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What is an interference engine?

What is an interference engine?

About the author

Chris Pollitt is an award-winning automotive word-wrangler, editor of the website Not 2 Grand and a keen collector of crappy old cars.

You may recall that a little while back we explored what would happen if your timing belt were to fail. When we did, we painted a picture of mechanical horror and bits of metal hitting other bits of metal. It served as a valuable warning.

However, there’s more to talk about. Namely, when your timing belt does break, or if your timing gets a great deal out of whack, why do things go so horribly wrong?

The answer is simple. It’s because we’re talking about the interference engine. This is an engine design which features space in which different mechanical will occupy at different times.

If they fill that space at the same time, you have, as the name says, interference. 

This interference comes in the form of the pistons and the valves. With correct timing, the valves are clear of the pistons when they reach the top the engine, or their stroke. I.e, the valves are closed, as they should be when the piston is at the top.

The valves should only be open when, in the case of inlet, the piston is going down and thus sucking in fuel and air. Or when the piston is moving back up to expel the exhaust gasses. This is the ‘squeeze’ and the ‘blow’ part of the old adage, suck, squeeze, bang, blow. 

What is an interference engine?

If the timing belt snaps, the valves ‘drop’ into the engine, because the cam is no longer connected to the action of the crank. But the crank is still going, and when it does, it smashes into the valves.

If you’re lucky in this scenario, it will be a case of replacing a few bent valves. If you’re unlucky, the valves could well smash through the piston and completely destroy the engine. Fun times. 

You’d think, then, that as time and technology have moved on, car manufacturers would have pulled away from using such a system.

Sadly, no, they haven’t. The interference engine is still by far and away the most common type of engine. 

But why? Well, it’s because an interference engine by its very nature allows for a greater range of movement. This has all sorts of benefits when it comes to tuning, especially in terms of compression, torque and overall economy.

A non-interface engine works by ensuring the piston never goes high enough to occupy the same space as the valves. The engine, therefore, is limited.

But if it helps explain it, if you wanted to change the compression of a non-interference engine, you may well end up turning it into an interference engine by altering the piston’s stroke. 

The bottom line, as we’ve said before, is to make sure your timing belt is changed regularly. There is nothing to be afraid of with an interference engine, it is a powerful, economical, reliable means of propulsion. It’s only when you neglect it, as with anything, that it bites you.