interferance vs non, correlation to “performance”?

[Erin]

Eric’s video about “What to do when timing belt breaks” got me thinking about this.

Me, I am just a home mechanic so I have not repaired 100’s of cars. maybe dozens in my life (especially the junkers I buy)

We know some car models have a “performance” edition. Cavalier had the Z24, other models have a “GT” version… Of course the term “performance” is used loosely here. Usually it is advertised by putting the letter Z and a number or calling it a “GT”

So anyways, how often is it that the non-performance model will have a non-interference engine and the “fast” model has an interference?
Sometimes whether or not an engine is interference can be a little hard to come by, sometimes there are even conflicting reports (when looking at forums. Yes, I know…)

Is there any correlation? Also on that note, does an engine having a timing belt or chain (if not both) correlate to whether or not it is interference?

Topic

[James O’Hara]

Interference has to with valve train and stroke. If you increase the stroke ie make the piston rods longer it will make the top of the piston closer to the valves. When it is interference if the valves and pistons are fully extended at the same time the piston will kiss the valve. Hence why timing is very important in an interference engine. Sometimes its gently and just bends the valve. Other times its like whale trying to french kiss you. and you bend the crap out of the valve, destroy the head, and mar the piston.

“Performance” engines and I use that term very loosely are sometimes just regular engines with the stroke increased or decreased. Depending on forced air and what not. So when the stroke is increased you get more compression. It also can take a non-interference engine and make it an interference engine.

This is why if a timing chain/belt jumps time it can cause a valve to be lower then it normally would be during a compression or combustion stroke. this causes the valve to kiss the head of the piston. Depending on the gear ratios depends on whether its like going head first into a windshield or just getting grazed by a bullet. Neither one is fun. Neither one is good. If you jump multiple teeth aka jump too far in timing head first into a windshield is inevitable.

If you just kiss a piston it will leave a mark. Sometimes if the mark is not too bad it will not be replaced. Though this is frowned upon as it will cause a hot spot in the piston. Basically as the wavefront forms from the combustion it is no longer even and smooth due to the piston no longer being smooth. This increases side loading, increase the chance of unburnt fuel being caught there, carbon buildup in that spot, and/or increased force on a sharp edge that is not designed to take that force. Meaning eventually down the road it will break. When it does it will not be pretty.

If the valve gets pushed over because it kissed to forcefully like hitting a deer head on. It will not just damage the piston and the deer but, will mess up the valve seat/cylinder head. This means the head has to be removed and machined flat so long as the digs are not too deep. Heads can be machined but, they are only given so much thickness to be machined normally its only a couple thousandths of an inch. This all depends on the manufacturer. Also the valve seat may be damaged so badily that there is no possibility of installing a replacement seat. If this is so the head is junk. Some people will weld new material cut it out and install a new seat. This can work but, is likely to cause issues due to the weld not expanding at the same rate as the rest of the cylinder head when the engine warms up. Causing it to crack again and you to be sol. This goes the same for filling in digs and dents. If that dig in the cylinder head exceeds the depth you can machine if you try to install it you will get a hot spot again. It will cause the same issues as a hot spot on the piston head. If you tried to make the cylinder head deeper in that one spot you would not achieve proper air/fuel ratio and cause a improper burn and f up the entire emissions system, cause overheating issues, or cause extra carbon causing a hot spot causing detonation and boom engine is dead again. Even though some injectors have individual fuel trim adjustments (mainly diesels) you can not adjust enough for that. If you had all the math you may be able to install a bigger injector at that point but, not likely.

[Mike]

There’s really no direct correlation or causality between engine output and interference/non-interference internal architecture. You can design a very aggressive non-interference engine or a very mild interference engine.

[Author]

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