Thanks to Scott Allard for saving this for me and helping me repost it.
A subject that has been of interest for some time is a distributor curve that is optimum for a given combination. I would like to open up a discussion here on this topic with an old school method that has been successful for me for many years. This is not complicated, and it is not one size fits all. There are many combinations that have a different need. Here are some generalities:
1. A heavy vehicle needs a slower advance curve. Heavy means, "perceived" weight, which varies by displacement and many other factors. But the load dictates the curve. This can be a 3500 lb car with a 2.75 rear gear, or a 4000 lb car with 3.25s. A lighter vehicle allows a quicker, more aggressive curve, again the load perceived is the key. A 3000 lb car with 4.56 gears can lock out the curve to the max, where a 5000 lb truck with the same gears will need the timing broken up.
2. Give the engine what it wants. If your compression ratio is too high, you need better gas. The timing can be retarded, but you can cause more problems doing this because retarded timing raises exhaust gas temperatures. You can crack heads, break ring lands and rings doing this.
3. You can determine the initial timing with a vacuum gauge . BTW, confirm TDC and use the proper timing tape for your balancer diameter, dial backs are OK in a pinch but I have found them to vary. Disconnect the vacuum advance and block the port even if you are using ported vacuum (and you should be). Advance the distributor until you get the highest steadiest vacuum reading and write it down.
3. Most wedge engines like 36°-40° total depending on certain factors like chamber shape, piston speed, effective squish and plug location. In this example I am going to say 36° total is the optimum. You found that you need 18° to get the most vacuum you can get. So, you need to limit the centrifugal timing to 18°. This can be done on a GM or MSD by swapping the centrifugal bushing in the advance system. On a stock Ford you have to weld the slot up that the advance pin is in and grind it to fit, or find a reluctor advance as close as possible to your desired timing. I measure the slot with a caliper, calculate length to timing ratio and size the slot accordingly. Ford arms will have a number with an L suffix on each side, i.e, 10L on one side and a 15L on the other. These numbers mean distributor degrees, see #4. Also, if you flip the reluctor from one slot to the other on a stock Ford, you have to re stab the distributor or your timing will be 180° out because the rotor is engaged to reference the side that represents #1 . The MSD Ford uses the same system as the GM, so the bushing is changed.
4. Keep in mind that the distributor runs at half speed to the crank. The change that you make in the distributor will be half of what you want to see on the balancer The 10L-15L nomenclature above is an example. These will yield 20° and 30° respectively at the balancer. The 10L in our example is too much for 36°, but would yield 38° total. If the engine will accept it, it will work, but if it really needs 36°, you need to fill the slot. Generally, since it would very rare to use the 15L side (30° centrifugal) I weld it in (or I used to, I can't see anymore well enough, so I have someone do it for me) and grind it to fit. The advantage of this is that if you find yourself wanting 38° total, you can pull the dizzy, swap the reluctor arm to the 10L side and you have it. On the GM and MSD style, various bushings are available to fit the need and are quite a bit easier to swap.
5. The rate at which you bring the timing in is important. I like to get it all in as quickly as it will tolerate it, but 2500-3000 rpms will fit most needs. You change this by changing the springs that control the advance rate. Lighter is quicker, and heavier is slower. Weight of the car, gear, converter will dictate this. You do not want detonation. After a change, pull the plugs and look for any shiny specks or uneven burn patterns on the plug. Even if you can't hear a ping, you can still detonate.
6. Vacuum advance. Not necessary on a race engine or one with the timing locked, but mileage and drive ability are important on the street. Hook it up, chock the wheels, and steadily rev the engine up slowly watching the marks on the balancer until it stops advancing. You should not see over 50° or you need to limit the vacuum advance travel. On old Fords you can washer them on the inside of housing the spring outboard of your spacer. Other cans are adjustable with an allen wrench. See your directions with the cannister to see what you need to do because they vary. The rate that it drops out under a light pull is adjustable too. Whether it is the allen wrench or the Ford early style that unscrews you can tailor the advance to drop out under light to moderate load. You don't want ping. Check the plugs again after a drive to look for evidence of detonation.
There you have it. Distributor tuning remedial grade. I hope this helps.
A subject that has been of interest for some time is a distributor curve that is optimum for a given combination. I would like to open up a discussion here on this topic with an old school method that has been successful for me for many years. This is not complicated, and it is not one size fits all. There are many combinations that have a different need. Here are some generalities:
1. A heavy vehicle needs a slower advance curve. Heavy means, "perceived" weight, which varies by displacement and many other factors. But the load dictates the curve. This can be a 3500 lb car with a 2.75 rear gear, or a 4000 lb car with 3.25s. A lighter vehicle allows a quicker, more aggressive curve, again the load perceived is the key. A 3000 lb car with 4.56 gears can lock out the curve to the max, where a 5000 lb truck with the same gears will need the timing broken up.
2. Give the engine what it wants. If your compression ratio is too high, you need better gas. The timing can be retarded, but you can cause more problems doing this because retarded timing raises exhaust gas temperatures. You can crack heads, break ring lands and rings doing this.
3. You can determine the initial timing with a vacuum gauge . BTW, confirm TDC and use the proper timing tape for your balancer diameter, dial backs are OK in a pinch but I have found them to vary. Disconnect the vacuum advance and block the port even if you are using ported vacuum (and you should be). Advance the distributor until you get the highest steadiest vacuum reading and write it down.
3. Most wedge engines like 36°-40° total depending on certain factors like chamber shape, piston speed, effective squish and plug location. In this example I am going to say 36° total is the optimum. You found that you need 18° to get the most vacuum you can get. So, you need to limit the centrifugal timing to 18°. This can be done on a GM or MSD by swapping the centrifugal bushing in the advance system. On a stock Ford you have to weld the slot up that the advance pin is in and grind it to fit, or find a reluctor advance as close as possible to your desired timing. I measure the slot with a caliper, calculate length to timing ratio and size the slot accordingly. Ford arms will have a number with an L suffix on each side, i.e, 10L on one side and a 15L on the other. These numbers mean distributor degrees, see #4. Also, if you flip the reluctor from one slot to the other on a stock Ford, you have to re stab the distributor or your timing will be 180° out because the rotor is engaged to reference the side that represents #1 . The MSD Ford uses the same system as the GM, so the bushing is changed.
4. Keep in mind that the distributor runs at half speed to the crank. The change that you make in the distributor will be half of what you want to see on the balancer The 10L-15L nomenclature above is an example. These will yield 20° and 30° respectively at the balancer. The 10L in our example is too much for 36°, but would yield 38° total. If the engine will accept it, it will work, but if it really needs 36°, you need to fill the slot. Generally, since it would very rare to use the 15L side (30° centrifugal) I weld it in (or I used to, I can't see anymore well enough, so I have someone do it for me) and grind it to fit. The advantage of this is that if you find yourself wanting 38° total, you can pull the dizzy, swap the reluctor arm to the 10L side and you have it. On the GM and MSD style, various bushings are available to fit the need and are quite a bit easier to swap.
5. The rate at which you bring the timing in is important. I like to get it all in as quickly as it will tolerate it, but 2500-3000 rpms will fit most needs. You change this by changing the springs that control the advance rate. Lighter is quicker, and heavier is slower. Weight of the car, gear, converter will dictate this. You do not want detonation. After a change, pull the plugs and look for any shiny specks or uneven burn patterns on the plug. Even if you can't hear a ping, you can still detonate.
6. Vacuum advance. Not necessary on a race engine or one with the timing locked, but mileage and drive ability are important on the street. Hook it up, chock the wheels, and steadily rev the engine up slowly watching the marks on the balancer until it stops advancing. You should not see over 50° or you need to limit the vacuum advance travel. On old Fords you can washer them on the inside of housing the spring outboard of your spacer. Other cans are adjustable with an allen wrench. See your directions with the cannister to see what you need to do because they vary. The rate that it drops out under a light pull is adjustable too. Whether it is the allen wrench or the Ford early style that unscrews you can tailor the advance to drop out under light to moderate load. You don't want ping. Check the plugs again after a drive to look for evidence of detonation.
There you have it. Distributor tuning remedial grade. I hope this helps.
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