Originally posted by Paul Ranson
I've never come across an engine with as low a CR as 7.5...
Paul
Historically, compression ratio has trended in interesting ways. From 1927 to 1934, the CR of the Cadillac V8 (a rather conventional sidevalve design) rose from 4.25:1 to 6.25:1, with the only major change being the increased availability of what was called at the time "high-octane" fuel...about 66 octane for "Ethyl," vs. 56 for "low test." Depending how you look at it, a piston engine with a CR of only 4:1 is a rather different machine than the one we know today. Here, when the piston is halfway down the bore on the power stroke, the combustion charge has expanded only 2x, eh.
Of course "Ethyl" was the marketing name for TEL, tetraethyl lead, which was discovered as an octane enhancer by Thomas Midgely and Charles Kettering of GM Research in 1921. Since lead was a well-known environmental toxin even then (Midgely himself nearly died from exposure to the lethal compound; in the first plant built to produce TEL in volume, five men died of lead poisoning in the first two months) it took the combined lobbying and marketing efforts of General Motors, Standard Oil and DuPont to put that one across on the public, and even then it took a number of years for TEL to attain wide acceptance and use. This is one of the more interesting tales in the annals of American capitalism, which folks may find illuminating.
Just before WWII the refiners learned, through regenerative catalytic cracking, how to produce high-quality, high-octane gasoline in mass quantities...this advance was largely the work of one man, Eugene Houdry, one of the true unsung heroes of both the auto industry and the allied war effort. (High-octane aviation fuel was key to allied air superiority.) After the war this production capability propelled the trend away from L-heads to the modern high-compression, overhead-valve V8. The 1949 Cadillac V8 came straight out of the box with a 7.5:1 compression ratio, a rather startling development for the time. Through the 1950's CR continued to climb, and by the 1960's musclecar era there were a few US production engines with 12.5:1 CR's, but emissions concerns put a stop to all that business.
At that point, the same company that had invented, developed, and popularized TEL for production cars then led the charge to eliminate it, when GM formalized its plans to phase in the catalytic converter, which made leaded fuels essentially obsolete: the platinum catalyst could not tolerate any amount of TEL in the exhaust and remain effective. But as there were no real substitutes for TEL available at that time, compression ratios would have to come back down.
All through the 1970's and 1980's in the USA, static compression ratio on production cars hovered in the 7.5 to 8.5 range due to emissions requirements, mainly the switch to unleaded fuels. With better chamber designs and electronic control systems (notably, piezo knock sensors) and improved unleaded fuels, they have since climbed back up to 10:1 and above.