Things like intake runners, exhaust headers, valves, and camshafts all play a big part in how efficient our engine air pump is at moving air from the intake to the exhaust. Much of the work in engine design and engine building is focused on improving this volumetric efficiency. If it was, we would say the engine has 100% Volumetric Efficiency. However, if we were to put a flow meter on the front of our air pump engine and actually measure the amount of air that was being pumped we would find it doesn’t always equal the engine’s capacity. It’s normally a little bit lower – that’s because our engine is not 100% efficient at moving air from the intake to the exhaust. The volume of air our engine pumps should be equal to the engine’s cubic capacity, so a 350 cubic inch engine should move 350 cubic inches or 5.7 litres of air. If we think of our engine as an air pump, we know that for every engine cycle our pump should suck in and pump out a fixed volume of air. ![]() Or simply, pumping air from intake to exhaust – thus the description of an engine as an air pump. ![]() ![]() You may have heard the saying that “an engine is just an air pump”, what that means is if we just forget for a moment about the process of combustion that happens when the valves are closed, for every engine cycle all we are really doing is taking air from the intake manifold – sucking it into the combustion chamber – and them pumping it out into the exhaust manifold.
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