In the 70s, EPA rules caused a decrease in compression ratios, for 2 reasons:
1. eliminate lead as “octane improver”
2. reduce nitrogen oxides emissions
BUT:
1. Ethanol as an octane enhancement? – some dragsters use pure ethanol @ 15:1 ratio
-also: more octanes only needed @ wide open throttle – which allows cylinder to be filled more completely. Did car makers switch to 4bbl carbs, to maintain power, by using more air/fuel, less efficiently?
2. If speed limits were reduced, smaller less powerful cars would gain practicality. combined with more efficient combustion, perhaps overall NOX output wouldn’t gain much. slower traffic would also get more people into bicycling.
? efficient high compression engines, as reason for cars used in other countries getting better mileage
– I think high compression is one reason diesels are more efficient.
– high power/weight applications use high comp. – motrocycles, airplanes
– high ratio allows more expansion of hot gases
nigel m – I am talking about a change which took place in the early 70s. horsepower ratings took a big plunge at that time – not the 90s .
at the time, carburetors were common – the change I refer to is that : before, most V-8s had 2bbl carbs, much smaller air flow. after: the 4bbl carb became by far the most common carb., on v-8s.
We’d at least get a higher power/weight out of engines, but I think they were also more efficient – or Why did increase carb size radically

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