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Why Should I Care About Compression Ratio?

December 1, 2010 | Troubleshooting & Repair
By Ron LaDow

If you are not contemplating or in the midst of an engine rebuild, you probably don't care about compression ratio. If you are, there are certain facts which should figure in your thinking.

Engine life is inversely related to engine RPM. The 'power' an engine makes is largely irrelevant to engine longevity. An engine that makes 100HP at 5,000 RPM, and limited to that speed, will far outlast an engine that makes 75HP at 6,000RPM, and regularly run to that speed. Loads increase linearly with power and geometrically with speed.

Engine speeds above 5,000 RPM are where almost all horsepower is quoted since it’s easier to make big horsepower numbers there. It's just not easy to make them at the RPMs commonly used; those big numbers are swapped for power elsewhere in the engine speed range. It's also easy to make noise, and this is entirely too often confused with power.

There are four ways of increasing power in the range between 2,000 and 5,000 RPM, where 99.99% of all driving is done:

  1. Increase displacement
  2. Modify for twin-plug ignition
  3. Fit slightly larger carburetors on 1720 engines
  4. Increase compression ratio
The 1st (increase displacement) is common; 86mm ('big bore') piston and cylinder sets. Cheap, easy (some more so than others), effective and durable. There are both old and new p/c sets of larger size; I have no knowledge of them. If they are durable, larger displacement remains the easiest and cheapest method of increasing power. The increase is directly related; +10% displacement = +10% power, assuming the engine structure can support the new parts durably. If you're considering increasing the stroke, you'll have to look elsewhere; it's not practical in 356 engines.

The 2nd (modify for twin-plug ignition) is very effective and also costly. My company and others offer twin-plug setups as an alternative. In concert with further increased compression ratio, the gains here measure in the range of 10-20% across the intended rev range.

The 3rd (install slightly larger carburetors) is expensive to come by now. The formulas for 1720cc engines running near 5,000 peak RPM call for 37mm carbs. Del Orto used to offer 36mm carbs; they're gone. My company offers 36mm Zeniths and others convert 32s to 34s; neither is cheap, but gains are in the 7.5%-10% range for the 36s. 40mm carbs have throttle bores which are almost 25% larger in area than the 36s; more suited to 2-liter engines or 6,000 and above RPM.

The 4th (increase compression ratio) is the result of careful work and can be done by a hobbyist and certainly by most any reliable 356 engine service shop. The gains here are more than standard textbook predictions, with some dyno increases showing over 10% for a 1:0 compression ratio gain. In other words, your motor delivering 60 #/ft at 3,000 RPM could be delivering 66#/ft; an amount you can feel in the seat of your pants—way more than any claim of an additional 15 peak horsepower.

Because the 356 engine is more sensitive to compression ratio than predicted, any variance between cylinders means a greater variance in the output of the individual cylinders. Simply stated, the engine is rougher than it needs to be if the compression ratios are not balanced between the cylinders.

Finally, increased compression ratio also increases efficiency; one of those rare circumstances where power, smoothness and fuel economy all benefit at all engine speeds.

Like all gains, increasing compression ratio costs. A hobbyist will need to buy or make various tools, make the measurements and then modify the various parts (or pay to have them modified). The specialist shop will have the tools but will have to put in time to measure and modify the parts. Those hours cost money, and none of the short cuts yet investigated delivers anywhere close to best-practice results.

"The first 90% of the project costs 90% of the money and takes 90% of the time, the last 10% takes the other 90%." Anon…

MEASURING COMPRESSION

A compression ratio is the ratio between the 'empty' volume in a cylinder with the piston at the top of the stroke (the “:1” in C/R notation, as 9.0:1), compared to the 'empty' volume in the same cylinder with the piston at the bottom of the stroke. (the left hand number; "9”:1or "8.5”:1 or whatever).

With the piston at the top of its stroke, there is some 'empty' volume left in the cylinder, bounded on the top by the chamber in the head, on the bottom by the top of the piston and surrounded by whatever portion of the cylinder remains uncovered by the piston. That is the "net chamber" and (as mentioned) the ":1" in the ratio. It is the most important number in establishing the compression ratio and the most difficult to measure. It is the sole component in the right hand number and some portion of the left hand number.

In some engines, notably those with hemispherical combustion chambers and central spark plugs, it can be measured directly in an assembled engine. Just pour in a measured liquid with the piston at top dead center and read the results. 356 combustion chamber shape and plug location makes this shortcut inaccurate. How inaccurate is anyone's guess; it is impossible to know.

Accuracy is important here. In the range of 9:1 compression ratio, a 1cc mistake changes the ratio by approximately .2:1, such that your 9:1 engine could be either 8.8:1 or 9.2:1. A 1cc left-over bubble isn't large and at best, you've given up some power, efficiency and smoothness.

To improve over that sort of inaccuracy, several parts must be measured in various ways and very carefully. With that data, you can build the engine accurately to the designed compression ratio.

With the proper tooling, measuring both the head chamber volume and the piston dome volume is not difficult, and they are both direct measures such that the numbers you get are used just as you get them. That's two of the three required numbers for the net chamber.

The third measurement is the most important of these three, as a .010" error means a 1.45cc error in a 1720cc engine. It is a calculated number, and is derived from careful measurement.

It is the volume defined by that portion of the cylinder left uncovered by the piston at TDC (known as the "deck height"), multiplied by the area of the cylinder.

The area of the cylinder is known, while the actual deck height is affected (in our engines) by:

  1. The length of the cylinder
  2. The "compression height" of the piston
    (the distance between the wrist pin centerline and the deck of the piston)
  3. The length of the rod
  4. The stroke of the crank
  5. And finally by the distance between the centerline of the crank and the surface where a cylinder seats on the case (see photo)
It is commonly and mistakenly measured by putting clay or solder on the piston top, rotating the assembled engine, and measuring the compressed thickness. The resulting dimension tells you something about the relationship between the piston dome and the combustion chamber, but has only an accidental connection with the deck height.

Accurate measurements here start with finding the piston deck. It really doesn’t matter where it is, so long as what you measure to for the deck height is the same surface you measure to for the dome volume. With the tail of a caliper against that established surface, cylinders and pistons in place on a mock-build engine, you can determine the height of the cylinder above the deck; deck height dimension.

Commonly, the values will wander a bit on the first measurements. Like using the tools to find the chamber and dome volumes, it takes a bit of practice to produce repeatable numbers. And you may find some inconsistencies as a result of those five variables mentioned above. You’ll have to find where the variance is and correct it.

As a final comment, you can successfully run 9.5:1 compression ratio, single-plugged, a 266* cam, Zeniths and stock exhaust on California pump gas, 91 octane. A recent (inadvertant) test suggests 9.75:1 may be workable; a test design is in the works.

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