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V6 Non-interference


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#1
snailman153624

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From the horse's mouth. End of debate. Period.


Is the engine designed as an interference or non-interference engine?

The engine is designed with a non-interference engine.


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#2
MazdaMan88

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Which means that if our timing belt breaks, there is no slaughtering of valves and what not. WHOO!

#3
Kutcht1

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All engines should be designed to be non-interference. There should be no reason to have an interference engine.
They are also fairly safe for a small sedan. Four stars for both front seats. The 626 rated the same as my Chevy Tahoe and it's over twice the size. B)

TomK

#4
vlad

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I don't understand why this has always been debatable. :unsure:

#5
snailman153624

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I don't understand why this has always been debatable. :unsure:


This topic just pops up ever few months, and it always seems there are some hardliners that refuse to accept the engine is of the non-interference type. I happened to come across this official Mazda FAQ, and will pin this up and point to it with my tongue hanging out next time this debate inevitably surfaces :P

Well, people who don't know any better (i.e. haven't actually messed with the engine internals or timing) will quote the Hayne's Manual (which states that the engine is an interference design....but the reader doesn't realize that Hayne's prints this same exact block of text in every repair manual, regardless of whether the engine is of the interference type or not).

Additionally, some auto parts' stores claim that the motor is of interference design so that they can scare you into buying an expensive timing belt kit every 60k miles. Sometimes even their computer database will indicate that this is the case.



I (and many others such as yourself) know that this simply isn't true. The pistons clearly have recessed regions to accommodate the valve should it happen to stay open when the piston approaches TDC. From an "optimal design" standpoint (as opposed to a reliability and maintenance standpoint), this is less than ideal, because these cutouts produce potential hotspots that can trigger predetonation.

In other words, the cutouts are there strictly to ensure the engine is of the non-interference type.


So, for those people that are book sticklers on this topic, they can rest in peace at night knowing Mazda officially states it is of the non-interference type :biggrin:

#6
vlad

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Thanks Marcin, I now understand why some many people have debated this :smile:

#7
mrslcom

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They are also fairly safe for a small sedan. Four stars for both front seats. The 626 rated the same as my Chevy Tahoe and it's over twice the size.

Yes, "four stars" is a good safety rating. But it is only with reference to a collision with a similar size and weight car. Physics will dictate that a bigger, heavier car will always flare better than a smaller, lighter car in an accident, even if both cars have the same safety designs and safety rating. This does not mean that the safety rating is unimportant because it is desirable to have the safest car in your size group. Safety rating is comparable within the same size group. They are less revelant when comparing between different size groups.

#8
Trebuchet03

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^^ I thought those ratings were based on the sled test - impact (I think 35mph) with a 'rigid' structure. So every vehicle has the same test in order to have similar comparison :blink: Kinda like the EPA fuel economy tests - you may never actually get that economy (cough prius cough), but its a decent way to compare one car to another :huh:

#9
mrslcom

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I thought those ratings were based on the sled test - impact (I think 35mph) with a 'rigid' structure. So every vehicle has the same test in order to have similar comparison

For such a test, an impact with a rigid barrier is equivalent to a head on collison with an identical car that is the same size and weight. Therefore, direct rating comparison between different size class is not quite accurate.

#10
snailman153624

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I thought those ratings were based on the sled test - impact (I think 35mph) with a 'rigid' structure. So every vehicle has the same test in order to have similar comparison

For such a test, an impact with a rigid barrier is equivalent to a head on collison with an identical car that is the same size and weight. Therefore, direct rating comparison between different size class is not quite accurate.

Not quite true, this is a bit of a complicated question. A car of the same weight/size class hitting head-on into a similar car will produce very different results than hitting a rigid barrier. Both cars absorb the energy, and it depends on the total energy of the collision, not just the individual speeds or weights/masses of the vehicles.

For example, getting hit by a truck at 35 mph head-on while you're going 35 mph head-on in a 4-door sedan could potentially produce a less forceful impact than hitting a rigid wall (in fact it probably would be less forceful). This is because both vehicles will be absorbing the energy of the impact, whereas a rigid wall does not (in essence) take any of the energy.

#11
Norm Peterson

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All engines should be designed to be non-interference. There should be no reason to have an interference engine.

TomK

I don't necessarily agree, even for a regular production state of tune. Depending on just what considerations drive the bore and stroke dimensions and the design of the head and valvetrain it may not be realistic in terms of useful compression ratio. Excessive valve clearance pockets represent regions where combustion may not always be complete, every power stroke (think: emissions). In many engine designs, a relatively close piston to valve clearance spec (aka "squish") is necessary for improving detonation resistance in addition to the benefits to specific power and economy.

That said, I agree that a conservative approach is wise with any belt-driven valvetrain arrangement. But I don't believe that such is necessary for more durable/reliable approaches (the chain-drives for the cams in Nissan's VQ series engines and those in most domestic OHV engines come to mind immediately).

From unfortunate experience, I can state in no uncertain terms that there is no piston/valve interference in these engines when a timing belt breaks or walks off the pulleys and through the covers with the engine running at 3500 rpm.

But it's at least conceivable that contact could occur if the belt broke at the precise instant that the valve(s) were in a state of "float", being tossed past the lift corresponding to the noses of the cams at some extreme rpm.


Norm

#12
dsstang25

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When a 3000lb car hits a 5000lb truck, the car instantly moves backwards while the truck continues to move forwards. That transfers a lot of force to the person in the car.

The crash barrier has padding on it so its not like hitting a truck. In a real wreck the vehicles tend to rip apart worse because things arnt spread out. So the frame on a truck will slice right through your car. And they leave those frames exposed. No framing covering it. The bumper sits up higher...

#13
Trebuchet03

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When a 3000lb car hits a 5000lb truck, the car instantly moves backwards while the truck continues to move forwards. That transfers a lot of force to the person in the car.

The crash barrier has padding on it so its not like hitting a truck. In a real wreck the vehicles tend to rip apart worse because things arnt spread out. So the frame on a truck will slice right through your car. And they leave those frames exposed. No framing covering it. The bumper sits up higher...


1. This thread is really old (but I've done quite a bit of research since then :P )
2. 3Klb v. 5Klb | non rigid impact | the 3Klb car does not move backwards "instantly". It happens fast (possibly faster than you can interpret) but not instant. Instant changes in direction indicate rigid collision. Cars are not rigid.
3. In crash testing, more than just a 'wall' is used. In fact, more recent sled designs have a more 'car-like' frontal shape. As far as padding is concerned: You could call it padding, but it is in no way like foam mattress padding ;) It actually is an Al honeycomb with a relatively high strength (NHTSA won't say what strength, but they did have a link to http://www.hexcel.com which makes barriers as high as 42 MPa)
4. The fact that the car is ripping apart is indicative of a lower energy (to passenger) impact. This is why a rigid wall impact is likely to be much more forceful than an impact with another car/truck/suv/etc.
5. Snail is going off his engineering experience ;) He's not one (and isn't) to talk from his ass ;)


------
As an additional related piece of information... No, you can't compare frontal impact testing (at least not more/less than 250 pounds vehicle weight). However, you can compare side impact testing:

The side crash rating represents an intersection-type collision with a 3,015 pound barrier moving at 38.5 mph into a standing vehicle... Since all rated vehicles are impacted by the same size barrier, it is possible to compare all vehicles with each other when looking at side crash protection ratings....Head injury, although measured, is not currently included in the star rating. An excessive head injury score (HIC greater than 1,000) is reported separately as a safety concern.

Good to know if you're shopping for a car ;)

-----
Finally... My new car has an interference design. As long as you keep up with maintenance (that is, replace T-belt) -you're fine ;) What Norm refers to is 100% correct. "Empty" space isn't so great. But notice how your engines have relief cuts in the pistons -- that's a compromise: hots spots for better emissions/non-I. That being said, I would feel more comfortable if my motor used a timing chain.

#14
modern

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That would be me, that is to talk from my ass.
The reason being I sold the cars and the best result I got was talking about people whose lives were saved while in these cars of wonderful design.
Head ons and getting out of the car and closing the door and the door sounding like it did before the head on...that usually meant the person was back to buy another one.
No science in that one.
Except the science prior to the production.
AHHHH I just made that up! (not)
You don't see that on every four star rated vehicle either.

And we sold the cars saying if the belt broke the valves would be okay.

#15
RoadRage

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If you were really worried about detonation, when you rebuild the motor have the face (Just the tops of the pistons) ceramic coated to prevent "hot-spots" and thus detonation. Fairly inexpensive procedure, you could also have the pistons cryo treated for stress reduction and durability after you have them ceramic coated, it if dose not fleck off or crack after they are cryo treated you should have one bad mother of a piston.




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