Another timing curve question
Detonation at light to moderate load will happen when trying to use vacuum advance on top a fast in advance curve like this.
The lightest springs I know of specificaly for Chrysler distributors are these:
I've not put one in the distributor, but by the dimensions they calculate to be even lower spring rate than the primary spring that comes in the race distributor.
race distributor advance springs
So one of those should get a real quick advance, then find a second spring that engages at 9 or 10 degrees advance. Sketching on the graph above for convenience. Initial is 13* and second spring is getting some tension around 22*, 500 rpm.
(this is just a rough sketch of roughly what can be done)
Just for comparison with a high performance setup with vacuum advance, lets look at the non-emissions A134 timing. For typical conditions, the timing at 1400 rpm would at most be 26* - with the pedal to floor!
At light to moderate throttle, it would be 26* plus vacuum advance, is 15* reasonable? At 41* that is far more advance than the race distributor. Perfectly safe because the load is light, and lighting it early should put more heat into useful energy instead of warming the cylinder walls and exhaust valve.
Lets look at the critical highway cruising range around 2600 to 2800 rpm, and figure factory 440 pulled at least 16"Hg when cruising.
29° initial plus centrifugal, plus as much as 24° from the vacuum advance. 53° BTDC
Now lets see what Direct Connection recommended as a starting point for vacuum plus mechanical.
Pretty close, 56°. Considering the factory tech had the leeway to advance or retard the initial a couple degrees as needed/wanted based on the local fuel and the driver preference. That could bring the factory setup to 55°
Also notice the MP distributor has a curve that's in and done a little quicker. I haven't tested a B/RB version to verify that, but it was true on the small block version.
I drove with that race distributor on the street. It's not going to ping driving around town. If it detonates at WOT, then it would be trouble.
Detonation at light to moderate load will happen when trying to use vacuum advance on top a fast in advance curve like this.
The lightest springs I know of specificaly for Chrysler distributors are these:
I've not put one in the distributor, but by the dimensions they calculate to be even lower spring rate than the primary spring that comes in the race distributor.
race distributor advance springs
So one of those should get a real quick advance, then find a second spring that engages at 9 or 10 degrees advance. Sketching on the graph above for convenience. Initial is 13* and second spring is getting some tension around 22*, 500 rpm.
(this is just a rough sketch of roughly what can be done)
Just for comparison with a high performance setup with vacuum advance, lets look at the non-emissions A134 timing. For typical conditions, the timing at 1400 rpm would at most be 26* - with the pedal to floor!
At light to moderate throttle, it would be 26* plus vacuum advance, is 15* reasonable? At 41* that is far more advance than the race distributor. Perfectly safe because the load is light, and lighting it early should put more heat into useful energy instead of warming the cylinder walls and exhaust valve.
Lets look at the critical highway cruising range around 2600 to 2800 rpm, and figure factory 440 pulled at least 16"Hg when cruising.
29° initial plus centrifugal, plus as much as 24° from the vacuum advance. 53° BTDC
Now lets see what Direct Connection recommended as a starting point for vacuum plus mechanical.
Pretty close, 56°. Considering the factory tech had the leeway to advance or retard the initial a couple degrees as needed/wanted based on the local fuel and the driver preference. That could bring the factory setup to 55°
Also notice the MP distributor has a curve that's in and done a little quicker. I haven't tested a B/RB version to verify that, but it was true on the small block version.