How to determine #1 TDC w/o distributor
 

Let's say that you took out your distributor to replace the seal, but you forgot where it was pointed.

I can line the pulley up to the Z1, but don't know whether it's TDC for cyl #1 or #4.

How can I tell if it's #1 or #4?

Pull off the valve covers over #1 valves and see if they are closed.

I would just try to start it. If it doesn't start, you are on #4.

I put two long plastic drinking straws together and took out the #1 plug. You will be able to touch the top of the piston. Turn the crank by hand, if the straw does NOT go all the way in, your at TDC, check your timing marks. Of course you dont want to drop the straw into the engine, thats why I made it longer than it needs to be.

my .02

The simpler the better, and the straw method looks doable as is feeling lucky (cyl. 1 or 4). Another easy method involves using a compression tester hose, the two-piece, separate from the gauge-type.

Remove no. 1 spark plug and attach the hose. Rotate engine manually or with remote starter switch until you feel air pressure from the hose. This signals no. 1 cylinder is on it's compression stroke. Continue to rotate the crank pulley until the TDC mark lines up with the split in the crankcase.

Sherwood

I know what you're thinking: "Cylinder 1 or cylinder 4?" Well, to tell you the truth in all this excitement, I've kinda lost track myself. But, being this is a 911 Porsche, the most awesome sports car in the world, you've got to ask yourself one question: "Do I feel lucky?" Well, do ya punk?

When the pulley is at the TDC mark, both #1 and #4 pistons are at TDC so it won't help with your problem.
When at TDC for #1, both valves will be closed. If you don't want to take off the valve covers you could see if the cylinder will hold air using a leakdown tester.
-Chris

Thanks Chris.

Correct me if I am wrong here, at #1 TDC both intake and exhaust are closed because the cylinder is firing. At the same time, #4 should have its intake open a bit at the TDC intake overlap value and #4 is at the start of the intake cycle right?

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anthony

I love that bit!

I of course thought I could be lucky and got it wrong. When I tried to start the car, I got the biggest backfire of all time. Everybody in my neighborhood came running out into their yards looking for terrorists. I am surprised the muffler survived. They must be made of tough stuff.

#4 is not an issue.
If the dizzy is out , when he puts it back in he will line it up on #1( the mark on the lip of the dizzy or pointing to #1 plug). It takes two revolutions at the crank to get one at the dizzy. If you dont have a compression tester you can try it my way.

If the straw doesnt go in as far as it did (and you are at the Z1 mark)than you know you are at comp on #1.

He can then static time it, to find out when the points open. If i'm not being clear let me know

Piston #1 will be at the top twice - once when the distributor should be pointing at #1 and again when the distributor should be pointing at #4. In other words, the straw test will only tell you if the TDC mark on the pulley is in the correct place.

Assuming that the TDC mark on the pulley is correct (the car ran before), we already know the position of the piston. Physically verifying the piston position (with a straw) will not give you any additional information. What you don't know is the phase of the cams i.e. if the valves are both closed on #1.

DarryID's animated 911 engine may help. See Here's something to stare at for a while...

http://members.aol.com/darryld/930.gif



-Chris

Since the poster said that he had taken the dizzy out to fix the seal I assumed that the car was running before he fixed the dizzy

I'm surprised you got a backfire. I've used the lucky method many times and never had an issue. In fact, many newer cars fire opposing plugs off of one coil pack, one during the power stroke and one during the purge stroke.

bt1211: What Chris is trying to explain is that piston 1 and 4 are always in the same relative position within the cylinder. When 1 is at TDC, so is 4. The only difference are the valve positions. The straw method only tells you piston position, not valve position. Hope this helps.