going to higher compression...
 

As some may know I love the little subes from the 80s and even before then.
my current has an ea82 3 main bearing boxer engine at 1781cc. Factory carb version. I have owned all versions but a turbo ea82, and had my foot on the throttle of one of those as well.

I found the most successful consistent fuel mileage was the loyales with SPFI (singular point injection) even with an old school high micron ECU making the electronics work.

For some reason I have associated the carb at 9:0.1 factory compression to make the engines weaker,built by subaru this way, as they are a snappy little engine to drive around with a natural tendency to make computers a complete idiot in the fuel/air world..

Recently I grabbed at a loyale engine and hope to complete my n/a carb project at around 10:1 to gain power with respect to: "Contains 10% ethanol" stickers on a lot of gas station fuel pumps today...

My anticipation leads me to ask here about the results, I have only had the ea82 up to 9.5:1 and it was the best one.
I can only assume this applies to many engines, even my v8s changed for the better with higher compression more so than large cams and crazy intakes.

A good idea? or am I going to be rumbling neighbors windows with a cessna AWD. (I have heard the 11:1 ea81 run with an incredible WW2 bomber/fighter type rumble) and am hoping the ea82 stays as calm as usual..

well, the octane of the fuel with 10% ehtenol is still what its claimed, so 87 E90 is still 87 octane, so you wont get better power by upping CR.

High compression alone shouldn't make your car louder. Maybe the engine would shake a little more. It should make the exhaust a little quieter because the gases would expand more in the cylinder before going into the exhaust pipe. That loud car probably had a high performace camshaft and muffler.

I couldn't tell you how Subarus work with high compression. In general it increases efficiency and power but could cause some knocking at low RPM and low intake vacuum (high throttle) if you don't do something else, like use high octane fuel or water injection.

I would think premium fuel would be a must for 10:1 compression. How do plan to increase the compression?

If you can make slightly higher compression work in your suby without detonating (pinging) it will definately make more power. It really takes a couple of compression points until you hear a real difference, but I doubt you will get away with much of an increase unless you change other things. Engine builders almost always use the highest compression ratio they can get away with, cylinder filling gets more effiecient, and you get more power. I will say it's real easy to get carried away, more than one guy I know has been unhappy with raising his compression ratio, making it necessary to now burn higher octane fuel, retard the timing, or both.

Higher compression engines do have more 'pop' is the word I use to describe the exhaust note difference. But, if you will be using the same exhaust and muffler as you have now, I wouldn't expect it to be any louder.