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Old 03-09-2006, 07:45 AM   #1
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Interference/Noninterference Engines

If you break a timing belt and your engine is an "interference" design, the pistons can smash the open valves and wreck the engine internally. But in "noninterference" engines, this damage doesn't happen. So I was wondering, is there some inherent advantage in designing an engine using the interference model that offsets the problems that can be caused by a broken timing belt? Otherwise it seems like a dumb design? Or am I just misunderstanding something about this?
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Old 03-09-2006, 11:58 AM   #2
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Re: Interference/Noninterference Engines

Designing a non-interference engine is a very difficult task to accomplish while maintaining a good compression ratio and not having very odd piston shapes and domes. One way of making an engine non-interference is by limiting cam lift to very low amounts and raising the roof of the combustion chamber so that they never would touch. But then you're left with a 8:1 CR and a cam with wimpy lift. The other way is to design most of your combustion space into the piston as valve reliefs. The odd shapes of the combustion space can limit power, and the tall profile of the piston can have negative affects on the rod length.

Instead the engineering choice is usually made to just design the bloody engine to run right and then design a timing belt that will last longer than the warranty
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Old 03-09-2006, 12:50 PM   #3
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Re: Interference/Noninterference Engines

Quote:
Originally Posted by curtis73
Designing a non-interference engine is a very difficult task to accomplish while maintaining a good compression ratio and not having very odd piston shapes and domes. One way of making an engine non-interference is by limiting cam lift to very low amounts and raising the roof of the combustion chamber so that they never would touch. But then you're left with a 8:1 CR and a cam with wimpy lift. The other way is to design most of your combustion space into the piston as valve reliefs. The odd shapes of the combustion space can limit power, and the tall profile of the piston can have negative affects on the rod length.

Instead the engineering choice is usually made to just design the bloody engine to run right and then design a timing belt that will last longer than the warranty
Or do what Saturn does and run a chain up to the OH cam.
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Old 03-09-2006, 04:00 PM   #4
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Re: Interference/Noninterference Engines

Thanks, Curtis. Hey LeSabre, I thought whether it was a belt or a chain had nothing to do with the "interference" property. What happens if the chain breaks?
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Old 03-09-2006, 08:53 PM   #5
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Re: Interference/Noninterference Engines

I don't think that's the point, its just that chains rarely break. They show signs of wear like sloppy timing long before they break so you know ahead of time. They also sometimes make noise. I guess with most chains, if it breaks and you ruin your engine, you've had to ignore several big signs for several thousand miles.
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Old 03-09-2006, 11:06 PM   #6
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Re: Interference/Noninterference Engines

Gotcha.
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Old 03-10-2006, 01:30 AM   #7
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Re: Interference/Noninterference Engines

already stated but:

inteference motor: something with power

non-interference motor, something that doesn't get out of its own way.

Although there is obvious safety factor with a non-interference motor, the main point is to not break such things in the first place. Most of the time, such things break because of someone being dumb. Don't be dumb, and you won't have a problem.
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Old 03-11-2006, 01:18 AM   #8
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Re: Interference/Noninterference Engines

sorry to semi-hijack this thread but if the timing belt is so essential to an interfearence engine that it will cause damage to the engine if the belt breaks, so why is it that the timing belt is in such a place you have to rip apart half the engine in most cases to get to the thing. In my Chrysler it would cost me $500 to have my timing belt replaced at a repair shop. well as long as I dont beat on my car Ie. heavy acceleration I should hopefully not have to replace the belt any time soon, btw what are the signs of near timing belt failure?
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Old 03-11-2006, 01:20 AM   #9
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Re: Interference/Noninterference Engines

thats because the engines are designed by engineers, not mechanics
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Old 03-11-2006, 02:02 AM   #10
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Re: Interference/Noninterference Engines

There is no set law that one makes power and one doesnt, but interference do tend to generally have more power due to reasons allready stated. But thats not to say that all of them make more power.

Timing belts are one of those things that isnt allways a given, you can run the hell out of a car and the timing belt is offen last as long as a car that was pampered. Belts also usually show no signs (other than visual... but they are not visable on most cars) of wear untill they break. Chains will start to get slack... but are noisy and harder to replace. If your timing belt breaks its more than likely time to replace the valve seals anyway.... (gotta have some possative outlook on everything... )
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Old 03-11-2006, 02:09 AM   #11
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Re: Interference/Noninterference Engines

only time I see broken timing belts is from either something siezing, something puking oil all over the timing belt, or because completely ignored the recommended intervals.

Another side note, just like any rubber-component, TIME is a factor. A 20 year old belt that has 2000 miles on it, will be deteriated. Mileage isn't the only factor, although for most car owners, thats not a problem.

The one that you can't forsee easily is pullies/tensioners/cams siezing. For example ford 1.9 and 2.0 SOHC engines are pretty famous for waterpumps siezing. They are non-interference motors though, so its not a major issue. But all the other situations, are very easy to anyone that pays even passing attention to their car.

But then, most people don't. My job security would be greatly endangered if people weren't so lazy/uneducated about their cars, so I'm not going to complain too loudly, although it will always mystify me.
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Old 03-11-2006, 04:59 AM   #12
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Re: Interference/Noninterference Engines

Quote:
Originally Posted by EnergyForce
In my Chrysler it would cost me $500 to have my timing belt replaced at a repair shop. btw what are the signs of near timing belt failure?
The reason it costs $500 is because of the $50 an hour labor. I'm currently replacing the belt on a VW jetta. The belt itself is exactly $14.53. I'm also replacing the tensioner which is about $17.50. The belt itself requires taking off a couple pulleys (three bolts each) and the timing belt cover (four bolts). I figure it will take me about 5 hours spaced out with a few beers.

But I'm buying the belt straight from the manufacturer, its my own labor, I'm half wasted while I'm doing it, and I'm probably going to get repaid with... uh... let's call it "favors". That $14.53 belt is probably $50 at the repair shop for the same thing, and they probably buy it from the same place I do.

As far as your question is concerned... there are NO signs of timing belt failure. Depending on the engine you might get incredibly lucky and have it slip one tooth. If you're even more lucky the valves don't collide with the pistons you'll just have an engine that doesn't really run well.. if at all. More than likely you'll be driving it for 60k miles without a hint of anything wrong, then an instant $1000 BANG!.
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Old 03-21-2006, 04:29 PM   #13
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Re: Interference/Noninterference Engines

i'm no pro on any of thid but doesn't the mr2 have a non-interference engine?
they get out of their own way i'd say. is this just a exception or is it some other factor that i'm leaving out that makes this car not slow.
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Old 03-21-2006, 11:27 PM   #14
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Re: Interference/Noninterference Engines

The MR2 has moreso the advantage of light weight. The motor by itself was nothing to write home about as far as HP/cc goes.

Unless you want to discuss the turbo'd MR2, which is even moreso the same thing. For a turbo'd engine, it wasn't anything impressive. But put in a light vehicle, it made OK acceleration.
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Old 03-22-2006, 05:10 PM   #15
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Re: Interference/Noninterference Engines

read the whole thread, it was explained that there is no deffinite law saying that interference engines make more power than non-interference engines. it's jus that interference engines are suck that if the valves are all the way down when the iston is all the way up, like if you broke a timing belt, the piston would hit the valve, a non intereference engine is just the opposite, the valves have no way of hitting the piston.
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