just a simple question

[operationcougar]

has anyone made a chain drive for the balance shaft and timing belt, i know its a little more work, but as most of the high end hondas and nissans etc engines ive worked on have kits to change them over to a chain.

i personally dont want one, due to the increased weight put into the spining volocity of the front of the engine but i was just wondering if it exsists

[SoCal Driver]

No.

[fast924S]

No, know one that I know of has made a kit like that because Of all the problems that could happen, Also you whould have to mod the front of the engine so the chain would have some type of lubercation. I work aat a toyota dealer and I aso work on many used cars of other types. The Mitsubishi eclips has almost the same setup as the 2.5L NA and it works just fine. Soo who knows

[MoonBoy]

Chains are louder, can't deal with the expansion of the block, and you'd still have to change it every so often, plus you would have to re-engineer how the whole belt system works. Not worth the effort.

[DannoXYZ]

Yeah, better done at the factory where they can make it part of the crankcase. Most motorcycle engines uses chains; some of them driven in the middle of cam to reduce torsional flexing. Also look at all the American hot-rod V8s with chain. Relatively low-maintenance compared to our timing-belts.

I've got a MB 190E and it's got a timing-chain, haven't replaced it in over 150k-miles... try that on a 944 timing-belt... heh, heh...

FWIW, on my previous Supra, I forgot to tighten down the cam-sprocket bolts after a belt-change and it spun off about 30-miles down the freeway. Engine died and I coasted to a stop on Hwy-1 along a nice stretch between Santa Barbara and Ventura. "Service Engineering" is a concept Porsche can learn from the Japanese. Someone had forseen this even and has valve-relief cutouts on top of the pistons. So, no banging valves on top of the pistons! Using tools from my bike-toolbox (I was on my way to bike race), I was able to put the cam-sprocket back on, properly tightened this time. Re-time the cams and put the belt back on and we were off in less than 30-minutes! Try THAT on a 944 when your timing-belt falls off...

With that in mind, I'm working with a guy locally to offer these for the 951:



A jig to center the cutting tool on the block with the pistons still inside is being made. You'll be able to do this operation while you're replacing the bent valves from the 1st incident. Figured people who've had catastrophic failure of their engine due to timing-belt problems might be interested in preventing future damage, even if the belts let go again...

[mitsumark]

Actually, the Mitsubishi G54B 2.6 that came in the Conquest/Starion model cars were the very first cars that used the chains and counter balance shaft design..They designed it,licensed it,and then sold it to Porsche..Porsche should of definetly went with a chain system.Especially on an interference motor.Ive built dozens of Starquests and never had any chain failures.I hate these rubber band belts...

[fast924S]

Danno thats a cool idea, make some for the NA too! Someone should at least make a better belt and a better tension setup. Why didnt they install a small hydrolic tensioner like toyota uses, then the belt would always have the proper tension.

There doesnt even have to be a chain kit, just want a kit to make the belt setup better. I bet some one with a Racing company Like Lindsay who makes a ton of stuff could if they wanted to, find a way to make it better

[SoCal Driver]

The later 944's and 968's do have a hydrualic timing belt tensioner. As long as you replace the belt every two years or so and don't over tension it there are no problems.

An improperly tensioned new belt is better than the old worn belt.

[NKN84944]

From talking with some engineering consultants last summer, I learned that there are a few reasons that belts are used. Supposedly it cuts out a lot of vibrations that a chain would transfer from the crankshaft to the camshaft. Seems like Porsche was pretty concerened about eliminating vibrations, and this was a logical thing to do. Chains also create more wear on the gears and rollers. I remember taking apart my friends oldsmobile after his timing chain failed, you guys should have seen the teeth on the gears, pretty much nonexistant.

Now as for improving the belts, I definately think it can be done. I am actually about to get my degree is polymer science and engineering here in a month Probably would have been cool to do this as my senior design project , but I am sure it would be interesting to look at the composite (I belive its some kind of fiber) reinforcements they currently use in belts and find a bullet proof alternative. I mean how many belts would we have breaking if they were kevlar (etc) reinforced. Just my thoughts, and are there any companies out there that already produce high performance belts? Would be interesting to look into.

[SoCal Driver]

You want a tad bit of stretch in the belts. Kevlar doesn't let that happen.

[herdman]

I remember an interview with Dr. Porsche, I think it was in an Excellence mag.. He said they would have used a belt in the 911 back in 1964 when they designed the flat 6 boxer but the tech. wasnt there yet so their only choice was the chain. The chain has had alot of problems through the years.

[Moneyguy1]

Moon:

Contrare.

1. Enclosed, chains are not noticably noiser than a belt. Other noises incurred in an automobile tend to drown each other out!! (exhaust, cooling fans, lifters, etc)

2. Chains are metal and all metals expand when heated. Therefore, the chain would cause less "stress" than a fiber belt which has a expansion rate close to zero. Chains are more reluctant to skip teeth and cannot lose teeth (they have none)

3. Properly engineered, a chain drive should last between 150 and 200k without any trouble (given proper maintenance and abuse kept to a minimum!!). Contrast that with a 30k interval for 944 belt changes at $600 to $800 a pop.(assuming you farm out the work because you do not have access to the P9201 tool) NK..I feel your friend's pain..that is what they get when non metal components "to reduce vibration". However, on older engines I have rebuilt, the only thing I ever found after 125+K was a slight strech in the chain. Everyone's experience may be different...

4. Unlike the 928 which also uses an even longer belt, there is no hydraulic tensioner or warning indicator on the 944.

5. The chains in the 911 series have actiually not been the culprit as much as the fancy adjusters and over engineering that went into them. A bigger problem in the early models was head studs pulling loose. especially on the 2.7 liter iteration.

Like a Porsche certified friend of mine said once "A lousy design, brilliantly executed".

The VR6 engine in the VW uses two chains with an intermediate shaft. I do not know personally of any failures of this design, although they may be out there. As for vibration, It makes one wonder why Porsche built such a large in-line four rather than something a bit less needy of balancing shafts such as a v configuration or a 5 cylinder in-line. The balance shafts introduce a second "serpentine" belt and more seals to leak and bearings to fail, not to mention the added weight...Just wondering..Every additional component that can fail multiplies the number of possible modes of failure.