Twin plug question

[super9064]

At what point do you need to consider twin plugging. And why not just split the spark from a standard distributor instead of a custom or very expensive dual distributor.

I'll be building a engine for my SWB at some point, I have 3 long blocks to chose the original Aluminum case, a 73.5 7R, and a 2.7 race prepped case.

[longhornchris04]

Electricity takes the path of least resistance... so splitting the spark doesn't ensure both plugs fire with the same energy, if at all. Additionally, providing multiple paths changes the resistance, which in turn changes the voltage across the gaps.

If the point of twin-plugging is to get better control of combustion propagation in the chamber and prevent secondary ingnition (knocking), then unpredicatable primary ignition seems to be the last thing you would want.

Put another way, if it were that easy then there wouldn't be so many other options for twin plugging.

[super9064]

I knew there would be a good answer to this, I just didn't know what it was, Thanks.

With the original 911R/ 906 spec motor they used twin plugs because the piston had a high dome and the head design not optimal. but what if you are using later heads, like from a 71T, that flow better, and compression that is 10-1 or less. Cost no object , great, but it looks like twin plugging adds at least $3,500-4,000 to the build. I'm trying to be a bit more economical these days while still being extravagant.

[longhornchris04]

Short answer - under 10:1 and <98mm bore you should be OK. A mild compression 2.7 should be fine. High compression not so much, same for a big-bore motor.

Of course, if you have the money you could always spring for the machine work for the twin-plug and just put old plugs in until you can splurge for your choice of spark options. If you are in the upper range of safe single plug compression (running conservative timing) you will benefit from twin plugging (and more aggressive timing).

Explanation follows:

Best info I can give you is out of Wayne's book + some general physics. The hemi-style combustion chamber (which Porsche's have, so do Harleys, it ain't just Dodge) with its high dome and off-center spark plug isn't great for flame front propagation, at least when compared to the more modern 4-valve center-plug design.

For a low compression engine, this is less of a problem. In order to increase compression, the piston dome is raised... to the point where it can actually prevent the flame front from crossing to the far side of the cylinder if the spark-advance isn't high enough. This of course raises the risk of spark knock. So, if you aren't running high compression (under 10 is usually the "magic number" but its actually dynamic compression which is also tied to cams, and the ability to keep the chamber temps in check) then the flame should easily cross the chamber.

With the large chambers (>98mm), the key problem is the distance between the spark plug and the far side of the chamber. Again, the options are either a controlled second flame front or more advance to get the flame to cross the chamber.

[Steve@Rennsport]

Originally Posted by super9064

I knew there would be a good answer to this, I just didn't know what it was, Thanks.

With the original 911R/ 906 spec motor they used twin plugs because the piston had a high dome and the head design not optimal. but what if you are using later heads, like from a 71T, that flow better, and compression that is 10-1 or less. Cost no object , great, but it looks like twin plugging adds at least $3,500-4,000 to the build. I'm trying to be a bit more economical these days while still being extravagant.

The later heads (2.2-2.4-2.7) are not as octane sensitive as the 2.0 heads so you can get away with a bit more compression before requiring twin-ignition on pump gas. Bore size & camshaft profile plays a big role here, as well.

People having access to 93 octane gas have more flexibility than ones with only 92, too.

More information on this subject can be found here http://www.rennsportsystems.com/2a.html

[afterburn 549]

If you have your heads off just have them drilled for double plugs, then you have options for later

[Cupcar]

Aren't there some increases in HP due to a reduction in negative work done during the cylinder pressure cycle (because there is less timing advance required)?

Given the above, it is harder to get that last little bit of compression with a second plug drilled into the combustion chamber, could this negate the above?

If the advantage only goes to decreased octane requirement, then if one is building a track engine for use on 110 or greater octane fuel why run twin plug at all - especially when the compression obtained could be higher with a given piston, not to mention doing the job with a cheaper, simpler, lighter ignition system?

Thoughts on these points...

[afterburn 549]

Originally Posted by Cupcar

Aren't there some increases in HP due to a reduction in negative work done during the cylinder pressure cycle (because there is less timing advance required)?

Given the above, it is harder to get that last little bit of compression with a second plug drilled into the combustion chamber, could this negate the above?

If the advantage only goes to decreased octane requirement, then if one is building a track engine for use on 110 or greater octane fuel why run twin plug at all - especially when the compression obtained could be higher with a given piston, not to mention doing the job with a cheaper, simpler, lighter ignition system?

Thoughts on these points...

I am not sure how a spark plug would take away from compression or C/R.
With Hemi style chambers the big problem is getting the flame front over the top of the piston crown.
With twin plugs this problem is circumvented and as a plus the timing can be back offed with out performance problems

[kenikh]

That said, twin plugging 9.5:1 and above will see great benefits in your ability to time the ignition ideally. With twin plugs, you will need much less advance than a single plug application and get snappier throttle response and some power increase.

This isn't my thinking, this is from Steve Weiner @ Rennsport Systems, so you can take it as gospel.

[longhornchris04]

Higher octane gasoline doesn't burn significnatly faster than the lower octane stuff... just more predicatably and w/ less chance of unintentional detonation. As a result, even with race gas you still have to get the flame front across the cylinder to the far side. With high domed pistons this requires a lot of advance - hence the negative work, or for high enough RPMs just can't be done - resulting in unburned fuel and lost power.

Ignition timing is complicated... almost a black art, but the science is fairly straight forward. The goal is to fire the plug early enough that peak pressure occurs just after TDC. This isn't, the only consideration though. Others include
- complete combustion - unburnt fuel/air is wasted power
- knock protection - from the spark side not allowing pressure/temps to exceed fuel autoignition conditions (higher octane, higher autoignition characteristics)
- Emissions - the higher the combustion temp, the more NOx produced.

To complicate matters, a lean fuel mix burns faster, a rich one slower. A rich mixture is also initially cooler, as the fuel absorbs heat during vaporization. This is why going lean is so dangerous - your chances of detonation go way up as the mix is at a higher initial temperature and generally more volatile. Its also why vacuum retard was added - it is partially tied to leaning out the idle/part throttle while the fuel system leans out the mixture for emissions & fuel economy reasons.

Additionally, the higher the RPM, the increased chamber turbulence better mixes the fuel/air. This is why spark advance is "all-in" early... the increased mixing increases combustion speed enough that more advance isn't needed.

Of course in a more modern spark tables take into account both RPM and either MAP or MAF inputs to correct the timing for RPM and load. These are tied to the fuel tables to run closer to the limits.

So, twin-plugging effectivly cuts the chamber in half which reduces the amount of advance needed. This in turn causes
- more complete combustion, especially at high RPMs - more power
- increased dP/dt - less pressure rise before TDC - less power lost
- more stable combustion - less mix for each plug to ignite

I'm sure its possible for you to run a "small" bore high compression in single plug configuration if you keep the timing conservative, but at that point you've created an unbalanced engine, with your ignition being the limiting factor.

In my opinion, this is no better than building a 400hp motor and trying to mate it to an unmodified 915 gearbox... you won't be able to use it to its potential.

[longhornchris04]

Originally Posted by kenikh

This isn't my thinking, this is from Steve Weiner @ Rennsport Systems, so you can take it as gospel.

Or what he said.

[Cupcar]

Originally Posted by afterburn 549

I am not sure how a spark plug would take away from compression or C/R.
With Hemi style chambers the big problem is getting the flame front over the top of the piston crown.
With twin plugs this problem is circumvented and as a plus the timing can be back offed with out performance problems

The second plug drilling looks to me to add about 1 cc to the combustion chamber volume. See below photo of a plug recess.

One extra cc in the last engine I built would have lowered the CR from 10.2:1 down to 10:1

[afterburn 549]

Originally Posted by Cupcar

The second plug drilling looks to me to add about 1 cc to the combustion chamber volume. See below photo of a plug recess.

One extra cc in the last engine I built would have lowered the CR from 10.2:1 down to 10:1

True, I do not see that as a detriment. ( with the plug installed and all,, are you sure it would come dwn that much ? Just flip the head over with the plug in the hole and see how much you can get in there B4 running over )

[Cupcar]

Response to ^^^

I don't have a head in hand to check right now. There is not too much plug projecting into the recess though, just the electrodes and a bit of the porcelain insulator.

One CC makes a 0.2 point difference at the 10.2:1 C.R. level of the example engine I gave, the sensitivity to volume increases as the C.R. goes up though, adding one CC would have dropped the C.R. by 0.5 points if my engine were at 11.5:1.

The twin plug engines I have built do have great throttle response and seem to run smoother, but that is a subjective thing.

For street engines, the sensitivity to knock is decreased with twin ignition and one can use lower octane fuel for a given compression ratio. This is a huge advantage I think for the hot rodder.

The lower NOx of of the twin plug is good for smog emissions- something hot rodders do not worry about.

So twin plug has definite advantages.

However, if an engine were built that was to run on 110 octane fuel only, where knock was not an issue, and the choice was 11.5:1 C.R. and single plug or 11:1 CR and twin plug, I am not sure which would make the greater power.

[cgarr]

We always use the 12mm plugs for the second and use a very small relief, one cc or less may be about right, then if you take off 3 to 5 thou off the sealing surface it may be about a wash.

[psalt]

To complicate matters, a lean fuel mix burns faster, a rich one slower.


Where did this idea come from ? This is incorrect, in fact the opposite is true. Lean mixtures burn slower. One demonstration of this is the lean intake backfire, which many CIS 911 mechanics are familiar with. The cold running mixture is too lean, for a variety of reasons, and the mixture is still burning when the intake valve opens and the fuel in the intake ignites. The fact that lean mixtures burn more slowly is the reason vacuum advance, not retard, is used. The less dense mixture at part throttle, low load cruise needs more time to burn completely and the engine can tolerate the extra advance because the cylinder pressure is low. This results in higher efficiency. Vacuum retard is strickly an idle H/C emission test device, it works mainly because of temperature, and has nothing to do with mixture formation. Ignition timing does not effect mixture preparation, it is adjusted in response to it. Conditions in the cylinder vary considerably under different loads and many absolute statement are false because of it. What happens at rest under idle conditions is largely irrevelent to what happens under load. Unfortunately, many misconception are created by playing with engines at idle.

[Cupcar]

Originally Posted by cgarr

We always use the 12mm plugs for the second and use a very small relief, one cc or less may be about right, then if you take off 3 to 5 thou off the sealing surface it may be about a wash.

I twin plugged a 2.7 head with the big plug recess volume like the one above once and ended up with a 69 cc combustion chamber volume. It's hard to make a high C.R. starting with that.

[eapcpa]

Paul wrote:
Where did this idea come from ? This is incorrect, in fact the opposite is true. Lean mixtures burn slower. One demonstration of this is the lean intake backfire, which many CIS 911 mechanics are familiar with. The cold running mixture is too lean, for a variety of reasons, and the mixture is still burning when the intake valve opens and the fuel in the intake ignites.

This is contrary to what I have known for years. Without getting into a long and complicated discussion, can you provide proof of your claim. I think you misunderstand the entire concept, but I have also been wrong before. I am eagerly awaiting further explanation of your theory.

I am quite surprised that someone has not responded to your claims already, unless you are correct. On the CIS engines, I thought the mixture burned too fast and backfired into the intake before the valve closed. ????

[psalt]

On the CIS engines, I thought the mixture burned too fast and backfired into the intake before the valve closed. ????


Hello Ed,

On a 4 cycle SI engine, the spark plug does not fire until after the intake valve closes at the beginning of the compression stroke. How does that fit with your suggestion ?

If you want to clear up your misunderstanding, I suggest a basic book on combustion science or BOSCH's "Gasoline Engine Management". Like I said, most absolute statements about SI engine are false, because conditions vary so much the answer is usually not a single point or linear relationship, but a peak with contradicting curves on either side. For example, the generalization that "leaner mixtures are hotter" by itself is false. A lean mixture by definition is above an AFR of 14.7:1. EGT actually peaks around stoich and falls off as the mixture gets leaner. One reason pilots are trained to use "rich of peak" and "lean of peak" fueling. "Leaner is hotter" is only true rich of stoich.

If you want to understand fuel burn rates, there is a good chart on page 127 of Jeff Hartman's EMS book. Almost any basic text on fuel and ignition timing will state that a rich mixture burns faster than a lean mixture. Flame speed can vary from 20 to more 100 ft/sec. Burn rate is highest at a rich mixture near 11.1:1 AFR and drops as the mixture leans. It also drops as the mixture gets richer than 11:1, but in a N/A SI engine, you are generally above this outside a subzero cold start. Understanding an ignition timing map can also help. The spark plug is fired to create peak cylinder pressure around 14 degrees ATDC to maximize work done. The burn rate varies with mixture, but is relatively constant for a fixed mixture. Obviously, as rpm increases, you need to fire the plug earlier, because the window of time to accomodate the burn speed gets shorter. Lets say you determine that 30 BTDC give best torque under WOT. Now, at part throttle, low load cruise, the same ignition timing map will give another 10-15 degrees of timing advance. Why ? Because the leaner, less dense mixture supplied under light load takes longer to burn and if the ignition point was not advanced, more of the energy in the fuel would be wasted in the exhaust. If a lean mixture burned quicker than the rich mixture supplied at WOT, the ignition timing would not need to be advanced.

[longhornchris04]

Paul - thanks for the correction. I appreciate having my mistakes/misunderstandings pointed out so I can learn.