Figuring Compression Ratios
Compiled by Mark Booker (Boatnik)
Back in June 2001, we discussed cc-ing cylinder heads and how to get an accurate measurement. The June article was Part 1 of 3 as we venture into understanding compression ratios, cranking compression pressures, and their effects on performance.
This article is Part 2 of 3, and its purpose is to discuss compression ratio. In the final installment of this series, Part 3, we'll crunch numbers for cutting the heads and make the actual cut. We'll also do some test-and-tune runs with our "pursuit heads" and document the performance improvements. We started with a set of stock 38cc Merc 175hp heads. Using the formulas presented in this article, we'll baseline my 150's compression ratio, then in Part 3, we'll mil these heads to achieve 200 pounds of cranking compression on gauge and a much higher compression ratio. Obviously with this much compression, we'll be running race gas (more on that in Part 3). The goal is a solid three mph increase and a speed of 85 mph on our test-bed 'Stream Viking (see Spotlight article for details on this boat).
To begin, the words "compression ratio" can often be confusing when discussed over bench racing beers because most of us associate "compression ratios" with the two-cycle's "enemy", the four cycle engine (big and small block Chevy's, Fords, etc). These two engine types probably hate each other because there's nothing like giving the "fist-in-the-air, medieval-Viking-ship-like" sign of victory as our outboard-powered screamer ride swoons past our buddies in their big-block powered Bajas. With all due respect though, the opposite can apply as well, especially when encountering those "Baja Bandits" with extra-high engine covers (blowers) and mountains of cubic inches (I've been dusted by a few of them…and I mean dusted!). Do we mean geometric compression ratio, or trapped compression ratio when discussing among our boat brethren?
Ever since the first internal combustion engines, regardless of whether the engines were two-cycle, four-cycle, diesel, etc., compression ratio numbers have always been the ratio of the volume of the cylinder with the piston at Bottom Dead Center (BDC) to the volume of the cylinder at Top Dead Center (TDC). The formula is as follows:
GCR = CV + CCV
Where GCR=geometric compression ratio, CV=cylinder volume, and CCV=combustion chamber volume.
The formula for cylinder volume is as follows:
CV = Pi/4 x (bore)² x (stroke)
Japanese two-cycle engine designers introduced another way of measuring compression ratios, which is important to note, because it sometimes gets mentioned during bench racing. It's called corrected, actual, or trapped compression ratio. The term I've heard most used is trapped, so we'll use trapped compression ratio for consistency in this article. The Japanese theory is that compression does not start until the exhaust port is closed, so now instead of using the entire cylinder volume, like in geometric compression ratio mentioned above, cylinder volume is only that space starting with where the piston just closes the exhaust port up to TDC. The formula is as follows:
TCR = TCV + CCV
Where TCR=trapped compression ratio, TCV=trapped cylinder volume, and CCV=combustion chamber volume.
All compression ratio values, either geometric or trapped, are the ratio of the maximum volume in any chamber of an engine to the minimum volume in that chamber.
Since we now know the formulas, let's do some geometric number crunching and baseline my 150 before cutting the heads. Specs are as follows (units in formulas will be cubic centimeters; to convert inches to cubic centimeters, multiply inches by 2.54):
Bore: 3.13", or 7.9502 cubic centimeters (cc's)
Stroke: 2.65", or 6.731 cc's
Total Cubic Inches: 122.3423
Total Cylinder Volume for all six cylinders: 2004.8322 cc's
Total Liters: 2.0048
Cylinder Volume for each cylinder: 334.1387 cc's
Combustion Chamber Volume (includes head gasket volume of 5.67 cc): 43.67 cc's
CV = .7854 (63.2057) (6.731) = 334.1387 cc's
GCR = 334.1387 + 43.67 = 8.6514:1
Don't forget to include the head gasket's volume in the combustion chamber volume. Without head gasket volume, compression ratio numbers will not be accurate.
There's no advantage of using either trapped or geometric compression ratio numbers when figuring compression ratio, just be consistent and use one or the other. I always use geometric compression ratio simply because it's what I've used for many years when building, modifying, or testing two-cycle or four-cycle engines.
One side note…
In the two-cycle engine, there's also the crankcase compression ratio. This won't be discussed in this article because, based on my research, it doesn't play as big as an impact on performance as does cranking compression. It's important in engine design, but the actual numbers have a very narrow window simply due to connecting rod length, bore, stroke, and a few other variables.
And finally, on a less technical note, if all this number crunching and formula frenzy becomes mind-numbing, another effective means is to simply throw away compression ratio numbers and just baseline our two-stroke wonders using gauge cranking compression numbers. That is, determine what cranking compression, on gauge, an engine will survive at using a particular grade of fuel, then shave the heads to meet this goal. Several posts on the Scream and Fly site from respected engine builders have informed readers that a particular o/b engine will survive without detonation at 'X' cranking compression psig on certain grades of fuel. I have found this advice to be quite accurate.
Hope this helps in understanding your engine a little better! See you in Part 3 where we'll finalize our "pursuit heads" mission.
Disclaimer: The knowledge in this article is not mine, but is info I've researched in various two-cycle books in pursuit of making my boat go faster. The authors of these books (way smarter than me) spent many years designing, modeling, simulating, testing, and racing various forms of the internal combustion engine, be it two-cycle or four-cycle. I used the info in these books, along with my own testing and tuning experience, to compile the information for this article. While it's great to win in racing, it's also fun just rippin' down the River together in "high-mph" mode. Any questions about this article may be emailed to me at firstname.lastname@example.org.
1) Design and Simulation of Two-Stroke Engines, by Dr. Gordon Blair
2) The Two-Stroke Tuners Handbook, by Gordon Jennings
3) Two-Stroke Performance Tuning, by A. Graham Bell
4) Auto Math Handbook, by John Lawlor
5) Boatnik's archived Test-and-Tune Notes