The first topic is the difference between a cam chain and a timing chain. Both involve chains and both set the timing of at least one camshaft. A cam chain however is just between camshafts, and the engine has a timing belt to drive the exhaust camshaft. A timing chain goes between the crankshaft and both camshafts. A little history may help.
Adjustable Valve Timing Cam Chain Systems
Engines dating back to the 80’s with dual overhead cams had a timing belt that drove the exhaust camshaft, and a cam chain that linked the two cams together to drive the intake camshaft. These early systems did not use a tensioner for the chain, as it was short and sweet, and simple. In the mid 90’s, engines began to use a cam chain tensioner on the chain between the two cams. The tensioner could vary the timing of the intake camshaft by moving up and down, but it had just two position settings: Down for retarded or up for advanced cam operation. So the intake cam changed position by a fixed amount of degrees of rotation. These systems were plagued by oil leakage at the gasket for the tensioner. The gasket was located directly above a soft rubber half-moon plug for the cam hole section of the valve cover gasket. Occasionally, a cam chain tensioner would fail due to plastic guide damage.
The real conundrum was, and still is, that the factory cam chain tensioners are very expensive, but the value of these cars has dropped significantly. There are many bogus and unreliable Asian brands for tensioners, some of which don’t even work once. Fortunately, as of about 2015, certain Asian brands have a reasonable track record, and cost half the price of a dealer unit. They are still a risk, and only have the manufacturer’s 12 month warranty. We cannot extend our 2 year / 24,000 mile parts warranty to cover such parts.
The only 4 cylinder gasoline engine produced with this cam chain and adjustable tensioner was the 1.8T 20V engine from 1996 through the first half of 2005.
The AHA and ATQ 2.8 V6 engines from 1998 to 2005 had an adjustable tensioner and cam chain system on each side of the engine. FYI: there is a booboo in the factory manual concerning the timing of these chains. It shows a 16 link distance between cam gear markings on either side of the engine, when in fact the left side takes 16 links but the right side takes 15 links. Ooooops.
The 2.7T V6 engine for the S4, A6 and Allroad models were also timing belt motors with cam chains and adjustable tensioners. Oddly, they seemed to have less frequency of tensioner failure than their non-turbo cousins, perhaps because their owners provided better overall maintenance.
Just to note, the 2.0 Tdi diesel engine has a cam chain with some very high tech gears, but no variable valve timing.
Timing Chain Systems
The first timing chain systems in the VW of US world was the VR6 engine in 1992. To help keep the engine / transmission assembly short, the chain system is on the transmission end of the engine. A lower chain sneaks out from under the flywheel / flexplate to an idler gear set just above the transmission. A larger chain goes up from there to turn the camshafts. This two section concept has been used extensively since then on L5, V6, VR6, V8, W8, V10, W10, W12 & WR16 engine configurations in both gas and diesel designs. The upper and lower chain system is referred to as a relay chain system. One exception is the V10 Tdi diesel from the 2004-07 Touareg, which has purely gear drive for the camshafts and all accessories. In Europe, an oddball VR5 engine was made briefly.
All these systems use roller chains, which, like it sounds, have roller shaped barrels that engage gear teeth with similar diameter radii. These essentially look like bicycle chains. Another type of chain called the gear chain or toothed chain has the likelihood of eventually becoming the chain system of choice. Gear chains use multiple plates stacked as thick as needed and sandwiched between side pates similar to roller chain side plates. The teeth of the gears involved have straighter sides instead of a rounded curve, and look more like gear teeth that could engage another gear right next to it. Overall, more tooth area is available for power transfer than with the same size roller chain. The power transfer has a much higher efficiency, and better wear characteristics. To date, gear chains are in use on just the EA888 4 cylinder 2.0T made since 2008 and the EA888 1.8T made since 2012, but used since 2014 in the US.
Variable Valve Timing: High Tech Power and Mileage Increase
Right at the turn of the century, Variable Valve Timing technology was introduced on gasoline engines. Variable Valve Timing is sometimes abbreviated as VVT. The hydraulic gear on the camshaft may be called an INA gear. These systems refer to the ability to vary camshaft timing in relation to the crankshaft in infinite intervals over a long range. The range of valve timing control is typically 40-52 degrees of intake cam timing and about half as much exhaust cam timing.
An electric solenoid controls the direction and volume of flow of engine oil to the two sides of a star-shaped cavity separated by a matching star-shaped vane inside the cam gear. The vane is attached to the camshaft, and rotates in relation to the gear. This allows hydraulic control of the timing of the camshaft in relation to the cam gear, and, through the timing chain, in relation to the crankshaft gear. Just as a fairly accurate approximation, variable valve timing is good for 15-20% better horsepower.
Variable Valve Timing With Timing Belt Systems
Fully variable valve timing was not just on timing chain engines, but also on 4, 6 and 8 cylinder engines with timing belt driven camshafts. These designs still use a tensioner on the chain between the 2 camshafts, but it does not participate in cam timing.
The timing belt driven 2.0T engine which started production in 2005.5 incorporates a cam chain with a fixed tensioner and a variable valve timing intake camshaft gear.
The 2.8 V6 30V in the Volkswagen Passat from 1997 to 2005 and all the V8 engines with timing belts have a variable intake cam timing gear and a cam chain on each bank of the engine.
The all-aluminum 3.0 Liter V6 engine in Audi models from 2002 to 2005 was timing belt driven. It had variable valve timing gears on both of the intake and the two exhaust camshafts, meaning the timing belt drove all 4 camshafts, and there were no cam chains. This was the only motor built of this design.
Variable Valve Timing With Timing Chain Systems
For four cylinder engines with a single gear type timing chain system of the pulley end, there are the 2.0T L4 engine that started in 2008 in the Audi line and 2009 in the VW models, and the 1.8T engine that follows later in the same EA888 lineage. These engines have variable valve timing only on the intake camshaft gear. Some 2.0T engines from 2008 to 2012 may have defective chain tensioners that can cause severe engine damage.
Karmakanix Knowledgebase Information on the 2.0T timing chain tensioner failure and links to the class action suit.
All V6 engines built after 2005.5 have been all-aluminum with stratified fuel injection. They are timing chain driven with variable valve timing gears on the intake and exhaust camshafts.
All VR6 engines have always been timing chain type engines with cast iron blocks and alloy heads. Older 12 valve VR6 engine have no valve timing adjustment. The 24 valve VR6 engines have been in use since the around the turn of the century. They have variable valve timing intake and exhaust camshaft gears on their timing chain systems.
Gunk deposits from old engine oil can clog up these cam control systems and cause an engine that will barely idle and cannot be driven. Failures in these systems are almost always related to insufficient oil changes, and often those clients are just following the manufacturer’s recommendations. These cam chain systems are one of the reasons that synthetic oil is required in most engines from this century. And the best synthetic oils should always be used.