960 Why oh why an interference engine!

[Bob 88]

Originally Posted by CliffHeaton

Correct me if I'm wrong but doesn't a 720 have a non interference engine. Also I'm past the point of no return for repair. The box and some other bits are sold. I still don't know why manufacturers make interference engines.

It's a very good question, and I can't think of an answer that doesn't imply some form of deliberate inbuilt life limitation.

[foggyjames]

Originally Posted by Paddy Wagon

it's a deliberate policy by some manufactures, it's what they call "planned obsolecence". There is no justification for not building a "safe" engine. The easiest and least costly way to do that is to use "dished" or "recessed" pistons. It's quite annoying, particularly when Volvo's B200 series can break belts without any danger of internal damage, the same applies to Ford's "Pinto" engine.

Much as I like to bash car manufacturers (), I don't think that's an particularly fair comment. If you want to be able to achieve specific outputs much north of 50bhp/litre whilst still achieving reasonable fuel economy, you pretty much do have to put the valves in harm's way, should the belt fail. Even saying that, I know of a B200K which bent valves after a belt failure. That was extremely unlucky, but it can still happen.

If the valves will be clear of the pistons at all times, it's unlikely you can use a hemispherical chamber, the cam will need to be low lift, and compression & 'squish' will be low. Nobody likes the destructive effects of valve/piston contact, but then very few people want the 110bhp and 25mpg I used to get from my old 240...and the two factors are closely related!

cheers

James