What is a Non-Interference Engine?

The modern internal combustion engine is a complex ballet of moving parts, but what happens if one of the ballerinas is not listening to the music? The answer is chaos. Depending on whether your car is powered by an interference or non-interference engine, that chaos could result in either catastrophic engine damage or minor engine repairs.

Interference vs. Non-Interference Engine Basics

For every two crankshaft revolutions in any engine, the pistons move up and down twice, while the camshafts travel one revolution, opening and closing intake and exhaust valves to allow for intake, compression, power and exhaust strokes. “Interference,” in engine terminology, refers to the paths that the pistons and valves travel, and specifically whether those paths cross.

Put simply, in a non-interference engine, when the piston is at top dead center (TDC), it will never go higher than a fully open valve; that is, the piston can never “interfere” with the valves. Conversely, in an interference engine, the piston could occupy the same space as an open valve. Precise valve timing is the only thing that keeps the piston and valve — the ballerinas in the opening metaphor — from trying to occupy the same space at the same time.

Most modern engines are interference engines for several very good reasons. In comparison to non-interference engines, interference engines “breathe” better because the valves can open earlier, close later and open wider. Interference engines can also achieve higher compression ratios. These designs extract more power, use less fuel and generate fewer emissions.

Facing Down Catastrophic Engine Failure

Whether maintained by a timing belt or timing chain, it’s critical to keep proper valve timing for power and efficiency. If the timing belt breaks, the valve train will likely stop nearly instantaneously, while the heavy crankshaft and pistons keep moving.

In a non-interference engine, a timing belt break will simply stop the engine. Because the ballerinas never cross paths, it doesn’t matter that one isn’t dancing to the same tune. The piston won’t contact the valve, and the worst you can expect is to re-time the engine and replace the timing belt.

In interference engines, milliseconds after the valve train stops moving, an out-of-sync ballerina crashes into an open valve. On some interference engines, a loose tensioner or skipped timing could lead to bent valves. It’s possible that only a few bent valves will have to be replaced, requiring the removal of the cylinder head. If a valve breaks off, though, it could bounce around in the cylinder, leading to far greater damage, possibly requiring engine replacement.

The only way to prevent either of these occurrences is to replace the timing belt on time. Most automakers recommend timing belt replacement around 90,000 miles, though some specify as little as 60,000 or as many as 120,000. Check your owner’s manual or maintenance manual to be sure. A timing belt replacement kit might include additional components, such as an idler pulley, tensioner pulley and water pump, which make sense to replace at the same time.

Very few automakers make non-interference engines anymore, since they can’t keep up with fuel economy and emissions standards. Thus, keeping up with timing belt maintenance can prevent breakdowns and possible engine damage.

Check out all the belts and hoses available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on non-interference engines and timing belt maintenance, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photo courtesy of Flickr.