the pin bore and the top of the cylinder piston. Also, find the piston volume from the car’s manual in centimeters cubic.

Negative CC is for the dish pistons, while a positive CC value is for a dome piston.

Step 5 – Get the piston – deck distance

Use the following formula to get the piston to deck distance. (Bore × 2 multiplied by 0.785 [conversion constant to cubic”] × gap from the piston to the deck at TDC).

Step 6 – Calculate the gasket capacity

Determine the gasket thickness and calculate the gasket capacity using the following formula. (2 × Bore × 0.785 × gasket height).

Step 7 – Determine the compression ratio:

Use the formula below to compute the compression ratio.

Compression ratio = {(swept volume + clearance volume) / clearance volume}

Whereby;

Swept volume = (π) x (bore radius squared) x (exposed cylinder height).

Clearance volume = 0.000cc [(bore/2)2 x π x deck height x 16.387].

Since the formula is long and tedious, click this link for an Omni compression ratio online calculator.

2. Use of a manometer

Using a manometer is much simpler, as you won’t have to open the engine block.

Necessary materials

  • A manometer
  • A partner to assist
  • Spark Plug opener

Step 1 – Run the engine

Ignite the engine for a few seconds to heat it. Warming up helps obtain accurate results.

Step 2 – Disconnect the spark plugs

Stop the engine ignition and disconnect plugs from the wire distributors. Use a wrench to unscrew the spark plugs.

Step 3 – Test the cylinder

Dip your manometer gauge in each cylinder at a time. Have a partner ignite the vehicle and then turn it off. Take the reading on each cylinder and record it.

Step 4 – Determine the pressure-to-compression ratio

Calculate the pressure to C.R. using the following formula. If you have a reading of 12 and your C.R. should be 10:1, your pressure reading is 120, or 12×10/1.

Frequently Asked Questions (FAQs)

Below are questions frequently asked by clients and are answered to best satisfy their need to understand the relationship between compression ratio and octane rates.

Is it advisable to refill with different octane gasses?

Different fuel octanes mean that the resultant fuel will be at an octane rating average of the two gasses. The engine will run efficiently, but you can consult the manufacturer’s manual for certainty.

Can I use a high-octane rating gas on a low-compression ratio engine?

High-octane gasses are expensive due to their high power. However, using it in a low-compression ratio engine is not economical. So, yes, you can use high-octane rating fuel, but it will not boost the engine’s power since it is a low-compression ratio engine.

Is it economical to use a low-octane rating gas on a high-compression engine?

Yes, it is economical to use low-octane gasses as they are cheap. However, such fuel on high-compression ratio engines will lead to detonation. Knocking can lead to engine damage resulting in expensive repair and replacement costs.

Conclusion

The efficiency of any engine depends on its ability to combust the air-fuel mixture in the combustion cylinder fully. The quality of the fuel and the capacity of the engine cylinders directly affect their efficiency too.

Ensure the engine cc capacity is supplied with the right fuel octane rating to avoid detonation that lowers its efficiency. Use the standard compression ratio fuel octane chart to know the fuel suitable for your engine.

Use low-octane rating gas for engines with a low-compression ratio. For engines with a high-compression ratio, use a premium or super-rated fuel.

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