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What does "Free Wheeling" mean, compared to "Interference"?

Below is some data on Mazda engines for purposes of illustration. This nomenclature confuses me. I always thought an engine was either “interference” or “non-interference”. " Non-interference" I thought meant the valves are separated enough from the pistons that the valves can never collide with the pistons even if the valve timing goes askew. “Interference” I thought meant the valves could possibly collide with the pistons if bad valve timing (caused by a broken timing belt say) causes a valve being pushed down by the camshaft at the same time when the piston was coming up. Bang. $5000 repair bill. No good.

But the data below separates these Mazda engines into “Interference” and “Free Wheeling”.

What does “Free Wheeling” mean? How is it different from “Non-interference”?

Mazda: Interference/non-interference

1.5L Free-Wheeling
1.6L Free-Wheeling
1.8L 4 Cyl. Free-Wheeling
1.8L V6 Free-Wheeling
2.0L Diesel Interference
2.0L DOHC (626, MX-6 & Protege) Interference
2.0L DOHC (Tribute) Free-Wheeling
2.0L SOHC Free-Wheeling
2.2L Interference
2.3L 4 Cyl. (B2300) Free-Wheeling
2.3L V6 Free-Wheeling
2.5L 4 Cyl. (B2500) Free-Wheeling
2.5L V6 Free-Wheeling
3.0L DOHC Interference
3.0L SOHC Interference

In this instance, ‘free-wheeling’ means ‘non-interference’. However, I was taught that ‘free-wheeling’ refers to Diesel engines, in that they have no throttle plate on the intake system, and therefore not much resistance to keep the engine from spinning when the gas pedal is released, unlike the engine braking on a gasoline engine. Engine braking on a Diesel is achieved through a ‘Jake Brake’ system, which puts a valve on the exhaust side of the engine to get compression braking, but is actuated by a solenoid and controlled with a dial on the dash.

“Free-Wheeling” means that if the cam belt or timing chain snaps or jumps, that even if the cam stops turning with some valves open and extended down into the cylinder, that the engine is designed so that there is enough clearance that the pistons will not hit them.

“Interference” means that the engine is designed such that the valves must be closed and out of the way when the piston reaches the top of its stroke or they will make contact, usually causing severe damage to one or both. The engine’s valves and pistons are normally synchronized such that they will never touch. But if the cam belt snaps or jumps out of time, the synchronization is lost and the pistons can hit valves that are open at the wrong time or no longer being actuated at all since the camshaft, which operates the valves, is no longer turning.

Hopefully this helps.

Free-wheeling is more of a mechanic slang term and the source of that info should probably not have used it to replace the non-interference term.
If they’re going to use the free-wheeler phrase then it should have been paired up with non-free-wheeler to prevent any confusion.

Free wheeling means that if the timing between the pistons and valves is lost due to a broken timing belt/chain, the engine is free to spin or wheel without interference.

Interference means the opposite.


Slightly off topic, but a long time ago SAAB used to have a free-wheeling mechanism in their manual transmissions.
There was a lever underneath the dash and the mechanism could be turned on or off as desired.

This would allow a car to coast without engine braking similar to coasting in neutral. The mechanisms were also a bit prone to whining and when using engine braking would often sound like a loud reverse gear whine when backing off the throttle.

Continuing off topic about free wheeling, the first Saabs had it as ok4450 describes. The reason for providing it was because they had two cycle engines that did not have oil injection but instead, the engine lubrication oil was mixed with the gasoline. On a long downhill with the throttle closed, little fuel-oil mix would go through the engine but it was mechanically safe to idle the engine as free wheeling would provide. 1933 Chevrolets and possibly other brands in the early 1930s had selectable free wheeling to help with gas mileage.

For the purpose of describing engine valve/piston clearance, free wheeling seems to be a better choice of words than non-interference to eliminate a possibility of misunderstanding.

No off topic for you. That 's the first thing I thought of having owned a SAAB two stroke many, many years ago. The reason you needed it was, if the motor was allowed to turn through the drive wheels with little,or no throttle, it was getting no engine lube through the gas oil mix which shortened the life of the motor if you did it too much… Later, with oil injection in motor bikes and other two stroke vehicles, it was unnecessary. A side advantage was that with out engine braking, it had better cornering traction on ice when you lift the throttle. Other than that, it was the worse motor in a car I have ever owned… Maybe great outboard, terrible car motor.

I didn’t see to many of those old SAABs but they did come into the dealer on occasion. My memory is very hazy on this but I’m pretty sure the SAAB Sonnetts with the Ford 4 stroke V-4 engines also had the free wheel mechanism.

Thanks for the explanation everyone. Seems like “free wheeling” and “non interference” are interchangeable for the most part, esp when referring to gas engines in modern cars.