advantage of twin plugging?

[rcwaldo]

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

[kristian85]

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.

[Superman]

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.

[Chuck Moreland]

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.

[boyo]

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

[Wayne 962]

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

[CamB]

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).

[Bill Verburg]

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

pedantic

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

[island911]

Originally posted by Superman
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, . . .

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

Not trying to sound sardonic.

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)

[Superman]

Enough facetiousness!

Chuck did nail it, didn't he?

[Zeke]

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?

[TimT]

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

[Superman]

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.

[CamB]

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

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

He even has pics

[Zeke]

Originally posted by Superman
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.

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.

[island911]

Originally posted by Superman
Enough facetiousness!

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"

Zeke, chemical kinetics, the burn rate snowballs.

[Superman]

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.

[Zeke]

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.