Timing Chains vs Timing Belts?
 

just a general question, I know the 928's timing belt job can be a pain in the ass and what not. but anybody hear of someone changing the belt over to a chain? just wondering.

Never heard of it but I suppose it would be like on the ford mustangs (or similar). The timing belt is not really that hard, I have however run into broken w/p bolts on removal. If doing it for the first time it can be a challenge, but just check everything twice when putting back together.

I never seen an external chain for timing. They are internal and are lubricated with the motor oil. I would think that without the oil the wear on the gears would be pretty bad. But I don't know about all cars just the ones I've worked on.

+1 I think you nailed this question.

he did. thanks. I was hoping it didn't sound stupid. lol

There were many engines with external timing gears and chains, Chev 427, 350, Dodge, AMC. The chains were under a cover and went around crank and cam but not in the oil pan. Some had nylon tipped Cam gear. They ran dry, not oiled. The 928 has a very long belt and the mass of any chain that long might be a problem. I am sure they looked into it in Stuttgart back then, you know, the same guys who gave us the V2, the Saturn 5 rocket, those guys.

350 chevy and 427 chain not lubricated? What year was that?

Gee, I could be wrong but when I took them apart I never saw any oil in them. I know the cover went around the crank/cam gears which was sealed. I never saw oil pour out of the cover when removed as I recall. Was there a passage for oil to get back in the pan from the gear cover?

Yes. On most a washer to sling, a slot to return.

Nylon coated aluminum gears ran quietly, but with age the nylon crazes and then disintegrates, then chain drags across the gear and you have a mess.

Cogged belt is a very good solution. It should be a minor worry with a 928.

It's been awhile since I had a timing cover off. Sorry about the mistake. I had only worked on older push rod motors back (60's) then. I guess they always looked pretty dry when I removed the covers, probably sludge blocked or something.

I remember now at least on my 67' 427 Vette, the bottom of the timing cover fit into the top of the oil pan so oil could drain or be lubed through that opening. It threw me until I thought about it more. Also some engines had timing gears instead of belts or chains, I think mostly on big inline 6's . Not sure if gears are still used today or not in that application.

Porsche was way ahead in many ways, big timing belt, 32 valve DOHC, Trans and gear box in rear, motor in front, and sophisticated rear suspension. Funny how Corvette implemented many of the same in later years. I did the T-belt on my 928 last year. Not too bad if you take lots of time.

I'm not an expert. Maybe there are other systems, too.
Kids bought a 64 Buick with the oddfire V6 (later the Jeep Dauntless engine).
Brokedown at FoodLion / timing issue.
Found hard chunks of nylon / polymer in the pan, along with aluminum.
Eventually I had to disassemble and completely rebuild it.
It had a slinger and return, which I think is pretty common.
That's why the timing covers have double lipped oil seals.

photo evidence. (looking at this Buick V6 chain, its hard to imagine a 7 foot chain driving a 928 engine, or even two shorter ones)

slinger washer is cone shaped, nearest the block, laying on the workbench.


Rebuilding the engine was interesting, rebuilding the 2 speed aluminum automatic transmission was fun.


http://forums.pelicanparts.com/uploa...1293994827.jpg

Hey Guyz this is one of those topics that the pros and cons of both systems are based on engine type and propose. Just talking about the 928 4.5L engine it's easier (for me) and better to stay with the belt. "why" if the belt brakes, my engine is still good. But when you get to the 4.7 and 5.0 the your valves and pistons take a dump on you when the belt brakes and your talking a lot $ to fix. I would love chains on those motors. What sucks is that they take longer to change and if you don't know what that engine noise is and your grinding you engine walls because of the chain having to much play that can be more expensive than valves and pistons.

These belts are actually pretty easy to replace.
Time consuming but easy.
Just go in on a rainy weekend every 30K miles or at intervals of every 3 years or so.

Metal timing chains do wear and stretch. The last one I replaced (that had jumped timing) had 105K miles on the engine. The chain was, at most, 26". If you figure that much wear on a 7' chain, you could expect to replace the chain about every 30K miles.

The 911 uses timing chains so I'm certain they gave it some thought in Stuttgart back in the day :)

But with all the accessories driven by the belt, and the expansion and shrinkage of an aluminum engine of V8-size, a belt is just an allround better solution.

The 32 valve engines have both, so best of both worlds maybe? :D

http://bilgalleri.dk/uploads_xl_wm/1...008%2009:42:23

I didn't know that. Great picture!

And the 911 chains with hydraulic tensioners and nylon guides and the like have been the sources of many problems, retrofits, upgrades, and outright failures over the years for 911 owners. I am not confident they are any "better" than our belts.....although the last of the air cooled 911's seems to have finally got it right regards the chain system

So, getting the chains to work properly on the 911 took 30 years - image if Porsche had gone with the solution on the 928!! :o

I own a 1987 911 Carerra as well as my 928 (and my wifes 944). Never had any issues with the timing chain on the 911 or the timing belt on the 928. It is all keeping up with maintenance on these great cars, and investing the time in research to learn how to do it right. The extra time spent to maintain them properly is well worth it. If you cheap out now you'll pay later.

True that. It's all about confidently doing those thrilling high-rpm blasts. :)

well the porsche cheyenne engine has timing chains and i personally feel they are more dependable and robust. I think however that they would increase the rotational inertia of the reciprocating assembly and lower overall RPM capability...something that i feel is one of the strong points of my 5.0 engine..it hits redline rapidly. If you were completly rebuilding an engine for performance you could lighten the internal assembly to compensate for some of the increased rotational mass..maybe...but overall you could get better results with a belt in terms of shear horsepower(all frictional losses take away from RWHP of course...). I have heard of someone converting to a chain drive from an Audi engine but could find no information concerning this...would be interested ...

One Piece Timing Chain -
 

The Porsche 911 engine has long had a bullet proof timing chain design for each bank of cylinders using engine oil pressured tensioners. These upgraded tensioners virtually eliminated all of their previous problems. It looks like the 911 engine photo that riber bentsen added displays the 1970’s non-oil-pressurized cam chain tensioners.

By the way - Mercedes used a single (extra long) timing chain design on their early 4.5 liter V8 engines. This design was basically similar in operation to the 928 timing belt. The extended length of these chains could cause LOTS of component failures, guide wear, centrifugal weight/stretch and valve train issues - especially if over rev-ed. It was not uncommon to replace both heads for bent valves if an aggressive driver missed a shift or spun the drive wheels in the mud.

The one piece, twin overhead cam timing chain design in some ways is better than the light weight timing belt but also has it’s drawbacks.

Michael

I don't see any problem with the timing belt as long as it's changed and properly installed. I see Gates has a heavy duty one out now for about $167. I put the Gates belt on my car last year, but the cheaper one as the newer version was not on the market yet. There had to be a good reason the Porsche engineers did not use a chain. All this had to be figured out before the engine was made. I am sure they did a lot of testing and looked into other options. As we know, the belts are widely used today. As pointed out here, the chain was not a good option.

The regular Gates belt, stock tensioner, new pivot bushings, new gears/rollers and new Laso water pump is all you need, regardless of year, interference or not.

Valve crash happens to cars that were neglected. And also, to ones where redneck "feel my way through the job" mechanics installed the belt without following exact procedures (getting washers out of place, not retensioning, not measuring tension, neglecting the plastic pivot bushings, running without covers, etc). And ones where the old water pump of unknown origin is left on the car (especially a pump that sat on a stored car).

So, buy a car. And unless it was serviced by Greg Brown or Mr. Merlin, the first thing to do is disassemble and inspect. Doesn't matter if it was serviced 5000 miles earlier, in fact, that's an indicator that you'd better tear it down to inspect.

I did everything you mentioned last February. I down loaded a "how to" list from that Rennlist forum. My car had 38K original miles on it. When I took the covers off, I saw the belt had been changed once, probably back in 1992. I changed the water pump but my old one looked quite new. I put a new Laso one on. I refilled the tensioner with new oil. Checked belt tension with the gauge and then again after 2K miles. I drove it about 7K miles since, until the snow hit here. Also Mrmerlin was a great help as others.