twin plug ????

[Norm Faustino]

hey guys heres one from a FNG! What is a twin pug setup and what is it for

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[Jack Olsen]

Since I didn't know what this meant until I bought my second engine, I'll give you my take on it.

It's using two spark plugs per cylinder to improve detonation in engines that have higher compression ratios. The most visible evidence that it's being used is a distributor with 12 wires coming out of it, or -- in the case of the 964 and 993 -- two distributors working in conjunction. The compression ratio refers to the amount that the engine's pistons compress the air-fuel mixture. Higher compression means reducing the air-fuel mixture's volume to a greater degree, creating greater atmospheric pressure, than an engine with lower compression ratio would. With the right kind of detonation (and detonation timing), you can extract more power out of an engine with higher compression.

At least, that's how my liberal-arts-educated mind imagines it. I hope somebody with more on the ball will correct my errors.

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Jack Olsen
1973 911 T (3.6) sunroof coupe
jackolsen@mediaone.net

[This message has been edited by JackOlsen (edited 06-06-2001).]

[3liter914-6]

Originally posted by Norm Faustino:
hey guys heres one from a FNG! What is a twin pug setup and what is it for

I had a whole treatise typed up but decided that this page
http://www.rennsportsystems.com/~porsche/2a.html
explains why Porsches tend to respond so well to twin-plugging more succinctly than I can.

the info's toward the bottom...

[This message has been edited by 3liter914-6 (edited 06-06-2001).]

[Norm Faustino]

thanks for the site address and thanks for the clarification guys

[campbellcj]

Here's what a (nauseatingly nice) twin-plugged 2.8 RSR spec motor looks like down under (there are not normally plugs here on a single-plug motor):




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Chris C.
73 914 2.0
70 911E Targa
campbell.chris@gte.net

Piston aircraft engines typically have two plugs per cylinder. One reason is because they have two independant ignition systems for safety. The vast majority of these are magneto type ignition.

As with car engines, the twin plugs also improve the combustion process inside the cylinder. They burn the fuel/air mixture more completely and also create a more even flame front of the expanding gases acting upon the piston crown. At least that was the answer in all those powerplant exams I wrote years ago.

This is well illustrated in an airplane because you can turn each ignition system on and off independantly when doing checks. There is a corresponding drop in engine RPM when one mag is switched off, showing that two plugs do work better than one.

This reminds me: Does any of the pilots on this board have or know of one of those Porsche powered Mooneys that were around years ago? That would look good in the garage beside your Porsche farm tractor.

[KTL]

From what I remember reading, twin plugs improve the combustion efficiency of the 911 motor due to spark plug location. Due to the orientation of the spark plug on the upper valve cover side, the single plug is somewhat shrouded within the combustion chamber and cannot light off the air-fuel mixture very well at higher than a 10:1 compression ratio. Single plug setups risk detonation at higher than 10:1 compression ratio. The addition of the second plug on the lower valve cover helps to light off the air-fuel mixture more effectively and reduce the chance of detonation occurring.

You'd like to have the plug located centrally within the combustion chamber, surrounded by the intake and exhaust valves. This setup is commonly seen on your twin cam inline fours and sixes. It's more effective in igniting the air-fuel mix and doesn't necessarily need twin plugs. Look at an Integra Type R (11:1 I think) or a Corvette ZR1 (11.25:1 I believe). Both are twin cammers and use only one plug.

Jack,

Detonation is BAD. Combustion is good. Detonation (also known as preignition) occurs when the air-fuel mixture ignites (combusts) when you don't want it to ignite. That's why the knock sensor is used on your 964 motor- to detect detonation and retard the ignition timing accordingly.

Compression ratio:

Volume of the combustion chamber & cylinder bore with piston at BDC divided by the volume with piston at TDC. Pretty much the same as saying how much the air-fuel mixture is compressed.



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Kevin
87 Carrera coupe

[Bob W.]

On a single plug engine, the ignition location is off to one side of the combustion chamber. When ignition occures the flame front has to travel across the chamber to the other side for complete combustion. This takes a specific amount of time and requires the ignition to occure sooner as the engine turns faster so that peak pressures occure near top dead center.
With a twin plug system and two flame fronts on opposite sides of the chamber, the amount of time for complete combustion is shorter. With a shorter combustion time, less ignition advance is required. With less ignition advance you will have less detonation problems.(you can run more boost or compression without detonation with reduced ignition advance.)

[Ted Stringer]

Can the 3.2 be setup with twin plugs? What all is involved in the mods. I am planing on overhauling the whole car in the near future (1 - 2 yrs) and have concidered going to twin plugs

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Ted Stringer
nuke3@juno.com
'84 911 Targa aka pocketrocket

[Doug Zielke]

Chris asked: "This reminds me: Does any of the pilots on this board have or know of one of those Porsche powered Mooneys that
were around years ago?"

Here's a nice image... (if you look closely, you can see a 2 distributors, one on the right, and one, hidden away on the left. Looks like 2 alternators too.
This motor is a work of art!)




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Doug
'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
members.home.net/zielke/911SC.htm