SimonExtreme

Loren

I looked at your link and I am even more confused! Your website states

Now, your website coinsides with my understanding, but your comments don't. Unless I am misunderstanding your website or what you have said above. What am I not getting?

Red rooster

SimonExtreme,
The stored energy is 1/2 L I squared ,or 1/2 L ( E/R) squared.
Charge time L/R.
So by adjusting L and R an acceptable energy level and charge time can be found.

CDi has a very fast discharge time so energy loss due to leakage is greatly reduced . One leakage source can be partly fouled plugs.
The drawback for CDi is its very short spark duration which makes it less effective when relatively lean mixtures are being fired.
The charge time can be made very short . A common application is very high reving Formula motors.

There have been systems designed to incorparate both CDi and inductive !

For a 964, a properly working inductive sytem is more than adequate.

Hope that helps,

Geoff

NineMeister

I'm like you Simon, when it comes down to the finer points regarding the specific workings of the Motronic system I am strictly a bystander because I only know what I have read or been told about by others, so partaking in a serious discussion of these details I have nothing to add.

However what I do have is a lot of practical experience of the result of running motronic systems on a 964 and a comparison with other systems. Without wishing to drag this thread into the depths of the recent Motec thread I think you are on the right lines to question what works in practice, rather than discussing what works in theory.

With respect to the standard ignition system on both the 964 and 993, I can confirm that if they are used on modified engines running at more than the standard rpm limit, even with standard spark plugs they will run satisfactorily up to 8000rpm, even with the dwell time limit that this implies (Geoffrey did the maths). Unfortunately just because it will run the engine at 8000rpm does not mean that the engine could not benefit from a better ignition system, and I know this from experience with my own 400+hp 8200rpm 3.8 engine after running it for 18months with the inductive system and then changing over to CDi for this competition year. To satisfy Loren, when I initially made the change I wired up both systems, tested them on the dyno back-to-back and found no difference in power. But as with the Motec/Motronic comparison, this is only part of the story.

What I did find was a subjectively more responsive engine with the CDi, which obviously encouraged further investigation. After trying different spark plugs and plug gaps I eventually found that the engine made a little more power at high rpm with improved performance past the power peak, and would also make the same power through the mid-high range with less advance. I suppose the real proof of the pudding is that the CDi is still on the car.

SimonExtreme

No, Colin. The real reason is you were too lazy to change it back. It typifies your slapdash approach to this season. No wonder you got the results you did

SimonExtreme

Geoff

This is why I love this forum. You have managed to put into technical terms what I already knew but didn't know how to say! I looked at these issues because of the trend to "upgrade" ellements of the ignition system, like high performance HT leads or even high performance coils. I concluded that there was nothing to be gained until you get to a very high level of tune and high revs. Seems like my amateur research came to the right conclusion. Shame, some of the parts are very pretty colours, which is probably the main reason a lot of people buy them................and a little hype

However, surely it is correct to say that on a STANDARD 964, the system is more than adequate. At some point, as Colin suggests,

Geoffrey

I thought there were 3 different maps for fuel and 3 different maps for ignition based on the TPS. Idle switch = idle map, WOT switch = WOT map, and anything inbetween uses the AFM. These are what I'd call base maps and they are modified by other parameters such as AT, short term lambda trim, etc. Do I hear you correctly when you say that the AFM map is the base map and the idle map is a modifier to the AFM map when the idle switch is active? If so, why do the Idle and WOT maps for ignition timing contain values that look like actual timing values and not modifiers to an AFM map?

I think the fact that the 964 coil is not fully charged does not matter with a stock or near stock engine because peak cylinder pressure will occur at peak torque where the ignition system is still capable of being fully charged. After 6000rpm the coil is not fully charged, but then again, the cylinder pressure is lower so it probably doesn't matter.

In racing applications, particularly with higher RPM engines, it does matter. And if you can increase the spark plug gap, then you will have a larger flame kernel and therefore the propagation rate will be faster, allowing you to run less ignition timing and therefore see lower pumping losses. I know this to be factual as I have experienced this on a dyno. And, my engine is running a high quality CDI system and I am running a .060" spark plug gap (I can probably go wider, I just haven't found an acceptable plug that can be opened up), and I can run lean AFRs without misfiring.

I agree with Loren that inductive ignition systems can be made for performance. Typically, though production systems are used by DYI guy. I have also found that 1/2 the v10 formula 1 engines used inductive and the others used CDI, so there are merit to both. It also isn't fair for me to be comparing a RACING CDI system to a production inductive system, but I do give that as an example.

We could delve into all of the forumlas and theory on ignition systems if you want to go there.

Red rooster

Geoffrey,
Interesting comment re 0.06" plug gap / advance . The only note of caution is that a gap that big seriously raises the running kV on leads/cap etc.
If your ignition system isnt 100% perfect and bone dry " dont try this at home" !

All the best

Geoff

Lorenfb

"These ignition systems are called transistor controlled ignitions (TCI). Because of the early transistor devices, the current and its rise time (the time required to deliver the spark energy to the plug) of the spark were limited."

- Website -

"Now, your website coinsides with my understanding, but your comments don't. Unless I am misunderstanding your website or what you have said above. What am I not getting?"

- Simon -

Good that you mentioned that Simon. The guy (me) that designed the website needed to put
an addition qualifying statement there:

The later semiconductor technologies, e.g. silicon bipolar power devices, have provided the inductive
discharge ignition systems with comparable spark rise times, energy levels, with the added benefit
of simplicity of design, better reliability, & longer spark durations."

"Charge time L/R." - Red Rooster -

Actually, the L/R is an approximation of the time constant for large values of R compared to L.
As R approaches zero, L/R becomes infinite and the differential equation defined earlier
better defines the charge time. The proper equation is the exponential current equation with both
L & R and then solved for time.

N51

What ignition system does Porsche use on their high rpm GT3 engines? The RSR at a purported 8500rpm.

Noah

Lorenfb

"What ignition system does Porsche use on their high rpm GT3 engines? The RSR at a purported 8500rpm."

Inductive discharge, as is the case for most/all late model engines. It's very rare that any OEMs
use CDI ignitions, i.e. less reliability, cost, & spark duration.

N51

Thanks for that and a good discussion. I'll sleep on it.

Noah

SimonExtreme

Excuse the "thinking out loud" mode, but I would guess that the time to chrage issue is why we see multi coil applications on road cars. That way they can stay with the more "reliable" inductive method and not run into trouble???

SimonExtreme

So those things actually work! I thought they were just sold, after market and in pretty colours with long worded promotional material, to the "fast and furious" brigade

Geoffrey

In an inductive ignition, the coil is going to be the determining factor for dwell time and the primary and secondary resistance will largely determine this. In a CDI system, the coil does not store any energy, it only transforms it so the primary and secondary resistance is very low. This is why you should not mix coils between the two systems. CDI systems like an MSD6A that can use a factory coil are not all that strong compared to a high quality racing CDI system.

An inductive ignition work on the following principle W=1/2L*(I*I) where w= energy, l=primary inductance, and I=primary current. Most inductive systems (production based) provide an energy level of 63mj (pencil coils like found in the GT3 can be as little as 50mj), are charged with 6-7a of current through primary inductance of 3.2mH. The ignitor is the component in the system that caps the current flow. So, to gain higher energy, you can either increase the primary inductance and/or the current flow, both will increase the dwell time or coil charge time. In the direct ignition system of the GT3, the coil only needs to fire once every 2 engine revolutions or once every 20ms at 6000rpm so there is plenty of time for the ignition coil to be charged and fired. The GT3 coils still require 2.5ms to fully charge. The Corvette LS1/6 coil per plug requires 5.5ms of dwell. I cannot recall from my testing, but I believe the Corvette charges the coils more than 7a.

CDI works on the principle of w=1/2C*(V*V) where w= energy again, c= capacitance and v=voltage. In a high quality digital racing system, the voltage is near 450 -500v and the capacitance changes from brand to brand, but the energy level is in the 105mj range, almost double the inductive style. Further, it can charge the capacitor in less than 1ms so its frequency is much higher and can operate in an enviornment where an inductive cannot.

Once the flame kernel has been started, the spark plug is no longer required. So, the fact that the inductive ignition system provides spark for a longer duration, only helps if the flame kernel is not lit right off. In a well tuned system, the short high energy spark of a CDI system will light the flame kernel.

A good example is a 993tt which uses the same fundamental ignition system (although only 1). If you increase the boost to 1.2bar it is very likely that the engine will begin to misfire at the stock spark plug gap once the engine is over about 6000rpm. Increasing the performance of the engine has uncovered a weakness in the production ignition system. A switch to a performance oriented CDI system like a Crane will solve the issue and allow a spark plug gap larger than the factory spec to be run without it misfiring.

"The only note of caution is that a gap that big seriously raises the running kV on leads/cap etc. If your ignition system isnt 100% perfect and bone dry " dont try this at home" ! "

Agreed, which is why you cannot look at just the individual parts of an engine, you need to look at it as a system of interrelated parts.

And, out of all of the ignition choices I had for my engine, I chose a distributor...can you believe that?

Make no mistake about it, the GT3 Cup engine is basically a production engine and therefore uses production ignition systems. The GT3R-RSR use them because of homologation rules, not because they represent the best technology. In reality, they are neither all that reliable, nor all that good. They are the same coils as on a Boxster.

Lorenfb

"Excuse the "thinking out loud" mode, but I would guess that the time to chrage issue is why we see multi coil applications on road cars." - Simon -

Just a side benefit, the main reason is elimination of the dist. & wiring.

"This is why you should not mix coils between the two systems." - Geoffrey -

Not completely true! Since most/all inductive ignition system coils can be used with CDI systems,
but NOT all CDI coils can be used with inductive discharge coils. The key, though, is the turns
ratio of the coil which should be about 100:1 for a CDI ignition. Turns ratios much greater than
that will produce too much spark voltage as most CDIs produce a peak primary voltage of about
400 volts.

"The ignitor is the component in the system that caps the current flow." - Geoffrey -

Not true! Most ignition modules (ignitors) DON'T have current limiting capabilities.
It's either the coil's series resistance or the ramp time limit determined by the
driver signal to the ignition module. The use of incorrect coils (series resistance)
on an inductive discharge system can damage the ignition module, which is less of
a factor on a CDI system.