WHY SHOULD I CARE ABOUT COMPRESSION RATIO?
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:
- Increase displacement
- Modify for twin-plug ignition
- Fit slightly larger carburetors on 1720 engines
- 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:
- The length of the cylinder
- The "compression height" of the piston
(the distance between the wrist pin centerline and the deck of the
piston)
- The length of the rod
- The stroke of the crank
- 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.
Questions, comments about compression?
Email Ron LaDow
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