I'd like to add another case...

When I bought my '91 C2 - actually a 91 version of the 90 C2 - I had a noticeable ping develop. It either was broke before I bought, or right afterwards. It took me a while to figure it out at the time but for about 3 or 4 weeks, had definite pinging anytime I really pushed the throttle over 3 or 4k RPMs. Being new, the car felt strong, but I was too concerned to really push it with the pinging going on. Finally found the belt problem and had it rebuilt by Rennsport systems in Oregon. After re-bolting in the repaired distributor, there was a NOTICEABLE increase in performance. It might not have been a dyno measurable power increase, but I guarantee you there was a throttle response benefit and a significantly "smoother trip through the rpms."

Let's not close this thread. I don't know how many 964 owners are out there reading this board, but I would be willing to bet that someone out there is completely unaware of their currently broken distributor belt. True - it might not kill your engine 99.99% of the time. True - if the secondary rotor stops at the right spot, some people may never notice a difference. But at the same time a degraded ignition system should be corrected ASAP. This discussion is bringing up some different aspects and observations of distributor belt failures. Who knows, if we keep talking about our own observations, maybe someone out there will say "hey, I have that problem too" and fix something on their own car. Or, we could just send it to arbitration, say "5 out of 6 mechanics agree that it doesn't affect anything at all," and end it right here again. Personally, I'd rather make my own decision about what is happening with my car based on as much information as I can gather instead of taking 83% of the shops word for it even though they have never seen my particular car.

Sort of like Bill, I think this discussion has gotten to me - more specifically - the ending of this discussion.

 

I have only had my 964 for two weeks and it didn't take but a few days for me to realize something wasn't right. Comparing my imperssion to similar symptons I had in past vehicles, the problem felt like an ignition miss or very minor series of flat spots. I noticed the tach needle tended to pause every-so-briefly as it revved, and under agressive throttle, I got a ping or two.

Sure enough, the belt had failed (missing teeth) and now I'm eagerly waiting for Steve@rennsport to ship back the distributor. I'll follow up with driving impressions after it's installed. I dearly hope I'll detect a noticable difference in performance.

FYI, with missing teeth on my belt, I realized it would be possible for the 2nd rotor to be way out-of-time if some of the good teeth on the belt "caught" on the drive gear. However, thinking further, I realized the drive gear of would turn at least 400 rpm when the engine was idle, so with missing teeth, I think it would be hard for the belt to "catch." All of the missing teeth seemed to be along once section of the belt as well. So, I think I was only experiencing 6-plugs. If the other rotor was being driven, it would be way out of time, and an think I would have massive backfiring, stumbling, etc. That has not been the case.

 

OK, I think it's hard to answer how much HP or Torque is lost when the dist. belt breaks. Why? Well, the belt breaking has different symptoms in different cars. I think this thread and the previous one has established this fact.

It seems that it one of Adrian's earlier posts he calculated the HP difference based on the 6 plug Carrera models. Interesting.

Maybe an interesting dyno comparison would be to dyno the 964 with both ignitions functioning, then disconnect the second ignition and dyno it again.

When my dist. belt broke, my mechanic ordered a rebuilt dist. While we were waiting for it to come in, I drove with the second ignition disconnected.

The symptoms of my dist. belt breaking were as follows:

1. One night on the way home from work, my car was missing terribly. It was a herky jerky ride home -- something was definitely wrong.

2. The next morning I was going to bring it to my mechanic, but it wouldn't stay started. It would turn over and promptly die. I had it towed to the shop on a flat bed.

So you see, in my case, driving the car with the broken belt was not an option. At least until I learned the "secret trick" of disconnecting the second ignition.

Hope that helps.

-- Melissa

 

Guys That was my fault. A little finger trouble. I mean't to close the thread but I should have left a message on the bottom to refer you to the new thread that contained all the necessary facts and details of what to do in th situation of a broken belt and how to avoid it. It was not mean't for any other reason but to return the subject back to topic.
The new thread is Tale of Woe No Compression. It currently has 15 replies.
I apologise for the inconvenience,
Ciao,
Adrian
911C4

 

Guys here is my answer posted under the other thread of which I did a very bad job of redirecting you all to,
Ciao,
Adrian

Since this thread blew onto the board I have done a lot of research and sought answers to my questions from many respected quarters. I have learned a lot on this subject.
I must change my blanket comment ( I am also guilty of making blanket comments) of breaking the belt will not cause damage to, "a broken belt will not cause damage in the short term" but if left unrepaired may well cause terminal damage if you ARE REALLY UNLUCKY. I mean by this, that if you detect a broken belt, you do not need to get towed to the nearest repair facility. However you cannot drive across America to get home first either. Let common sense prevail.
Now some basic questions and answers.
Can a broken secondary distributor belt burn a hole in the top of a piston?. Answer: Yes it can.
How?. If the distributor rotor stops inline with a cap contact and stays there and keeps the spark plug firing all the time and the spark plug survives this etc etc.
Is there protection built in for this?: What protection is installed is not adequate for the job if the spark plug keeps firing non stop forever.
Is there a fix for this?. Yes there is, the vent kit and a regular 12,000 mile inspection.
Is this fix adequate?. In the data I have received back, yes it is but a belt replacement is recommended by USA based people at 50,000miles and here in Europe at 60,000miles or every 5 years REGARDLESS. Absolutely no different to the overhead cam belt recommendations that are often ignored as well, especially on 944s and 968s.
How Many Holes in Pistons have been found?. Again from the data I have received back, very few. All reported to myself have been on the race track..
Can the average 964 driver identify the problem quickly?. The answer is no!!!. They feel something may not be right but do nothing about it. THIS IS THE SINGLE BIGGEST MAJOR PROBLEM. IF YOU THINK SOMETHING IS WRONG WITH YOUR 964 GET IT LOOKED AT.
How long can a 964 be driven in the worst case scenario of the rotor and cap contact aligned before terminal damage occurs?. For the average 964 driver who drives on the road estimates vary but the mean average seems to be around 5000 miles. On the race track, maybe the next race. For the rest, somewhere inbetween.
Which 964s are most likely to be affected?.
(1) USA versions.
Why?. Technical Service Bulletins to rectify problems were rarely incorporated. Only people in the know had this work carried out. However any 964 anywhere in the world without the vent kit installed runs a higher risk of having a broken belt and no knowing it and Possibly damaging their engine over time.
(2) 964s which have been modified to improve performance, including installation of hot plugs but do not have the basic TSB improvements carried out. No vent kit "Very high risk" of damage if belt breaks.
(3) Racing versions that retain the secondary belt drive system.
Conclusions:
Porsche made a failure when they designed this system. It took time to show itself. Porsche came up with a solution which works. (They should have done more but this is only my opinion) Vent kit and regular scheduled inspections.
The experts around the world agree that the vent kit does solve the problem for reliability purposes and to manufacturing standards.
So what are we left with.
964s without Vent kits installed.
964s which do not get the full service done properly every time.
964s with a problem but not looked at.
Bascially it is a care and maintenance issue in 2001.
Is it a serious problem?. To me this problem is no more serious than running out of oil, or leaving the cam belt (not on a 964) until it breaks. Purely from a statistical point of view of course. The destruction of any individuals engine is a major serious problem for that person but not necessarily for the whole series.
It really is "very rare" for normal average 964 owners to trash an engine because of this problem.
This is an easily avoidable and preventable problem, not a 964 "nasty Porsche hidden design failure" problem.
Recommendations:
1/. Install Vent Kit and New Belt Right Now, today, make this go away right now. Call Steve Weiner and spend your 150 bucks or install the belt and kit yourself.
2/. If you have the vent kit installed already but have no receipt for a distributor belt, and you have more than 50,000 miles on the clock, call Steve Weiner and spend your 150 bucks or install the belt yourself.
3/. If you have more than 50,000 miles on your secondary distributor belt, REPLACE IT.
4/. Ensure when you have the normal service (and please remember in the USA you guys go 3000 miles more than we do here in Europe)
that the #1 and #2 ignition functional check is carried out correctly.
5/. If you notice or feel something is wrong. CHECK THE BELT FIRST. Pop off the distributor cap and check it.
6/. If you install performance enhancing mods and regularily use the race track for any reason, CHECK THE BELT every 3000 miles and change it every 15,000miles.
7/. If you race a 964 full blown, you are on your own. If the belt breaks you are going to more often than not blow a hole in your piston. This is called BAD LUCK.
Summary:
Care and Maintenance is the key. Look after the 964 and it will look after you. If something is wrong, please fix it. This is not a hidden problem unless you hide it.
Ciao,
Adrian
911C4

 

Thanks Adrian,
Your support of this list and others on the 964 has been a great help to me as I redicover the Joys of 911 ownership. No joke here, I know the cars are special, and with that come special maint efforts & "challenges". With your summary of the belt issue we can all rest easier. If Porsche had just put this in a service notice. we would just know - like any other service item, the belt needs attention. My car had 4 owners - all in the Seattle area - who drove an average of 3500 mile yearly. My guess is though low mileage is attractive ( like new body), lurking in the engine was a ventless distributor that poped the first time the car was driven resonable hard. So I would just add to your list...1)don't wait for 50k if that vent kit is not there - expect a belt failure if your car is io years old as mine was (and I suspect driven lightly in the city in the summers...and 2) the quick check AND emergency get home trick is to pull the Amp connector! That takes ONE minute with no tools. I could not get the distributor caps off till it was out of the car on a bench. ?Am I right?
Thanks again for your support...
Regards, Jack S in Seattle '91 C2 Cab Tip

 

Dear Jack,
Point 1/. in my recommendations covers your first point. No Vent kit do it now. If you replace the distributor with a vented version you will get a new belt.
I also say under "Is the fix adequate" that the belt should be changed every five years.
On the servicing issue. It is actually in the 1994 Official "Porsche" 20,000km or 12,000 miles service. A functional check of the #1 and #2 ignition belts should include a physical check of the belt. One problem is that many 964s are serviced without using the manual and more importantly with knowledge. As I have said, the distributor belt is the same as a cam belt.
It is very hard to say what somebody should do to get home. There are many varying levels of expertise. Your suggestion is a good one. Pull the connector. Of course one could always just pull the centre lead off the secondary distributor and make sure it is not hanging near any metal.
The servicing data is contained in my book. I have made up some "authors recommendations" and added them.
Ciao,
Adrian
911C4

 

what does the vented distributor look like?

 

Vilhem,
Take a look at the dizzy feature on the 'Technical'page of my site for pics etc. www.porsche964.co.uk

John