Finding TDC

[zimmy]

Here's a question for you engine guys. Does anyone know an accurate and easy method for determining the exact TDC of an engine when it is fully assembled in the car. I'm guessing that the only way is to determine the travel through the #1 spark plug hole. Any suggestions and gadget help would be appreciated.

Here's the reason for the question. Inglese has been telling me to increase my timing for the Webers. They said that they need as much as the engine can stand. I was told the best way to determine this is to increase the timing a bit at a time until the car is fighting the starter. Then back it off a degree or two. Well according to my timing marks on the balancer, I am at 30 Degrees initial and it starts easily and quickly!!! The car actually runs better and I don't think there is any pinging with 18 degrees of mechanical (48 degrees total!!). This can't be right. My guess is that the timing marks are wrong. The other thing that I haven't checked yet is the fact that I am timing it at about 900 RPM. I have the weakest MSD springs in the MSD distributor. I am going to check to see if I am actually getting a bit of mechanical thrown in at that rpm. I'll try it down around 750 or so.

Any thoughts???

Zimmy

[SCOBRAC]

Zimmy,

You must have a bent pointer or something got twisted on installation.. Not that they would be the same, but I get pinging at about 2200 rpm to about 2600 rpm using the light springs when I run 12 degrees of initial with 26 degrees of mechanical advance.. I use two blue springs currently with the smallest mechanical stop (26 degrees). I could likely use more initial but it seems to work best overall.. No pinging but I'm all in at 2800..

If it isn't pinging, get it set to the best "feel" and check it again using a reliable advance timing light.. see if the light at whatever rpm and number of degree points in your curve you check match what you expect the distributer to based on the curves provided by MSD.. They should..

Another thing to consider.. Are you sure, 100% sure you are timing off # 1 cylinder? I know this sounds stupid but with the billet distributer you can easily think you are using #1 based on the location of the #1 terminal end (typical ford #1 spot) and be taking your readings off another terminal.. better to take off the plug end of the wire if there is any question..

Otherwise.. I might be inclined to pull #1 plug and watch it cycle, be sure you are at TDC of the compression stroke and look at your damper.. you will be (should be) about 5 degrees BTDC on the marks..

Regards,

[Redd Snake]

I'm interested in validating the pointer setting on my FE.

What's the best way to watch the cycle on cyl #1? A dial indicator with an extension into the #1 cylinder?

Michael, (newbie question) why should you be 5 deg BTDC on the marks while at TDC of the compression stroke? Shouldn't the marks read 0 deg at TDC?

[Bruce Botti]

A dial indicator fixtured into the #1 cylinder is the best way to determine TDC. It helps to be good at Origami!

If you have the crank pulley that is set in rubber, you might want to check to make sure that it has not moved. This caused me untold grief on an FE until I found the problem. It can also be dangerous as it could separate and cause great damage under the hood.

Take care,
Bruce

[zimmy]

Thanks to everyone that has offered help. I have confirmed that I am definitely using #1 cylinder. I have also checked that the timing marks are correct at TDC (within a degree or so) and that the damper appears to be okay and also correctly set for TDC.

When I checl the timing marks against what the MSD curve should be it is right on the money. Begins to climb right about 1000 rpm and increases 18 degrees as expected. Maybe the PowerMaster high torque starter is changing the rules. Maybe this starter makes it hard to tell when the advance is starting to hit against the starter. Who knows? I'm still confused so I put my initial back to 20 degrees with a total of 38 degrees for safety.

ZImmy

[SCOBRAC]

man that's a lot of initial timing.. but if it isn't pinging.. what is the compression ratio? I cannot get that much and I am about 10:1 or just over that.. maybe 10.3.. and I get it kicking back at 15-16.. and then it pings at mid throttle under load.

A good "dial indicator" is a 1/4 inch wooden dowl carefully inserted into the chamber and cranked by hand to determine TDC. I said five degrees as an example. Within a few degrees plus or minus might have been a better example.. Zero is ideal, but it almost never happens that way.

Regards,

[zimmy]

I agree that this sounds like an amazing amount of initial, but there is no pinging or starting problems. I think that my engine is about 9.5:1 compression. I am also using the individual runner custom manifold for the 48 IDA webers from Inglese. I also have a custoim grind cam designed for the manifold and webers. Maybe all of these things considered allows the higher timing advance. The car does seem to run a lot stronger and cleaner with this advance.

Any other feedback is appreciated.

Thanks all.

Zimmy

[SCOBRAC]

What did you finally settle on.. 20 inital and how much over all? 40, 42? Those are numbers you might see on a 429/460.. Interesting.. Motors always run strongest prior to detonation(pinging) if it isn't detonating you will be fine..

[68gt500]

The easy and accurate way of finding TDC is thread a bolt into the plug hole. Turn the engine (don´t use the starter for this !!!) until the piston contacts the bolt. Mark the damper. Then turn the engine the other way arround until the piston hits again. mark the damper again.

The precise TDC is halfway in between the marks.

Mike (Germany)
68 GT500
(not a Cobra but the same heart)

[Clois Harlan]

There is no shame in owning a GT 500!!

I wish I had one, great car.