Builders,
One of the things that sets our work apart from other engine options is the amount of direct interaction we have with builders. A salesman can spend all day talking to a guy about buying something and still have no real measure of what that guy’s understanding of the product is. Conversely, I can spend 10 minutes showing a single step to a builder, and then ask him a few questions. I will then have a good gage on his understanding, and tailor the delivery of information to his level. Although we have several hundred builders, I am an ‘idiot savant’ when it comes to perceiving and remembering our builders development of mechanical skills and understanding.
In conversation, one of the things I have consistently seen is that many builders don’t have a good picture of how ignition timing works on an engine. There are three levels of understanding on this: 1) being able to follow the directions on our website and use a timing light. 2) Knowing a little more about the why of timing in our engines, and 3) having a grasp on the factors affecting the behavior of combustion chamber reactions. With the help of the graph below, I want to move a number of builders from 1) to level 2).
.
.
Above is an ignition graph I ‘borrowed’ off the MSD ignition website. Although this timing ‘curve’ is not an exact match for a Corvair flight ignition, it is close enough to support a good explanation and aid builder understanding.
.
First, some language translations: What they are calling ‘initial’ timing is what I call ‘static’ timing. On a Corvair we usually start with 8 degrees to get the motor to fire up on the stand. But this number isn’t important, ‘total’ timing, or what I call total advance is. This is the number that your engine will fly around at, and it is the one you must check with a timing light. Also, people use the term “ignition curve” to describe the advance section of the graph, because theoretically it would graph as a curve, but in practical reality it graphs as a straight line.
When an individual cylinder is coming up to fire at idle, the spark takes place about 10 crankshaft degrees before the piston gets to the top of the bore. This allows the peak pressure and the effort of the cylinder to take place after the piston passes over top dead center onto the power stroke. As the engine rpm comes up, the spark needs to happen sooner because there is less physical time for the combustion to take place. This change in timing is called ignition advance, and in the Corvair this is done with mechanical flyweights inside the distributor body. Cars have a very complex set of running parameters, but airplanes do not. With fixed pitch props the power required by any rpm is fairly consistent. Thus the mechanical weights can cover the task that cars must have complex systems like vacuum advance (then) and computers (now) for. Airplanes like ours can use a very simple timing setting where the engine stays at one setting most of it’s airborne operation.
.
William,
Nice work; now everyone has no excuse to still be at level one!
“starts loosing power when they are at 300 feet climbing out.” Should be ” starts losing…”
Dan, Thanks, I corrected it. I appreciate having you send in your editing notes, I can read this stuff many times and still miss stuff. Everyone likes it better when it is clearer.-ww
Thanks William – an excellent description of the reason for and consequences of spark advance. If you can get across those basic concepts – flame propagation rate is constant, as engine moves faster the time available is less, so you have to fire it earlier, and too soon = detonation, then you will have knowledgeable engine owners. Great teaching description.
One of the things I accel at is trying to follow your instructions to the T. That’s my personality. However, that personality trait doesn’t keep me from doing stuff like leaving off a piece of the distributor lock-down mechanism. Won’t do that again. I’ll never forget the look on your face.
“Can’t fix stupid.”
Can you go into a bit more detail on the quench area? I understand the result, but not why it has that result. Looking for a little bit of ‘Level 3’…
Dave,
You can search the term on my websites and read more about it from past stories, and for general information you can read this story, minding the fact it is about liquid cooled V-8s, the specific numbers are different, the principle is the same: http://www.superchevy.com/how-to/engines-drivetrain/94138-piston-head-clearance-guide/
William thanks for keeping your articles up on the web. I was getting a little frustrated with troubleshooting my installation. A 5 minute search of your site and I had a swift kick to my ego and the answer.