The chain tensioner can only keep the chain tight up to certain point. Even if the chain tensioner was able to keep an elongated chain tight, The cam timing would be off with a severly elongated timing chain, thus the chain will have to be replaced to have the cams timed correctly.
The chain elongation on the V8s is greater because of the many more pins and rollers and the wear of each. Since the second driven cam in a V8 is so much farther away from the drive sprocket (measuring along the chain travel) than in an inline engine, the timing chain must be replaced more often in a V8.
The plastic chain guides are better than the old rubber coated aluminum chain guides. Possibly it was difficult to do accelerated life tests on the plastic guides. Maybe a better material has not been developed yet.
But just think how much better the double roller chain is than the the rice burners with a belt that has to be changed every 60,000 miles. Although the belt costs less than a chain, the total cost is more because of the labor to replace the belts many times.
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