OddOne replies:
"An increase in compression ratio -always- requires more fuel, requires fuel with a lower burn time, and reelases more energy in a DISproportional amount. A 350 smallblock, for example, might make ~225HP at 2400 RPM with 9.5:1 compression and get 22 MPG off 87 octane gas. Replace the pistons (ONLY) and bump the ratio to 10.5:1 and you'll drop to like 14 MPG but the horsepower at the same rotation speed will go up to around 350, and as a result you'd need to run 92 octane or it'd ping as the 87 won't burn slowly/long enough to drive the pistons through the now longer (in terms of ratio) power stroke."
I understand the issue of needing a higher octane fuel to properly power a high-compression engine -- removing this from the discussion, lets just say that the low compression and high compression versions of your hypothetical engines were both running high octane gas.
You state that "an increase in compression ratio -always- requires more fuel". What is the SCIENCE behind this statement? A higher compression engine will perform more work in a given combustion cycle than a lower compression engine -- assuming, in your example, that the only change made was to replace low compression with high compression (however it may be done -- head milled, thinner gasket, domed pistons, or a combo of the three), and assuming that the change was not significant enough to alter the fuel-to-air ratio (I know -- hang with me here) -- then what is the SCIENCE driver behind a raised compression requiring MORE fuel, by virtue of simply raising the compression? Are you suggesting that raising the compression ALWAYS leans out the mixture, requiring an additional fuel amount to be added? Why, if so? What if I change the cam to increase the amount of overlap and decrease the cam centerline, thereby lowering cylinder pressures in the high compression engine, and also lowering the propensity of the high compression engine to ping?
Interesting thoughts you have provoked -- I currently own a high-compression muscle car, and have owned several different makes/models musclecars over the past 30 years -- all were at least 10.5:1 or higher compression -- all were/are carbureted, and all have/do get abysmal gas mileage, by the simple virtue that none were designed for gas mileage -- strictly point and shoot, straightline performance. And, I am VERY familiar with the need to run racing gas (even though the car is not raced, per se) since 92 octane pump gas is wholly incapable of preventing pre-ignition in older high compression engine/cam/induction system designs.
Your argument brought to mind a story -- my teenage friend's father's owned two similar Oldmobile 98s -- one was a 1970 with a hi-comp 455, and the other was a 1971 with a low-comp 455 -- essentially the same car, weight was similar -- hi-comp 1970 Olds car was able to get 15 MPG on the road, if you kept your foot out of it -- low-comp 1971 Olds car, driven on same trip and in similar fashion, could only manage a paltry 11.5 MPG, and was noticeably less powerfull -- carb and plugs showed proper mixture -- tried changing metering rods and hangers -- not much improvement. Decided to change cam in low-comp 1971 Olds car and replace with a specialty grind -- power jumped noticeably, making it nearly as fast as the hi-comp 1970 Olds car, once carb was tweaked out -- but gas mileage only climed to 13.5 MPG.
Above rebuttal is admittedly unscientific, with too many other variables unidentified -- but, as a general rule -- most of my friends and I noticed that once the compression ratios were dropped in all new cars in the 1971-72 timeframe, the gas mileage tended to drop also -- as did the horsepower (obviously).
Or had our foots simply become heavier?
Hit me with the hard science -- comments?
intothegoodnight