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?
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.
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.
I own a 2013 Volkswagen Golf TDI. Just recently, I drove a one year NEWER 2014, VW, with a 2.5, 5 cylinder Sportwagon. The 2.0 TDI, and the 2.5 Gas Engine, get the SAME Fuel mileage in town. Its the TDI that saves fuel on the freeway. Yes, the 2.5 Gas engine has the timing chain, and will last the lifetime of the car. The TDI is economical on the freeway, but its HIGHER cost in the beginning, @ purchase, and it's timing belt maintenance/replacement, is why I am going to trade my diesel in, for the gas engine with its timing chain, very soon.
- Quieter operation
- Weaker than Chains, higher chance of breakage/damage.
- Made of elastomeric material which could degrade if comes into contact with petroleum products or other chemicals.
- Chosen by car manufacturers due to economical reasons.
- Replace @ every ~40k miles or so (very rough estimate)
- Belts stretch and loosen and cause whining noise before breakage (especially when the car is cold). This is an indicator of a failing belt, pulleys, & tensioners and will let you know that you need to service your belts.
- Easier to replace. Less time/labor charges. Tends to be less difficult. Usually manufacturers will design cars (with timing belts) to be able to service and replace the belts easily because belts inherently fail more easily and more often.
- Cheap parts
- Much Stronger, Less chance of breakage/damage.
- Chains are somewhat known to be able to last the entire life of the car.
- Replace when rebuilding or >100k miles (very rough estimate)
- Harder to tell when the chain needs to be replaced. Will tend to start stretching, but unless severely stretched/damaged, the chain should not cause severe issues with the timing because the teeth will still keep the chain in the right relative location. A worn chain could start rattling. A severely stretched chain could jump teeth and you will have timing issues at that point and if you have a interference type engine, you risk damaging or breaking your valves.
- Requires replacing/servicing the chain tensioner relatively often
- Harder to replace. More time/labor charges. Tends to be more difficult. Chains aren't expected to break or require as much servicing, so manufacturers tend to not put as much priority on making it easy to access/service.
- More expensive parts.
Theres also Direct Drive Gear.
In the end:
The chain is a stronger, longer lasting product, that is harder to diagnose and harder to replace. When and if it breaks, it will break catastrophically without as much warning.
The belt is more regularly replaced and is more telling when it is near its end of life. It is usually easier/faster to replace.
Balance between Durability / Maintenance / Costs