Distributor Recurving?
The simple things about distributor curve
You have initial timing, what you set with a light at idle
You have mechanical advance
You have vacuum advance
You add them all up for TOTAL timing, IE
10* + 23* = 34 initial and mechanical
plus
say 15* vacuum at highway cruise, light throttle = 49 total
Many talk of total and leave off the vacuum because they do not run vacuum, and the initial + mechanical is the "foot on the floor" figure, because vacuum is not "in" with heavy throttle
CURVE is this:
The springs --- how heavy as well as the "slack" (some springs) that allow some movement against the heavy spring, I.E. sometimes a light spring that allows lower RPM movement, up against the "slack" in the heavy spring, which requires a lot more RPM to move the heavy spring.
The above is illustrated better at the webpage posted below. Go towards the bottom of the page, the photo of the various springs. NOTE the slightly longer, heavier springs. These are what I was referring to, allow the weights to move "so far" in the "slack" of the longer heavier springs
http://users.erols.com/mathewg/Dist.html
(The above would be a typical stock application)
So "recurve" springs are lighter, and often one OR BOTH springs are left out. Back in the "points" days, some of us used to run with NO springs. The friction of the points drags the weights to retard for starting, and as soon as the engine starts, you have full mechanical advance.
Springs, then, affect the RATE of the mechanical advance Several race parts outfits, Summit, Jegs, Mopar Performance, etc, sell "spring kits" with a selection of springs. You just have to fiddle with them until you get what you want.
SLOTS. The LENGTH of the slot determines the AMOUNT OF DEGREES, which for a performance / street dist. is usually between 18-22* or so. Factory ones -- especially smoggers -- can be much much LONGER, so if you try to run a stock smog dist. at more INITIAL timing, you may end up OVER advancing at the top end, because there is simply too much advance in the distributor
So you have to braze up the ENDS of the slots and file them back to what you want (trial and error) to get the AMOUNT OF DEGREES you want. This is made much easier with the use of a distributor machine.
ALSO be aware that some people talk of DISTRIBUTOR degrees while others talk of CRANK degrees. Crank degrees are double dist. degrees. Here for example is a chart I found "at that other place" and this shows slot length versus DIST degrees. So for say, 20* at the crank you will want to use the 10* figure in the chart
Modifying advance slots
degrees / slot size
6.............. .340
7................355
8................375
9................390
10...............405
11.5 ...........420
12...............435
13...............445
14...............460
15...............475
16...............490
17...............505
18...............520
Here is a good example of a factory setup, 67 426 Hemi, right out of the factory manual. NOTE that these figures are in DISTRIBUTOR degrees
If you take the left column, go down towards the bottom, TIMING says 12.5BTC, so this is your INITIAL IDLE setting
Now up towards the top see "Advance--Centrifugal (Distributor) it says "7.5 to 9.5 @ 1400 RPM. THIS MEANS you get 19* AT THE CRANK (9.5 x 2).
(You can also see in this chart how RPM affects the mechanical advance, at different RPMs the SPRINGS are controlling this function)
Go down further and see Advance -- Vacuum (Distributor...shows a max. of 11*, this again is 11 X 2 for 22 CRANK degrees
So you add 12.5 and 19 (mechanical) so with your foot on the floor, the advance will be 31.5*, and I'm sure many of these could have used a little more initial back then
And then for cruise, you add the vacuum of 22* for 43.5* total.
see? easy.
The simple things about distributor curve
You have initial timing, what you set with a light at idle
You have mechanical advance
You have vacuum advance
You add them all up for TOTAL timing, IE
10* + 23* = 34 initial and mechanical
plus
say 15* vacuum at highway cruise, light throttle = 49 total
Many talk of total and leave off the vacuum because they do not run vacuum, and the initial + mechanical is the "foot on the floor" figure, because vacuum is not "in" with heavy throttle
CURVE is this:
The springs --- how heavy as well as the "slack" (some springs) that allow some movement against the heavy spring, I.E. sometimes a light spring that allows lower RPM movement, up against the "slack" in the heavy spring, which requires a lot more RPM to move the heavy spring.
The above is illustrated better at the webpage posted below. Go towards the bottom of the page, the photo of the various springs. NOTE the slightly longer, heavier springs. These are what I was referring to, allow the weights to move "so far" in the "slack" of the longer heavier springs
http://users.erols.com/mathewg/Dist.html
(The above would be a typical stock application)
So "recurve" springs are lighter, and often one OR BOTH springs are left out. Back in the "points" days, some of us used to run with NO springs. The friction of the points drags the weights to retard for starting, and as soon as the engine starts, you have full mechanical advance.
Springs, then, affect the RATE of the mechanical advance Several race parts outfits, Summit, Jegs, Mopar Performance, etc, sell "spring kits" with a selection of springs. You just have to fiddle with them until you get what you want.
SLOTS. The LENGTH of the slot determines the AMOUNT OF DEGREES, which for a performance / street dist. is usually between 18-22* or so. Factory ones -- especially smoggers -- can be much much LONGER, so if you try to run a stock smog dist. at more INITIAL timing, you may end up OVER advancing at the top end, because there is simply too much advance in the distributor
So you have to braze up the ENDS of the slots and file them back to what you want (trial and error) to get the AMOUNT OF DEGREES you want. This is made much easier with the use of a distributor machine.
ALSO be aware that some people talk of DISTRIBUTOR degrees while others talk of CRANK degrees. Crank degrees are double dist. degrees. Here for example is a chart I found "at that other place" and this shows slot length versus DIST degrees. So for say, 20* at the crank you will want to use the 10* figure in the chart
Modifying advance slots
degrees / slot size
6.............. .340
7................355
8................375
9................390
10...............405
11.5 ...........420
12...............435
13...............445
14...............460
15...............475
16...............490
17...............505
18...............520
Here is a good example of a factory setup, 67 426 Hemi, right out of the factory manual. NOTE that these figures are in DISTRIBUTOR degrees
If you take the left column, go down towards the bottom, TIMING says 12.5BTC, so this is your INITIAL IDLE setting
Now up towards the top see "Advance--Centrifugal (Distributor) it says "7.5 to 9.5 @ 1400 RPM. THIS MEANS you get 19* AT THE CRANK (9.5 x 2).
(You can also see in this chart how RPM affects the mechanical advance, at different RPMs the SPRINGS are controlling this function)
Go down further and see Advance -- Vacuum (Distributor...shows a max. of 11*, this again is 11 X 2 for 22 CRANK degrees
So you add 12.5 and 19 (mechanical) so with your foot on the floor, the advance will be 31.5*, and I'm sure many of these could have used a little more initial back then
And then for cruise, you add the vacuum of 22* for 43.5* total.
see? easy.
