turbovanmanČ
11-13-2011, 02:12 AM
Discuss, :eyebrows:
The timing belt tension had to be set so high to overcome "tow roping" of the timing belt, i.e. the timing belt going into negative tension. Tow roping is a belt killer. We found that this problem was caused by the extremely low valvetrain friction from using roller rockers combined with the DOHC set up. As soon as an exhaust valve rocker goes over the nose of the camshaft, there is no friction to slow it down and it tries to close the valve even faster, causing the exhaust cam sprocket to rotate clockwise faster and decrease the tension in the belt span between the sprockets. With a bucket tappet, which is used on most DOHC 4 cylinders, there is friction. On the 8 valve SOHC engine, there is an intake lobe on the same camshaft coming up to help out! So we had to crank up the initial belt tension to solve the problem.
An automatic belt tensioner would probably have helped. However, belt life is probably improved if people watch their belt tension and keep it within spec.
The timing belt tension had to be set so high to overcome "tow roping" of the timing belt, i.e. the timing belt going into negative tension. Tow roping is a belt killer. We found that this problem was caused by the extremely low valvetrain friction from using roller rockers combined with the DOHC set up. As soon as an exhaust valve rocker goes over the nose of the camshaft, there is no friction to slow it down and it tries to close the valve even faster, causing the exhaust cam sprocket to rotate clockwise faster and decrease the tension in the belt span between the sprockets. With a bucket tappet, which is used on most DOHC 4 cylinders, there is friction. On the 8 valve SOHC engine, there is an intake lobe on the same camshaft coming up to help out! So we had to crank up the initial belt tension to solve the problem.
An automatic belt tensioner would probably have helped. However, belt life is probably improved if people watch their belt tension and keep it within spec.