Grumpy

Final awhile since I've done a Final Answer, so with some recent discoveries, I'd thought I'd ramble on about sparks for a bit.


In the days of the flatheads, and large combustion chambers, the idea of more timing was a good thing took hold. With 60 Octane gas, and 160cc combustion chambers, there was just not alot that you could do other then add fuel and timing to try and make more HP then stock. But times are changing.


Just to review the basics.
For a spark to occur there must be an ion trail. As the voltage builds at the center electrode, it begins to want to find a ground. So some molecules that have develeoped a charge, flake off the senter electrode and begin floating around, as the voltage builds more do, and finally as the voltage reaches a critical level, and there is enough charged particles (ions), the voltage jumps the gap and there is a spark. Now, for an instant there is critical point that starts the reaction in the chamber (note, flame is used just as a matter of convence, while there is *fire* involved it is but a small part of what's going one). As the spark actually developes, into a flame *kernel*, it takes time. Actually getting this flame to where it is about .1" in diameter takes alot of time, once the flame gets out of the kernel stage it is about instantaneous for the reaction to develope across the chamber, thou chamber size has alot to do with how long it takes to do that.

While we start the fire way before TDC, we want peak pressure in the chamber to occur at 12-18d ATDC. So this makes it rather easy to see that if we could run the timing at say 10d ATDC we could recover alot more power since we weren't just wasting time for the flame to get going. Trouble is there is just no practical way to get the fire to the level of being serious that fast. So all we can do is try and MINIMIZE the amount of advance we need to run.

1. Smaller chambers, it's only obvious that the smaller the chamber the less time it takes for the flame to reach across it. there are some other benefits, ie less surface area means less heat goes into the coolant, thus more energy to push the piston down.

2. Often you can run a flat top piston, and then have the center of gravity of the piston more in line with the wrist pin so the piston doesn't try and rock as much, so the rings seal better.

3. Less carbon forming in the chamber. While a thin whispy coating is good (ie it acts like a thermal barrier), the outter edges with a smaller chamber tend to develope less carbon.

4. If in doubt, take a look at the turbo v6 with there 38cc chambers, and how well they can be made to run. Also, GM trend to make the SBC chambers smaller.

OK,

now back to the spark.
There are several imprtant issues with the spark generation. You want the voltage to build really fast, so that the ion trail forms and you can get the fire started. This is where CDI ignitions have there advantage. The run a primary voltage of about 400 volts instead of 12, so they have a head start on getting the flame going. Also, note, when you change to a CDI, if it's really working well, you should be able to retard the timing some.
Now, the trade off in the CDIs, is while they get the spark started faster, that means it just doesn't have the time, to develope much amperage so the spark fades out really fast. So what some of the CDIs do is start several sparks to *MAKE UP FOR THIS WEAKNESS*.

So how can we get the best of all worlds?.
All you have to do is look at the latest GM releases (ok, there are others), but the answer is Coil Near Plug.
Why, is the CNP so neat?.
Well, any coil gets hot, from all the electricity flowing thru it, with a CNP you only fire the coil 1/8 as often, so it is on less time, and off long enough to really have time to cool off!.

What brings this all to mind for me is that I just went to an eDIST. The allows the conversion to a CNP setup.

With this, brings needing some reprogramming, to run less timing.

I hope, this better deminstrates, why you want to run the min amount of timing consistant with a given HP level. With proper design and EXECUTION, there is just no reason for the more is better nonsense to continue.

Again, this is a very short, distilled set of notes for a very complex process.

Bruce
Less is more..........

Beast4

Very nice discussion of the high points of spark ignition of the fuel air mixture. Have you ever played with the Electromotive ignitions? Had one on several engines and really liked it.
For those not familiar with it:

Not quite as nice as coil near plug but has 4 coils firing two cylinders at a time (like on a sportbike) and potentiometer adjustable timing curve and rev limit. Has crank trigger without the need for a cam sensor.

TC602

Grumpy, I am glad you posted this, because it conferms my suspisions about timing. I am using the ARAP bin as my base. It had way to much timing for my little 305. I log my runs, then adjust the timing acording to where I get knock. I have found lots of power and throttle responce in lowering areas of my timing table. When I get knock at a particular part of the timing table I retard it .5 degrees, burn a chip and do it again and so on. I am up to 25 burns just for my timing table and it keeps getting stronger. My question for you is how much timnig can I take out? Do I keep retarding the timing table until all knock is gone, or do I keep taking it out until performance starts to suffer? At lower LV8 I have higher timing as RPM and LV8 goes up my timing starts to come down. My timing varies from 34.5 at the most to 17.6 at the least. When is not enough to much?


Thanks
Tom

Grumpy

Over the years, I've spent a ton of time with various ignitions. The DIS that electromotive does, while better then oem still leaves some to be desired.
That is:

When you use any reluctor/ pickup timing set up you have the problem of always figuring the A/C generator effects of the system, the reluctor setup is an A/C generator and while they use a 0 crossing electronics setup there is still some **creep** to the timing. If you look at some of the gm disassemblies you'll see a timing correction for this.

Now if they were using a halle effect sensor that would be tns better.

But, the really (in my opinion) part of the DIS scheme is using the capacitance of the *plug at overlap* to force the at ignition to fire.

Having just changed from DIS to CNP I can tell you the results are noticeable.

Grumpy

The answer is a stop watch.
G-Tech or a roll your own (there is one using an audio setup tied into a spark plug, but we're getting off thread here).

When your tuning for performance, you just have to use a timer. Your buttometer is just too often wrong.

What you might try is adding a slight amount of fuel BEFORE the onset of detonation. Once detonation starts, it takes a big pull of taking timing out to stop it.

formula5

I have found that with my car liked less total timing than stock but it wants to come in by 3600rpm vs 4800rpm.Yes less than stock.Actually about 30d total is just right.Much more and there is detonation.But bringing it in my total sa by 3600rpm has knocked off a bit off my 1/4 mile time.I would say finding @ what rpm to bring the total timing in by is a major part of tuning that is overlooked.

Grumpy

Just looking at where your peak torque is should answer that question rather well.

Grim Reaper

Thanks for the "Final Answer' post, I've missed these.

I am not feeling well right now, but I have some more "thoughts" on this subject that I would like to add later. Hopefully, we can keep this thread alive as the Spark article for future references.

I am going to have to chat with Tim about taking some of these posts and incorporating them into a FAQ for Eprom burning and tuning. Unfortunately, people seldom read those and it turns into a lot of work for nothing.

Matt87GTA

Well, you can count on me to point people to a FAQ section if one is created. If we work together on this, we can really save on the typing and re-stating of the same things.....

BTW, good stuff Grumps .

ryn28

iTrader: (1)

I really like reading this kind of info. It helps greatly with understanding the inner workings of what's going on which makes tuning that much easier.

While I slip up once in awhile asking a question that maybe I should have searched the boards a little harder for, I see it's not as easy to find Traxion's Intro to Prom Burning as it was before. I must have gone back to that thing a dozen times in the very beginning and it was helpful each time. It has to be accessible and easy to get to. I know that doesn't guarantee people will read it but it helps.

Beast4

The comments about getting to the info are right on target. I was fortunate to stumble across the main site first and then the forums and was really impressed by the amount (and quality) of the the techical articles/info there. Clearly many of the posts on this board are by people who have not seen them - the classic example is the number of T-5 and T-56 swap posts, most of the issues being well addressed in the tech info section.

Having done a lot of work to an '85 IROC in '85 and '86, when I had to "wing it" on just about everything I can REALLY appreciate the advantage of having the kind of people on this board that we are so fortunate to have. To Grumpy, Traxion and all the others who have contributed so much, thanks and "Well Done".

Ken73

Grumpy, this goes back to what you and I had discussed in e-mail - the MSD 8610 that I found in the Smokey Yunick book. It's basically a "coil selector" according to the text. Looks identical to a distributor with the exception that it doesn't have plug wires coming off of it.

I certainly would love to see this make a comeback! Here's a pic as proof it existed at one time...



I don't know the downfalls of this system though, if there are any. I had thought about replacing the contacts in the plug towers on a normal distributor with hall effect transistors, and putting a magnet on the tip of the rotor. Use the pulse from the pickup to fire the selected (via magnet/hall effect sensor) coil.

If I'm not mistaken, this allows the use of much smaller coils, doesn't it? More recovery time, less heat generated...?

Ken

Grumpy

Like most of Smokey's stuff he was a decade or two ahead of most everyone else.
Yes, with the less heating of the coil, it can be smaller. Also means less Cu is used. You'll note the newer GM have reduced the wire size on the ecm wiring.

The advent of the Smart Coils is really something. It is amazing what the spark looks like off these lil coils. That and there is little fear of generating RFI.

boostcreep

Got a question here...if a cd applies 400v to the coil, and a regular ignition only uses 12v, does that mean that the cd ignition can develope more voltage at the plug? And could someone explain how heat affects the performance of an ignition coil in a little more detail? I'm a little cornfused here. Thanks

Grumpy

In simplistic terms:

An ignition coil has 100:1 ratio, at 100:1 that would mean the secondary voltage would be 1,200 volts (just making a point here).

What happens is as the **point** open siganl starts the primary field collpases, and as the field collapases it starts a secondary voltage, but it isn't enough to jump the gap on the plugs, so it collapses again thur the primary, and that field collapses a this **RINGING** eventually raises the voltage to a high enough level to fire the plugs, mind you this only takes a .1 msec but it does take time.

Now with the DCI ignitions and 400+ volts across the primary with that same 100:1 coil you have 40,000 volts instantly on the first collapse of the igntion to fire the plugs. This raising of the voltages is called the *raise time*.

This fast raise time is of particular value when starting and at high rpm, and by high rpm we're talking 8K+ RPM.

The problem with the last raise times, is that they don't have anywhere near the same amperage output as a slower system, so to make up for that, they fire the plug several times.

The *extra* voltage available is called reserve voltage. For any given circumstance the plug takes X amount of voltage. A CDI *MAY* allow for better firing of fouled plugs, but that in it's self can be more of a problem, then a cure. ie driving around with a plug that is firing in a chamber with a problem that you can't feel, means it will be at a higher level of failure before you look for the problem. While a exhaust valve that needs adjustment with conventional system, and just adjusting the valves would cure, would easily go to where the valve is actually burnt before you'd really feel it with a CDI. So if you do go with a CDI, you have to be especially sensitive to misses and curing them immediately.

formula5

Sounds like the spark tables will need to be reworked after cdi.What would you say grumpy, less overall timing?

Grumpy

If you want to actually figure out how much, go to MSD or some other CDI manufacturer and compare rise time (as they claim), and see how far the crank moves in the difference in rise times.