An engine is an air pump, and every number in this category describes some part of how well it pumps. Get the geometry right first — bore, stroke, chamber volume — because compression ratio and displacement fall out of it, and everything downstream is built on those two figures.
Compression ratio is the one worth being careful with. It is the ratio of total cylinder volume at BDC to the volume left at TDC, and it depends on five things people routinely forget to include: chamber cc, gasket bore and thickness, deck clearance and piston dish or dome. Miss the deck height and you can be a full point out, which is the difference between pump fuel and detonation.
Cam timing is where an engine gets its personality. Duration sets how long a valve is open, lobe separation angle sets how far apart the intake and exhaust peaks sit, and together they produce overlap — the window near TDC where both valves are open. Overlap is why a big cam lopes at idle and pulls hard at 6,000 rpm: it scavenges beautifully at high rpm and lets exhaust back into the intake at low rpm.
Airflow ties it together. Displacement × rpm × volumetric efficiency tells you how much air the engine wants, which sets carburettor or throttle body size, injector size and turbo sizing. Be honest about VE: a stock engine is nearer 80% than the 100% people like to assume.
FAQ
What compression ratio can I run on pump fuel?
On modern 95-98 RON pump fuel, a naturally aspirated engine with good chamber design and knock control commonly runs 11:1 to 12.5:1. Iron heads, poor chamber shape or a lot of ignition advance pull that down. Boosted engines run lower static compression because boost raises the effective ratio.
Does advancing the cam change overlap?
No. Overlap is set by the two durations and the lobe separation angle, all ground into the cam. Advancing or retarding moves all four valve events together — it shifts where the powerband sits without changing how much overlap there is.