LS1-450

iTrader: (33)

Purpose of the discussion is to determine the of effect spark on combustion. I'm confused by the actions of some contradicting the statements of others.

1) I know that there are Drag racers whom change plugs to re-gain a couple of lost tenths, even after they started on new plugs that haven't fouled & there is no high RPM missfire while racing.

a) Apparently, the resistance of the plug increases; therefore causing reduced voltage in the combustion chamber which equals lost tenths?

2) Why do guys dimple the electrode of a spark plug & then drill a hole in the ground strap above it to improve combustion? I know that this modification increases fuel economy.

3) Why do so many go to great lengths to reduce the resistance between the coil & the combustion chamber? Although, it should be mentioned that this usually ends up being only the wire while the spark plug resistance is completely ignored (even though it's the largest resistor in the circuit).

4) Why go through the trouble of indexing plugs so that the open end of the electrode is toward the center of the combustion chamber, if spark is not really important? Why are indexing shims available if indexing isn't important?

5) Why do so many claim that spark is spark & the power or application of the spark makes no difference as long as there is spark in the combustion chamber?

6) Why have auto manufacturers gone to coil on plug designs?


It is my experince that spark does effect combustion. Would truly like to confirm how power of the spark effects engine power.

V=I*R, I=V/R
---------------------------
7000 Ohm spark plug
750 Ohm plug wire
4000 Ohm from the plug gap
----------------------------
R=11,750 Ohm
V=20,000 V coil (for conversation)
I=1.7 Amp


Scenario 1: Change plug wire to low resistance type of 25 Ohms

750 Ohm x 1.7 Amp = 1,275 lost Volts
25 Ohm x 1.7 Amp = 43 lost volts
-----------------------------------
= 1,232 Volts gained in the combustion chamber

Does this Voltage increase result in added power &/or combustion benefits?
If not, how much would?


Scenario 2: Reduce the spark plug resistance to 500 Ohms, if this is possible.

7000 Ohm x 1.7 Amp = 11,900 lost Volts
500 Ohm x 1.7 Amp = 850 lost Volts
-----------------------------------------
= 11,050 Volts gained in the combustion chamber

How would this combustion chamber voltage effect power &/or combustion? Seems to me that @ minimum, there is an advantage in measuring plug resistance & then using a plug, within the selection of plugs with the least amount of resistance.

How do these voltage increases effect cornel shape/size?

Am not sure how to proceed w/ a gap change scenario. That gap seems to be a leading cause of missfires & what works as a set gap doesn't seem to be changeable without compression, timing, or temperature chamges.

Someone must have @ some point in time sent all of the voltage they could into a combustion chamber, on an engine dyno & measured the results. How 'bout just a chamber pressure or chemical measurement made under the same "higher voltage" scenario?

DanO

Ok lets start with the basics... of sorts

Spark ignites the mixture, this much is understood

Spark timing is set so that the maximum amount of fuel (chemical) energy can be extracted into useful mechanical work... through pressure converted to mechanical energy.

With a given engine (combustion chamber, compression ratio, fuel properties, in cylinder mixture motion) there is an optimal spark advance that results in the most torque. This advance point is known as MBT (maximum brake torque) timing. Advancing or retarding the ignition would result in lower output torque.

Now there is alot that must happen and can alter the events between spark and torque.

First you have to look at ignition delay, there is some finite amount of time between the spark plug firing and the flame front developing.

Next, the flame travels at a certain speed across the combustion chamber. This is highly dependant on combustion chamber design and fuel properties. Some fuels burn slower, some burn faster.

This flame results in high pressure being formed in the combustion chamber. You ignite the fuel so that the peak pressure point occurs after top dead center. This is reffered to as LPP (location of peak pressure). Typically the optimal point lies somewhere between 10-15 degrees after top dead center. So spark timing, therefore is only set so that the LPP is set properly. This is why some engines make the most torque with 30 degree spark advance and others make it with 15. This means that the combustion chamber design on the 15 degree advance engine allows for a quicker burn yet still has a LPP at the optimal point.

Now on to your question.. First i will say that as long as the fuel can be ignited properly, there is no benefit in many of the gimicks that are seen in aftermarket.. HOWEVER..

Spark and resistance across the gap is much higher during the high pressure compression event. Sometimes, the ignition system cannot overcome this and does not ignite the fuel, this is sometimes refered to as spark blow out, which is why people look into high power ignition systems, but modern ignition systems (ls1, ls3, etc..) are relatively high power already and you dont see much aftermarket in spark systems on these engines.. They can consistently ignite the fuel mixture under many condiitons

Now for the orientation. the theory is that they do not want to have the electrode disrupt the flame front formation. They want the flame to form and travel to the heart of the combustion chamber with little disruption.

To take that one step further, you will see many race engines that utilize 2 spark plugs. This is to ignite 2 flame fronts at the same time, thus allowing the fuel to burn quicker. So the ignition advance can be decreased, yet still maintain LPP and hopefully increasing the "peak" pressure because it burns quicker.

LS1-450

iTrader: (33)

DanO, Thanks for the reply.

Regarding your last two paragraphs:

[QUOTE]"Now for the orientation..." [QUOTE]
I am interpreting this paragraph (2 sentenses) to be an explanaition for the importance of spark plug indexing.

[QUOTE]"To take that one step...."[QUOTE]
So, if 2 simultaneous flames burn fuel quicker, wouldn't a single larger flame accomplish a faster burn as well? Probably not as fast as the double flame, but.....


What about the dimple in the electrode & drilled ground strap modification? This does change something because I have experienced an increase in fuel mileage.

Why do racers change plugs after a few runs?

LS1-450

iTrader: (33)

DanO, or anyone else whom wants to chime in;

Everything asked in my reply to your post are questions. All questions (ignoring the fuel mileage increase comment), are asked in an effort to gain a better understanding & are not writtne as "matter of fact."

Please comment.

DanO

yes the orientation was the explanation of indexing. Its the theory behind why they do it.. the true sensitivity of plug indexing would require testing using in-cylinder pressure measurement.. i.e. it may be of marginal impact, or may play a significant role.. (if i were a betting man it may have less impact than one would think.. likely not measureable)

as for the "larger flame" commment... Spark ignites the mixture.. you cannot get a larger or faster flame by having a higher voltage spark. The key for spark is to have enough voltage to ignite the mixture on a consistent and repeatable basis.. the fuel and air chemical reaction provides the flame..

If you have a poor ignition system, there is benefit in upgrading.. the main reason is that you may not be igniting the fuel air mixutre at the right time, consistently, or even may missfire.

If you have a modern ignition system, coil near plug, coil on plug, etc.. there is little benefit in upgrading. OEM's have an interest in fuel economy and emissions.. ensuring the mixture is ignited consistently and properly leads them to have fairly decent ignition systems now (especially since the days of points)

The spark is like the finger that starts the domino effect.. as long as you tip the first domino over thats all the finger can do.. The trouble is.. you need to make sure the finger is strong enough to tip over the dominos, make sure there are the correct amount and spacing of domino's, and make sure you push them over at the right time (your friends always get mad when you push it over too soon..). Also 2 spark plugs is like starting your line of domino's from both ends.. the total time is significantly reduced

However, you can also use another ignition source besides a spark.. i.e. use burning fuel directed into the chamber to ignite the mixture.. this has benefits becasue it can ignite more than one location.. but this topic is meant for another thread..

1 FMF

iTrader: (1)

http://ngkaz.home.att.net/wsb/html/v...ome.html-.html
check out the white papers for starters, if i can find some more reputable tech papers on the subject i'll post them.

The V = I * R does not necessarily apply or isn't that simple. Because of the spark plug gap, it is an open circuit. As the coil discharges voltage builds in the circuit between the coil and the spark plug gap, there is no loss because the circuit is open and no current is flowing. It isn't until the voltage potential is great enough that the electric arc forms across the plug gap, and it ionizes the air/fuel mixture between those two points to form a conductive path. That's all i can remember at this point,

4: the "theory" behind indexing was either to face the open portion of the gap towards either the intake or exhaust valve depending on the head design, or just towards the center of the bore in the hopes of better combustion. It is debatable. I've never seen conclusive proof that it helps. the most valid logical reason for indexing was to gain extra clearance between the ground strap and piston in really high compression engines.

5. because it is not the spark that results in the piston moving down resulting in work done, it is the combustion of the air/fuel mixture and the expansion of those gases. It is easily understandable to think that if you have a really weak spark then you will have poor combustion, all the way to the point of misfires where combustion doesn't happen. But I fail to see the logic where greater spark energy keeps on increasing combustion. There is a point where more than a certain amount of spark energy is nothing but wasted energy, only melting and wearing and overheating the spark plug electrode.

6. very simple: cost & reliability. no wires means less maintence, less cost to build. Also means less radio frequency interference since the spark plug wires are a major cause of RFI.

Silver2000WS-6

iTrader: (1)

Not trying to hi jack but I am also interested in spark/timing issue.

First off I am pretty educated in Thermodynamics so if it gets deep just keep explaining cause I will probably understand it. My question lies in why you want more timing. Lets say you have and engine and you have two tunes for it. One tune is for 87 octane and the other is for 93. Lets pretend that both are dead on where the highest pressure due to combustion occurs directly after TDC. Lets say that the 87 tune needs 22* of timing to reach this point and the 93 tune needs 28* of timing. Why is the 28* tune superiour to the 22* tune. Both fuel/tune combinations allow the highest pressure to happen at the "Magic Spot" so why is one superiour? Is it because the 28* tune allows for a longer combustion there for allowing the pressure to be higher? Thanks for any help I just want/need to be put in the right direction. Again don't be afraid to get to technical.

,Chase

yomama

iTrader: (9)

[QUOTE=LS1-450;10464106]Purpose of the discussion is to determine the of effect spark on combustion. I'm confused by the actions of some contradicting the statements of others.

1) I know that there are Drag racers whom change plugs to re-gain a couple of lost tenths, even after they started on new plugs that haven't fouled & there is no high RPM missfire while racing.

a) Apparently, the resistance of the plug increases; therefore causing reduced voltage in the combustion chamber which equals lost tenths?

Fresh plugs always help, and they may be reading plugs to adjust for mixtures
I dont think its bad practice to change them after every run when your racing for big dollars.

2) Why do guys dimple the electrode of a spark plug & then drill a hole in the ground strap above it to improve combustion? I know that this modification increases fuel economy.

This can be done for several reasons
1. The flame kernal produced by these modifications has a better burn rate and could aid in a more complete combustion.
2. The spark will only jump from the edge of its electrode, the more edges you have, lessens the chance of a high rpm misfire.

3) Why do so many go to great lengths to reduce the resistance between the coil & the combustion chamber? Although, it should be mentioned that this usually ends up being only the wire while the spark plug resistance is completely ignored (even though it's the largest resistor in the circuit).

4) Why go through the trouble of indexing plugs so that the open end of the electrode is toward the center of the combustion chamber, if spark is not really important? Why are indexing shims available if indexing isn't important?

Again flame kernal and flame propagation is the main advantage of indexing.
Where the flame starts is important, especially when you have dome or popup pistons.

5) Why do so many claim that spark is spark & the power or application of the spark makes no difference as long as there is spark in the combustion
chamber?

For the most part this is correct, once you get past a certain voltage it really doesnt matter how much more voltage you have as long as the spark is timed and doesnt blow out(ie forced induction). Higher voltages will just burn components up. Look at high end dragsters with dual magnetos

6) Why have auto manufacturers gone to coil on plug designs?

Better design, higher voltages, no spark plugs wires to leak/replace. Also they needed to be able control/monitor each cylinder individually for emmisions and driveability.

DanO

The problem with lower octane fuel is that you cannot usually get the "proper" advance for MBT timing. Higher octane fuel lets you get closer to MBT.

Your tune for 87 octane will likely have to sacrifice the location of peak pressure to prevent knock. thus its maximum advance is 22 degrees and the LPP could be 20+ degrees ATDC

The 93 tune will likely come alot closer to having MBT timing thus can run 28 degrees. and bring the LPP to maybe 20 degrees ATDC.. but your maximum advance may still be limited by knock

Then you can run race fuel and run MBT timing which could be 30 degrees for that engine and rpm. That would pull the LPP to the proper location.

(the numbers i used here are arbitrary, but show my point)

More timing isnt always good... but most people think so because with regular pump gas, you run into knock before you pass MBT

Silver2000WS-6

iTrader: (1)

Thanks a lot this is what I wanted to know. Where I was wrong was I was thinking that with either tune/fuel you would be able to reach MBT. Again thanks for clarifing things. How common is it to be able to reach MBT with 93 on a stock say ls1 engine?

,Chase

DanO

Totally depends on what operating condition, airflow, rpm, air temp, barometric pressure.. etc.. but either way i cant give you a hard number.

but another note, when an engine is designed.. it is designed with the fuel octane rating in mind. thus the lower grade fuel can get closer or hit MBT timing in that engine design. However, the engine that is designed for the higher octane fuel takes advantage of that will make more torque at MBT timing (if somehow both engines are at MBT timing).

99blancoSS

iTrader: (115)

I've got some new (new company) iridium plugs on the way and all the testing info. As soon as I get it I will post the data. They are suppose to be better and have proof. I believe its pertinent info to this discussion.

Silver2000WS-6

iTrader: (1)

Is this because the combustion takes longer "because of the higer octane" there for the pressure is higher hence the torque is greater? Again thanks for the help.

,Chase

DanO

no, its because you can run a higher compression ratio and get a higher cylinder pressure.. for example.

flame speed, "burn time", etc.. are all relatively similar between 87-93 octane fuels (or close enough for this argument) They are not as interrelated as you think.

Silver2000WS-6

iTrader: (1)

Ok I think I am getting closer to understanding now. Lets say we have an engine and MBT can be reach on 87 octane and 22* adv timing. Lets assume that with the same engine on 93 octane and 28* adv timing MBT can also be reached. Would either fuel have an advantage since they can both reach MBT with different tunes?

,Chase

Sbertolone

iTrader: (5)

the difference in the fuels is that the 93 will tolerate a higher cylinder pressure before detonation, the higher cylinder pressure can be moved closer to mbt, therefore more timing advance.

Gh0st

iTrader: (3)

Oh ok, i guess energy plays absolutely no role

WeaponX_Perf

It plays a role, even the location of the spark and the way the flame front propagates plays a role. I have quite a bit of information, including SAE papers, to post in regards to this.

As a simple test, anyone with a flash tuner can drop dwell from factory to ~1.3mS and see the effects of less spark energy in the combustion chamber. If what some say is true "a spark is a spark", people should gain horsepower because the energy charging the ignition is drawing less then half of it's previous load from the alternator. I would wager there would be a power loss meaning ignition energy has a direct effect in "in cylinder" engine performance.

Gh0st

iTrader: (3)

Yeah that makes sense. If your dwell is lessened you're not providing enough spark time burn the higher energy(slower burning) gasoline.

EdmontonSS

I think, along with the other posters this is mostly done for reliability purposes, new plugs are cheap insurance against blowing out a spark on a run, that being said, new plugs take a lot less voltage to jump the gap than even slightly used ones. I imagine that consistency in paramount.

The arc likes to jump from edge to edge, as mentioned above. I believe drilling the ground strap may allow the flame kernel to propagate from the hole as well.

There is very little point in reducing the resistance in the wire or plugs. The secondary windings of the coil build up voltage until the spark plug gap is ionized (technically the air and fuel molecules across the gas are ionized). There is no current flow until enough energy is put into these molecules to ionize them, once the current does start flowing the ionized gap has the equivalent of resistance, as does the secondary of the coil itself (especially considering inductive loads). The resistance put into the plugs and wires tune out EMF noise. There would be higher current flow without resistance plugs and wires, this would be a "hotter" but shorter "duration" spark, but the total spark energy would not change. I like premium wires for the better materials and insulation, but don't expect power gains. My guess is changing out a 5000 ohm wire for a 10 ohm wire would only make a few percent difference in current flow across the plug gap. Negligible gains in most cases.

Indexing is important as it allows a consistent flame front to occur cylinder to cylinder. The ground strap of the electrode blocks the flame front from propagating in that direction. In a street car, I'd never bother, but in any class racing, you'd be crazy not to index the plugs, IMHO.

Spark is not always spark, a hotter, longer duration spark across a bigger plug gap will cause a larger flame kernel to form in the same amount of time. This is somewhat equivalent to advancing spark timing, but not truly. Having a smaller, weaker spark, and subsequently advancing timing back to MBT is not as effective as retarding timing a couple degrees and having a "hotter" spark. This is because, as the flame kernel is propagating, the pressure in the rest of chamber (caused by the kernel over and above the pressure increase caused by the decreasing chamber volume) is rising. The earlier the spark occurs the more time for the rest of the chamber to get to the temperature (caused by the increase in pressure) for auto-ignition to occur. This would then typically lead to detonation.

I believe the manufacturers need a large high energy spark to get large flame kernels and clean burns at low loads (hence the large factory plug gaps). Also I believe making the plug wires short has kept the reliability of the newer engines way up. Extra reading found here: http://www.wellsmfgcorp.com/pdf/counterp_v4_i3_2000.pdf

If you wanted, non-resistor plugs are available. The resistor materials used in the plug don't resist the same using a 9V multimeter vs. how the material acts while conducting high voltages (this information direct from Champion). You will see very little variance in new plugs anyways.