Timing chains and interference - just thinking

[FloydPink]

Hey all, wondering if there’s a reason that Porsche used timing chains and whether or not old 911’s have interference engines.

Are any/all of the air cooled engines interference engines? If they are, do timing chains reduce the risk of valve/piston contact in the event of an over rev? I’m thinking a belt might slip and cause contact, whereas our timing chains would stay “in sync” with the teeth on the gears and prevent the valves from hitting the pistons (unless the valve springs take too long to expand/compress?).

I’m just a novice with a curiosity about all of this air cooled Porsche knowledge. Hoping the board can enlighten me.

[john walker's workshop]

They interfere quite nicely. chains trump (pardon the expression) belts any time.

[FloydPink]

What if an engine is over-revved or a shift is missed?

[fanaudical]

A couple of comments/opinions:

- Remember that the original 911 engine was designed in the very early 60's - timing belts were still somewhat "emerging technology" at the time, I think.

- Neither timing chains nor belts truly protect an engine during an over-rev.


- 911 rocker shafts are supposedly designed to snap easily should things hit; as many can attest to here, that doesn't always prevent expensive damage...

[dannobee]

All 911's will bend valves if the chain breaks (interference engine).

What happens during a missed shift is the valves float and hang open enough so that the piston smacks it. On occasion, the later 911's with cast rockers will break the rocker arms. But that usually just adds to the carnage list, depending on the area of the valve reliefs on the piston.