advantage of twin plugging?
 

Hello all.

While the benefit of twin plugging is obvious, how exactly does it reduce detonation? I know you increase CR a full point with the addition of two plugs. Is it because most systems fire the plugs one right after the other for a more complete burn?

What do you think?

TIA

Chris

Nope, they're fired simultaneously and ignities the mixture in two different places, allowing for faster, more complete burn--more power, less pinging. Does wonders for torque.

No. They fire at the same time.

To understand how this works to avoid detonation, you need to understand detonation. I'm not trying to sound pedantic, so I hope I don't irritate anyone. Detonation is where the fuel/air mixture in the cylinder reaches a temp/pressure combination high enough to spontaneously combust. When the spark plug fires, it creates a fire that spread out in a spherical pattern from the source (spark plug). As the flame front moves across the cylinder, it builds up pressure in front of it. Detonation can occur if the critical temp/pressure combination is achieved before the flame front reaches the other end of the combustion chamber. Detonation, when it happens, is the last bit of fuel spontaneously combusting (pop) under the increased pressure.

I think twin plugging avoids this high-pressure region. Since there are two ignition sources, the fuel burns quicker, and also it allows for a cushion between the two flame fronts. Someone will probably understand the twin-plug effect better than I, but I hope this description of detonation at least helps.

Maybe I'm slow but think the benefits are anything but obvious.

Two spark plugs mean the flame front travels a shorter distance. Shorter distance has two benefits: the burn happens in less time and the burn happens at a more consistant rate (longer flame propagation means more variation in burn time).

It is the consistancy that allows a higher compression ratio without uncontrolled combustion.

The faster burn helps too because you can now reduce the timing advance while maintaning complete burn. Less advance means the charge is resisting the compression stroke of the piston less, thereby creating more power.

Awright, so I just get a drill press and a tap & die set, and pop a new spark plug hole into the head, right? ;)

Seriously though, is there a reasonably affordable way to upgrade heads, and is it really worth it? Let's talk actualy HP or torque numbers for 1 vs. 2 plugs. Any one have these measurements?

-Boyo

From the new book:

The 911 cylinder head does not lend itself to ideal, uniform ignition. The two valves in the center of the head take up the majority of the room, forcing the spark plug to be located off to the side. The ideal place for the spark plug location in the head would be in the center of the cylinder head, slightly offset to the exhaust side which is the hottest part of the head. When the spark is fired and the mixture is ignited, the flame front expands across the combustion chamber. Installing two plugs per cylinder increases the efficiency of combustion, and reduces the time it takes for complete combustion to occur. Having this ideal combustion also means that the ignition timing advance can be reduced, because the spark can be fired closer to the top dead center. Reducing the ignition-timing advance can reduce the operating temperatures of high compression engines. Most twin-plug engines will achieve their peak horsepower output at about 23-24 degrees of total advance, compared with the typical 30-35 degrees required for single plugged engines. Because the combustion process is more efficient, it doesn’t need to be ignited as early.

With respect to the 911 engine, the high-domed pistons that are typically used to increase compression ratio have the unfortunate side effect of splitting the combustion chamber in half. On a typical single-plug head, the flame front will start from one side of the combustion chamber and travel across the center of the head to the opposite side. This delay in complete combustion means that the ignition timing on the engine must be advanced so that complete combustion can occur by the time the piston is about 25-35 degrees past TDC. For more information on flame-fronts and detonation, see chapter two of “The Sports Car” by Colin Campbell.

Installing two plugs per cylinder means that the combustion process is initiated on both sides of the cylinder head. This accelerates the ignition process and allows you to retard the ignition timing by 10 degrees or more at higher RPMs. An engine produces power when the combustion process exerts force on the piston right after the piston has past TDC. A pressure increase that occurs before the cylinder has reached TDC pushes back on the cylinder as it’s compressing the mixture –doing negative work and possibly knocking in the engine. The accelerated ignition sequence from twin-plugging gives a net power increase because there is less energy wasted on pushing back on the cylinder while it’s approaching TDC. The gasses in the combustion chamber expand later in the power stroke, and allow all of the pressure that is built up to act upon the piston during its downward stroke.

On an engine with a compression ratio less than 10:1, twin-plugging really doesn’t have too much of an effect, primarily because these engines are not running at unusually high compression. However, on a race or high-compression street engine, the twin-plug system can increase horsepower primarily in the high RPM range by creating more efficient and quicker combustion in conjunction with the increase in compression. In basic terms, twin-plugging is used to gain the maximum effect of increasing the compression ratio. Without the implementation of the twin-plug system, a high-compression motor may encounter significant detonation at mid-range RPM with an open throttle. Adding the twin-plug system decreases detonation, decreases your cylinder head temperatures, and also will decrease the octane requirements of the higher compression engines. The addition of the twin-plug system is typically required to allow the car to fully utilize another set of engine modifications that increase the compression ratio.

-Wayne

Nice Wayne :)

The major cost isn't getting the heads machined - it is finding a twin ignition source...

Prices start at at least $1000 for the ignition side (a lot of DIY and some second hand stuff for that).

Or you could buy an engine that was twin plugged from the factory. I wonder where one of those could be found hmm....

????????
Hey Jim, Ain't that what Pee Wee Herman got in trouble for?

Not to worry . .. To sound pedantic you need to know what your talking about. :D

Not trying to sound sardonic. :rolleyes:

EDIT: BTW, I think Chuck nailed it.

Sometimes I feel moronic, but I'll always consider you, Jim and you, Island, as iconic.
M



(btw, this is a good thread. I always wondered what was the advan of twin plugs)

Enough facetiousness!

Chuck did nail it, didn't he?

I seem to be missing something here, so if you guys will indulge me. You are talking about detonation. I assume that is ignition that is unwanted at the time it occurs. You are talking about flame spread. This, I think, is the delay in getting all of the fuel/air mixture ignited for full power effect. If the flame spread is the problem, doesn't that occur after the spark whereas detonation would occur before the spark? How does having spark in two places at once prevent a problem that occurs before spark?

I know it works, but why?:confused:

One issue twin plugging allegedly cures is incomplete combustion. With a single plug on one side of a high CR hemi combustion chamber, the flame front never makes it past the top of the piston dome, and all the fuel isnt burned. If you dont burn all the fuel, you dont extract all the power from the fuel. Ideally the plug should be smack in the center of the combustion chamber, indexed with the "open" part of the electrode facing the intake valve.

Or you could add another plug on the other side of the piston, 2 flame fronts=more complete combustion=more power.

Thats how it was explained to me.

and thermodynamics gives me headaches :rolleyes:

Milt, the thing we're talking about is that rattling engines sometimes make, usually under load. Common in many cars at lower rpms. can happen at high rpms.

Too-advanced timing can contribute to this, as can low octane fuel, heat and other factors. Advanced timing can even be caused by carbon or other sharp solid that can glow hot enough to ignite the fuel. But usually detonation is not caused this way. Usually, the flame front creates so much pressure in front of it, that the fuel there is spontaneously exploded. Fuel in the cylinder is not supposed to explode. It's supposed to burn. So, twin plugging and other strategies are used to avoid the explosions.

Steve Weiner at Rennsport has written some of this stuff up pretty extensively:

http://www.rennsportsystems.com/2a.html

He even has pics :D

http://www.rennsportsystems.com/plugsA.jpg http://www.rennsportsystems.com/plugsB.jpg

That was what was piquing my interest in your theories. "The flame front creates so much pressure in front of it that the fuel there is spontaneously exploded." An event that occurs after spark. So then, to understand, you retard the spark with the twin plug and burn the fuel, but not explode it, to control detonation. This is safer than having a too advanced spark with only a single plug. And lastly, by ensuring complete burn, you control events that otherwise may have caused the fuel to ignite too far before TDC so as to apply all the power to the downstroke and none to the compression stroke, which is the knock, or "rattle," right?

In lay terms, logic suffers a little here, but I realize science is not easily explained in common terms. I'm a carpenter/contrator and I can hang a door with the best of them. But it can't be explained to someone who has never hung a door.

Yes, I suppose I should clarify: Spontaneous Combustion is defined as the ignition of any material that is not initiated by the direct application of a flame. It is brought on when an exothermic reaction has a sharp rise in reaction time and starts to openly burn.

I see you have since changed your wording to "spontaneously exploded". As chemical kinetics go, that is much closer to describing detonaion.

There are far too many websites that either say; detonation and pre-ignition are the same. . .AND. .detonation is "Spontaneous Combustion":rolleyes:

Zeke, chemical kinetics, the burn rate snowballs.

You've got it right. The epiphany many folks have at this point is that the fuel does not explode. In fact, at high rpms, the spark happens at, say 35 degrees BTDC, and the last perhaps 20 or 30 percent of fuel burns right after TDC. The flame does not chase the piston down. Instead, the fuel burns over about 50 degrees of crankshaft rotation at the top of the stroke, building as much pressure as possible just past TDC. Again, at high rpms.

Differentiating between pre-ignition, dentonation and spontaneous combustion helps considerably. This all may seem rather trite to some, but to many, this thread will be a foundation to understanding some fudamentals that are widely (and wildly) misconstrued.

But the gist of it all is twin plugging WORKS, provides more power ( ever drive a 964 with a broken dist belt). Porsche did it on there early race cars.. and the collective Porsche braintrust blows all of us out of the water onsome of these subjects..

Im sure POrsche has quite a bit of internal data and documents regarding twin plugs........... it would be dry reading Im sure but quite informative.

In a previous thread titled Why do lean cars run hotter? I attempted to put it in lay-terms.

Here we go:

Capturing the "BANG"

Air+fuel+spark=BANG
gunpowder+spark=BANG . . .that all seems simple enough.
Or simplistic?
Pressure is the missing ingredient in the above equations.
For example; If you make a little trail of gunpowder (or pour a trail of gasoline) on the ground and light it, you get no "BANG" ; just a fizzle or a flame.
If you take a bit of the powder and wrap it in paper (firecracker) that little containment will cause the pressure to rise fast; which in turn causes the chemical reation to go even faster =BANG.

Pressure management:
The piston engine wants to have a fairly fast burn, but not to the point of "bang" (detonation)
Think about the forces you put on a bicycle pedal. You wouldn't want to jump on the pedal only at TDC, and sit back on the seat after 15° of rotation.
So too, the engine wants to capture the burn pressure to extract a nice even push (work, hp).
The engine geometry is infuencing the burn rate by the changing volume (effecting pressure by giving more space).
The advance is infuencing the burn rate by where in the geometry (bicycle pedal) the burn starts building pressure. Too early and the pressure rises fast, puts some of its work into pushing the piston, and a lot of work in to make heat for the head.
So there *is* a lot of balancing going on there beyond just lighting it off.

since getting a 993, I have done a lot of research about the car and Porsche's (at that point in time) latest and greatest advancements. I thought reading about how the secondary ignition is fired microseconds after the first to burn what was not burned from the primary ignition. Maybe I dont remember correctly.

:confused:

If I recall thats what the top fuel cars do since they have trouble burning the fuel they pump through those things. I realize they burn 90% nitro-methane, which burns (explodes) at a much slower rate than gasoline, but none-the-less it is a internal combustion mechanism.

Thank you all for contributing!

Chris

Im gonna buy either a twin plug 3.6 or a single plus, what gives here, they are both rated at the same horse power, is it worth the money for me to spend the extra on a twin plug motor, Kevin

Kevin, as little or as much as I understand here, one thing is accepted. Twin plugs allow for higher compression ratios and higher CR is more HP. So, it's like the old adage, "How fa$t would you like to go?"

Now here's a thought - several types of cars use multiple coils (one per plug). For example, the RX-7 had 6 coils, 2 for each rotor. Presumably fired by low voltage pulses from the electronic ignition... See where I'm going here?

Maybe some potential to hack together a working system... In essence you'd be going from a mechanical (distributor) to electronic ignition system. Scavenge parts from a couple RX-7s, plug in a Motec engine control unit - might as well upgrade to electronic fuel injection while you're at it...

Never mind! Sounds expensive too...

-Boyo

Yeah Island!!!
 

I think you summed it up beautifully in lay terms Island. The bicycle analogy is right on the money. It shows the correlation between timing and a the burn rate of fuel to extract maximum power.

Advance is a compensation for the burn rate of fuel not being fast enough to complete on the combustion stroke right?

To much advance and detonation occurs from too quick a rise in tempurature on the compression stroke.

Twin plug to the rescue. Quicker controlled burn rate at a later timing.

I never quite completely got it until now.

Thanks
Bobby

P.S. It also shows why higher octane achieves the same thing.
Your a genius Dr. Island!!!

Boyo they also do this with a kit that uses a crank trigger, ots of Porsche use this at the track, Kevin

So, if I can turn the discussion to the practical side . . .

- How much does it really cost to twin-plug a 3.2? What with the parts, the machining, engine drop, assembly, I'm wondering if we're talking $2,000 or $6,000?

- How much more HP and torque does it give, on its own? Assume the 3.2 has the stock US C.R. (9:1?) and you don't increase it.

- How much more HP and torque does it give, combined with a C.R. increase? Assume you now increase the C.R. to something that works with Calif gas, say 10.5:1?

- Beyond HP and torque, are there other advantages of twin-plugging? Better MPG, cleaner emissions, etc?

- Beyond the initial cost and the added cost at tune-up time (replacing 2x the plugs, wires, rotors, etc), are there disadvantages of twin-plugging? Trouble with Calif smog, have to relocate heater duct or other things in the engine compartment, etc?

Great thread..... v. interesting.:p

following on from jyl.... does twin-plugging affect reliability and can it result in increased engine wear (assuming that engine internals ARE modified to deal with the incresaed C.R) ???

Can't comment on most of the questions except price...

Typical machine price -

Modify heads for twin plug - $330-350 (seen as high as $670 for changing both plug sizes from 14mm-12mm)
Modify lower valve covers - $50

Ignitions:

RSR Twin plug diz - $unobtanium
Electromotive - $1500
Modified 964 dual diz w/ MSD (rennsport) $1900-$2500 (depends on donor parts)
Programmable systems $1000-How much you got?)

Labor 0-? depends on if you disassemble

So I would figure on $2000-$3000

I've done this and this is the parts costs:
- Andial splitter box: ~$600
- Machining heads and valve covers for twin plug: ~$400
- Used 964 or 993 distributor: $250-400
- Custom chip: $450
- 2nd set of plug wires: ?
- 2nd coil: ?

Knowledgable people will say that there is a power improvement and other knowledgeable people will disagree. My take on twin plugging is that it will allow you to increase compression and thus make more horespower but I think it is a waste of money on a stock motor.

One down side is that I have to remove the headers to change my lower plugs.
See my web site for what was involved in my own twin plugging project.

-Chris

What About Those JE Pistons Anyway?
 

Chris, thanks for the good content on your website.

Sorry about the pistons.

A slight detour of topic - I occasionally see stories on this board of JE pistons failing but don't recall ever seeing such a story about Mahle pistons. Is this because the JE pistons are inferior? Or because hot-rodders / budget racers tend to use JE pistons (for cost reasons?) so those pistons are more often put "in harm's way"?

Thoughts? Maybe you've talked to other racers, etc, about this.

Re: What About Those JE Pistons Anyway?
 

I haven't updated my web site in a while but I am not prepared to lay the blame on the JE pistons. From what I can tell, the motor went lean. Possibly because of a fuel tank venting issue. Mahle's do fail but perhaps they are used by more competent engine builders who avoid the problems that would make any piston fail.
-Chris

Just to add some more inflection to an already complex discussion...

People often say that twin plugging adds HP. Most of the knowlegeable people I've seen or read say that it improves torque, but doesn't make a big difference at peak HP. This is confirmed by the data mentioned above. In all the data that I've collected; Peak HP seems to be overwhelming related to the amount of air moved through the engine at high engine speeds, so it's a function of engine capacity, engine speed, cam spec's and porting and valve size. Note that neither ignition timeing nor number of spark plugs appears has much of an impact on the flow of air through the engine.

Increasing the CR does increase the torque somewhat, but if you look at the changes in torque/liter of the different 911 motor's in the 70's, you won't find a big drop in the torque/liter as the CR's dropped rather dramatically. So there are obviously other things that factor in with a greater impact, such as cam timing and intake/exhaust tuning as well as combustion quality.

Which now brings be back to partially rebut myself...

Twin plugging should improve the combustion quality by ensuring that more of the fuel/air mixture gets burned. I think that this was the primary motivator in the case of the 964 since the improved burn quality significantly reduced emissions. We shouldn't underestimate the importance of emissions to an auto manufacturer such as Porsche. Anyhow, Hemi-headed engines such as the 911's, Ford's SOHC 2.0, Jag Straight 6 and Alfa's inline 4-Cylinder have always responded well to twin plugging because the large combustion chamber requires a large "intruder" on the piston crown to get a reasonable CR. This intruder gets in the way of the flame front. They also lack a "squash" zone which also improves the combustion quality. ("Modern" penteroof 4-valve heads have none of these problems.) The very large bore of the 964 also contributed to the poor combustion quality since it just took sooooo looonnnggg (in engine terms) for the flame front to reach the extremes of the cylinder from the single plugs off-set position. As far as 964's with broken distributor belts, keep in mind that the distributor timing was set for twin plugs. When the belt breaks, you are left with a single plug engine with the spark timing far too retarded. I suspect that if you were to take that hypothetical 964 and advance the timing (as a "battlefield repair") that you would gain back a lot -- but not all -- of the missing performance.

Just some more things to consider.

So what about aftermarket multiple spark ignition systems? I believe manufacturers such as Saab are using this technology as standard on their engines.
I suppose if the sparkplug is in a poor location then multiple sparks will have little benefit.

What interesting reading this thread makes- thanks to all contributors for clarifying the twin spark theory.

I agree with jluetjen. Unless you have variable cam timing, the benifits are, maybe a bit, in the mid-range, and mostly for emissions. But only if match w/ a higher compression ratio.

From the thread The effects of higher CR is this chart, showing the relationship between CR and thermal efficiency.
(just one more factor in understanding the engineering balancing act)

http://forums.pelicanparts.com/uploads/thermeff.jpg

Actually the reality is different. This I can state as a fact after installing an MSD on my 2.0 (With S pistons). MSD's do make a difference under certain conditions. As far as I can tell these are:

1) Sub-obtimal or large combustion chamber designs (such as a 911's).

2) Relatively slow engine speeds. Since MSD's only do their MSD thing at speeds of 3000 RPM and below, I guess that this is the range.

Why: As the fuel/air mixture flows into the chamber, it (hopefully) does not sit still. This is know as "Swirl" and "Tumble". In the range of head designs (from Hemi at one end to bath-tub chambers like in a small block V8 at the other. We'll ignore Heron Heads), Hemi heads apparently have great flow and poor swirl and tumble charactoristics. Bath-tub and 4-valve Pente-roof designs apparently have much better swirl and tumble charactoristics. The same applies to squash zones which come into play as the piston approaches TDC. All of this movement inside the chamber ensures an even distribution of mixture within the chamber as well as affecting the flame front. Even in a poor hemi-head's chamber, there is some movement. So if the ignition does not initiate a good burn on the first try, the 2nd or 3rd time that the the plug fires will often work better if the conditions around the plug are different. As the engine speed increases, there is less and less time for multiple sparks, plus the increased gas velocity apparently increases the mixing. MSD doesn't do much with engines on the other design extreme since the mis-fire much less. Basically the extra sparks are waisted on already burnt mixture.

BTW: 911's hemi heads and big valves flow a lot of air, but have compromised combustion design -- especially the 2 liter cars with S pistons. On the other hand, when Keith Duckworth designed the Cosworth DFV, he consiously chose to use smaller valves then he could have in order to improve the swirl and tumble charactoristics. The increased velocity passed the smaller valves helped with this. The result is that the engine had excellent torque/liter and pretty much pushed out all of the other F1 engines for over 10 years with the exception of the Ferrari 312 engine.

Just to add to Chris' (Bennet) comments, I think you need a second Bosch CD box (or two MSDs and sell your one Bosch CD box to recoup the cost!).

Looking at some of these prices, I start to feel a bit less crappy about how much I spent...

I have a 964 dizzy modified to fit a 3.0 case and with a chopper plate inside to provide a crank signal for a MoTeC. I did none of the work, but incessantly pestered the guy who did.

If I was just after twin ignition, what would I do....

....wait for it (I have been thinking about this).

To back it up, here is my understanding:

- no point in twin ignition unless you have high compression. I don't see the benefit on a stock Carrera (I think you would fail smog too). For a rebuilt 2.7 or SC with high compression pistons (and in particular more exciting cams) I see a benefit, in particular because a high compression piston with deep enough pockets for S cams and beyond has a REALLY high dome (have a look for Chris Streit's pic of his new pistons). Thus you need carbs, MFI or fancy EFI with individual throttle bodies (cause these cams won't run well with CIS or airflow governed EFI).

- if you want twin ignition, you need the heads and lower valve covers modified - approx US$400

- then you need the cheapest way of getting a pair of sparks to each cylinder. It seems this is Electromotive (~$2000) unless you are good at DIY and can find a cheap 964 distributor and donor SC one (for points, advance mechanism and drive/gear, forgotten the name of the bit at the bottom). You still need two CD boxes then (ie MSD)

But what I would do is think about a fully mapped ignition with a crank angle sensor. Ignition only assumes you have carbs or MFI. It isn't too expensive - eg:

NZ made IgnitionLink is approx US$500 (NZ$990) plus 2x ignitors at about US$110 each and a PC link at $40. Issues are figuring out the crank angle sensor and finding suitable coil packs (eg the RX7 idea above). This could be CHEAP as well as good - I can see under US$1000 and it should be pretty easy to program (just copy the factory advance curve for starters.....)

I might give it a go on my BMW (but single plug) as the feeling of accomplishment would justify the $$$.

Other stuff I can think of:

I've used a full set of 964 plug leads.

My lower spark plugs are "little" ones.

I still run 98RON (93 your gas) with 10.3:1 c/r.

Wow, this has turned into quite a discussion. Without going into too many details the following statements are true with respect to twin-plugging on the 911:

- The 911 head does not give good combustion in it's original form. This is because the spark plug is off to the side. Theoretically, it should be in the center of the head, like it was on the 959 (four valve heads).

- Twin-plugging a stock engine will buy you almost no performance improvements. Twin-plugging is an upgrade that needs to be combined with either:
a) an increase in compression ratio or total compression from forced induction
b) a signifianct increase in the piston diameter (like 100mm or greater). This is because the time it takes the flame front to travel across the piston is significant at these larger diameters.

Curiously enough, the factory did not twin-plug the Turbo cars, yet I would suspect that they could have benefitted from it. Cost savings?

The additional power from the 964 does not come from the twin-plugging, but instead from the higher compression. The twin-plugging allows you to run that higher compression safely and effeciently, giving you the power increase...

-Wayne