danglerb

People focus so much on the TB, which is 8 to 10 hours of work every 60,000 miles, thats what about 5% of the overall PM for the same period? Doesn't the 944 have an even shorter interval for replacing the timing belt?

Anything that could be done to improve the system would be nice, but not essential.

ceedee

that's will always be a mystery to me

UKKid35

Ask the Citroen SM owners about that one...

Ron_H

Maybe one reason for choosing a belt over a chain is the experience Porsche had with that "other model" they have been making since 1964 or so: the 911.
Lots of problems with chain tensioners and I carried spare tensioners and parts to change them in my 914/6. Big problem when they fail. Some guys use mechanical tensioners and scrap the oil filled stock ones but then they must be set with shims and probably result in more wear 'cause they don't absorb any energy or shock. Then the factory went to pressure fed tensioners and everyone thought their problems were over. Wrong!! Pressure fed tensioners can also fail, particularly when you lose oil pressure for whatever reason, but that is a moot point in a 911 engine; I have broken a crank in a matter of seconds when I lost pressure due to an oil leak. And let me tell you about changing chain tensioners in a 911 engine, particularly one that's turned around and stuck up against a firewall like on my 914/6. Carry lots of band aids. And a few C clamps to clamp the chains while you pull the tensioners and,......and...... lots of swearing and grunting and groaning. Particularly when you do it on the side of the road and the sun is going down...... and it is getting cold.....and you're about 400 miles from home. At least in the 928 your tensioner light will come on (hopefully) and you'll have time to do something about it at the next town. With the 911 chain tensioners, you are either on spec or not. No adjustment except with the solid tensioners. But what did Porsche learn by making that glorified VW, right?

Chazz

The Cayenne uses a duplex chain cam drive.

ceedee

is that what happens to them?

UKKid35

I have also read that if the air con was switched on the cam sprocket, which is a press fit, would then slip due to the sudden increase in load and cause the interference engine to crash. Not surprisingly knowledgeble owners now have a pin fitted to the sprocket.

docmirror

I agree about the maint statement. My Espada has chains, and they are a constant concern to me. Nothing is 100% safe except non-interference. there is no reason any engine has to be built as interference for the public, non-racing crowd. Cutting the recesses in the piston for valves would have saved many millions of $$$ in repair bills over the last 30 years for the fleet of cars that are belt driven. sometimes I think there is an evil conspiracy with mechanics to keep interference engines beign produced.

Ferrari used chains for many years. Some great engines do have chains.

fabric

DOHC engines have better high RPM performance, pushrods do better at the low end. I'm guessing this is a function of 4 valves vs 2, and that you can have higher redlines with the DOHC. Looking at the Z06 engine, redline is 7K rpm, and even at 7 liters they're getting 16/26 for mpg. Some of that is the ridiculous 1-4 forced shift if you don't really gas it, so if you drive to avoid that I bet fuel economy is horrid.

THere's probably more to it than just the weight, I'm pretty sure the DOHC is just a more efficient engine, and if you look at it in a 4 cylinder application it's a different ball of wax.

danglerb

If I understand it correctly a big advantage of 2 vs 1 exhaust valve is heat dissipation from the hot gases. Large valves absorb more heat, but are limited to the same size stem for cooling.

I don't see it talked about that much, but I get the feeling that high lift cams put a HUGE stress on valve springs and they wear out much faster, from heat I think.

On topic to the thread, I have no general opinion on belt vs chain, but the "way" Porsche did our belt is a bit annoying.

JKelly

My two other cars, and the previous one before them (all non-porsche), have all used a belt and the interval of service was the same as the 928 at 60,000 miles. After doing a very thorough 928 timing belt service myself, I realize that the best way to think about it is as a whole "system" and not just a "belt", which of course is how Porsche thought of it. If a person understands and maintains the different components of the "system", I don't see how it can fail. How often does a 928 belt just snap? very very rarely....that's not including the ones that shred. I would bet that most of the shredded ones could have been prevented. An occasional peek down through both the left and right timing belt cover vents can tell you a lot about the health of the "system". The only thing I wish Porsche would have done differently is to somehow make visual inspection of the components behind the center timing belt cover easier; maybe by using a two or three piece center cover.

danglerb

Clear TB cover? With LEDs

Normy

Well, my understanding is that the Gilmer belt has less friction, is far quieter, and makes it possible to drive other items other than the camshafts.

-Frankly, I'd prefer a chain over a belt. My understanding is that while chains do break/skip, it takes hundreds of thousands of miles or heavy high-RPM use to cause this to happen at the same intervals that normal belt changes on M28's take place. In short, the chain isn't infallible...but it is far more reliable.

VW and Porsche in the early 1970's went on a Gilmer-belt spree. All their engines had these, but when the problems started to crop up down the road, they changed their tactics: modern Porsche engines use chains, as do all the VW starting with the VR6. VW specifically installed three timing chains on this V6 because chains are more compact than belts.

N!

Mike Frye

There is one other indicator you can look at relatively easily. One 'lister recently used it as his early warning and he says it saved his engine.

If you regularly check your airbox you will have a way to tell if your belt is shredding. The fine black residue will collect on the air filter and you can easily get to that without any tools.

Obviously this is the type of thing that only works in one direction (meaning the reverse isn't true that if there's no residue there's no problem ) But it is one additional indicator for the arsenal.

hacker-pschorr

Friend of mine started taking his ZO6 track car to work (410+rwhp) to save money over driving his Yukon every day.