Timing belt???
09-05-2012, 03:03 AM
#31
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So the Porsche designed DE-tensioner WAS designed by Dr Bob's Great Aunt Claire Vouyante!
09-05-2012, 03:49 AM
#33
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When the 928 was developed, Porsche had been running chains in engines, with very poor results, for quite some time. In 1976, the chain tensioner problem with the 911 engines, had still not be solved and these engines "ate" chains, tensioners, and valves on a routine basis. There were literally tensioner "updates" faster than we could keep up. The heat generated from the turbo location, on the 930 engines, made these problem with the chains and tensioners even worse. To say that the timing chain and tensioner problems made many people very wealthy would be an understatement. I personally can think of at least 8 versions of aftermarket chain tensioners and chain tensioner "stops". Many of those "solutions" turned the chains and associated parts into junk and required changing the chains every 30,000 miles.....yet this was much better than the alternative of having the tensioner fail and having the valves hit the pistons, for a huge amount of people.
Compared to the 911 series engines, the 928 engines, with a timing belt, were way, way more reliable and required much less maintance.
Of course, Porsche finally engineered a solution that was to prove to "fix" the tensioner problem, by 1984. Certainly Porsche was not going to redesign the 928 engine and go back to using internal chains...especially when the external belt was working so well.....this far into the production life of the 928. The belt worked great and the problems were largely simple issues solved by routine maintainance.
I'd think that when faced with the alternative to use a big chain or an external belt, in 1976, this probably wasn't even discussed in the engineering meetings. I'd imagine that the instructions to use an external timing belt came from the very top of Porsche. There was simply no way they were going to design another engine with chain driven cams, at that point in time.
Looking at the problems that Porsche has had with the internal chains on the Boxster, 996, and 997 engines, I'd guess that if they had the room, they would make the engines slightly larger and abandon the chains, even today. These engines all have terrible problems with the internal chain drive mechanism and have had multiple supercessions. Some of these supercessions are so extensive that if you need a chain for an early 996 engine, you need to buy a new crankshaft, a new chain, and a new countershaft!
I'll take a belt that can be inspected and replaced at regular intervals, over an internal chain, any day.....every day.
Now think about how much a timing belt reduces the harmonics that are transfered to the crankshaft.....that's an entirely additional advantage....and another discussion.
Compared to the 911 series engines, the 928 engines, with a timing belt, were way, way more reliable and required much less maintance.
Of course, Porsche finally engineered a solution that was to prove to "fix" the tensioner problem, by 1984. Certainly Porsche was not going to redesign the 928 engine and go back to using internal chains...especially when the external belt was working so well.....this far into the production life of the 928. The belt worked great and the problems were largely simple issues solved by routine maintainance.
I'd think that when faced with the alternative to use a big chain or an external belt, in 1976, this probably wasn't even discussed in the engineering meetings. I'd imagine that the instructions to use an external timing belt came from the very top of Porsche. There was simply no way they were going to design another engine with chain driven cams, at that point in time.
Looking at the problems that Porsche has had with the internal chains on the Boxster, 996, and 997 engines, I'd guess that if they had the room, they would make the engines slightly larger and abandon the chains, even today. These engines all have terrible problems with the internal chain drive mechanism and have had multiple supercessions. Some of these supercessions are so extensive that if you need a chain for an early 996 engine, you need to buy a new crankshaft, a new chain, and a new countershaft!
I'll take a belt that can be inspected and replaced at regular intervals, over an internal chain, any day.....every day.
Now think about how much a timing belt reduces the harmonics that are transfered to the crankshaft.....that's an entirely additional advantage....and another discussion.
09-05-2012, 05:22 AM
#34
Three Wheelin'
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I also own a relatively old (2002) Cadillac with the “Northstar” engine. It has a layout similar to the 928 engine but it is not an interference engine (somewhat lower efficiency) and the cams were driven by a highly reliable chain. I’ve got a lot of miles on that engine and have never had to service the chain.
So, here’s my question: What was the reason that Porsche went to such a relatively inferior design (at least from my outlook). I’d love to hear from folks who would like to defend the timing belt.
.
So, here’s my question: What was the reason that Porsche went to such a relatively inferior design (at least from my outlook). I’d love to hear from folks who would like to defend the timing belt.
.
Belt is external, quieter, easier to service, lighter, not in the oil bath. Just so many benefits .
Save those chains for those SOHC motors.
Oh and the Northstar is 'ok' but not really as nice IMHO as the 928 V8
(and chains do need to be serviced, they stretch and effect the cam timing.(even with the tensioner working.).
09-05-2012, 04:23 PM
#35
Burning Brakes
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This is only true if you want to conserve energy. If the goal of the flywheel is to simply supply inertial mass to the rotating assembly to smooth out revving/deceleration, then the mass controls the degree of buffering offered by the flywheel. On an energy storage system, heavier flywheels are better, because you DONT want them to slow down, but for an engine that is put under dynamic loads, lighter flywheels are better since they produce crisper response. As long as the flywheel has sufficient mass to achieve the smoothing effect you are looking for, then extra mass is not necessary. Also, the torque converter does act as an "additional" flywheel (as does the gear train, technically, when connected to the engine). Typically they are designed to be light because the engine is designed around a given flywheel specification, and they want to minimize the perturbation to the system caused by increased weight bolted to the flywheel
09-05-2012, 06:21 PM
#38
Captain Obvious
Super User
Super User
This is only true if you want to conserve energy. If the goal of the flywheel is to simply supply inertial mass to the rotating assembly to smooth out revving/deceleration, then the mass controls the degree of buffering offered by the flywheel. On an energy storage system, heavier flywheels are better, because you DONT want them to slow down, but for an engine that is put under dynamic loads, lighter flywheels are better since they produce crisper response. As long as the flywheel has sufficient mass to achieve the smoothing effect you are looking for, then extra mass is not necessary. Also, the torque converter does act as an "additional" flywheel (as does the gear train, technically, when connected to the engine). Typically they are designed to be light because the engine is designed around a given flywheel specification, and they want to minimize the perturbation to the system caused by increased weight bolted to the flywheel
