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?

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 :)

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.

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?

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.

Gotcha.

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.

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. :nutkick: :nutkick: 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?

thats because the engines are designed by engineers, not mechanics :icon16:

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... )

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.

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!.

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.

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.

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.

read the whole thread, it was explained that there is no deffinite law saying that interference engines make more power than non-interference engines.

"Definite law"? No....but the simple variables necessary to produce high HP/cc is going to lead an engineer towards making the engine interference.

You look at the absolutely highest HP/cc engines being mass produced right now, sport bike motorcycle motors, there isn't a single one made in the last 20 years that was non-interference motor. There's a very obvious reason why.

not always, it depends on what the engineer is designing. if he's looking for low end torque, he's not going to design an engine with tonnes of airflow and no air velocity. and motorbikes rev up to 12000rpm, they need shitloads of airflow, that's why they need so much lift and compression, that's why they're interference. and saying that a non-interference engine can't get out of it's own way is completely wrong, they can and they do, they just might not rev as high or have the power of an interference engine of the same size.

not always, it depends on what the engineer is designing. if he's looking for low end torque, he's not going to design an engine with tonnes of airflow and no air velocity. and motorbikes rev up to 12000rpm, they need shitloads of airflow, that's why they need so much lift and compression, that's why they're interference. and saying that a non-interference engine can't get out of it's own way is completely wrong, they can and they do, they just might not rev as high or have the power of an interference engine of the same size.

find me a "high HP/cc" engine that is aimed for low end torque.

Like I said....

if you read my post you quoted you'll see that i said interference engines don't always make mroe power than a non-inteference engine. if you can only run a certain ammount of compression because of fuel preignition limitations, and you increase flow to the maximum, you could still find youself with a non-interference engine. just because the valves can hit the pistons doesn't make them more powerful, it just means they probably allow more flow, which could be a bad thing.

another thing, you can have your pistons shaped so they don't hit your pistons, that doesn't decrease power very much and makes them non-interference.

my point is that just becase and engine is concidered an interference engine doesn't make it more powerful than a non-interference engine. it's true that most high horsepower/cc engines are interference engines but they do that because they need the extra air flow. if you can't benefit from the extra flow there's no point in adding it, it might even remove power and torque.

I read your post and understand it very well.

That doesn't change that your points completely miss my points, and seem to continue to miss my points.

I never said that an interference motor MUST be high hp/cc, only that the variables involved in designing motors, makes it more likely.

Imagine it like this, since you didn't like my generic answer: imagine a sliding scale of engine design. At the very tippy top of the scale, is the highest HP/cc engine in existance, which would be over 200HP/liter (we'll keep this limited, only NA (no power adders) engines, only on pump gas). These will absolutely most definitely be interference motors.

Then as you go down the scale, around 100HP/liter, you will start getting to motors that are FEASIBLE to be non-interefence motors, and then as you go down from there, you will find more and more motors that are non-intereference motors.

Do you agree with that?

... Its like two monkies fucking a football.... both have good intentions, and are horny as hell.... but in the end your fighting over a football in which to fuck.

Ladies... you're both pretty.

The point here that I think we're both trying to make is that designing a non-interference engine can be done with the same level of power, but the design characteristics make it intensely difficult to achieve that without killing octane tolerance, quench, flame front speed, rod length, etc. Anything can be done, its just that in general the bean counters have decided that $900 in service work every 60k miles is a good thing for the service department, so why worry about going to the extreme expense of designing one?

I disagree.

Name me any motor, modified or otherwise, that is internal combustion 4 stroke (don't try to use the wankle motor...you know what I mean), runs on pump gas (93 octane at most, E85 doesn't count) that makes over 100HP/liter and is non-interference with no power adders.

This isn't about bean counters, this is the simple relationship to port flow (which requires large valves, large lift, 4 valve realistically) and high compression ratio. You can't possibly have both and keep it non-intereference. Ain't gonna happen on pump gas with no power adders.

Actually, even on race gas, it won't likely happen, to take advantage of race gas, you need high CR which unless you neuter the valve lift, you'll still be in the same boat.

Name me any motor, modified or otherwise, that is internal combustion 4 stroke (don't try to use the wankle motor...you know what I mean), runs on pump gas (93 octane at most, E85 doesn't count) that makes over 100HP/liter and is non-interference with no power adders.

Well, first of all, 100hp/liter is pretty lofty for a non interference motor. That wasn't what I was saying. I'm going to make a few drawings and get back to you on this one. Basically what I'm saying is that it IS partly a bean-counter thing. They CAN be designed, but there is just no motivation to do so. Pictures forthcoming...

ah crap... server space is full.... pictures after I clean up my website :)

I read your post and understand it very well.

That doesn't change that your points completely miss my points, and seem to continue to miss my points.

I never said that an interference motor MUST be high hp/cc, only that the variables involved in designing motors, makes it more likely.

Imagine it like this, since you didn't like my generic answer: imagine a sliding scale of engine design. At the very tippy top of the scale, is the highest HP/cc engine in existance, which would be over 200HP/liter (we'll keep this limited, only NA (no power adders) engines, only on pump gas). These will absolutely most definitely be interference motors.

Then as you go down the scale, around 100HP/liter, you will start getting to motors that are FEASIBLE to be non-interefence motors, and then as you go down from there, you will find more and more motors that are non-intereference motors.

Do you agree with that?

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.

actually you did say that interference motors have power and you were saying that a high horsepower engine MUST be interference and i was trying to say that it doesn't have to be. as curtis said, there's just no motivation to do it but it CAN be done.

OK, a picture to help demonstrate:

http://www.curtisandkim.com/interference.jpg

Its important to note that these are only generalizations. Not all interference engines look like the one on the right, nor do all non-interference engines look like the one on the left. But for the sake of discussion we'll be using these two for now. We'll assume several things for this lecture... I mean discussion :)

both engines have the same dynamic compression
both engines have the same bore and stroke
both engines have the same cam timing events and parameters.
in all comparisons, you can assume apples-to-apples generalizations. For instance, one of the correlations I will make is comparing flow and valve angles, but you can assume that I mean it as a generalization.A very common way of creating a non-interference engine is to cant the valves at a greater angle. That way for a given linear lift value, the valve only moves a fraction of that distance relative to the stroke. A .500" lift in the left engine may only take the valve .300" closer to the piston face, whereas on the right engine a half inch of lift means a half inch closer to the piston. While valve gross lift isn't the main factor in overall valve-piston clearance in a normally operating engine, it certainly is a factor when the timing belt breaks.

In general, in order to design a non-interference engine, the following happens:

The valves must be canted on a greater angle which makes a poorer flow path. Greater concessions will generally have to be made to velocity in order to get the same mass flow which affects shape of the torque curve.
The piston must be of some kind of hybrid dish/dome design to maintain compression. This oddly shaped piston is generally a KILLER to flame front speed and homology.
The shape of the dome/piston often necessitates a greater distance from the spark kernel to the piston face. This requires more ignition lead which can hurt power, MPG, NOx emissions, etc.
The shape of the combustion chamber often leaves very little room for effective quench which can also adversely afect MPG, ignition lead requirements
The shape of the piston often requires very tall compression heights in order to fit the rings in a logical space. Tall pistons are heavier and they also require shorter rods which leads to less piston dwell at TDC and increased side loads and wear.The interference engine is ultimately simple, and its incredibly easy to take advantage of all of the benefits of the engineering that come with its simplicity. Flat or dished pistons for a good flame front, quench areas, shallow combustion chambers to keep the plugs closer to the piston, and straighter valve angles to help flow, velocity, and swirl.

Other generalizations are abundant. Typically straighter valve angles also means that larger valves can be used, but that's only a benefit if your engine's design requires larger valves.

You can make a non-interference engine that makes the same kind of power that an interference engine does, however you have to engineer it in such a way that ALL of the problems become addressed in the bulleted list above. In truth they're not all that difficult to overcome, but that's where the bean counters come in. They look at the engine on the left that you spent two years developing to make the same power as the one on the right and they say; "but it will cost 20% more to make and we won't make as much money on preventative maintenance at our dealers' service departments."

Its all a matter of what you're trying to design. If you're targeting drivers who know their stuff (like we do), do the non-interference engine. That way you'll have every grease monkey buying your car. That accounts for about 5% of the new car buying population. Not a smart demographic to play to. On the other hand, you have John Q Commuter who is buying a name like Toyota or Honda, who cares? You're not going to sell any more cars if you spend all the extra on the non-interference engine, and you won't get any residual money when JQ Commuter does $3000 worth of damage for not changing his timing belt when he should have. Shame on you, John. Regular maintenance is right in your owner's manual. You should have read it before you bought. No one blames the car, its a necessary evil of ownership just like changing the oil. Even if they ask, "when should I change the timing belt?" when they buy it new, they're not thinking about that $600 repair bill 5 years from now. They want the car.

As a former bean-counter for several car companies, trust me. Its a bean counter thing more than you know. Everything is a bean counter thing in the car industry. Designing an engine for its long-term reliability is not a foreign aspect by any means. BMW, VW, MB, and several other companies often design their engines with 300k mile or more reliability, but many of them still use an interference design with a belt. Its just not a concern when you're selling new cars.

One of the terms we use in the business is Temperate Elasticity. Basically its a parameter that shows the number of your new cars people are buying vs. how many used cars they're buying. Basically, if you have a die-hard Chevy fan, they are more likely to buy new than a 100k-mile used chevy. If you are a die hard BMW driver, a 100k Bimmer is a great way to get a much cheaper car that still has 150k of its life left. Its a double edged sword. Although its nice to know that the people view your car as a bulletproof tank, used car sales don't make money for the car company. So, tuning your company's Temperate Elasticity is always a battle between making cars that are reliable enough for your target demographic, but not so reliable that you lose too many sales to used autos of your own brand.

Basically what that means is if Chevy suddenly came out with a vehicle that lasted for 500k and cost $60k, no one would buy it. Inversely, if Volvo came out with a Corvette-killer that had reliability issues and died at 100k, it would be corporate suicide. You build cars that fit your buyers, and almost never do you build high performance non-interference engines. The target buyers who would appreciate it only account for about 5% of drivers. Of that number, only a small percentage are new-car buyers, and of them a percentage would do their own repairs anyway. Not worth spending extra money that won't make you any money.

SO, long story longer... on the topic of non-interference and Temperate Elasticity: Designing a non-interference engine wouldn't add any appreciable sales on the new side, but it would add appeal to the used side of the equation. It would skew your TE numbers toward the bad side. Instead they spend money on things like designing a timeless style or body shape that will help numbers on BOTH sides, new and used.

In all of that you didnt explain why non interfiernce motors use rings that are crooked? :evillol:

Nice, you should put that in the FAQ thingy.

Uh.... crooked rings help the uh, offset piston conversion from dis-centered head tolerance.... I'm retarded.

Well, first of all, 100hp/liter is pretty lofty for a non interference motor. That wasn't what I was saying.

I'm still looking for someone to tell me that I'm wrong. All I've gotten so far is "you're mostly right" (add generic "but" here)

First off, if you're talking about a 2 valve per cylinder design, you're already talking about low HP/cc without severe modifying that would require you to use race gas most likely.

If you're talking about 4 valve per cylinder, which is needed to increase head flow, you have more valve area vs bore area. The more valve area, the less lift you can get away with and still achieve non-interference status. You need the valve lift to acheive good head flow also, which will greatly cut into your compression ratio.

Seems rather simple to me. Greater performance REQUIRES reduced combustion chamber tolerances.

It doesn't have to be a difficult answer. Yes or no will work. The answer is rather obvious if you ask me, but it seems many here want to make it a difficult one.

actually you did say that interference motors have power and you were saying that a high horsepower engine MUST be interference and i was trying to say that it doesn't have to be. as curtis said, there's just no motivation to do it but it CAN be done.
you are correct, that first post I made, my tongue was thoroughly planted in my cheek. I shouldn't have expected anyone else to figure it out though. I know those things are very hard to translate over the net :screwy:

But...."doesn't have to be" I definitely disagree with. There's a fine line between that, and NOT FEASIBLE. Which I've stated. Still waiting for someone to prove me wrong. No one has yet. I assume its because there's too many choices out there to chose from...? I'm patient. Take your time.

Someone really smart once told me "if nobody else see's things your way... maby you should try seeing it theirs" Just some food for thought.

Someone really smart once told me "if nobody else see's things your way... maby you should try seeing it theirs" Just some food for thought.
Since you've yet to offer anything useful to this conversation other than biting comments, maybe you should consider your own advice?

I don't think anyone really disagree's with me, I'll be charitable and say that, most people don't consider my level of performance that I'm discussing worth discussion on stock engine applications.

But IMO, that would be true 10 years ago, but performance is always increasing, and motors are getting smaller due to fuel consumption and emissions concerns, so you can be sure that future cars will continue the same trend. And to go back to my point they will be intereference motors BECAUSE of that (higher performance). What is the exact, specific line in the sand that non-interefence motors isn't feasible anymore isn't really an issue, but my original point is still very valid: high performance (as I define it) requires reducing combustion chamber clearances, which makes non-interference motors pretty much impossible to achieve.

Do you have something to offer to dispute that?

My mommy once told me...junk food is bad for you

I offered my part of the conversation on page 1. Retorical argueing is pointless, both motors can make power, but you see it more so in interfernce motors. End of story. There are alot of factors involved in designing a motor, some less obvious than others. End of story. Timing belts sometimes break for no apparent reason. End of story.

Your new here, and endless arguing and calling people out on stuff over and over even though both sides have allready been covered and there is no definative Yes or No answer to the question- is not a good way to start off.

I'm still looking for someone to tell me that I'm wrong. All I've gotten so far is "you're mostly right"

I'm sorry to say, but AF isn't a place where we dwell on right and wrong, black and white, or left and right. This is a discussion about benefits and drawbacks and there is no room for right and wrong. In case you didn't notice, my post (which was based in experience) was not biased toward any one person's opinion in this thread.

You need the valve lift to acheive good head flow

No, you need good head flow to achieve good head flow. If you're relying on valve lift to get head flow, then you've abandoned low-lift flow characteristics in favor of mass flow at peak VE. Great for a race engine, but terrible for the street.

Seems rather simple to me. Greater performance REQUIRES reduced combustion chamber tolerances.

Huh?? chamber tolerances? I'm not sure where you're going with this.

It doesn't have to be a difficult answer. Yes or no will work. The answer is rather obvious if you ask me, but it seems many here want to make it a difficult one.

Yes or no will NOT work. That's the whole point. Each one has trade-offs and benefits. If you entered this thread just to fluff your ego then you have joined the wrong forum. If everything in the automotive world were a yes or no answer, we'd all be driving the same car in the same color with the same options.

Its obvious that you are just here to press the benefits of multi-valve, small-displacement engines. In the Engineering/Technical forum (which, in case you have failed to realize... I am the moderator) we objectively discuss automotive engineering and technical things without prejudice, bias, or childish tantrums. There is no right or wrong when it comes to such a broad topic as how an engine designer configures an engine. Especially when YOU aren't the one designing them and singing the checks to put them in production.

This has just become argumentative..... Closed.