Re: How to fine tune spark advance with dwell angle changes
We now have a consensus on the dwell-timing relationship and test results jibe with theory.
Yesterday I contacted Lars Grimsrud and asked him if he could run the tests. Some of you may know of Lars - an aerospace engineer out of Colorado who is also an excellent tuner and "distributor guy". He got right on it and ventured out to his shop with a test engine. He actually had to install a carburetor on the test engine to get it running, but was back to me in a couple of hours with results.
Initially he confirmed the 1:2 dwell timing relationship that I initially reported, but I suggested he try again as after thinking about it I concluded that it should be 1:1. (Someone on the CF opined that this was the relationship and another found it was 1:1 on a Sun distributor machine.) Lars was back to me shortly reporting that he confirmed the relationship is 1:1.
So here's the bottom line, and if you want the theory read beyond.
1. Increasing dwell angle (which would be a narrower point gap) retards timing one degree for every one degree increase in dwell angle.
3. Decreasing dwell angle (which would be a wider point gas) advances timing one degree for every one degree decrease in dwell angle.
3. One half turn of the Allen wrench changes dwell angle by 11 degrees, and the relationship is linear.
4. Turning the screw CW increases dwell (retards timing) and CCW decreases dwell (advances timing)
Now on to the underlying operation. Each cylinder cycle occupies 45 degrees of distributor rotation. A dwell angle of 30 degrees means the points are closed for 30 degrees and open for 15 degrees on each cycle. Increasing dwell means the points are closed longer. They close earlier and open later, which fires the spark. Thus increasing dwell retards timing and vice versa.
If dwell angle is increased by one degree the points close one-half degree earlier and open one-half degree later. Since it's points opening that determines timing and the distributor rotates at half crankshaft speed, one-half degree advance or retard at the distributor is once degree at the crank. Thus the 1:1 relationship.
Lars went on to describe that even FAA A & P training manuals have it wrong and I quote from the CF:
"In fact, the FAA even has it wrong in its A&P Powerplant Training Advisory Circular, stating that a 1-degree change in dwell will change timing by 2 degrees. Additionally, I just dug up my notes from Airframe & Powerplant school where it states that, "dwell is the number of degrees of crankshaft travel that the points remain closed." As we know, this is also flat wrong: Dwell is the number of degrees of camshaft travel that the points remain closed. Since camshaft rotation = distributor rotation, dwell is also the number of degrees of distributor rotation that the points remain closed."
Understanding the dwell-timing light relationship is handy to know along with the sensitivity of the adjustment. You can easily fine tune the timing with just an Allen wrench to test SOTP engine response and find the detonation limit. In an emergency on the road if you add insufficient octane fuel or encounter higher ambient temperatures or lower altitude, all of which can exacerbate detonation, you have a quick and easy way to make approximate timing adjustments to quell any detonation.
The nominal dwell spec is 30 degrees with a range of 28-32, but the engine will certainly run in the range of 25-35 if not more in a pinch, so you have a maximum timing adjustment of 10 degrees and +/- at least five if the starting dwell angle is 30.
If you want to look at the discussion on the CF - all 171 posts at last count you can find it here:
Even though I got egg on my face I'm glad I started these threads and everything got sorted out. I wasn't the only one to screw-up. Even the FAA training materials don't have it right!
Here is Lars' final results and summary from the CF:
We now have a consensus on the dwell-timing relationship and test results jibe with theory.
Yesterday I contacted Lars Grimsrud and asked him if he could run the tests. Some of you may know of Lars - an aerospace engineer out of Colorado who is also an excellent tuner and "distributor guy". He got right on it and ventured out to his shop with a test engine. He actually had to install a carburetor on the test engine to get it running, but was back to me in a couple of hours with results.
Initially he confirmed the 1:2 dwell timing relationship that I initially reported, but I suggested he try again as after thinking about it I concluded that it should be 1:1. (Someone on the CF opined that this was the relationship and another found it was 1:1 on a Sun distributor machine.) Lars was back to me shortly reporting that he confirmed the relationship is 1:1.
So here's the bottom line, and if you want the theory read beyond.
1. Increasing dwell angle (which would be a narrower point gap) retards timing one degree for every one degree increase in dwell angle.
3. Decreasing dwell angle (which would be a wider point gas) advances timing one degree for every one degree decrease in dwell angle.
3. One half turn of the Allen wrench changes dwell angle by 11 degrees, and the relationship is linear.
4. Turning the screw CW increases dwell (retards timing) and CCW decreases dwell (advances timing)
Now on to the underlying operation. Each cylinder cycle occupies 45 degrees of distributor rotation. A dwell angle of 30 degrees means the points are closed for 30 degrees and open for 15 degrees on each cycle. Increasing dwell means the points are closed longer. They close earlier and open later, which fires the spark. Thus increasing dwell retards timing and vice versa.
If dwell angle is increased by one degree the points close one-half degree earlier and open one-half degree later. Since it's points opening that determines timing and the distributor rotates at half crankshaft speed, one-half degree advance or retard at the distributor is once degree at the crank. Thus the 1:1 relationship.
Lars went on to describe that even FAA A & P training manuals have it wrong and I quote from the CF:
"In fact, the FAA even has it wrong in its A&P Powerplant Training Advisory Circular, stating that a 1-degree change in dwell will change timing by 2 degrees. Additionally, I just dug up my notes from Airframe & Powerplant school where it states that, "dwell is the number of degrees of crankshaft travel that the points remain closed." As we know, this is also flat wrong: Dwell is the number of degrees of camshaft travel that the points remain closed. Since camshaft rotation = distributor rotation, dwell is also the number of degrees of distributor rotation that the points remain closed."
Understanding the dwell-timing light relationship is handy to know along with the sensitivity of the adjustment. You can easily fine tune the timing with just an Allen wrench to test SOTP engine response and find the detonation limit. In an emergency on the road if you add insufficient octane fuel or encounter higher ambient temperatures or lower altitude, all of which can exacerbate detonation, you have a quick and easy way to make approximate timing adjustments to quell any detonation.
The nominal dwell spec is 30 degrees with a range of 28-32, but the engine will certainly run in the range of 25-35 if not more in a pinch, so you have a maximum timing adjustment of 10 degrees and +/- at least five if the starting dwell angle is 30.
If you want to look at the discussion on the CF - all 171 posts at last count you can find it here:
Even though I got egg on my face I'm glad I started these threads and everything got sorted out. I wasn't the only one to screw-up. Even the FAA training materials don't have it right!
Here is Lars' final results and summary from the CF:
Final Results and Summary:
- You can make minor timing adjustments using the dwell adjustment screw.
- 1 degree change in dwell changes timing 1 degree.
- Turning the screw IN (clockwise) INCREASES dwell and DECREASES timing.
- Turning the screw OUT (counter-clockwise) DECREASES dwell and INCREASES timing.
- Turning the screw 1/8 turn changes dwell and timing by about 3 degrees.
- .019" point gap = about 31 degrees dwell.
- Increasing point gap decreases dwell and increases timing, and vice-versa.
- A 2 degree change in dwell changes point gap by .003", so every .0015" change in gap changes dwell and timing by 1 degree.
Comment