Timing Chain vs. Timing Belt

Question

What is the service life of a timing chain and how does having a chain drive cam benefit from a belt drive cam and vice-versa?

Answer 1

Belts are quieter but are often less obviously worn before they break (chains stretch and loosen, belts tend to stay tight without damaged belt teeth up until the moment they snap). Timing belts are also less complicated (chains require an oil bath). Chains are generally stronger, so vehicles with timing chains often drive more than just the cam with the timing chain. Due to stretch, when you change a chain you MUST replace all the timing gears, as the teeth will wear to match the chain stretch. While not necessary on a belt driven system, a failed gear will cause a belt failure and, depending on your engine, could be very bad, so it's generally recommended to replace all idlers and gears in a belt system, too.

The amount of damage caused by a belt breaking vs a chain breaking entirely depends on the type of engine. Engines with really high compression ratios (performance cars that require high-octane fuel, diesels, etc) tend to have the valves and piston heads move such that they could come into contact if the cam stops opening and closing the valves. When a piston head smashes into a valve, you ruin the head, valve, and potentially other cylinders if pieces of shattered valve travel through the air intake system to the other cylinders.

There's a good section on the TDI Club's FAQ about timing belts, as that's what Volkswagon uses on all their turbo diesels. A belt breaking on a TDI is catastrophic. As far as I know, most cars use chains and compression ratios low enough that there's no risk of piston heads smashing into an open valve should the chain break. You can't retrofit a belt system to use a chain, you wouldn't want to do the reverse, but I have seen people replace belts and chains with direct gear drives.

Answer 2

I think we covered this before, but I can't find it... Timing chains have much greater service lives at the expense of slightly more noise and friction, as well as a lot more damage if they break (but odds of breakage are much less than a belt).

Answer 3

I may not be 100% accurate but I think belts have expected lift time of 60k-100k and at 100k they should definitely be changed.

Chains are claimed by a lot of manufacturers to have lifetime duration and shouldn't need to be replaced.

They have been known to break but as Brian mentioned, that's very rare. I've also heard of a more common failure mode with chains where over time the metal stretches which alters the timing, so although everything runs, your car becomes de-tuned and to fix it, requires a chain replacement.