Technical Interference or not?

[KPat]

Watching this vid and the professional mechanic states it's a non interference engine. I questioned this and he replied "Im sure they are not interference as we had one in with a snapped belt and no bent valves." Lots of info on FF about these being interference engines. Could this just be a myth? Is there actual evidence of damaged engines with snapped cam belts?

 

[portland_bill]

Yes, and No.

The 60hp 1.2 is non-interference, later 70hp has variable valve timing, and is interference. The pic of the video is definitely an interference engine, as that is the variable timing pulley we're looking at.
All 500s are interference.
Panda became interference when it went 70hp, around 2010.
If an interference engine is running slowly, tickover, when the belt breaks, it is possible that it stops without valve damage. It could be that the engine changes between non-interference and interference as the valve timing changes. There'll be no data on that, although engineers at Fiat will know. Difficult to find a volunteer to find out. But if it is non-interference at low revs, that could be why some have experienced failure without valve damage.
Belts have also been known to fail as the engine stops. When you turn the engine off, it stops with a bit of a 'bump', and this can cause a belt on the edge to break. If found before the engine is turned, there'll be no internal damage.

 

[jrkitching]

It could be that the engine changes between non-interference and interference as the valve timing changes.

Valve timing won't affect whether the engine is interference or not, but valve lift will. It's basically just a question of whether there's clearance between the valves at maximum extension, and the piston crown at TDC.

AFAIK the VVT system only changes the timing, not the valve lift.

That said, common wisdom is that all VVT engines (and hence all 69HP 1.2 engines) are interference. IIRC it's a higher compression engine than the 60HP 1.2, and the cam profiles may be different, hence less clearance between valve and piston. It's possible that these engines are only just interference, so whether or not you suffer damage when the belt gives way may be down to the specific tolerances and valve clearances of your individual engine. Just because you've read somewhere that someone has 'got away with it' doesn't necessarily mean yours will be the same.

The reason there's so little information out there is probably because these belts very rarely break in service.

 

[Pugglt Auld Jock]

From my, mostly Panda related, experience. If it's got the VVT cam pulley - as seen in KPat's post above - then it's definitely interference. If the pulley is solid (pressed steel on earlier engines and cast on later) I believe they're non interference. I also believe the smaller capacity engines are all non interference. However this is all knowledge acquired mostly from talking to people who "know" or from forums and videos.

My attitude towards all this though is that I don't want to find out, by having a belt break or skip a few teeth, whether it's interference or not because you can bet your boots it's going to do it somewhere inconvenient or dangerous - like the outside lane of a motorway or miles away out in the country on a dark wet and windy night. So I would never let a belt go much over the recommended interval before changing it.

 

[KPat]

From my, mostly Panda related, experience. If it's got the VVT cam pulley - as seen in KPat's post above - then it's definitely interference. If the pulley is solid (pressed steel on earlier engines and cast on later) I believe they're non interference. I also believe the smaller capacity engines are all non interference. However this is all knowledge acquired mostly from talking to people who "know" or from forums and videos.

My attitude towards all this though is that I don't want to find out, by having a belt break or skip a few teeth, whether it's interference or not because you can bet your boots it's going to do it somewhere inconvenient or dangerous - like the outside lane of a motorway or miles away out in the country on a dark wet and windy night. So I would never let a belt go much over the recommended interval before changing it.

Well as I am not one to skimp on servicing. Oil, coolant, filters, belts, brakes etc all get attention well before the mileage intervals. So interference engine shouldn't be an issue. For me anyway...

 

[portland_bill]

Valve timing won't affect whether the engine is interference or not, but valve lift will. It's basically just a question of whether there's clearance between the valves at maximum extension, and the piston crown at TDC.

When a piston is moving towards the top on the exhaust stroke, the exhaust valve is already closing, and the inlet valve is opening, but in most cases, neither will be fully open. Timing will determine whether either are open enough to meet the piston. Nothing sits still, until the belt breaks.

 

[jrkitching]

When a piston is moving towards the top on the exhaust stroke, the exhaust valve is already closing, and the inlet valve is opening, but in most cases, neither will be fully open. Timing will determine whether either are open enough to meet the piston. Nothing sits still, until the belt breaks.

That's true, of course - and the engine is designed in such a way that the valves won't make contact with the pistons in normal service, whatever the position of the variator.

Once the belt breaks, then the positional relationship between camshaft and crankshaft is lost completely, and if any valve is protruding into the cylinder beyond the safe clearance limit, then something will break when the piston next reaches TDC.

My point was just that the engine is either an interference engine, or it isn't; it won't come into or out of being an interference engine as the variator moves. You can't make an interference engine into a non-interference one by changing the valve timing. On most 4cyl engines, there is usually at least one valve fully open for any given position of the camshaft and if the belt breaks, that's going to make contact with a piston on an interference engine.

On engines with variable valve lift (the TA is such an engine), then they may indeed only be interference in those power regimes where the valves are fully opened.

Whatever, it seems foolhardy to play russian roulette with a cambelt, and I certainly wouldn't want to rely on something I read somewhere about someone who had one break some time in the past and seemingly got away with it. Even the best case means waiting some indeterminate time at the side of the road for a breakdown service, and that's not somewhere I would want to be.