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Compression Ratio and Combustion Chamber Volume

More on Blueprinting

By Mark Langford, EAA 424508, N56ML@hiwaay.net

Mark Langford in his Corvair-powered KR-2S

When you buy an engine “off the shelf” from one of the mass production manufacturers, the compression ratio should be a known quantity. For Lycoming and Continental, this is certainly true. But if you’re converting an automotive engine for aircraft use, you’re probably going to want to lower the compression to increase your safety margin. High compression ratios increase the probability of detonation, an insidious “meltdown” process which is most likely to occur during high-power modes such as takeoff, resulting in large power losses and potential engine failure in a matter of minutes or even seconds.

Compression ratio is pretty simple; it’s just a measure of how much the air-fuel mixture is being compressed by the pistons. In very simple terms, the formula is:

Cv + Ccv + Ov
CR = -------------------
Ccv + Ov
Cv = cylinder volume
Ccv = combustion chamber volume
Ov = other little volumes such as deck volumes in the gasket area

The formulas are easily found on the Internet, as well as compression ratio calculators that will do all the hard work for you. Just fill in the blanks. Cylinder and deck volumes are easily calculated from measurements, but combustion chamber volume (including piston crown if not completely flat) must be measured.

Optimal compression ratio varies for different engines, but for carbureted, air-cooled engines, 9:1 should be considered the upper limit for burning 93 octane auto fuel, though 100LL allows much more margin before detonation sets in. For higher safety margins, the 8:1 range is highly preferred and specified by many engine manufacturers. Great Plains Aircraft Supply Company, one of the VW engine folks, prefers 7:1, which is similar to Continental and Lycoming numbers. Do the homework on your particular engine type and application to arrive at a safe number, but the lower the better from a detonation standpoint.

Of course, more power can be gained by increasing the compression ratio, but it’s not a proportional relationship. Don’t think that you’ll get a 12.5 percent increase in power by going from 8:1 to 9:1; it’s more like 5% and perhaps not worth the risk. My Corvair engine is set for 9.3:1 and has particular attention paid to combustion chamber design, yet it’s still on the ragged edge. Trust me when I say that all it will take is one detonation experience during climb-out to convince you that lower compression is better!

Measuring Your Combustion Chamber
If you’re going to mix and match aftermarket parts, you’re certainly going to want to know your resulting compression ratio, and that starts with measuring your combustion chamber volume. The process is simple and costs virtually nothing, but it’s somewhat time consuming. As with most areas of airplane construction, cutting corners and relying on wishful thinking is rarely a plan for stellar success. Spending a little extra time on attention to engine detail is good insurance for your first flight.

Combustion chamber volume
Here’s the technique: Simply fill the combustion chamber with a preciously measured amount of fluid. That amount, measured in milliliters or cubic centimeters (or converted to cubic inches), is your combustion chamber volume.

Measuring combustion chamber volume starts with the head removed from the engine, cleaned of all carbon deposits and gasket material; valves installed with their springs and retainers; and spark plugs installed. It’s best to apply a thin film of grease to the valve faces before installation to ensure a good seal. Then a thin, clear, acrylic disk is placed into the chamber against the cylinder sealing surface, again with a thin film of grease to prevent leakage. This disk will have two holes drilled through it to allow for filling with a pipette and venting of the air bubble inside.


The 10-ml (milliliter) pipette (which is also 10 cc) can be bought from chemical supply houses, but one source is www.McMaster.com, part number 41405T5 for $10.49. You can also buy 1/8-inch disks while you’re there (part number 1221T17 for $6.17) or cut your own out of some scrap acrylic - the thicker the better. It doesn’t have to be perfectly round, but it must be perfectly flat. So cutting it on a bandsaw or even sanding it to size on a belt or disk sander will work fine. Of course, the acrylic disk doesn’t work on all varieties of cylinder heads, but it works on most air-cooled engines with separable pistons and cylinders, and most water-cooled heads as well. The working fluid should be something easily visible while measuring, so a 50/50 mix of automatic transmission fluid and kerosene is commonly used.

Getting Started
Start with the cylinder head set up on the bench and leveled in both axes; the acrylic disk, lightly greased, is then placed on or inserted against the cylinder sealing surface, depending on your conditions. Next, draw more than 10 ml of fluid into the pipette, allowing the fluid to slowly drop back to exactly 10 ml, and then place the tip of the pipette into one of the two holes in the disk. It’ll probably take four or five iterations of this process to arrive at the point where you’re herding an ever-decreasing air bubble around under the acrylic surface, which can be done with thin shims strategically placed to herd the bubble to the “escape” hole. At the instant the fluid meets the breather hole, stop releasing fluid and read the meniscus level. Add this reading to the previous number of pipettes, and you’ve got your chamber volume in cubic centimeters. I’ve done the same chamber three times in a row using this method and come to within one-tenth of 1 cc (0.1 cc) each time, so the operation is accurate and repeatable.

No bubbles
This is what it looks like when you’re done: no bubbles.

Now that you have the combustion chamber volume, the volume of your cylinder, the deck height volume (distance between piston crown and chamber’s cylinder sealing surface), you’re ready to calculate compression ratio as outlined above, or better yet, using one of the online calculators.

Sticklers for perfection will want to carry on and measure each of the combustion chambers in the engine, especially if it’s clear that the cylinder heads have been resurfaced or if the valve seats have been reworked. Believe it or not, machine shops don’t care nearly as much about your safety margins as you should!

Once armed with all the combustion chamber volumes, any large variance is cause to remove some noncritical areas of the combustion chamber in order to equalize the chamber volumes, which will result in a smoother running engine, and now you know exactly what your compression ratio is on all cylinders.

Combustion chamber
With a little grinding from noncritical areas and potentially some unshrouding of the valves, each combustion chamber can be brought within close tolerances of each other.

For all the gory details, more photos, and a detailed how-to article on this subject, including links to a spreadsheet calculator that I used when building my Corvair engine, visit http://Home.HiWAAY.net/~langford/corvair/valvejob.html.

Editor’s note: Here’s a video that shows a less-accurate technique, but it will help illustrate the method Mark outlined. www.YouTube.com/watch?v=K6c2cg0TFkU

Mark Langford started his career working on guided missiles while serving as an active-duty member of the U.S. Air Force. Later he became an automobile mechanic, specializing in German autos. Using his GI bill, he earned a bachelor of science in mechanical engineering from AuburnUniversity in 1988. Mark has been working as a design engineer in the aerospace industry for Teledyne Brown Engineering in Huntsville, Alabama, ever since. He earned his private pilot certificate in 1993 and promptly started construction on a KR-2S. Mark assisted Larry French and solid-modeled the 450-hp Lionheart Staggerwing design in the mid-1990s. An article detailing his role in the development of a new KR-2S airfoil was published in the Sport Aviation December 2006 issue. Mark is probably approaching a high-time record for flying behind Corvair engines and is certainly one of the most experienced with building and flying Corvair engines throughout the experimental aviation community. His website www.N56ML.com/corvair is a cornucopia of information, and his willingness to assist other builders with his experience and information is unparalleled. He enjoys complete support from his family and has certainly put his KR-2S to good use as a time machine.
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