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Anyone know when the 3.5 moved from a non interference engine to an interferance engine? I thought my 97 was a non-interference. looking at a 2000 300M today and was curious...
 

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That's when your wife tells you the grass needs cutting but you're watching a game.

Just kidding. An interference motor is one in which the valves and pistons can make contact if the valve timing gets too far out of whack. That's what happens when a timing belt breaks; the cam(s) are no longer in synch with the crankshaft, so a piston comes up while a valve is fully open. When they meet, the result may be a bent a valve or broken piston or both.
 

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98+ 3.2/3.5 are interference. 93-97 3.5's are not, and will survive a timing belt failure.
 

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98+ 3.2/3.5 are interference. 93-97 3.5's are not, and will survive a timing belt failure.
I've always wondered, why make an interference motor (or as my Dad calls them, self destructing motors) If it's possible to make it so they don't interfere, then why would you change that? My random guesses would be you could make the engine physically smaller, and thus lighter and cheaper, or you could control the combustion better. One of these days I'm going to beat up a powertrain engineer and ask them. Everytime some one mentions interference and motors in the same sentence I just want to go throttle an engineer at a car company and ask them: why?!?
 

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I've always wondered, why make an interference motor (or as my Dad calls them, self destructing motors) If it's possible to make it so they don't interfere, then why would you change that? My random guesses would be you could make the engine physically smaller, and thus lighter and cheaper, or you could control the combustion better. One of these days I'm going to beat up a powertrain engineer and ask them. Everytime some one mentions interference and motors in the same sentence I just want to go throttle an engineer at a car company and ask them: why?!?

You are pretty much on track with your guessing. Engines are made interference to tighten compression ratios and improve performance for the most part; allowing a more precise burn. They are smaller and lighter also, but thats not the main reason. Many diesels are interference also, they run compression ratios upwards of 20:1.

Interference engines are fine as long as you change the timing belt before it breaks. On engines with a chain, such as the 2.7, there are hardly any problems at all with pistons contacting valves, unless the timing chain breaks or the guides wear out -- which is usually not common -- and will exhibit obvious symptoms before it blows.
 

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Timing belts are out. They won't come back until they are good for 1 millions miles, so when carbon nanotube based timing belts are readily available...
plus, you have to separate the oil in the camshaft phasers & the camshaft drive. Audi did it in the 3.0 V6 [continuous intake, discrete exhaust]
http://www.audiworld.com/tech/eng110.shtml

The little valve relief cutouts, which allow for maximum valve lift while the piston is at TDC, are just breeding grounds for unburned fuel. Granted they are still around on some engines, but massively reduced in spite of ever increasing valve lift, and wider camshaft phaser angles, partly due to lower valve angles and greater expected life of timing chains.
 
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