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post #16 of 31 Old 07-27-2010, 06:29 PM
MilitaryJK4455
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Quote:
Originally Posted by Farmdiesel View Post
With Interference engines the valve will come in contact with the piston if the engine gets off timing, ie timing belt/chain breaking. So if the piston is at the upstroke and valve is open +Crash+ Bent Valves. My understanding in Non-interference engines there is enough room so the valves and pistons will not contact even if the timing goes.

I wasn't aware the 3.8 was a interference...I'm not sure what indicates an engine being one or the other? I assume efficiency and the need for higher revs
Oh ok... Well that makes sense... You taught me something

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post #17 of 31 Old 07-27-2010, 07:10 PM
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I was under the impression the 3.8 was not an interference engine, does anyone know for sure?

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post #18 of 31 Old 07-27-2010, 07:36 PM
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It's chain driven, not a belt so does it even matter? I haven't seen a bad chain on an engine w/ under 175K~ miles in years!

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post #19 of 31 Old 07-27-2010, 08:09 PM
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Originally Posted by Bionicrooster View Post
I was under the impression the 3.8 was not an interference engine, does anyone know for sure?
According to the service manual, "The engine does not have provisions for a free wheeling valve train."

I interpret that as being an interference engine.
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post #20 of 31 Old 07-27-2010, 09:35 PM
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Quote:
According to the service manual, "The engine does not have provisions for a free wheeling valve train."

I interpret that as being an interference engine.
Thanks! BTW I just got back from Maine, and already miss it. Have a camp down east and love it. Great wheeling up there too

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post #21 of 31 Old 07-27-2010, 09:48 PM
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Thanks! BTW I just got back from Maine, and already miss it. Have a camp down east and love it. Great wheeling up there too
Most of my wheeling has been in northern Maine. Usually to get to a place to start hiking.
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post #22 of 31 Old 08-13-2013, 09:25 PM
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great info guys thank you those of us new to Jeeps like it alont
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post #23 of 31 Old 08-14-2013, 07:07 AM
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Free wheeling engines can also have catastrophic failure if belt breaks. This would mainly occur in high rpm situations. Some motors have good chain and guide designs where both will last as long as the motor. Others have design not so good where the guides will wear out and come apart causing chain to have slack and allow motor to destroy itself. Most all chain drive motors have a tensioner that is controlled with oil pressure. As chain stretches tensioner can keep constant pressure on chain to compensate. With this being said you can see how using a good quality oil and performing scheduled oil changes is critical on these motors. All chain drive motors I have ever worked on give a very early warning before a tensioner fails or a guide. If it gets to the point of engine destruction then the driver must have just turned the radio up and ignored the early warning noises.
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post #24 of 31 Old 08-14-2013, 09:19 AM
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Anyone know if the 3.6 has a belt? I have a 2013 & haven't thought about it until I saw this thread. Tks
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post #25 of 31 Old 08-14-2013, 10:00 AM
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Originally Posted by offroadjeepjk View Post
Anyone know if the 3.6 has a belt? I have a 2013 & haven't thought about it until I saw this thread. Tks

"Valve System: Chain-driven DOHC, 24 valves and hydraulic end-pivot roller rockers"

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post #26 of 31 Old 08-14-2013, 11:15 AM
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Sweet, one less thing to worry about.
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post #27 of 31 Old 08-14-2013, 12:11 PM
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Timing belts definitely had a bad reputation but as technology improves, so does belt performance, materials and longevity. Remember when fuel injection first came out and it was viewed as the worst thing ever?

I remember as a kid being really torn between timing gears or a timing chain on my 351W so that's proof that I've certainly come a long way. The truth is, modern motors with timing belts have better mileage, less parasitic drag, and the belt helps to damp valvetrain action a bit for a smoother running engine.

I'm truly amazed at the Pentastar V6 which is why I bought two vehicles that both have them. It's a workhorse of a motor and has a great feel to it. If it came with a belt vs a chain, would that have stopped me from buying it? Nope.

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post #28 of 31 Old 08-14-2013, 02:22 PM
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Yea but in non interference (freewheeling? Never heard that one before) engines the combustion chamber is either A) HUGE with a related low power output or B) Above the valve like in a briggs and stratton or Flathead. Even the Dodge Hemi will touch valve to piston with a broken chain!

The primary reason for belts is: they can be used externally. A chain requires oil and thus gets buried in the crankcase behind the water pump etc. And on a OHC or DOHC chain drive - the chains are so long even the slightest stretch can retard the valve timing as much as 2degrees!

A belt can be changed in ~30 minutes with no loss of coolant or oil. A chain takes about 7 hrs (to do it correctly) and requires draining and filling the radiator, the oil and disposal of both...

The secondary reason is (believe it or not) they DON'T stretch. That is until they wear out which occurs faster than a chain towards end of life. A chain wears from day one. If it stretches .005 - in 200K miles - that was at a rate of .005 divided by 200K AND any gain never left (ie if it stretches .002 it STAYS stretched .002).
A belt will have ZERO stretch until around 45K and may gain .003 by 90K. It CAN get stretched momentarily under hard acceleration - but returns back once inertia is more equal between crank and cam gear. Like a rubber band returning to its shape after use.

As a machinist I prefer chains. Even though on a OHC or DOHC if you surface the head - you have moved the cam closer to the crank and that is then "made up" by the chain - resulting in retarded valve timing. A lot of rebuilders forget this as two degrees does not make it run bad - just lower economy and lower power but compared to the pre rebuild it is BETTER so overlooked!
As a consumer I prefer belts - (and have even belted some chevies for drag racing) just MUST do the maintenance on time!

Whenever I have seen an interference engine with bent valves - it was NOT because of "a broken belt". It was because of failure to perform scheduled maintenance. THEN the belt broke! I have never seen an engine come in with all of the valves bent a broken belt and less than 40K on it. I am sure there are some, but VERY rare.

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post #29 of 31 Old 08-14-2013, 02:31 PM
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Daewoo cars were bad about braking belts before their scheduled replacement and they would destroy themselves. Then they went out of business probably a contributing factor.
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post #30 of 31 Old 08-14-2013, 04:53 PM
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Quote:
Originally Posted by jwmbishop View Post
Yea but in non interference (freewheeling? Never heard that one before) engines the combustion chamber is either A) HUGE with a related low power output or B) Above the valve like in a briggs and stratton or Flathead. Even the Dodge Hemi will touch valve to piston with a broken chain!

The primary reason for belts is: they can be used externally. A chain requires oil and thus gets buried in the crankcase behind the water pump etc. And on a OHC or DOHC chain drive - the chains are so long even the slightest stretch can retard the valve timing as much as 2degrees!

A belt can be changed in ~30 minutes with no loss of coolant or oil. A chain takes about 7 hrs (to do it correctly) and requires draining and filling the radiator, the oil and disposal of both...

The secondary reason is (believe it or not) they DON'T stretch. That is until they wear out which occurs faster than a chain towards end of life. A chain wears from day one. If it stretches .005 - in 200K miles - that was at a rate of .005 divided by 200K AND any gain never left (ie if it stretches .002 it STAYS stretched .002).
A belt will have ZERO stretch until around 45K and may gain .003 by 90K. It CAN get stretched momentarily under hard acceleration - but returns back once inertia is more equal between crank and cam gear. Like a rubber band returning to its shape after use.

As a machinist I prefer chains. Even though on a OHC or DOHC if you surface the head - you have moved the cam closer to the crank and that is then "made up" by the chain - resulting in retarded valve timing. A lot of rebuilders forget this as two degrees does not make it run bad - just lower economy and lower power but compared to the pre rebuild it is BETTER so overlooked!
As a consumer I prefer belts - (and have even belted some chevies for drag racing) just MUST do the maintenance on time!

Whenever I have seen an interference engine with bent valves - it was NOT because of "a broken belt". It was because of failure to perform scheduled maintenance. THEN the belt broke! I have never seen an engine come in with all of the valves bent a broken belt and less than 40K on it. I am sure there are some, but VERY rare.
Wow good stuff, thanks for posting. Totally didn't even think about machining affecting the distance and causing the timing to be off. You learn something new everyday!

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3.8 liter , JK Wrangler , question , timing belt , timing chain , wrangler

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