CALLING ENGINE GUYS/GALS

EDIT to post #61
I do not mean to imply that you need less than 4% to have a decent driver. I'm just saying that it is possible to get to less than 4%. I'll bet there are plenty of driver's out there at 10% LD. The thing is this; on a GOOD day, a good low-mileage 318 might make
135psi CCP at sealevel at 4% leakage. Which means, That at zero LD, the pressure would be ;
135/96%= 140psi.. If you had one at 10%leakage, that would then be
140 x 90%= 126psi
So then from 135 to 126 may not seem like much difference, intuitively , but giving the Wallace calculator a workout, I see that at 8/1 Scr and 900ft elevation, that amounts to a V/P index of 114.
At 126psi, the V/P is equivalent to what you would get at 2700ft elevation with still 8/1 Scr, or
a drop in Scr from 8/1 to 7.5/1, which at 900ft equates to a V/P of just 106 .
106/114 equates to a V/P loss of 7%, and so a loss of performance of that same 7%, at WOT, beginning at stall rpm, and diminishing with rpm to break even somewhere north of 3000 rpm with the stock cam.
This translates at part throttle to always having to press the gas pedal harder than what would be possible with a higher VP. And of course that also sucks gas mileage.
All in all, low cylinder pressure is a bad thing. As is large Leakage.
Large leakage also means combustion pressure is gonna blow into the pan. When the engine cools off, water that came in thru the carb as humidity will now condense in the pan, forming acids with the blow-by, that are then gonna be in the bottom of the oilpan, ready to be pumped all thru your engine on the next warm-up cycle, where they attack anything not made of iron or steel or rubber. Not to mention that the PCV is sucking it all up as it turns to vapor.
And almost finally, most of this scenario happens in the first inch of ring-travel.
And finally; in that last inch, the ring gap shrinks and grows by 3.1416 times the change in bore size. So if your bore at 1" down is 3.910 and at the very top just under the ridge is 3.917, that is .007 , and so the gaps are changing .022! and while they are doing that, they are scrubbing the ring-lands, wearing both the rings and the pistons.
So no, you do not have to target 2% or even 4% Leakdown, but you can clearly see the problems at 10% which, intuitively, does not seem like much; but from the engine's point of view is huge.

Read about VP here; V/P Index Calculation