First interference engine?
Discussion
Pretty sure an Alfa Nord Twin Cam engine would 'interfere' if the timing chain broke. Well it did when I had the cam timing out of phase, so that would certainly occur without a timing chain.
Surely it can't be a particularly unusual situation for any engine apart from the dead basic things.
Surely it can't be a particularly unusual situation for any engine apart from the dead basic things.
I have always understood that the Fiat 125 had the first twin-cam production engine with a cambelt, and I'm pretty sure it would have been an interference design.
They came out in late 1967 and even had 4-wheel disc brakes which was quite advanced for the time.
However they had other less impressive features - my Dad bought a 1970 one in 1972 and spent some time soon afterwards removing rust from the front inner wings!
Still I got to buy it in 1977 and it was a revelation after my dreadful MkII Cortina 1500.
They came out in late 1967 and even had 4-wheel disc brakes which was quite advanced for the time.
However they had other less impressive features - my Dad bought a 1970 one in 1972 and spent some time soon afterwards removing rust from the front inner wings!
Still I got to buy it in 1977 and it was a revelation after my dreadful MkII Cortina 1500.
imagineifyeswill said:
If you mean an engine where you get piston to valve damage in the case off a broken timing belt, then in UK I would think it was probably the Ford 1.6 Pinto engine, 2.0 litre was a safe engine 1.6 interference.
I broke a couple of cam belts on 1.6 pintos. The second one I changed by the roadside. 
PositronicRay said:
imagineifyeswill said:
If you mean an engine where you get piston to valve damage in the case off a broken timing belt, then in UK I would think it was probably the Ford 1.6 Pinto engine, 2.0 litre was a safe engine 1.6 interference.
I broke a couple of cam belts on 1.6 pintos. The second one I changed by the roadside. In times past, many (most?) "ordinary" car engines had vertical, parallel intake and exhaust valves, flat-topped or recessed pistons, pushrods, lowish compression ratios, and timing chains. That meant the chance of tangled valves was nil, that it was unlikely that the chain would break, and that if it did there was no piston/valve interference.
Interference became more likely with timing belts and inclined valves. I can remember two occurrences from the early/mid '70s. A mate who broke a Jensen Healey timing belt and a work colleague who ignored the belt replacement intervals (that was a FIAT twin-cam if I remember correctly).
Having said all that, there must have been interference engines ever since there were inclined valves, hemi heads, and domed pistons.
Interference became more likely with timing belts and inclined valves. I can remember two occurrences from the early/mid '70s. A mate who broke a Jensen Healey timing belt and a work colleague who ignored the belt replacement intervals (that was a FIAT twin-cam if I remember correctly).
Having said all that, there must have been interference engines ever since there were inclined valves, hemi heads, and domed pistons.
a8hex said:
PositronicRay said:
imagineifyeswill said:
If you mean an engine where you get piston to valve damage in the case off a broken timing belt, then in UK I would think it was probably the Ford 1.6 Pinto engine, 2.0 litre was a safe engine 1.6 interference.
I broke a couple of cam belts on 1.6 pintos. The second one I changed by the roadside. 
the 1.6 pinto is interferance but only just in that sometimes it misses (as can the CVH but not as often as the 1.6 pinto) where as the 2.0 cannot physicly contact it's valves and pistons.
The 1.8 TD used in the escort/orion is a wierd one, it is interferance but the valves usually survive and snap the cam shaft instead.
The Don of Croy said:
Could someone give a short explanation of why this design is now so popular? Is it packaging?
You want the volume of the cylinder to be minimal with the piston at TDC. This allows you to run a relatively higher compression ratio without having to run a long stroke. The disadvantages of a comparatively long stroke compared to bore are longevity, smoothness and deck height which has a big impact on packaging. With a higher compression ratio comes a more complete and therefore efficient burn, so more power and efficiency and lower environmental impact. High compression ratios used to be the preserve of finely fettled race engines back in the days of mechanical distributors but with the advent of adaptive engine management the manufacturers were ready to exploit the increasingly widespread availability of high octane pump fuels.
Basically you could never make a 'safe' engine with the same efficiency as an interference engine, not with poppet valves anyway.
gareth_r said:
Are variable valve timing engines more likely to be interference?
To be honest by the time VVT arrived on a large scale, the majority of engines were interference anyway. That said, since VVT engines effectively have a longer cam duration they are even more likely to bend valves than non-VVT engines, some models of which will very occasionally snap a belt without bending any valves through shear fluke. If this happens one should probably buy a lottery ticket though. BritishRacinGrin said:
The disadvantages of a comparatively long stroke compared to bore are...
I thought it was because piston speed/ acceleration was limited, a long stroke means limited revs. An engine performance is related to the air pump, so the volume over time, or cc/minute. So make the volume bigger or the revs higher. In F1 they went to very short strokes. The main advantage was back in the day the RAC hp and hence annual road fund tax was lower. The torque was also better at low revs.
In my 1920's ohv Alvis both valves are timed to be open at DTC, but it is low compression and has a 69mm bore and 110mm stroke. The sports model had a 103mm stroke to get it in to the 1500cc class.
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