Interference engines

A but of a build up, but I *do* have a question...
The timing belt on my '87 16V Scirocco stripped last Tuesday.
This is the second time I've had one strip out on a 16V Scirocco. And the
third time I've had a timing belt strip out. The first time was many years ago on a Fiat X-1/9. The result was that the valves became works of modern art (bent in interesting ways) with all the attendant repair work required. I was told back then it was because the engine was an "interference" type. The top of the piston fits so closely to the head that if the a valve is open when the piston comes up they will "interfere" with each other and something will get wonked.
Fine. I was also told that the 1.8L 16V's in the Scirocco's were interference engines and that there were only two possible positions the engine could be in where they would not mash things up.
When I lost the belt on the first Scirocco some years back I happened to be at idle, at a stop light. Everyone said that that was what saved me because it (somehow) increased the probability that the engine might be in one of those two position when the belt went. The shop put in a new belt, timed the engine, and it ran fine for another five or six years. In fact, it was running fine until I took it off the road a few months back.
When I lost the belt Tuesday I was coming down an exit ramp, clutch on the floor, engine at idle. The shop put in a new belt, timed the engine, and it's running fine (so far, anyway).
My question is, was I misinformed that the 1.8L 16Vs are "interference" engines, or have I just been really really lucky twice?
- Bill
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my 2 cents, at low RPM the oil pressure is lower and the lifters aren't pumped up as at higher RPM.

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I hadn't considred oil pressure at all. In fact, I guess I hadn't even considered that they were hydraulic lifters.
- Bill
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I call really lucky twice.
Don't forget at higher RPMs you are usually accelerating in gear. This gives the rotating mass in the engine more momentum which allows it to continue rotating after it dies. Also the force of being in gear will force the pistons around for another HIT.
I have seen 1.8T motors survive broken timing belts while at idle & those are most definatly interferance engines.
Craig

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Twice lucky. ;-) Change that belt every 40K miles and then it won't strip or break on you. I have seen them break after a meer 44K miles. Owner's son was driving it like a "Bat out of H#LL"............well you know. ;-) Caused damage to the valves, but the pistons survived. Also worked on a VR6 where the chains had jumped. That wasn't pretty either!
I have seen 2 belts snap while the interference engines were at idle. Owners were lucky like you.
Don't push your luck and change that belt more often and before it breaks!
JMHO
--
later,
(One out of many daves)
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Even more so. About 30 seconds earlier I'd been up around 4,000 RPM on the highway I was exiting from when this one went.

Thanks. I was trying to remember if it was 40K or 60K. I have the service records for the car and it claimed it had been done about 30K back. But I've also just discovered that the odometer slips during warmer weather, so that "30K" could be 50K for all I know.

No. I'll have to fix the odometer so I don't miss doing it.
Thanks.
- Bill
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Some belts have age as well as miles listed in their service interval.
wrote:

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Thanks all for your comments and observations.
I'll look into the age thing.
And that caused me to wonder something else. The car has fairly low mileage. Depending on how much faith I put in the odometer, between 120 and 140K. But it spent several years sitting in a garage. I'm wondering if sitting there, not moving, for all those years might not have also aged the belt and contributed to this failure.
- Bill
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Absolutely. Age is just as big a factor for belts as mileage.

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I was running around 4,000 RPM about 30 seconds before this too.

I'd figured on momentum, but didn't think the cam shafts would really have enough to keep it spinning. On the other hand, as you mention, at high RPM there's a lot of rotational energy stored in there.
Someone mentioned that, if it's going slow enough, there's a "snap back" from the valve springs which would tend to take the valves to one of the safe positions.

Related question: Are the 8V (as in 1990 Corrado, 1980's Sciroccos and Rabbits) also interference types?
- Bill
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snip

AFAIK If they are 8v, they are not usually an interference engine.........unless they are diesel engines which are interference.
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usually, unless the owner is at or near the upper RPM range, then things go bad real fast!
"> AFAIK If they are 8v, they are not usually an interference

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The older 1.8s are non-interferance. You can break belts all day long at redline & they wouldn't care. The newer ones AEG & newer technically aren't but they are extremely close & I have seen them hit due to carbon buildup on the pistons.
Craig

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