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View Full Version : Interference vs. Non-Interference


Chad
04-10-2002, 06:35 PM
Ok.

I get in my car, get on the freeway on-ramp, give it about 1/2 throttle and it dies.

Won't restart.

Pull off the cat. (was told it may be plugged, and had been having power issues)

Won't start.

Have the car towed to a local mech. He thinks it skipped a tooth. It backfires out the intake.

Looks in his book. It says the 3.3 is an "Interference Application" He tells me that it probably has a bad valve or head.

He tells me to take it to Subaru.

Any thoughts?

Green1995SVX
04-10-2002, 07:54 PM
Our engines are non-interference. Simply put, no damage can be done to the engine from a broken cam belt.

-Mike

dromano
07-31-2002, 07:55 PM
Chad, what was it? Sort of sounds like timing belt.

nvrmore100
07-31-2002, 10:41 PM
As stated, its NOT an interferance engine. My timing belt literally ate itself and the engine is fine. Not all that hard to change yourself either. Check out my howto if you feel like giving it a go:

http://www.ryanmacdonald.com/

Under Sunfire/SVX > SVX Howto's

kuoh
08-01-2002, 07:07 AM
Ask him to show where it is in the book it says that. Either his book is wrong, he looked up the wrong car, or...he's just giving you the "excuse of the day".

KuoH

Originally posted by Chad
Looks in his book. It says the 3.3 is an "Interference Application" He tells me that it probably has a bad valve or head.

1994SubaruSVX
08-05-2002, 01:51 PM
engine get damaged is when water got in the intake and the damn thing hydrolocked. basically it sheared a piston rod and damaged my block.

breaking timing belts do nothing to our cars.

svxpert
08-25-2002, 08:44 PM
<<Ask him to show where it is in the book it says that. Either his book is wrong, he looked up the wrong car, or...he's just giving you the "excuse of the day". KuoH>>

I know this might stir things up but "technically" the Subaru DOHC 3.3 is an interference engine. For those who have SVX engine knowledge, you will know why they classify them as interference. Lets see who can explain why. I'll put another SVX Pace car Poster up for grabs.

Boone
08-25-2002, 08:56 PM
Because if the timing belt breaks, it interferes with your plans. :p

dromano
08-25-2002, 09:01 PM
when it breaks it interfers with getting to where one was going!!!!!!!!

svxpert
08-25-2002, 09:15 PM
<<Because if the timing belt breaks, it interferes with your plans. >>

<<when it breaks it interfers with getting to where one was going!!!!!!!!>>

Nope

Nope

Trevor
08-26-2002, 12:30 AM
May be because it is possible, but unlikely, that a piston could come into contact with a valve. Has this happened, you are the expert ?

svxpert
08-26-2002, 05:18 AM
<<Has this happened, you are the expert ?>>

not to me or anyone else I have ever heard. The question is why Subaru classifies this as an "interference" engine. Even though it is almost impossible to have "interference" when the timing belt breaks the question to you guys is why? There is a specific answer (reason) I'm looking for...

Earthworm
08-26-2002, 10:03 AM
I'm curious where Subaru says the 3.3 is an interference engine. (I'm skeptical :D )

...unless Subaru's interpretation of interference/non-interference is backwards from what we interpret it to be.

dromano
08-26-2002, 08:23 PM
If we all leave this thread for a week will you tell us? You are way more patient than me:rolleyes: :rolleyes: :rolleyes:

svxpert
08-26-2002, 08:58 PM
do you need hints?

dromano
08-26-2002, 09:15 PM
OBVIOUSLY:confused: :confused:

huck369
08-27-2002, 06:07 AM
If I were to guess, I'd say that if the belt broke and a cam stopped with a lobe at it highest point (which is not likely) it would cause the valve to hit the piston.
As this is the only way that it could be an interference engine, unless the gear drive between the intake cam and the exhaust cam stripped then the intake valves would get into the exhaust valves and cause damage.

Do I get the poster?:rolleyes:

svxpert
08-27-2002, 07:15 PM
.................................................

huck369
08-27-2002, 07:25 PM
I WON....I WON....:D ......I WON....I WON....:D ......I WON....I WON....:D ......I WON....I WON....:D ......I WON....I WON....:D ......Not that I want to gloat or anything;)

dromano
08-27-2002, 07:42 PM
Originally posted by svxpert
We have a winner. The simple answer is cam gears. All Subaru DOHC engines are interference, the only difference in the EG33 from other DOHC engines is it has cam gears to also keep it in time. (amoung other things) So when you break a cam belt the 2 OHC's in each head keep it from destroying itself. Huckster, send me you address.
subarueast@yahoo.com

Sooooo...,,, is the premise bogus or not?????????
Will the gears keep the pistons & valves apart if a timing belt goes south?:( :( :mad: :confused:

Trevor
08-28-2002, 04:53 AM
Please, explain how " cam gears keep it in time " when the belt is broken.

As I see it in order to be truly safe there should be clearance at all times between valves when fully open and the associated piston.

Are you saying that this is not so and with the shafts coupled the relationship between valve spring pressures means that the shafts will not stop or run out of time resulting in a fully open valve coming in contact with a piston at TDC even though clearance does not exist ?

Please explain in detail as this an interesting concept.

huck369
08-28-2002, 05:40 AM
The belt only turns the lower(exhaust) cam, which has a gear on it that turns the upper(intake)cam, so if the belt breaks the two cams on each head stay in time with each other, just not with the crank(and pistons).
The only part that can be "interference" is the intake and exhaust valves, and since the belt doesn't affect thier relationship, a broken timing belt can't hurt the engine.
The only way to cause a problem is if one of the cam gears strips and allows the 2 cams to get out of time, thus allowing the intake and exhaust valves to collide, causing warped valves.
Clear as dirt, Huh :rolleyes: Well that is my best stab at it:confused:

Beav
08-28-2002, 10:40 AM
This is really a *looking for a kind term or word here* silly discussion. If you wanted to call an engine an interference engine because the valves can collide when their drivetrain disintegrates that's fine. But you may as well go ahead and call all engines interference engines as a broken connecting rod will allow a piston to kiss a valve as well. A shattered valve keeper or retainer, a broken valve head, a broken valve spring, or any of a hundred different situations can cause a clash inside.

Interference and non-interference are colloquial terms used to indicate if the pistons and valves could/would collide in the event of a timing belt/chain failure. Everyone should understand that failure of other parts can cause catastrophic repercussions due to the meshing of parts that weren't designed to mesh. Any other use of the term is merely a semantical play on words.

huck369
08-29-2002, 05:47 AM
I guess it has been a "silly” discussion, but it started with someone saying the book said the 3.3 was an interference engine, were as some of us have found out that it isn’t an interference engine in the normal sense of the term, and we wear trying to clear the air as to why the book would say it was.
I know it was probably clear to you, as you’re a mechanic, but many on this board don’t have a clue when it comes to the internals of an internal combustion engine, so we were trying to help them understand the best we could.

And, Hey, I got a poster out of the deal:D :D :D