I'm not sure where you are getting 20 atmospheres from. An 8.5 CR engine will have a PR of around 8.5. The density ratio will be less however as a result of the temperature rise from compression and losses from having a VE less than 100%. Density would be the key factor here in required spark energy though the compression temperature rise would have a slight offsetting effect as the spark would have to input less energy into raising local temperature of the mixture between the the plug electrodes to initiate combustion.
A compression tester will illustrate what pressure ratio your engine develops at cranking rpm at least and this will be lower at 2500 rpm BTW, due to VE loss. You certainly don't see 294 psi on a Lycoming with a compression tester.
I totally agree on the last two paragraphs. Introduce a 2-3 inch spark gap somewhere in the system is bad news for a coil. It will arc internally which can damage some coils in short order. Have seen that more than a few times.
Thank you, Ross, for your sage observations. Both you and Dan among others here are artisans in both talk and walk. Very few with that combo. Thx.
Re: the 20 bar pressure for 8.5 CR.
The justification for that figure is just straight thermodynamics.
Verbally, when you trap a volume of air in a cylinder and then reduce the volume, two
separate things happen. First, the pressure increases as the volume is reduced. Second, the temperature increases as well, which adds additional pressure.
Getting into a little math, the exponent of the first is 1, the exponent of the second is .4 for air. Combined, it is 1.4
So,
The relation of CR to PR is CR^1.4, or in the Lyc case, 8.5^=20.0072 PR
That pressure derived is of course, only a starting point, but is the proper starting point when we then roll in all of the other well known factors that will tend to reduce the number.
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As an interesting reverse example, those amazing jet engines that push the tubeliners across the sky have pressure ratios(PR) near 40. In terms of compression ratio(CR) it would be 40^(1/1.4)= 13.9CR
Ron