All timing belts tend to have dynamic tensioners in recent years. There is less tension variation than with static types. Belts tend to use similar tensions with regard to design (trapezoidal or spherical tooth design). Too loose, they rattle and teeth fall off, too tight they whine and weaken the valley area).
The Honda belts do have the rounded tooth profile (if memory serves), and the Alfa TS engines do have the more square section - trapezoidal - teeth (can't recall with certainty what the Fiat Twin Cam belts had, but I think was squarish section teeth). I can't imagine how the tooth shape would affect what tension a given belt may require.
I do note that the Honda engines of which I'm speaking are single cam engines, perhaps twin cam Honda engines of similar era (e.g. the H22A series) have a stiffer belt tensioning spring, but looking at online photos suggests they are similar to the (weak) tensioning spring on the single cam engines...
The Honda engines I've mentioned are quite 'modern' in the evolution of fundamental engine architecture, and still used 'weak' 'static' belt tensioning quite successfully (I'm sure Honda could have used 'dynamic' tensioning had they so wished or deemed needed, and / or stronger tensioning). By 'successfully' I mean very reliably with extended change intervals (unlike the TS Alfa engines with substantial 'dynamic' belt tensioning). Whether or not later Honda engines retained 'static' tensioning I don't know.
I can say from experience that the Honda belts I've seen are barely tensioned by the tensioning spring. The spring is so weak it would hardly take the slack out of a new belt, hence my practice of giving the spring a 'helping finger' when installing belts, just enough to ensure that all slack was eliminated (but still not 'tight' by any stretch of the imagination). I therefore do still wonder why Alfa found it necessary to use such substantial belt tension, since it obviously isn't needed with other engines of not disimilar basic design, and whether or not it may actually be problematic...
Modern engine belt routing has become more compact. Smaller pulleys and faster bending of the belt increases loading on the belt.
As I (think I...) understand this, smaller pulleys or sprockets don't impart greater tension into the belt or chain (i.e. the tension that results primarily from driving the camshafts against the stiffness of the valve springs, not the as fitted belt tension, which is also unaffected). The crank / camshaft pulley ratio is always a leverage ratio of one to two regardless of the pulley sizes (given that the camshafts must rotate half a rotation per one crank rotation). This means that if the drive pulley rotates 180° then the driven pulley must rotate 90°, and the camshaft pulley 'lever arm' must be twice the length of the crankshaft pulley 'lever arm', so no matter the pulley diameters the end of one lever arm must move half the distance that the other lever arm moves (or vice versa), or else the leverage ratio becomes other than one to two.
With a smaller radius crank pulley, the crankshaft can pull against the belt more strongly (i.e. the belt tension has less leverage acting against the torque at the crankshaft, and so the belt moves less per degree of crank rotation). But the camshaft pulley will also be smaller (to maintain the one to two leverage ratio), so there is less leverage acting on the camshaft via its' sprocket, so the camshaft is less able to resist the force imparted by the smaller crank pulley. So, as long as the pulley ratio does not change, then the force (tension) imparted into the belt will not change, even if the forces 'seen' at each pulley does change, it all comes out in the wash...
I do agree that smaller pulleys (in particular the inherently smaller anyway crank pulley) may well shorten belt life due to repeated 'over' flexing of the belt around the smaller radii sprockets, creating somewhat greater physical stress (kevlar fatigue?) as well as some slight increase in running temperature (the kevlar belts probably aren't affected much if at all, but bad for the rubber component of the belt, given time).
Twin cam engines produce greater camshaft speed fluctuations and greater reverse bending which accelerates belt fatigue.
"Greater camshaft speed fluctuations"? Not sure about that. Twin camshaft engines do impart stronger fluctuation in the
load the belt experiences, because each camshaft only has half the total engine valve loadings associated with that camshaft (i.e. only the inlet valves or only the exhaust valves), meaning that each (belt loading) valve opening event is more separated from the other loading events, i.e. less overlap between them. This creates greater spikes in the loadings experienced by the belt, even if the highest loading level that the belt experiences may not increase.
As I understand it, this increase in tension variation is only seen in the belt run between the two camshafts, the long belt run between the 'second' camshaft (whether it be inlet or exhaust camshaft) and the crankshaft experiences
all the valve opening events with less fluctuation between them (unless perhaps the section of belt between the camshafts is experiencing fluctuating loads to the point of becoming loose tight loose tight, which I suspect unlikely unless the belt has some actual slack in it...).
Single cam engines have all the valve loadings associated with only the one shaft, so there is more loading overlap and so the belt tension fluctuates less. Either way, strong fluctuations in belt tension are probably bad, so minimising them should be a good thing, but it will be what it will be for a given architecture.
Alfas and Fiats use the Pratola Serra family of engines. The Alfas use hotter profiled camshafts. That's why Fiat never had as severe timing belt issues.
The FIAT 'Twin Cam' engines are quite different and much earlier than the Pratola Serra engines (replaced by the Pratola Serra series). They were developed from an (earlier still) OHV pushrod block by Aurelio Lampredi (ex Ferarri engine designer), and the first (or at least among the very first) production engines to eschew chain for belt drive.
It may be that the more 'ambitious' valve timing associated with variable valve timing at higher rpm may be a factor in reduced belt life, but the lift and duration of each individual opening / closing event isn't typically affected by this, so I tend to doubt it...
Usually it is only diesel engines which have HTS (High Temperature Stable) belts. More heat escapes down the exhaust pipe on petrol engine but more heat radiates from the combustion chambers on diesels. That's why diesels have HTS belts.
I'm sure belts don't like higher temperatures, I wouldn't if I were a belt...
Regards,
John.
