As engines age their effective compression ratio increases due to carbon deposits in the cylinder & on the pistons.
Essentially, the built up crud inside the engine forces the fuel/air mixture inside each cylinder to get squished into a smaller volume, raising the compression ratio.
Start with an 8.5:1 compression ratio, subtract a small amount from each (Exaggerated, but I'll use .2) of the volume from each side to get 8.3:.8 = 10.3:1 new compression ratio.
The crud reduces starting volume too, but it is more important how much volume is missing when the piston is at the top of its stroke and the fuel/air mixture is at its densest.
"Essentially, the built up crud inside the engine forces the fuel/air mixture inside each cylinder to get squished into a smaller volume, raising the compression ratio"
Do any of those STP or other brand cleaners really help on that crud?
Essentially, the built up crud inside the engine forces the fuel/air mixture inside each cylinder to get squished into a smaller volume, raising the compression ratio.
Start with an 8.5:1 compression ratio, subtract a small amount from each (Exaggerated, but I'll use .2) of the volume from each side to get 8.3:.8 = 10.3:1 new compression ratio.
Sounds reasonable but every car manual I've ever looked at recommends doing a compression test on old engines looking for compression drops. Sure you have stuff building up on the piston heads etc. but on the other hand the clearances between the rings and cylinder walls are increasing causing compression leak. Wouldn't these two effects balance out or even shade towards compression drops? The crud reduces starting volume too, but it is more important how much volume is missing when the piston is at the top of its stroke and the fuel/air mixture is at its densest.
How well the combustion chamber is initially designed can affect the resulting octane rating need also.