rebuilding the 7A soon, Oil burning, higher CR and MPG Options

[stephen_lee]

So its time to rebuild one of my 7AFEs because of the all too common oil burning issue, and early signs of rod knock.

SO i want to go fuel economy way since its my DD.

Would a higher CR(12:1ish) make for a better mpg rating when i dont get on it since more torque = less throttle needed?
or is my logic flawed?

[Bitter]

higher CR does not need more fuel, it needs a more knock resistant fuel. 10-11:1 is perfectly fine on pump gas with the right timing curves, even 12:1. but the 7afe pcm may not like that much and running timing retarded all the time will hurt fuel mileage. more fuel can be added to help prevent knock by quenching the combustion chamber and cooling things by running rich, but thats not because you need more fuel for a higher CR. honda's run high CR and get great mileage

good example is my GF'd 50CC moped. it came with a dismal 8:1 CR which we increased to 14:1 by simply shaving a little metal in the right place. we checked the AFR by doing plug chops and using a colortune test plug before and after. we did not have to change carb jetting one bit with the nearly doubling of the compression ratio. however we did need to use a higher octane fuel. she netted not only more power but a slightly better mpg number as well. you're extracting more power from a given amount of fuel.

an 1800cc engine is going to burn X amount of fuel at Y rpm and Z amount of load no matter the compression ratio, assuming all things are equal. the difference is that a higher CR will need a higher octane to keep the fuel needs the same, otherwise you'll need to run rich to prevent knock. the pcm does this already at WOT by going very rich to be on the safe side. well it goes way overboard and the afr will dip to as low as 10:1! thats way too rich, the only purpose of going soo rich is to prevent knock.

if you're looking for mileage the best thing to do would be to port the head and intake manifold as you rebuild, a small bump in CR or slight timing advance, some AFM tuning on a dyno, and a slight increase in cam duration. you'll net a good increase in power and should get a slight bump in your mpg as well. of course, only if you can stay off the gas pedal.