Why twin plugs?

[HardDrive]

Why is it nesscary to use twin plugs on some engine builds?

[svandamme]

911 heads have the spark plug positioned to one side
this works well for stock engines , but on high compression engines this isn't ideal

high compression engines get a second plug, on the other side of the combustion chamber, and this helps preventing detonation

it is highly recommended for engines with 10:1 or higher compression
if not , the engine will not run on pump gas, you would need higher octane fuel then what pumps have...

there is only a modest performance and milage improvement from twin plugging so it doesn't make a good cost effective upgrade on an otherwise stock compression engine

[livi]

Stijn, is it just an anti-ping device, then ?

And yet another avatar..

[svandamme]

at least as far as i understand it, it seems to be the main use for twin plugging, TP'ing itself isn't the wonder upgrade when it comes to producing power

new avatar, well yeah , got a new car, so got a new avatar , seemed like the right thing to do...

[livi]

New car ?!! How could I miss that ?

Where are the pics, oh His Holy Prophet ?

[svandamme]

haven't got any yet, busted my digital camera

[javadog]

911 engines are limited in the amount of compression they can safely use on a given grade of fuel. The detonation problem occurs when the pressure rise, in the combustion chamber after the spark plug has fired, causes the fuel/air mixture on one side of the combustion chamber to spontaneously explode before the flame front of combustion reaches that area. Adding a second spark creates a second flame front and the fuel/air mixture is burned before the pressure rises too high, as there is a shorter distance for the flame to travel from each plug to the farthest reaches of the combustion chamber.

The power gain comes from the ability to use more compression, or more boost in the case of a turbocharged engine. Or, you can run a lower octane fuel for a given compression ratio/boost level. It also means you can change your ignition timing, which can help power.

JR

[Chuck Moreland]

As above the reason is to reduce the distance the flame-front has to travel, from spark plug to the perimeter of the cylinder. 2 plugs = shorter distance = shorter time for full propagation. This lets you run higher compression for a given octane.

You also run less advance. Consider that when the spark plugs fire with advance, the piston is still moving up. The expanding explosion is working to push the piston back down until it reaches TDC.

If twin plug lets you dial back total advance from 32 degrees to 20 degrees, that's less time for push back. This alone should yield HP gains. In practice it doesn't yield that much, it is primarily the higher compression the brings the HP.

[syncroid]

Originally posted by Chuck Moreland
As above the reason is to reduce the distance the flame-front has to travel, from spark plug to the perimeter of the cylinder. 2 plugs = shorter distance = shorter time for full propagation. This lets you run higher compression for a given octane.

You also run less advance. Consider that when the spark plugs fire with advance, the piston is still moving up. The expanding explosion is working to push the piston back down until it reaches TDC.

If twin plug lets you dial back total advance from 32 degrees to 20 degrees, that's less time for push back. This alone should yield HP gains. In practice it doesn't yield that much, it is primarily the higher compression the brings the HP.

I've been wondering about this. When you do the TP conversion with a standard 964 distributor on a 3.2 (motronic) is it necessary to modify the distributor to change the timing or is it done in the dme control unit itself? Do you really need to change the timing at all??

[Carrera3.5L]

Originally posted by syncroid
I've been wondering about this. When you do the TP conversion with a standard 964 distributor on a 3.2 (motronic) is it necessary to modify the distributor to change the timing or is it done in the dme control unit itself? Do you really need to change the timing at all??

The 964/993 distributor is a "dummy" unit that simply distributes the sparks. The timing maps are in the EPROM and can be modified by a chip programmer.

It is essential to have a new chip programmed when twin-plugging as the stock advance is way too much as explained in a prior post.

Ralph

[syncroid]

Thanks Ralph.

[fastbacker]

I'm no expert here but I think twin plugging is also used in large bore engines (98mm+) due to the large distance the flame has to travel.

[svandamme]

Originally posted by fastbacker
I'm no expert here but I think twin plugging is also used in large bore engines (98mm+) due to the large distance the flame has to travel.

it depends on the head

for a 944, 100mm bore, 10.6:1 compression , it works with a single plug , because the head is completely different in shape:flatter and smaller then the actual bore, and the plug is positioned to the center of the smaller head , but offset to the bore of the cyllinder

the 911 is a different beast because of the hemispherical design of the head, which is round and has more depth to it...

so on a high compression 911 engine , you would usually find domed pistons, making it even harder for the flame to propogate from one end to the other..

at least that's my interpretation...someone else might explain it better...

[Carrera3.5L]

Originally posted by svandamme
it depends on the head

for a 944, 100mm bore, 10.6:1 compression , it works with a single plug , because the head is completely different in shape:flatter and smaller then the actual bore, and the plug is positioned to the center of the smaller head , but offset to the bore of the cyllinder

the 911 is a different beast because of the hemispherical design of the head, which is round and has more depth to it...

so on a high compression 911 engine , you would usually find domed pistons, making it even harder for the flame to propogate from one end to the other..

at least that's my interpretation...someone else might explain it better...

Well, I think we're all assuming that we're talking about air-cooled 911 heads.

Twin-Plugging not only provides a safety margin for larger bore motors, it will also make (or in my instance retain) horsepower even with stock level compression ratios (i.e. 9.5:1).

One of my hybrids on my Andial signal splitter recently failed, thus only one set of plugs was firing (the lower bank). After swapping the leads from the splitter to the coils so the top bank were now the ones firing, went to the dyno before replacing it (actually Andial replaced under warranty) just to see what the measured difference in power was as the car idled noticeably rougher and felt down on power. It essentially ran like a highly modified 3.2L.

With one set of plugs firing and the twin-plugged chip programmed with less advance swapped for a chip programmed for single plug and thus more advance, the motor was about 15 horsepower down and didn't like the advanced timing with the larger bore and poor 91 octane used.

The splitter was than replaced and the twin-plug chip reinserted and voila, power levels back up where they should be in this configuration. Motor idles smooth once again and doesn't seem to strain itself as it pulls effortlessly up the rev range.

Ralph

[89911]

Nice description Ralph. At least I feel better about going the twin plug route. I think I have the same setup as you with an Andial splitter. I can cut out either bank of plugs if I want and the power and idleling are noticable changed.

[island911]

Originally posted by HardDrive
Why is it nesscary to use twin plugs on some engine builds?

because the 'expolsion' (burn) progesses spherically (for the most part.)

A piston in a cylinder approximates that spherical shape. (note: most engines have the stroke close to the diameter of the cylinder.) THe spark is best started in the center of the volume, such that the explosion is most efficiently captured. . .. if ya have big valves in the way, placing opposing, symetric plugs is close to the same.

[randywebb]

Other than what's been said above, the lack of water cooling in the 911 engine means that 4 valve heads cannot be used. 4 valve heads can have a central spark plug, thus reducing the distance the flame front must travel to ignite all the fuel "at once." That, with the head design, means that as displacement and compression increase you have problems. Hence the 2 plugs.

Even w/o add'l comp. the engine will 'run better' - i.e. feel better. IIRC, Bruce A. cited about a 4% hp increase. As noted above, real adv. is to get a higher comp. ration and thus more power. Remember, it is the 'push' of each piston that turns the crank and all the stuff attached....

2 plugging is spendy, but if you are rebuilding the motor anyway, it makes sense to spend the small amt. extra for the drilling. You can then do the more spendy electrics stuff at your leisure w/o pulling the engine and tearing it down again.

[Jack Olsen]

Here's something that's always puzzled me. We all agree that higher-compression combustion chambers usually benefit from twin plugging. But for some reason, it's not common (or all that necessary?) to twin plug turbocharged or supercharged motors, where you're typically running lower compression, but the stuff you're compressing has, in effect, already been pre-compressed by the blower. I would think that the actual volume of the compressed air in a turbocharged motor's cylinder is greater -- even with lower contribution to that compression from the combustion chamber -- and yet the need to control the burn in the combustion chamber is less critical.

Can anyone 'splain this to me?

[9dreizig]

I was just wondering the same thing, since all a turbo does is increase the effective compression..

[island911]

Originally posted by 9dreizig
I was just wondering the same thing, since all a turbo does is increase the effective compression..

different tho' . . . when the charge is cool(ed) the bang is slower.