Interference/non interference engines?

[J4CKO]

I was wondering why most cars with cambelts have such a critical relationship between the valves and pistons given the carnage that ensues if the belt brakes, I know some engines dont have
the possibility of the valves and pistons colliding so it can obviously be done, is it that is isnt normally a consideration if the belt is changed to the intervals or is it an engine design, packaging or compression thing, i.e. is a non interference engine a bit taller, lower compression or something ?

[aka_kerrly]

J4CKO said:

i.e. is a non interference engine a bit taller, lower compression or something ?

This must play a part in it.

On NA engines if you can get the valves/pistons as close as possible you can run high compression which equals more power.

I guess as well there is a link between the shapes of the combustion chamber and the head design which may influence the design for example when you see pistons notched on just the inlet side.

[Mastodon2]

When you have a non-interference engine, I'm fairly sure the gap means it will not be able to run much compression, meaning low power. My experience of non-interference engines is also dire specific output, my old 1.3L Fiesta with the Endura engine only had something like 55bhp.

[Mave]

Don't forget, valve lift is part of the equation

[eltax91]

One assumes the lower compression ratio makes them marginally less economical too

The 1.8 mx5 I have has 140bhp and gets about 30mpg. Given its engine size and low weight, that's not great.

[anonymous-user]

The engine in the Mk1 and Mk2 MX5 is a non interference engine, and as described here has a low compression ratio and fairly low specific output. It's origins as a turbocharged engine in the 323 is usually blamed. Nice to not have to bother changing the belt until you feel like it though!

[LocoBlade]

Non interference means low compression and/or low valve lift, neither of which are desirable features of a modern engine that needs to produce good power and burn fuel efficiently.

[Krikkit]

eltax91 said:

One assumes the lower compression ratio makes them marginally less economical too

The 1.8 mx5 I have has 140bhp and gets about 30mpg. Given its engine size and low weight, that's not great.

140bhp from an N/A 1.8 isn't bad at all, I presume they've sacrificed efficiency for a bit more power.

[rigga]

Valve angle plays a part too along with the amount of valves, 2 valve heads can run the valves vertically in line with piston travel in the bore, 4 valve heads need the valves to be angled in order for them to fit in the combustion chamber, this limits clearance between valve when open and piston at tdc.