Yes I have a concept for an autotensioner, but needed to finish the belt first to ensure the loadings are correct. That is nearly done but I now need an engine to play with to get the Autotensioner in the right place. I dont really want to remove the engine from the Esprit for that purpose, but my Sunbeam is already out and in bits. Problem is that the Esprit get jealous then keeps looking for attention stopping me from getting on with the Sunbeam, but it is moving up the to do list quite fast..
With regard to cam timing error, this will depend on the engine the layout and the belt used. When we set our timing we set it statically. Unfortunately when the engine is running the system becomes dynamic. In our engines, there are torsion inputs coming from the crank, eg after the spark plug fires, pressure builds and subsides in the cylinder causing different rates of angular acceleration during each engine revolution. we also see positive and negative torsional effects from the camshafts as they open and close the valves plus the oil pump is also a source of variation as the oil pressure in each part of the rotors rises and falls as part of the process. When you put all this together at the same time like it or not, each cam will see a plus or minus variation (cam ref crank) from the nominal position it was originally set as the engine cycles. I have seen variations of several degrees on some engines if a system resonance is encountered making it very important to ensure the cam timing is accurate. I have seen on engine that was set up using the correct timing tools which had seen better days (a bit worn) and the valves hit the pistons. There is sometimes as little as 5° or 6° allowable before terminal damage can become reality. Note that once the belt tension drops the potential for dynamic error increases as the magnitude of cam movement can increase. The transition from slack to tight also accelerates the damage to the belt which is why autotensioner work so well, as they limit this.
With regard to the adjustable pullies, they are a good way to set the cam timing to be accurate. The cam, pullies etc are all made to a tolerance so in theory there is a lot of potential variation on the timing in addition to the above. If you set the timing to be accurate using adjustable pullies then some of the variables are reduced