Interference/Noninterference Engines

[UncleBob]

Originally Posted by beef_bourito

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.

[beef_bourito]

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.

[UncleBob]

Originally Posted by beef_bourito

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....

[beef_bourito]

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.

[UncleBob]

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?

[TheSilentChamber]

... 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.

[curtis73]

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?

[UncleBob]

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.

[curtis73]

Originally Posted by UncleBob

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...

[curtis73]

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

[beef_bourito]

Originally Posted by UncleBob

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?

Originally Posted by UncleBob

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.

[curtis73]

OK, a picture to help demonstrate:



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

1. both engines have the same dynamic compression
2. both engines have the same bore and stroke
3. both engines have the same cam timing events and parameters.
4. 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.

[TheSilentChamber]

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

Nice, you should put that in the FAQ thingy.

[curtis73]

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

[UncleBob]

Originally Posted by curtis73

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.