Top Dead Center...well, nearly

[JWPATE]

Fellows,
I offer this recent experience as a caution to other members on the subject of engine TDC. Most of us take the attitude that one absolute certainty we can rely upon, is the belief that when the Z1 pulley mark is aligned with the case split - We can expect that number one and four pistons are at the full extension of their stroke, about to reverse direction, one starting a power stroke while the other is starting an intake stroke. BUT as in the familar lines from Porgy and Bess.............."it ain't necessarly so".
When I was doing a leakdown check on a 964 engine, the result of which precipitated a rebuild, I was somewhat puzzled when the engine would spin over, each time I put the air pressure to it, even though I was right on the index mark. To finally achieve a pulley position which would allow the cylinder to accept air pressure without spinning over, I was consistantly at a spot on the crank pulley about 1 3/4 inch to the right (early) side of the Z1 index?? What is going on here. It was the same, of course, on every cylinder.
After the testing was complete, I took measurements, through the spark plug hole, of piston distance, and sure enough........it was at its stroke end at the same unmarked spot on the pulley, well to the early side of the index.
The engine had been running fine so I concluded that the valve timing must also be set to this actual TDC spot, and not to the Z1 index. Today, with the engine on the stand, I confirmed that this is the case. Sooo, all suspicion points to the pulley being positioned on the crank wrong. Never seen that before in my limited experience, and how would it be possible? The 964 pulley has a single locating pin and relies upon the friction/clamped fit of the tapered joint. Not easy to screw that up unless the locating pin were completely missing... It was not.
Sooo... Only one possibility remained on my list. That 964 (not turbo) pulley is a combination three-belt pulley integrated with a rubber vibration damper. The sections are fastened by ten allen bolts and I found that somewhere, sometime, they had been removed and replaced with the holes shifted by one number. KILROY WAS HERE.
The 964 technical data book cautions - "Do not disturb Allen bolts visible through recessed front of pulley". Perhaps this kind of result was one reason for the caution. It seems an easy trap for the well intentioned guy, attempting to get the pulleys off without the holding (9290) tool. Except for the leakdown test, I wonder whether I would have caught the error. Perhaps it would have been noticed when the pistons were visible after the heads were off.........and perhaps not. Would have made for some interesting valve timing don't you think?
Regards,
James

[goldgunner]

If ever I work on a 964 engine and timing that will be very important and useful info - thanks James~!

[snowman]

Always use a piston stop to determine tdc. A spark plug is removed and a substitute spark plug with a slug that will stop the piston is inserted. The engine is rotated both ways to the hard stop provided by the spark plug stop. TDC is exactly halfway between the forward and reverse stops as indicated by a degree wheel. There is NO substitute. TDC can be up to 5 degrees off from the factory!, On a Porsche its more likely to be less than 3 degrees, but 3 degrees is a LOT.

Both cam timing and spark timing should be set by the ACTUAL TDC, not the indicated. Z1, TDC.

Note. The engine may seem to run fine, with the timing advanced or retarded by 5 or more degrees. Only a dyno will tell the truth as to which is better. I would go with what the factory recommended. FInd the real TDC and set it from there.

[JWPATE]

OK Snowman,
Thank you for responding. No, I do not believe that you comprehend the issue here.
James

[911pcars]

Good advice with the piston stop. I noticed that Stomski Racing sells an extended version of a piston stop due to the depth of the sp hole.

Sherwood

[JWPATE]

Gentlemen,
Calm down please. I have already located and corrected the the TDC issue. The story on this thread was only offered as an effort to help other members (964 owners).
In some ways, I wonder if it is still possible to offer simple advice on this forum, without starting an unwanted fuss, which I, for one,, am just too old for.
As there is no interest in such nonsense from old James, for the time being, consider me off the line, though I will still watch the other listings.
Best regards,
James

[911pcars]

"Most of us take the attitude that one absolute certainty we can rely upon, is the belief that when the Z1 pulley mark is aligned with the case split.......... "

No fuss. It's up to the individual to decide how much to assume and how much to check. But they should also understand the potential areas of error and/or tolerance stack. For example, if you hadn't noticed, you wouldn't have probed deeper into the situation. Good observation from you. However, not every learning experience is a result of chance. If snowman was around when you put it together, he probably would have mentioned it and saved you some head-scratching time.

And if other DIYers read this, perhaps they too won't assume all is well and instead double-check the variables before any installation procedure.

Sherwood

[snowman]

The TDC issue is basic to all engines. Everyone assumes that the indicated TDC is the true TDC, NOT SO. One of the most fundamental engine checks is to confirm and if necessary remark the TRUE TDC. Everything is based on this mark. If this mark is off you will set up everything else wrong. Its sort of engine 101 class. Any true aficionado will check this first, everything else second. Engine timing is generally pretty forgiving, plus or minus 3 to 6 degrees and most will not notice. Cam timing is significantly more sensitive than ignition timing, but if you want max performance, you MUST be right on.

An example of cam timing is as follows: The 7805 cam from Elgin for the 912 engine. Only 2 degrees off this cam will produce a major suckout at 4000 rpm, with the cam right on, it will produce more power than thought possible from a 912, no suckouts anywhere. This suckout can even be confirmed with engine simulation software. Just 2 degrees!!!! MOst engines will just degrade slightly, or improve slightly, but you never know.

[JWPATE]

Yes Snowman, you are correct. Also Sherwood, you are correct.
Without also climbing on a soapbox, TDC is basic to any engine adjustment, be it Bentley or Opal, Merc or VW. Thanks, for your knowledge and support make this site worth visiting.

Forgive me please, for going out of bounds.

You have pointed out also that even a correctly set up engine can be off the TDC index 1 or maybe even 2 degrees. Never knew it, but I accept your findings, and in the world of racing - or high performance this fact is certainly an issue! Thanks.

The subject of my original explination of this particular thread though, has no focus on the world of racing engines, nor any attempt to squeeze out that last power of horses.

No, I was only attempting to caution the very much larger world of 964 owners that there is a very real possibility of wide TDC issues on those cars, made possible by the "do not touch" torsional vibration dampers built into the cranc pulley.

In this case, NO, we are not concerned here with 1 degree. Nor with 2, and not even with 5 or 10. I am here to warn about real possibility of break-down problems for the owners of these cars, should they use official manuals to guide them through, say, a distributer service. How about anything from 30 to 180 degrees from where the manual instructs? Line it up according to the manual you paid big funds for, and no start. New parts and adjustmenmt. No start. There is NOTHING to prevent or warn you of it. Just Be cautious. In this case, or any, yes go back to basics, and find the true DDC for your engine. No fancy tools needed - just where is that piston? A plastic straw will do.

Of course "true aficianodas" (I believe that was the expression used), will know such things already), Never mind that, If you ESPECIALLY have a model with the ring of
Allen bolts around the front pulley drives............check it out. There could be problems there for ordinary owners.

And don't forget......You never again can be just some new owner, No you are in it too , so don't be shy ............Maybe that poor guy by the roadside still wants to know where his hood opening release is hidden, or maybe where the spare tools are.. Give him a hand. Looks like he already has a cell phone? OK, stop anyway, he/she is one of us. This is not a stopwatch.....oh no,,,,,,in time - you too will be tne great Snowman or any of the other experts.

Welcome aboard.
Nothing ever lasts................forever ! well maybe love does
James

[Henry Schmidt]

Originally posted by JWPATE
..........................In this case, NO, we are not concerned here with 1 degree. Nor with 2, and not even with 5 or 10. I am here to warn about real possibility of break-down problems for the owners of these cars, should they use official manuals to guide them through, say, a distributer service. How about anything from 30 to 180 degrees from where the manual instructs? Line it up according to the manual you paid big funds for, and no start. New parts and adjustmenmt. No start. There is NOTHING to prevent or warn you of it. Just Be cautious. In this case, or any, yes go back to basics, and find the true DDC for your engine. No fancy tools needed - just where is that piston? A plastic straw will do. ...............

James, good for you. Very well put.

[philippeF]

I've just check that on my 2.7RS stock engine on piston 1...
with a dial indicator firmly attached to the case, the result is the Z1 mark on pulley is within 1 degree of real Piston1 TDC...

maybe a nice hazardous situation......

cheers Philippe