interference or non-interference

86porsche · 03-11-2009, 06:07 AM

Hi there,

Can someone please tell me what 928 models are non-interference? I have tried to find this out but nobody seems to know for sure. In my case I have an 1986 4.716v Euro spec car.VIN WPOZZZ92Z65840885 if anyone could tell me for sure I would be greatful. Cheers

Dean_Fuller · 03-11-2009, 07:48 AM

An 86 model is Interference.

Danglerb · 03-11-2009, 07:51 AM

Yours is interference, some question about the earlier Euro S models, but by late 84 all were interference motors. The non S models are non interference, but even the early S can bend valves if carbon builds up in the piston valve reliefs and the timing belt breaks.

928SROCK · 03-11-2009, 08:52 AM

what exactly does Interference and non interference mean to a engine? and why did porsche switch? thanks

MPDano · MPDano's Garage

Timing Belt Breaks and all hell breaks loose. Interference motors will smash valves and piston heads when TB Belt breaks. Non-Interference motors will not.

Danglerb · 03-11-2009, 11:04 AM

On a non interference motor the valves and pistons can never touch even when the timing belt is broken and valve is stuck down as far as it can go. The valves are up in a sort of pocket in the heads, and pistons have valve reliefs, half circle cuts for more clearance.

With interference motors the damage from valves hitting pistons can be pretty severe. Bent valves are typical, but damage can also occur to pistons, cylinder walls, and camshafts. Other times just a few bent valves.

86porsche · 03-11-2009, 11:05 AM

OK. that is the answer I was loOKing for, have the parts just gotta get the job done now. In the mean time I wont be driving it much.Murphy's law! thanks for the reply lads

rhjames · rhjames's Garage

Quote:

Originally Posted by 86porsche

OK. that is the answer I was loOKing for, have the parts just gotta get the job done now. In the mean time I wont be driving it much.Murphy's law! thanks for the reply lads

Hopefully you're saying the belt is still intact and you intend to do a renew before you have to rebuild a set of heads....and pistons....and------

--Russ

lizard928s · 03-11-2009, 08:41 PM

when you have the belt off for the job you can test carefully.

However I had an 85 euro 4.7L that was non interference.

rich · rich's Garage

My 1980 M28/11 broke valves off, cracked pistons and split the cylinder wall. But my heads were shaved.

m21sniper · 09:04 PM

Quote:

Originally Posted by 86porsche

Hi there,

Can someone please tell me what 928 models are non-interference? I have tried to find this out but nobody seems to know for sure. In my case I have an 1986 4.716v Euro spec car.VIN WPOZZZ92Z65840885 if anyone could tell me for sure I would be greatful. Cheers

84 and earlier US are definitely non interference. 300hp and earlier Euro motors are 'sometimes' non interference (meaning you can get lucky).

Yours is 310hp, and is interference.

Make sure you rebuild the tensioner and replace the idler roller. Also a very good idea to change the water pump while you're at it.

Quote:

Originally Posted by rich

My 1980 M28/11 broke valves off, cracked pistons and split the cylinder wall. But my heads were shaved.

Well gee.....

86porsche · 03-12-2009, 05:29 AM

Thanks again. I have all the bits to rebuild. water-pump, tensioner etc. Don't want to have to go back and do this job again! at least now I know that if i dont get the job done soon I'm taking a big risk.

928SROCK · 03-12-2009, 08:42 AM

Why didn't porsche have enough smarts to use a timing chain? I mean for something as critical as this you would think that they would of. Even American cars back then used timing chains. I'm not to impressed with porsche and there belt design. Its to bad a car that cost that much is that sensitive to a major motor break down even after replacing the belt I hear the tensioner could still screw things up and the belt could still have problems. It makes me not want to drive it knowing it could break and cost the same or more then what you could by a older used 928 for. I herd of people changing theres just because there afraid of it breaking and then they still have problems and the car never runs right again. Does anyone else have the same thoughts?

lizard928s · 03-12-2009, 08:47 AM

timing chains are alot harder to seal, and make alot more noise!

redrider044 · 03-12-2009, 09:24 AM

I was told by a mechanic friend of mine that some American cars are going back to timing chains, I just don't remember which ones. I think as well as others that timing belts were & still are "cash cows" for the dealers as well as "sealed" automatic transmittions. The auto manf. had to do something since the cars were requiring less maint. Eg. spark plugs generally go 50-100k before needing replacement, oil changes go well beyond the normal 3k, coolants last about 5 years, just to name a few. Gone are the days when you opened the hood of a car (1955 Chevy) & you could see the spark plugs & actually see the ground looking straight down. Granted, there is no comparison in saftey, comfort & sometimes power, but I still miss my old big block '69 Camaro. Sorry guys, just some rambling from an old hot rodder.

rhjames · rhjames's Garage

Quote:

Originally Posted by 928SROCK

Why didn't porsche have enough smarts to use a timing chain? I mean for something as critical as this you would think that they would of. Even American cars back then used timing chains. I'm not to impressed with porsche and there belt design. Its to bad a car that cost that much is that sensitive to a major motor break down even after replacing the belt I hear the tensioner could still screw things up and the belt could still have problems. It makes me not want to drive it knowing it could break and cost the same or more then what you could by a older used 928 for. I herd of people changing theres just because there afraid of it breaking and then they still have problems and the car never runs right again. Does anyone else have the same thoughts?

When GM was in the planning stages for Saturn, they did a lot of research about timing belts versus timing chains.

Their conclusion was that even though there is more noise (however slight) associated with chains, the reliability was hands down in favor of the chain.

That said, if you look at what's involved with encasing the chain (TB) with a 928 V8, the cost would have been substantial.

As with most car manufacturers, reliability isn't always the most profitable way to go.......

Frankly, I would have preferred a "gear drive".

--Russ

928SROCK · 03-12-2009, 10:02 AM

but for the price that these cars cost reliability should be what your paying for.

Normy · 12:39 PM

The "Gilmer" belt used in our cars was picked over chains because they are quieter, lighter, and they can also be used to drive things such as the oil and water pumps. This made the engine shorter as well. Also, belts require no lubrication- making the engine somewhat cheaper to build since lubrication sprays and extra oil gaskets were not required. I think I read all this in "Project 928".

The Gilmer belt is very typical '70's German/European construction technique. VW finally quit using these in their 1.8 and 2.0 liter four cylinder engines recently, but Ferarri continues to use them to this day.

I suspect that a timing belt could be produced that would last the life of the car, but it would require much stouter construction than is currently available. Belts don't "break". These things are similar to the drive belts on Harley-Davidson motorcycles, and have kevlar fibers embedded in the rubber to give tensile strength and they are very strong. The typical failure mode of a Gilmer belt involves the engine either skipping a tooth or two due to looseness, or several teeth being sheared off altogether upon startup, allowing the crank to skip forward. The teeth are nothing more than rubber bumps along one side of the belt. When they get old and dried out...these bumps can simply be ripped off. The solution is a belt that is wider, or two belts working as a team, or kevlar loops within the teeth themselves that prevent them from being sheared.

-I think [?] the general consensus on this board and Reutterwerk and Rennlist is that the "Porkensioner" that Ken sells over on Liftbars.com is probably a vast improvement over the factory tensioner, and this part would allow the belt to remain serviceable for a far longer period of time.

As long as it is changed every 45k miles [and done correctly], the timing belt is every bit as safe as a chain or a set of [noisy] gears. Let it go, however, and you now have a cobra under your hood waiting to bite...your wallet!

N!

Danglerb · 03-12-2009, 01:03 PM

People say timing belt failure, but my guess is the belt never is the root cause.

Cam gears wear through the anodizing and starts searing off teeth from misalignment.

Water pumps fail.

Idler pulleys fail.

Shoulder bolt bends.

Tensioner fails (likely in the too tight direction, which damages one of the above)

m21sniper · 05:31 PM

Quote:

Originally Posted by lizard928s

timing chains are alot harder to seal, and make alot more noise!

I will make that trade off any day of the week.

As a guy who's turned wrenches for a living, IMO timing chains are far superior to belts for durability. And gear drives are even better still.