question on ignition timing: CDI vs BREAKER POINTS

[champ20B]

My question is... does a CDI ignition require a bit less advancement (simply regarding the position of the mag plate) than it would with the old points type?

The reason I ask, is that a while back I took an old 6NHR hotrod and put on a OMC ignition (like the American HRs used). All I had to do was to shim one side of the flywheel key slot, reshape the key, and use a thinner "wave spring" washer to give the nut full thread. It worked out perfect. The way I timed it was to set the old Phelon breaker at .018, detect where it breaks, then set my home made piston stop at that precise point. After switching the ignition system over with the OMC wheel, I put in the piston stop, then I made a mark on the block and a equal mark on the wheel at the stop point. I then used a timing light to get the marks aligned via the strobe indicator while spinning it over. Well, when I went to try it out, the darn thing kicked back and nearly snatched my fingers out of joint. I then retarded the mag about 7-degrees, and that problem was solved. It ran like a dream. I just wonder if it is really at the right point as now, compared to the old like this? BTW, I found a surplus HR16B Tilly pumper carb, rebuilt it, and put it on my hotrod and it is great!! All I had to do was to open the carb side of the original carter adaptor to match the tilly as it is about 1/16" larger. I have the diaphragm side facing downward and rigged it with the throttle cable coming in on the front. It actually works well like this and doesn't get in the way, looks good to.

[hydroplay]

You are comparing apples and oranges. You should have run the Phelon mag and found where it sparked with the timing light and then set the OMC to spark at that same point. That eliminates all the extraneous time factors.

[Aeroliner]

We have been using the XDI ignition system on our Mod engines with great success. The nice part is that you can take advance out as the RPM increases. We take out about 4 degrees with our 40 and 44 Merc's. you can find some photos of our system in the technical section under the 40-H thread. I can supply a system for a 2 cylinder engine if your interested. Actually we have systems for 2, 4 and 6 cylinder Merc's.

Alan

[Ron Hill]

We have been using the XDI ignition system on our Mod engines with great success. The nice part is that you can take advance out as the RPM increases. We take out about 4 degrees with our 40 and 44 Merc's. you can find some photos of our system in the technical section under the 40-H thread. I can supply a system for a 2 cylinder engine if your interested. Actually we have systems for 2, 4 and 6 cylinder Merc's.

Alan

In the old days, 55-H and 30-H days, I would only give the motor half throttle out of the corner but then advance the timing by squeezing the throttle.


But, maybe, now, with higher compression, you give it high timing until it just about melts, then retard it???

I guess I never knew the timing on the D ALKY CROSS FLOWS or my Konigs...my dad did all the work...

[zul8tr]

Since you need to start from scratch why not bolt a flywheel pointer at a convenient place on the block. Then use a dial indicator in a plug hole (pick one) and mark various piston positions on the flywheel at the pointer (I assume you know a range of these positions?). Then using the timing light adjust the mag to a piston position and lock it down. Test, observe plugs and piston crown and adjust timing and high speed jet as needed.

[Bill Gohr]

You are comparing apples and oranges. You should have run the Phelon mag and found where it sparked with the timing light and then set the OMC to spark at that same point. That eliminates all the extraneous time factors.

If the engine was running correctly prior to the change, you should have run the engine with the original parts, created a mark and a pointer to verify your timing with a light. Then, mark tdc on that wheel and use a degree wheel and a dial indicator to figure out what your actual timing was, the transpose that info into your new system.

[Aeroliner]

IGNITION TIMING

Combustion of the fuel/air mixture does not happen all at once like an explosion does. It happens as a progressive burning of the fuel, starting from around the spark to the outermost combustion area. When the spark happens in relation to crank degrees before top dead center (°BTDC) of the piston is something that needs to vary according to engine rpm so that maximum combustion pressure of the burning fuel/air mixture happens between 15 and 20 degrees after top dead center (ATDC). That is necessary for the most power to be transmitted to the gears/sprockets/tire. As engine rpm's increase, the amount of time from the piston being at a set amount of °BTDC to being at top center decreases. But the amount of time that the fuel/air mixture needs to burn remains the same (with the exception above 7000rpm when the extra turbulence caused by the squish band shortens the time needed to burn). Going only by this aspect of combustion it would be ideal for the CDI to increase the °BTDC of the timing as the rpms increase, in a linear fashion. This is more or less true for a 4 stroke engine, but for a 2 stroke there are other factors to consider:
1. Since a 2 stroke has combustion once every crank cycle (whereas the 4 stroke combusts once every 2 cycles) then the pistons top temperature can get high enough to melt it at high engine revolutions with the timing advance equal to that of a 4 stroke engine.
2. At high rpms the delivery ratio of fuel/air becomes less and the pumping loss of engine power becomes more with increased timing avance so that it is accumulative enough to weaken engine power at high revs. "Pumping loss" refers to the power detracted from the engine because of the force needed to push the piston up to top center against the pressures of the compressed gases and the pressure caused by gas expansion due to combustion.
3. Making the ignition and combustion happen later at high rpms increases the powerband of the engine equipped with an expansion chamber because the exhaust exiting the exhaust port is still burning and is therefore hotter than it is at lower rpms. That increased exhaust temperature allows the pressure/sound wave to travel faster. A pressure wave that returns to the cylinder sooner than normal better matches when the piston upstroke happens to prevent loss of fuel/air charge out the port. That in effect it allows a wider powerband of engine power.

The following is from Eric Gorr's paper "Basic 2 Stroke Tuning" where he talks about advancing or retarding the whole ignition curve of an advancing/retarding CDI on a dirt bike by changing the stator position CW or CCW:

AFFECTS OF THE IGNITION TIMING

Here is how changes in the static ignition timing affects the power band of a Japanese dirt bike. Advancing the timing will make the power band hit harder in the mid range but fall flat on top end. Advancing the timing gives the flame front in the combustion chamber adequate time to travel across the chamber to form a great pressure rise. The rapid pressure rise contributes to a power band's "Hit". In some cases the pressure rise can be so great that it causes an audible pinging noise from the engine. As the engine rpm increases, the pressure in the cylinder becomes so great that pumping losses occur to the piston. That is why engines with too much spark advance or too high of a compression ratio, run flat at high rpm.

Retarding the timing will make the power band smoother in the mid-range and give more top end over rev. When the spark fires closer to TDC, the pressure rise in the cylinder isn't as great. The emphasis is on gaining more degrees of retard at high rpm. This causes a shift of the heat from the cylinder to the pipe. This can prevent the piston from melting at high rpm, but the biggest benefit is how the heat affects the tuning in the pipe. When the temperature rises, the velocity of the waves in the pipe increases. At high rpm this can cause a closer synchronization between the returning compression wave and the piston speed. This effectively extends the rpm peak [power] of the pipe.

Below is a dyno print of the horsepower difference in a 50cc engine that had the tinming retarded 4 degrees which resulted in the powerband extended 1000 rpm more.

Alan

[Aeroliner]

Getting ready for the new season with a fresh 44 build. Installing the XDI ignition system and will test run in a week or so.

Alan

[champ20B]

That merc 4cyl is a really great looking engine! Thanks Aeroliner for the info on timing....

I guess it would have been a good idea to have used a timing light on my old ignition first as some suggested. But what I did in detail was to adjust the breaker at the recommended opening. This ignition was rebuilt before I got it so the points aren't worn down.
The points break indicator I used wasn't a buzz-box, but a small light powered by a AA-battery attached to wires with alligator clips. The way it works on all of my points systems is that when the points crack open, the light goes dim. Interestingly, it doesn't go out completely but just stays dim until the points close and it goes bright again. Attached to a points set removed from the plate, it will always go completely out. But on the plate hooked up, just dim....
when the light bulb goes dim, I took a homemade piston stop and adjusted it to where the piston would touch precisely at the point to where the points break indicator would go dim. I made the mark on the wheel and the block. I then put on the other ignition and wheel and marked the new wheel even with the mark on the block at the piston stop. I then took a timing light, and after removing the stop spun it over. I positioned the mag until the marks of the wheel/block aligned.
I was told once that the later CDI systems (Motoplat in particular) were very strong, and didn't require as much advance as with the old phelon. I assume that the OMC ignition hotrods used is like the motoplat in that respect, given the result of cranking and kick back. I only retarded it slightly and it stopped doing that. But I always like second and even third opinions to be sure!