Capacitive or Inductive Ignition?

[scarceller]

Originally Posted by 911pcars

I don't think there's such a phenomenon as a cool or cooler spark. As it relates to jumping an air gap, current systems either produce a single spark or a longer duration spark. Wider gaps require higher voltage to jump the gap and requires an ignition box and components with this capability.

Sherwood

Sherwood,

Good point on the use of cool vs hot, maybe a better way to ask the question is:

What's better, a long duration spark with lower voltage?
or Short duration spark with higher voltage?

Thanks.

[T77911S]

you bumped the idle UP to 920? what was it?
if you dont get spark knock at 5BTDC, i would leave as is unless you want to go to MSD. alot of people on here say the bosch units are better, but when it comes to racing, the MSD are what most are using, like he said above, look at the nascar boys.

[scarceller]

Originally Posted by T77911S

you bumped the idle UP to 920? what was it?
if you dont get spark knock at 5BTDC, i would leave as is unless you want to go to MSD. alot of people on here say the bosch units are better, but when it comes to racing, the MSD are what most are using, like he said above, look at the nascar boys.

Early Carrera cars 84-85 idled at 800RPMs while the later 86-89 cars idled at 880RPMs. Seems the factory decided to bump the idle up to 880RPMs for some reason, most likely to help with coming back to idle from a higher RPM.

So I only really bumped idle up 40RPMs from stock 86-89 setup. Idle is set in the chip and can be adjusted in 40RPM increments, I simply moved it up 1 notch.

Also, the 5deg Advance did help at idle not sure why? I tried 0, -5 and +5 with the +5 giving the best idle for my configuration. Ignition timing can be altered in 0.7 deg increments in the chip.

[911pcars]

Originally Posted by scarceller

Sherwood,
What's better, a long duration spark with lower voltage?
or Short duration spark with higher voltage?

As long as the spark can ignite the A/F mixture that's all one should be concerned with. The ignition boxes with higher voltage capacity merely insure there's enough voltage should it be needed.

I think it's been shown that engines with carbs and/or MFI can produce A/F mixtures on the rich side; sometimes preventing the plug from firing. Higher voltage and/or a continuous spark can reduce the chances of a plug fouling due to excess fuel and/or oil. ECU-controlled engines are much less likely to afflicted with rich A/F mixtures unless the system malfunctions or is way out of whack. The solution is to repair it rather than to fix with a band-aid high voltage box.

Also, the 5deg Advance did help at idle not sure why? I tried 0, -5 and +5 with the +5 giving the best idle for my configuration. Ignition timing can be altered in 0.7 deg increments in the chip.

Retarding the ignition (typically anything ATDC) is emissions-related and thus quite common today. Thus, it should be no surprise the engine idles and runs better (and dirtier) with more ignition advance. However, make sure the total ignition advance isn't excessive which could lead to high-speed detonation and either short term or long term engine damage. Not sure if/how the DME box compensates for non-spec static timing setting.

The answer is in the Bosch book or on the tip of someone's tongue on this forum.

Sherwood

[scarceller]

Originally Posted by 911pcars

However, make sure the total ignition advance isn't excessive which could lead to high-speed detonation and either short term or long term engine damage. Not sure if/how the DME box compensates for non-spec static timing setting.

Sherwood

Sherwood,

The setting of ignition timing in the DME I know very well. The Motronic cars have 3 ignition tables in the chip:
1) for idle
2) for Part Throttle
3) for WOT

You can simply set the idle ignition to +5 and not affect anything else. It's far more flexible than simply rotating the distributor in the older cars.

You can build ignition curves anyway you wish, it allows you to set timing for a given RPM and load range.

In the end I simply altered the idle ignition without affecting PT or WOT ignition.

[911pcars]

Originally Posted by scarceller

Sherwood,

The setting of ignition timing in the DME I know very well. The Motronic cars have 3 ignition tables in the chip:
1) for idle
2) for Part Throttle
3) for WOT

You can simply set the idle ignition to +5 and not affect anything else. It's far more flexible than simply rotating the distributor in the older cars.

You can build ignition curves anyway you wish, it allows you to set timing for a given RPM and load range.

In the end I simply altered the idle ignition without affecting PT or WOT ignition.

And your car is still able to pass emission inspections? If so, that's good.

Sherwood

[scarceller]

Originally Posted by 911pcars

And your car is still able to pass emission inspections? If so, that's good.

Sherwood

Sherwood,

Very funny you ask this. In 2006 I bought the car and it failed emissions at idle with HC=1200 lots of un-burned fuel! Bottom line was that at 800RPM idle with ignition at -3 the car had a lean misfire. Then couple this with O2 Sensor closed loop at 14.7AFR . Luckly, RI is nice and gave me a free pass because of the age of the car.

But this still had me worried so I moved idle up to 920RPM and set ignition to +5, then disconnected the O2 sensor, set AFR at about 14.3AFR and then had the test re-run, and the car passed! Also I have NO CAT! but they only tested HC, O2 and CO they did not require NOX for this car. It's nice to know these cars can pass emissions even without the O2 Sensor closed loop and NO Cat.

[ischmitz]

Originally Posted by scarceller

Ingo, when you say 4000RPMs do you mean crank RPMs?

Yes, all my RPM figures are crank-RPM. And I understand that my setup subjects the single spark plug to a six-fold increase in spark frequency simply because I do not use a distributor. That should not matter to the upstream components though in a first approximation.

The coil for both the CDI and the Motronic setup is doing exactly what it is doing in a car. So the differences for my setup are the ambient pressure and the spark frequency at the spark plug. And with that being said there is a very noticable and significant difference in how both systems act on the bench at higher RPM.

Originally Posted by scarceller

In theory, as RPM increases in an inductive ignition you tend to run out of Dwell time to charge the coil. But as RPMs increase the coil also starts to act more like a transformer, meaning (in Theory) if you drive the coil at say 1000 cycles per second (1000hz, on and off very fast) then even though the input is a DC square wave the coil still tends to act like an AC Transformer. It simply induces the primary signal into the secondary coil. So if the coil is wrapped 100 secondary turns for each 1 primary turn (1/100 ratio) then the 12v square wave on the primary tends to produce 1200v on the secondary. I wanted to mention this theory because it may be the reason that the spark looks like a constant beam on the bench in free-air at hi-RPMs. Just a theory of course.

I understand all this and RPM effects are very obvious in both systems: In case of the CDI setup the main capacitor voltage does never reach the saturation voltage of the DC/DC converter at high RPM's simply because the DC/DC converter can not deliver infinite currents.

In case of the Motronic the available dwell time is reduced resulting in less spark energy as well. But again, these effects are not effected by my single-spark bench test setup. So my results reflect some other differences that I believe are spark-duration related.

ingo