School me on timing - Vintage Mustang Forums
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post #1 of 9 (permalink) Old 08-03-2018, 07:59 AM Thread Starter
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School me on timing

I've set my initial timing at 12 degrees advance, but the factory specs are 6. I know setting the advance higher "wakes the car up" a little, but I've also heard people setting it as high as 15. I don't really understand the science to setting the advance that much higher. It would seem to me if the ignition happens when the piston is closer to top dead center and the compression is at it's highest point, the resulting explosion would be stronger at six degrees vs fifteen. And if the setting can vary so much between six and fifteen degrees, why use a timing light? Why not keep advancing until you hear pinging at high RPMs and then back it off? One last question. Is it better to advance or retard the timing if the engine is running a little hot?
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post #2 of 9 (permalink) Old 08-03-2018, 08:20 AM
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Flame front propagation. It takes time to burn the mixture. As far as how much depends on variables such as compression and chamber design. A poor chamber design will delay the burn rate so you’ll need more timing to start the flame front earlier. The more advance you have, the more pumping losses you have as the engine has to fight the early burn pressure build up. The sbf seems to like about 38* total mechanical timing. On my GT40P the heads are designed to be very efficient and need very little timing, 30* tops. My compression rated by Ford is 9.5:1 and runs great on 87 octane with stock cam that builds pressure early. Recently someone commented they were running a higher compression on their GT40P with milled heads on 87 octane with no issue and it wouldn’t ping or knock with more then 30* timing, only loose power.
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post #3 of 9 (permalink) Old 08-03-2018, 08:41 AM
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Okay, there is a lot of reading to do on this topic, but also a lot of misinformation on the web. The short version is that fuel takes time to burn, think of a fire burning something up more so than an explosion. To burn all the fuel completely is the goal, and that’s the way that your engine will run most effectively and efficiently. So, given that the fire takes a little while to burn, you have to ignite it with some time to spare before the piston is at the top of the travel. This is “advance”. As the engine speed increases, the time the piston takes to complete the travel is less, so the ignition has to happen sooner, that’s where the mechanical advance of the distributor comes in. The centrifugal force of the faster engine speed moves a set of weights outward and results in the spark firing sooner. At full speed, stock distributors will advance to almost 40* BTDC. This comes from the base timing (what you set with the timing light) plus the distributor’s built in advance potential. You have to set the base timing with regard to the total timing, as it is affected directly by it. The reason you hear about people increasing the base timing today is that modern gasoline is very different than it was in the 60s, and it takes longer to burn than the old stuff. So the old specs just don’t work for peak performance anymore. Most guys set initial timing around 12* BTDC on their small-block Fords, that seems to be the sweet spot for today’s pump gas. Cars will generally idle better with even more advance than that, but if you go too high, the total timing at high RPMs will be too much, and the engine may experience knocking. This can be played with by using the vacuum advance unit, but that’s a whole other topic.

Also, in general terms, retarded timing equals running hotter, more advanced will run cooler. Has to do with when the combustion is happening relative to the stroke. Really late timing will make your exhaust manifolds turn red hot—not good.
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post #4 of 9 (permalink) Old 08-03-2018, 09:02 AM
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[QUOTE=gjallen3;10001336]School me on timingQUOTE]


Timing is everything...

Working on old cars teaches us patience... and every curse word imaginable!
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post #5 of 9 (permalink) Old 08-03-2018, 09:16 AM
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Quote:
Originally Posted by gt350sr View Post
Timing is everything...
It is...

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post #6 of 9 (permalink) Old 08-03-2018, 09:32 AM
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Things like the heads combustion chamber, cam timing, and fuel can all affect the amount of timing and engine will like.
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post #7 of 9 (permalink) Old 08-03-2018, 10:38 AM
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It is nice to use a distributor that allows the amount of mechanical advance to be adjusted so that initial and total advance are not locked in step. The springs holding the mechanical advance can be changed for different points of total advance. A common issue is when the springs become loose, something to check. Usually 28* is the best point of total advance being reached.

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post #8 of 9 (permalink) Old 08-03-2018, 06:03 PM Thread Starter
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I am impressed by how much knowledge is in this forum The mystery is solved for me and it all makes sense. Thanks so much

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post #9 of 9 (permalink) Old 08-03-2018, 06:24 PM
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Examples given of modern heads not needing much total advance are correct. Conversely, if a person is using the stock iron heads, then you are going to need a lot more total advave . Stock 289 HiPo advance is 40 degrees BTDC. Try running 28 total on that engine and you will see a major loss of power.

Stock A Code 289 engines are going to need 36 degrees BTDC.


So no one specification is going to be right for every engine, but as a general rule of thumb, if you have 1960 era cylinder heads, then you'll need more timing. With more modern heads, you'll need less. With either one, initial timing. can be 12 - 18 degrees depending on the engine. On my HiPo and Shelby's I used 18 initial and 40 total with Weber carbs. With a vintage Paxton, I had to reduce total to 38.

Note: stock FoMoCo distributors do have provisions for changing the amount of mechical advance. There are two slots to choose from, and using the narrow slot will let you run more initial timing and still not have too much total timing.

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