Crane XR700 = capacitive discharge

[SorenDK]

Hi guys!!

One quick question which puzzles me. Does the old Crane XR700 ignition unit replace a capacitive discharge unit like the CDI from Bosch or the MSD 6AL units?... or is it simply a optical ignition system designed to replace points?

besides the optical points in the distributor, i have a golden colored crane unit ... it actually looks a bit like the MSD unit with cooling fins and everything... except its gold and not red...
IMHO i think its crap and is considering other alternative ignition options... what do you guys think?

Thanks for a great forum :-)

[304065]

http://www.cranecams.com/uploads/instructions/90000700.pdf

The XR700 is an inductive discharge system with an optical trigger wheel to replace the points. It's not a CDI.

Optical triggers can be messed up by dust and grease inside the distributor cap. Probably better than points, which can bounce and arc (although points aren't terrible when they are only switching a tiny current such as in the Bosch 3-pin CDI context).

Anyway, think of the XR700 as electronic ignition, which uses transistors (maybe an SCR) to switch high current to an inductive-style coil to charge the coil and then stop, resulting in field breakdown and high voltage on the coil secondary. Contrast this with the MSD or Bosch CDI, which build up energy in a capacitor and dump it into a smaller coil which then steps it up to ignition voltage.

Looks like it only switches 4.5 amps, however. Using their recommended coil with 1.4 ohms primary resistance, and inductance of 7.5mH, that's good for about 75.94mJ of ignition energy, which is plenty. I wonder if it will have sufficient time to charge. I see that it says don't use if you are going over 6500 RPM. Their performance coil has only .4 ohms of primary resistance and 5.3mH inductance, for 53mJ, but it should charge faster.
http://www.cranecams.com/402-407.pdf

Do you have the equipment or technical facility to measure the duration of the secondary ignition pulses? (Careful, it's dangerous and can kill you) It would be interesting to note the spark duration of the XR700. It should be longer than a CDI of equivalent vintage.

[Lorenfb]

"I wonder if it will have sufficient time to charge."

T (charge time) = L X I / V, for one time constant (63% of max.)
R = V / I then, T (for 63%) = L / R

If L = 7.5 mh and R = 1.4, then T = 5.4 ms to reach 8.6 amps (12V / 1.4 ohms)
times 63% or 5.4 amps.

For a 6 cylinder engine @ 3K RPM, each spark occurs every 6.6 ms and
for a 50% dwell that leaves only 3.3 ms to charge the coil, i.e. 3.3/5.4
or 60% of 5.4 amps (~ 3.2 amps). At 6K RPM it becomes apparent that
there will be a problem with reduced spark energy (~ 1.6 amps or about
10 mj of spark energy and reduced voltage).

[john walker's workshop]

i round file them.

[304065]

Loren, that is very helpful. Let me be sure I understand.

The coil will eventually approach 8.6 amps after five time constants or so, but we're only concerned with the current at the first time constant or 63% of the way to max. (1-(1/e))
At the first time constant, the elapsed time is 5.4ms, and the current is 63% of 8.6 or 5.4 amps at this point.

Using a 50% duty cycle, the coil can only charge for half of that time, or 3.3ms out of 6.6ms per ignition event, and it takes 5.4ms to get to 5.4 amps. So at 3.3ms into a 5.4 ms charge time, we're 61% of the way in, or only 3.3 amps. 3.3 ^2 * 7.5 mh * 0.5 = 40mJ, but at half of the 3.3 amps: 1.6^2 * 7.5 mh *0.5 = only 10mJ of energy.

Here's the relationship graphically. It's pretty dramatic.



Now, I notice you used a nominal 12V, do you think it's reasonable to expect 13.8 volts coming off the alternator? And in terms of Dwell period, the factory specified 38 degrees +/- 3 degrees for the Bosch Distributor and 40 degrees +/- 3 degrees for the Marelli. (I have a Bosch BIM 137 module that does active dwell control, more on that later). So can we plug 38 degrees dwell or a 63% duty cycle?

Here's a "stupid" question for you: what happens when we use a coil with less than 1.0 ohm primary resistance, like the Carrera inductive coil (0.4-0.7 ohms primary resistance)? At 12v and the midpoint of that range (0.55 ohms) I get 21.8 amps??

[304065]

Originally Posted by john walker's workshop

i round file them.

For a minute I had an image of you with a round (like rattail) file. Then I realized you meant "circular file!"

[Lorenfb]

"Let me be sure I understand"

You do, completely! Nice graph.

"do you think it's reasonable to expect 13.8 volts coming off the alternator?"

But under worst case with voltage drops, something less than 13.8.

"Here's a "stupid" question for you: what happens when we use a coil with less than 1.0 ohm primary resistance, like the Carrera inductive coil (0.4-0.7 ohms primary resistance)? At 12v and the midpoint of that range (0.55 ohms) I get 21.8 amps?? "

The question is not stupid. That's a good point and there's a trade-off that occurs,
e.g. Charge Time = L / R, so the lower the R the longer the charge time but the
current increases too. The lower L also reduces the charge time but the energy
is reduced. So the values have to be tweaked for optimum energy, voltage, and
charge time based on the max RPM. With the CDI system, it's easy - it's all about
the capacitor voltage and the capacitance, less of a problem.

[304065]

Originally Posted by Lorenfb

"Let me be sure I understand"

You do, completely! Nice graph.

"do you think it's reasonable to expect 13.8 volts coming off the alternator?"

But under worst case with voltage drops, something less than 13.8.

"Here's a "stupid" question for you: what happens when we use a coil with less than 1.0 ohm primary resistance, like the Carrera inductive coil (0.4-0.7 ohms primary resistance)? At 12v and the midpoint of that range (0.55 ohms) I get 21.8 amps?? "

The question is not stupid. That's a good point and there's a trade-off that occurs,
e.g. Charge Time = L / R, so the lower the R the longer the charge time but the
current increases too. The lower L also reduces the charge time but the energy
is reduced. So the values have to be tweaked for optimum energy, voltage, and
charge time based on the max RPM. With the CDI system, it's easy - it's all about
the capacitor voltage and the capacitance, less of a problem.

Quite true and the more I have thought about the trade-off and done various models, the more I understand how much the current limit of the ignition module becomes a factor in the design.

Let's take for example a Bosch BIM 137 ignition module. I have one of these sitting on the bench ready for testing, max current is 8-10 amps, the module does "active dwell" which means it will limit the current at low RPM.

We know that at the first time constant that the coil will only be at 63% of the max current. The dwell period is only longer than the first time constant at very low RPM (depends on coil choice of course, for my example I used a Carrera TCI, .7 ohms and 3.6mH, just like your web page says). So, the coil current should be at least 10A at the first time constant, which means that the max current should be about 15.9 amps (10/.63), let's call it 16 amps. To get 16 amps out of a nominal 12V coil, that implies a primary resistance of .750 ohms (12/.750= 16). (That's pretty close to the Carrera TCI coil, in yet another proof that the Factory knew what they were doing when they designed the Carrera's DME-switched ignition system)

Of course the coil never gets to 16 amps, it only gets to 10 before the module cuts it off or the dwell period ends, but 10A * 12V is still 120 Watts, that's going to take a big heat sink. Resistive heating will be a function of the 63% duty cycle at 38 degress dwell.

[Bo_66]

Better then points and condenser. Worked great on my 2.0 up to 7 krpm with .028" gap. And starting improvement also it trigged a -66 tach.
I'm not sure its better than à CDI.
Bo

[Lorenfb]

"for my example I used a Carrera TCI, .7 ohms and 3.6mH"

So for the 3.2 TCI system, the charge time constant (L / R) is ~ 5ms.
At 6K RPMs the sparks occur ever 3.3 ms, so ideally the current only
reaches about 15 amps times 63% times 3.3 / 5.0 or about 6 amps.
The energy is still over 60 mj, though.

Now if one uses the Bosch Blue coil, a different result occurs:
For that coil; L = 12 mH and R = 3.5 ohms, so T.C. = ~ 3.4 ms.
I at that T.C. (~ 6K RPMs) is 2.2 amps (63% of max @ 12 volts).
The energy is then about 29 mj, where 30 mj is considered minimum
necessary.

[moneymanager]

After reading this thread I remain puzzled. Two questions:
I understand that the Crane doesn't replace a CDI. But lots of people apparently use it (both the xr700 box and the optical trigger) AND an MSD or other CDI. What is the point of using both? Is the result a stronger or more robust final output?
Second, do we think there is a problem above 6k or not? The discussion seems inconclusive.

[John at J&S]

If an MSD is added to a Crane or other inductive system, the MSD does all the work, the inductive system merely provides a nice stable trigger.

I believe Crane was using an ST L497 ignition control IC:

http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000079.pdf

These are very good when used with a coil with primary resistance around 0.5 ohms. This allows the coil to reach full charge in about 2.5 to 3.0 msec.

Rather than use a dwell control chip, I use software to control dwell in the SafeGuard.

Video of me testing the spark energy of a unit I may offer at some point. This version increases coil current from seven amps to ten amps, as the boost goes from 0 psi to 20 psi:
Ignition Energy Varies with Boost - YouTube

[304065]

The reason for the "redundant" systems is that the ignition mode of one (kettering, inductive, CDI, multi-spark CDI) is not necessarily in the same box as the desired trigger mode (points, pertronix, hot-spark, reluctor).

If somebody would come up with a hall-effect trigger that fit inside the distributor cap of an early car, that ran a modern solid-state coil driver up to about 10A, that would solve all our problems. Oh AND make it original looking or indistinguishable from concours.

[porschetub]

I have now experience with these units but I do know they have worked well on air cooled type 1 motors for sometime.
Have used Pertronics Ignitors before and they are good, but not sure if they make a module to go with the ignitor and their high power coil ,research time.
As mentioned the resistance of the coil is an important consideration, a system like this will be on my shopping list soon so thanks for tech info,some of it went over my head but I understand the theory.

[HawgRyder]

I remember my old 427 Ford engine had a Thunderbolt ignition system...it consisted of two electronic packs in the glove compartment area..firing through a split coil wire into the top of the distributer.
One day on a run down the track...the engine went "soft"..would not rev over 6000 RPM....it turned out one of the packs quit on me...I had to install a new transistor (heavy duty...did both packs) and got it running at peak efficiency again.
I guess that proves the theory of charge time as per one pack - one coil not being quick enough for high speed sparking.
Bob

[davidbir]

I use a Crane XR700 on a six cylinder engine-British. I occasionally run it over 6K and it is fine-it always starts very well and has done for eight years now. I notice in the instructions that they suggest cutting a hole in the distributer cap and checking the "lead" on the spark with a timing light--I haven't done that yet...

[Fishcop]

I've also run the Crane XR700 in my 69T for many years (originally fitted with Kettering ignition). It has been a terrific upgrade for a car that didn't come with factory CDI. I've run it happily at the track to 7k without issue. I have also picked up a Crane Hi6 CDI which I will soon introduce to the system.

[lab racer]

I am new to the air cooled 911
I want to get rid of the points. Other posts say the magnet based systems do not deliver consistent timing at higher RPM.
With that in mind I'm looking at the Crane optical.
This post states the OEM CDI does all the "Work"

Since a 0.5 Ohm coil is needed for the XR700 how much lower does that make the mJ spark energy from the CDI?

Also are the sparks from the XR700 and the CDI additive?

I would presume the spark duration would also be slightly longer as the XR700 would optically detect and trigger before the signal to fire reaches the CDI.

Since the CDI does the work, would you recommend the XR 3000 for improved high RPM performance?

[lab racer]

Can someone post a link on how to install and set up the Crane xr optical system?

[swade]

http://www.cranecams.com/uploads/instructions/9000-0700_.pdf