Timing Explanation

[Cool67]

I recently converted an Autolite 2100 to an Edlebrock 1406 + Performer intake on a stock 289. I couldn’t get the timing correct initially because the idle was running too fast, even with the idle screw all the way out. But the car ran great. It was a lot more responsive than with the 2V. The tires even screeched a little when the auto trans shifted from first to second.

When I finally figured out why the idle was running too fast, corrected it and set the timing to 6 degrees (and also tried 12 degrees) BTDC, the car ran the same as with the 2V (which is what I originally expected). No more screeching tires. I believe that when the timing was incorrect, it was more retarded because the vacuum hose on the distributor was pointing more towards the left (8 o’clock). Or, I guess it could've been off by one tooth. Right now, it’s pointing almost straight (6 o’clock) towards the front of the car.

Does anyone know why the car ran better when the ignition timing was off ?

I’d like to get responsiveness back but am not sure what the timing was and if it was OK to run it that way.

What should I set the timing to for the best low end response?

I have a stock 289 (67 mustang, auto trans), a stock distributor with Petronix electronic points.

Any explanation and/or tips would be greatly appreciated.

 

[rbohm]

small block fords usually run best when the timing is set around 10-14 btdc giving a total timing of 36-40 degrees. the other thing you need todo is make sure the vacuum advance line is disconected and plugged before you set your timing.

 

[10secgoal]

rbohm is right. Although I like more inital timing. Really livens a car up. If you can get it to start that is. It was night and day difference for my car with 18 as opposed to 14. If you move it up, be sure to remove total timing from the vaccum advance.(small allen screw in the vaccum port will adjust that.)

 

[Cool67]

small block fords usually run best when the timing is set around 10-14 btdc giving a total timing of 36-40 degrees. the other thing you need todo is make sure the vacuum advance line is disconected and plugged before you set your timing.

Yes I did plug the vacuum line to the distributor and also the carbuerator side. Thanks for the info.

 

[Cool67]

rbohm is right. Although I like more inital timing. Really livens a car up. If you can get it to start that is. It was night and day difference for my car with 18 as opposed to 14. If you move it up, be sure to remove total timing from the vaccum advance.(small allen screw in the vaccum port will adjust that.)

Can you explain more about this: why would I want to remove the total vacuum advance and how exactly do I do this? Thanks for the info.

 

[67CoupeDriva]

Initial timing + vacuum advance = total advance. Too much total advance = knock. So if you add one, you may need to subtract from the other (or run higher octane gas).
SBF do like slightly advanced (initial) timing over the way they came from the factory. Better throttle resonse, pep, and a little better mileage will result, but you have to be careful not to go too far.
So to have your cake and eat it too, adjust the initial timing and back out some of the total (vacuum advance). Not all cars will have that adjustment, if not you have to play around with the springs and weights inside the distributor.

 

[Cool67]

Initial timing + vacuum advance = total advance. Too much total advance = knock. So if you add one, you may need to subtract from the other (or run higher octane gas).
SBF do like slightly advanced (initial) timing over the way they came from the factory. Better throttle resonse, pep, and a little better mileage will result, but you have to be careful not to go too far.
So to have your cake and eat it too, adjust the initial timing and back out some of the total (vacuum advance). Not all cars will have that adjustment, if not you have to play around with the springs and weights inside the distributor.

OK. Thanks for the additional info.

 

[SoCalCruising]

Your distributor probably has initial, vacuum and mechanical advance. Both vacuum and mechanical advance amounts may need to be adjusted if you advance initial timing too far.

 

[67CoupeDriva]

Your distributor probably has initial, vacuum and mechanical advance. Both vacuum and mechanical advance amounts may need to be adjusted if you advance initial timing too far.

You're right, of course. I got hung up on vacuum advance from reading the earlier posts.

 

[Cool67]

Your distributor probably has initial, vacuum and mechanical advance. Both vacuum and mechanical advance amounts may need to be adjusted if you advance initial timing too far.

Thanks for the clarification. I know how to set the initial advance (~12degrees @ ~550rpm) w/ the vacuum to the distributor plugged, and I believe that the mechanical advance is adjusted by springs, but I don't know how to adjust the vacuum advance on a stock distributor. What should the vacuum and mechanical advance be set to and how do you measure them?

I still haven't seen any replies on why the car ran better when the timing was off. Is it more likely that I had a lot of advance timing rather than retarded timing?

Is it easier to set the correct timing with one of the newer aftermarket distributors like the Petronix brand?

 

[ddonaca351]

Rather than 12, you might try 15-16... and also just for an experiment, bump the idle speed up about 100 rpm. 550 seems reeeaaalllll low to me. Even my 5.0 was 660 from the factory, and my 70 was about 700 with the stock cam.

Yes it is likely that you had the timing advanced, not retarted. Retarted timing runs lazy, and can also cause overheating problems.

DD

 

[67CoupeDriva]

I still haven't seen any replies on why the car ran better when the timing was off. Is it more likely that I had a lot of advance timing rather than retarded timing?

Is it easier to set the correct timing with one of the newer aftermarket distributors like the Petronix brand?

The reason timing is important is, at the risk of sounding flippant, all about the timing. The car runs better when it is right because the mixture is burning at the point that it exerts the most efficient pressure on the piston. When it's too early, it can be burning when it should be pulling more mixture into the cylinder. When it's too late you can be losing compression because the exhaust valve is opening already. Optimal timing will therfore vary from engine to engine depending on the specifics of cam, degree, etc.

Is Pertronix offering an entire distributor now? I know I installed the Pertronix I module. It won't make a difference in setting the timing (you still do that by rotating the distributor) but it will remove the variable introduced by the point dwell setting (and subsequent adjustment required due to wear). You also won't run into points bounce which can cause erratic timing.
If they are offering an entire distributor, setting the initial timing will probably still be the same, but there may be some easier adjustments for vacuum and manual advance.

 

[Cool67]

Rather than 12, you might try 15-16... and also just for an experiment, bump the idle speed up about 100 rpm. 550 seems reeeaaalllll low to me. Even my 5.0 was 660 from the factory, and my 70 was about 700 with the stock cam.

Yes it is likely that you had the timing advanced, not retarted. Retarted timing runs lazy, and can also cause overheating problems.

DD

Thanks for the info.

 

[Cool67]

The reason timing is important is, at the risk of sounding flippant, all about the timing. The car runs better when it is right because the mixture is burning at the point that it exerts the most efficient pressure on the piston. When it's too early, it can be burning when it should be pulling more mixture into the cylinder. When it's too late you can be losing compression because the exhaust valve is opening already. Optimal timing will therfore vary from engine to engine depending on the specifics of cam, degree, etc.

Is Pertronix offering an entire distributor now? I know I installed the Pertronix I module. It won't make a difference in setting the timing (you still do that by rotating the distributor) but it will remove the variable introduced by the point dwell setting (and subsequent adjustment required due to wear). You also won't run into points bounce which can cause erratic timing.
If they are offering an entire distributor, setting the initial timing will probably still be the same, but there may be some easier adjustments for vacuum and manual advance.

Thanks for the additional explanation. And yes, Pertronix does offer a complete distributor. http://www.pertronix.com/

 

[ddonaca351]

To add to the above.... as the charge burns inside the chamber, the pressure increases astronomically as the flame front moves across the top of the piston.

This is the actual force that makes the power... the pressure pushes the piston down, which turns the crank, etc, and so-on.

Now it takes time (although its thousanths of a second) for the flame to burn across, and this varies with chamber design, mixture, etc...
This is why you change the timing for optimum power, you want the highest cylinder pressure, juuuuusstt as the piston is crossing TDC and starting down. That way you get the longest push possible on the crank (more power).

EDIT: Too much advance, peak pressure is too soon, the piston has to push up through that pressure to reach TDC, too little advance and the piston is already headed down, and you loose that high pressure push.

With new style chambers, most good aftermarket heads have a very fast burn, so they make peak power with 5-6 degrees less timing than the old iron heads did. The lower compression you have, also the more timing you'll need, cuz the space in there is bigger and takes longer to burn...
Unfortunately low comp also usually leads to poor turbulance, and hot-spots, which can cause spark knock, causing you to back the timing off, and loosing power again. (catch 22)

Clear as mud, right...

DD

 

[Cool67]

To add to the above.... as the charge burns inside the chamber, the pressure increases astronomically as the flame front moves across the top of the piston.

This is the actual force that makes the power... the pressure pushes the piston down, which turns the crank, etc, and so-on.

Now it takes time (although its thousanths of a second) for the flame to burn across, and this varies with chamber design, mixture, etc...
This is why you change the timing for optimum power, you want the highest cylinder pressure, juuuuusstt as the piston is crossing TDC and starting down. That way you get the longest push possible on the crank (more power).

EDIT: Too much advance, peak pressure is too soon, the piston has to push up through that pressure to reach TDC, too little advance and the piston is already headed down, and you loose that high pressure push.

With new style chambers, most good aftermarket heads have a very fast burn, so they make peak power with 5-6 degrees less timing than the old iron heads did. The lower compression you have, also the more timing you'll need, cuz the space in there is bigger and takes longer to burn...
Unfortunately low comp also usually leads to poor turbulance, and hot-spots, which can cause spark knock, causing you to back the timing off, and loosing power again. (catch 22)

Clear as mud, right...

DD

Thanks for the additional info. I'll keep playing with the timing. Hopefully, I'll find a setting that gives me the more "peppy" response that I enjoyed for a brief moment.