# '66 389 timing marks



## relmers (Jan 7, 2009)

I have a 1966 GTO, 389, mostly stock. The engine was rebuilt about 20 years ago and they said they put in just a little bit more of a cam than stock (I have no idea what it is, and I don't get any lurching in idle so it is small). 

In high school auto mechanics many years ago, we were taught basic engine tune ups but never really got into the performance. Well, at my older age I'm trying to learn and want to make my car just a little bit snappier. I've been reading about "Total" timing which I never new about, in school we were taught to use what the book said. 

If I'm not mistaken, 6 degrees, BTDC is the mom and pop or book timing for this car. As mentioned before, I want to try and do the total timing somewhere around 32-35 degrees BTDC, or so I think; correct me if I'm wrong. Anyhow, I'm looking at my timing mark (picture attached) and I've always used the bottom mark as my 6 degrees. The tab has been scratched and I'm pretty sure those marks show -4 on the top, 0, in the middle and +6 below, can anyone confirm this?

I have an adjustable timing light, but I've been taught they are not always accurate and not to trust them. My thoughts are to measure the distance between the zero mark and the 6 degree mark and mark the damper 5 additional times using that distance, the 4th and 5th will be 30 and 35 degrees, I think anyway. 

Am I on the right track here? Be easy on me, I know just enough to be dangerous...

Bob


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## BLK69JUDGE (Jun 10, 2010)

somebody has an a/c car ...... 3rd pulley clue
what rear gear are you running ?
still have the super turbine 300 ? 2 speed 

Scott


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## relmers (Jan 7, 2009)

A/C, not yet, getting ready to install it next month, Florida summers to warm not to have it. The rear end, I don't recall what the gears are, it's been years since I figured then out, at which time I pulled the powerglide and installed a 700R4, now I can cruise the Interstate and it doesn't sound like I'm going to throw a rod.


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## Lemans guy (Oct 14, 2014)

Relmers, yes that is 6 degrees Before Top Dead Center. BTDC…

And the -4 would be After top dead center ATDC.

There would be Centrifigal timing inside the distributor with weights and springs that would bring that timing up into the 30’s along with the 6. Of course by now that has changed from wear or changed by somebody and so you need to find out what it is.

I would use the advance timing light, I have found them OK. You can use timing tape which goes on the harmonic balances with the timing marks and then you easily verify your timing light.

The first number that you need to get is the Centrifigal timing number. To get that easily do this procedure.

1. with engine off remove dist cap and rotor and remove one spring on the weights that you see on top of the distributor. Be careful not to lose the spring and wear safety glasses as they sometimes fly out.

2. Now replace the rotor and cap. Get a helper, put on the parking brake and chock the wheels. park or neutral.

3. Hook up your timing light, you can preset the advance it about 30 degrees which isshould Enable you to pick up the mark. Or you can advance it as you go.

4. have you helper start the engine and begins to read the timing. Now have him apply throttle slowly and watch the timing mark on your light. Your advance light will show the mark at zero while the advane is read on the light. So if the light is set at 30 and the mark shows zero, that is 30 degrees.

5. The important thing here is to keep revving up UNTIL THE TIMING MARK STOPS ADVANCING. Note that number. It will stop much earlier than normal because you removed one spring. You will not have to rev it to 5000 RPM. .or more.

6. Now take the number you have and subtract your base number. You had it set at 6 degrees. Let’s say that you wound up with 30, now subtract 6 and you know that that your Centrifigal. Timing is 24 degrees.

7. Now replace the spring. ( Also don’t do this with a conventional HEI distributor. In that case just leave the two springs on and do it.)

That is the number that you must have to start to work on getting a performance curve on your timing.

Good luck and let us know how you do.


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## Lemans guy (Oct 14, 2014)

Sorry I did not mention the basic step of removing and plugging your vacumn advance and knowing your base timing setting when you begin the procedure


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