more chain tensioner stuff

fitchn · 12-16-2006, 11:15 AM

In upgrading the chain tensioners on a 911 sc, why aren't mechanical tensioners as used by racers back in the day recommended for street driving? They can never collapse (since they are solid aluminum). There can be only two issues that I can think of: 1) Whoever sets the chain tightness really has to know what they are doing. 2) As the operating temperatures change the length of the chain will be slightly different resulting in varying tension. With regard to the second problem, I did some rough thermal expansion calculations and noticed that because the tensioners themself will also get longer it shouldn't really be a problem for reasonable temperature ranges. So why aren't these used as they are much cheaper than the carrera pressure fed tensioners and are essentially bulletproof?

Zeke · 12-16-2006, 02:03 PM

Chain wear (stretch) is the answer. You have to adust the solid tensioners every so often and that's not something poeple other than racers (who might have the motor out more frequently) want to deal with.

safe · safe's Garage

Quote:

Originally posted by milt
Chain wear (stretch) is the answer. You have to adust the solid tensioners every so often and that's not something poeple other than racers (who might have the motor out more frequently) want to deal with.

Do you mean that you will have adjust it due too wear or that it wears the chain more?

Zeke · 12-16-2006, 05:16 PM

Shouldn't have any effect on the chain unless adjusted too tight. I think it's been noted that the chains will have a tight spot and unless you find that spot to make the final adjustment, they will run tight. Also, my experience with chains (not in a Porsche engine) is that they loosen more at the beginning. So, by my thinking, one would have to drop the motor, or fight the exhaust thing, and adjust after a few hundred miles.

I bought some, said I was gonna run them, and changed my mind. If I were to have built the motor I had in mind, I would have used rebuilt tensioners, or 930 tensioners, and/or collars.

fitchn · 12-16-2006, 06:05 PM

Thanks for that, but its not like you have to drop the motor to replace the chain tensioners (or in the case of solid tensioners adjust them every so often). Once you remove the muffler and a few bolts (with the oil out of the car) all that stuff is very accessible. How often would you have to adjust the chain tensioners anyway? It seems like it would take many thousands of miles for the chain to stretch an appreciable amount.

Walt Fricke · 12-16-2006, 06:38 PM

Well, if these parts were self-adjusting due to a happy coincidence of thermal expansion, I think Porsche would have been more inclined to abandon hydraulic tensioners.
It seems, however, that expansions do not so happilly work out to offset each other. The result being that with solid tensioners your chains are too loose to start with, just right as a certain temperature is reached, and then too tight. Or perhaps it is the other way around. In any case, that seems to be why Porsche used these infernal devices. They were a source of disasters at first until some design bugs were worked out, but, by the time of the 930 (and then the Carrera) tensioners, it seems they had become very reliable.

In any event, I think these serve more as shock absorbers than anything. The tension part comes from the big spring inside the tensioner. The oil serves to dampen the oscillations. So if the dampening part stops working, things jump around. No dampening with solid tensioners.

I am told that if you jack the car way up you can run the engine and watch what the chain does without making too much of a mess (unlike removing a valve cover, where you will wish you hadn't tried). No guarantees, but those who have done this say the amount of chain movement, even with everything working as designed, amazed them. The tensioners smooth things out.

Race engines, due to frequent disassembly and, at the factory level and a couple of levels below that, ample budgets, can have new chains and sprockets frequently installed.

You don't have to drain the oil to open the chain covers if you jack the car up from the rear. But it is a real pain to pull the muffler, pull sheet metal, pull the covers, and reverse the procedure. Plus you'd have to know how tight is tight enough. That certainly isn't something a car company would inflict on its mostly street driving customers.

But solid tensioners are cheap as things go (the simplest you could make yourself out of your existing tensioner cores and some nuts and bolts and little more), so I nominate you to try this for a year or so and report the results. Some folks do swear by this.

Not me, though. I have a set in my spares box for the track in case a tensioner fails. I found I had to machine them when the time came to use one, because mine were made for the early thinner idler wheel carrier, and thus were too fat for the later, better wider based carrier.

And if you have the Carrera cover plates and switch to solid tensioners, you'd best figure out a solid way of plugging the hole. It is amazing how much oil can come out how fast. Don't ask how I know (can you say oiled the whole track in one lap?).

Walt Fricke