At what point should I consider twin plugging
 

What are the driving factors and at what point do I consider twin plugging?

As I understand it in the auto world twin plugging gives you a faster, more even burn, reducing the likelihood of detonation due to the length of time the combustion process takes as well as allowing a reduction in spark advance further reducing likelihood of detonation without reduction in power, and even increase in CR or boost with lower likelihood of detonation.

Did I get that right?

So, where does it start being something worth looking at?

For example, I'm probably going to end up with 3.4L 7.5 or even 8:1 CR, SC cams, Franken CIS, currently have a K27/29 and could consider replacing with a GT35 series or something more suitable, with probably coil on plug (EDIS) or coil near plug (LS1/2) wasted spark driven by microsquirt.

Street car, not a track sled.

Is twin plugging beneficial at this point? Or not?

I went to twin plugging just for the ability to run less timing at the same boost level and detonation prevention. Also it was a classic case of WYIT, the heads were sent out to be rebuilt sooooooo...

could you tell any difference after the twin plug.

I would love to do the same for the same reason.

The car hasnt turned a wheel in almost 2yrs. The back of napkin research I have done however says that I wouldnt expect to 'feel' a difference.

And I have changed too much in the rebuild to really compare once it runs. New cams/bigger turbo/crank-fired ignition etc.

For me, if I needed it, twin plugging would be more for increased safety factor because I'm hotting up the car, rather than keeping the same (or less) safety factor and gaining a little performance.

I seriously doubt you could "butt dyno" the difference between single and twin plug setups without adjusting timing/boost/CR to take advantage of it. Possibly you'd see a marginal increase with real instrumentation.

So the question stands, when is it time to start considering whether I need the added safety margin afforded by twin plugging?

The answer is simple..
When you can afford to do it.

They don't really provide a safety margin. The timing is less advanced because two plugs can burn everything in sight much faster than 1. For an open chamber hemi, with no quench area, the chance of detonation is about the same.

Fortunately, that chance is low. Unfortunately, the chance of pre-ignition is high from hot spots like the valves. Most Porsche aircooled engines are damaged by pre-ignition, if they are ever damaged.

Two completely independent plugs might be useful in that you could have some ignition redundancy. Some of those old Porsche 935's would hydrolock if they didn't burn.

Remember, the 993 GT2 EVO was over 600HP for 24 hours with single plugs.

For NA engines, it's a different story. You get a significant power improvement with twin plugs because they solve the problem of the flame propagation being interrupted by the piston dome.

Thinking aloud, firing symmetrically should enhance an even flame pushing more evenly on the piston crown. Would assume this does enhance power slightly, no?

But to your point Speedy, with flat top pistons, there's no dome to jump. I see your point.

Maybe it truly isn't necessary on flat top piston'd engines?

When tuning (to the limit) whatever combination of cams, compression, displacement and turbo you are running, twin plugging will enable you to run more boost than single plugs can for any given octane fuel.

Said the man that did my heads,Thanks Ollies

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[QUOTE=Tippy;9581365]Thinking aloud, firing symmetrically should enhance an even flame pushing more evenly on the piston crown. Would assume this does enhance power slightly, no?

That's an interesting theory, but the pressure wave is pretty fast. One does pick up 2 to 3% in power because the fuel is all burned faster. Other than that, no data suggests any benefit regarding detonation improvement, said the guy with no dog in the twin plug hunt. Like I said Porsche engines are seldom killed by detonation.

I'm a big fan of twin-plugging WYIT. Even if you have no plans to go twin right away, you won't have to disassemble the engine in the future to do it.

Wouldn't say there's a specific HP or boost threshold at which twin-plug is required, too many variables.

When I finish transferring the data from my old PC to the new one, I'll pull up the dyno of a built 3.3L Turbo that had an ignition amplifier fail and drop out the secondary plugs. It lost a lot of power and torque. While we waited for the replacement part, we tuned it as far as it would go on that inductive single-plug setup and 91-octane pump gas. It was no where near the 500whp it finally did with all 12 plugs firing.

My take from that is twin-plug is a benefit.
Just my experience. Results may vary, etc.

I agree concerning the advantage of redundancy, too. You may end up down on performance if you lose half the ignition, but you'll still get home.

Ollies did my heads as - super nice to work with if there is anyone questioning that these days.

Interesting info about dome vs flat top pistons re: the flame front. I handnt factored that into my bench racing.

Slightly off topic but squish isnt impacted by piston shape right?

My twin ignition was on the car when I got it so nothing to compare with. I do feel it has saved my a** a few times.
Had this in my file, not sure where it came from:
Twin-Ignition:
Porsche has used twin-ignition in their air-cooled racing engines since the 4-cam Carrera engines. The high-domed pistons necessary for high compression ratios requires another spark plug to start another flame front on the other side of the piston dome. The best place for the spark plug is the middle of the combustion chamber. From the center, the flame front will have the least distance to travel for complete ignition. This reduces the need for as much ignition advance to start and finish the combustion process when the piston reaches Top Dead Center. Since the spark event is starting closer to TDC in the compression cycle, there is less pressure from the beginning of ignition that is pushing ‘back’ down on the piston crown as the combustion event progresses. This lessens the ‘negative’ work done by the expanding gasses and allows all of the pressure building in the cylinder to push the piston in the correct direction, making the engine more efficient.

Single plug combustion pattern:
The offset-plug position on a 911 delays the combustion process. By installing two spark plugs per cylinder, you will increase the acceleration of the ignition sequence. This can reduce the required advance by 10 degrees or more thus lowering cylinder head temperatures. In terms of power, twin-ignition will add some 3-4% or more depending upon compression ratio, over a single ignition system. RPM can increase as much as 700 RPM at top speeds. If high compression ratios are to be used, twin-ignition allows all of the power benefit to be gained from the increase. Twin-plug equipped 911 and 930′s run much crisper and cleaner with lower cylinder head temperatures and improved throttle response. Plus, a twin-plug 911 is much less prone to plug fouling with today’s fuel.

Twin plug combustion pattern:
Besides the damage to one’s bank account, there isn’t one drawback to installing this system and enjoying the benefits in throttle response, power and great drivability that twin-ignition adds to any 911 or 930. Quite a difference!. In some cases, it is a necessity to realize the maximum gains from a complete set of engine modifications.

The water-cooled engines have a much more optimal spark plug location at the top and center of the combustion chamber that negates the need for two spark plugs per cylinder.
Twin-ignition Hardware
There are 3 basic methods to installing and using dual plugs in these engines:
1) Custom billet Distributor or stock distributor converted to RSR configuration. The first option is a brand new, billet unit utilizing an American-made magnetic trigger & advance mechanism with a Marelli-type cap and custom rotor. These have three ball-bearings to support the shaft and are quite durable. The second option utilizes the OEM distributor to mount a billet adapter ring to accept the Bosch 12-nipple RSR cap and machining the distributor shaft to accept the RSR rotor. The cap and ring must be properly phased to the rotor alignment using a distributor machine and the advance curve should be modified to suit. Point-type units can be converted to breakerless by installing a Pertronix Ignitor. This setup can trigger either the OEM Bosch CD boxes or a pair of MSD 6AL’s with MSD matching coils for best performance. This setup is available for all 2.0 through 3.8 litre engines with carburetors, MFI or EFI systems.
CIS-equipped 930’s should use make sure the distributor retains that all-important vacuum advance/pressure boost retard feature that is critical to engine life and proper throttle response.
2) 964 Dual-distributor converted for 2.4-2.7-3.0-3.2 litre Engines. This one is done by installing a trigger from a donor SC distributor and using the appropriate crank gear, depending upon the engine being used. This unit will not fit a 930 due to interference with the boost plumbing. Again, these can be triggered by OEM Bosch CD boxes, MSD’s or a proprietary splitter unit for Motronic-equipped engines. This setup is ideal for any Motronic motor and works very well in carbureted or MFI-equipped engines. The 3.6 crank gear must be used for these conversions.

So the 3-4% power gain is due to delaying timing mitigating "blow back" of the piston. Forgot about that major fact!

That is more or less accurate. No notible improvement in throttle response or drivability though. Spark is fast. It takes much longer to get fuel and air into the cylinder than it does for spark to set it off.

Rug CTR2 700+ Hp all day long on single ignition. Just sayin'....

...not running CIS. :)

Yup, the black CTR2 Sport we worked on (ex-Beddor car) was single plug. Those were modified 993 Carrera heads. Lower holes had a set of dummy plugs in them, cover with 993T lower valve covers...

...and then there's ANDIAL twin-plugging their 3.8L B-Turbos. They were cozy with Porsche Motorsport.

And some purportedly original 993 GT3 Evo engines have 993 twin distributors and 2 coils, while other photos show single head distributors.
Is it possible the factory went back and forth on some of their own builds?

chris,
if you could only do one mod at time what would be the order. (not to go OT as this may interest the OP).

twin plug
cams
a true header like rarely8 header.

It is legit to match the domes of certain high compression piston designs ( >10) to come very close to the chamber walls, creating a squish area. I suppose if that were altered, it could effect squish.

I have headers, exhaust, upgraded turbo, and will be doing cams and clutch this winter along with probably EDIS+COP ignition driven by Microsquirt v3.

Based on what I'm hearing its worth doing the twin plugging if/when I yank the heads to do p's and c's.

i agree. would love to twin plug.
for me its not so much the work and machine work, its the ignition sys.

I have a "header", 7006 and RUF IC but would love a true header, cams and twin plug.
also trying to get a perspective of the upgrades from someone that really knows the facts of the upgrades, chris.

Way back in the mid eightees IMSA was changing rules to try and slow down the Porsche 962 because it was winning every race again and again. They made the 962 run with more weight to try and slow it down.
The cars came with Titanium nuts and bolts throughout the chassis so they were removed and steel ones installed in place of them to increase weight. That wasn't enough so next IMSA forced them to install a 70 pound lead weight on the floor to the left of the pedals and drivers feet. Then IMSA told Porsche to remove the twin ignitions and dual plugs and go back to single plugs to try and slow them down.
I don't remember what else they might have done but two valve engines do run a little better with twin plugs then one off center single plug.

I have an old 980CC BMW twin cylinder air cooled motorcycle with dual ignitions and dual plugs. It's sort of like half a Porsche 356 or 912 motor.
If I pull one spark plug cap off one of the cylinders while it's idleing at 800 rpms so it goes from dual plug ignition to single plug ignition the motor slows down a little.

Since squish was mentioned earlier and JFairman mentioned the later IMSA 962s, if you look at one of those later 962 cylinder heads, they actually have a lot of material added to the combustion chamber opposite the spark plug, in an effort to speed up the combustion by adding turbulence via squish. Not sure how well that worked compared to the twin plug set up, but there is a nice picture of one of these heads in Bruce Andersons book.

Love this discussion.

Well, the 3.0L 962 never had twin plugs because it was a four valve engine. As far as pulling the plug, you effectively retarded the timing when you did that, so of course the rpm went down.

There is no magic. Physics rules. There is only a 2 to 4 percent gain to be had, strictly from the shorter combustion duration. Every Ruf CTR2 is making 700HP with a single plug.

I was actually thinking of that. I know people weld aluminum heads to repair them. I wonder if you could build a squish area up without hopelessly distorting the head. You could do a 95mm bore with 6 CR 930 pistons, and bring it to 8 CR with squish area.

http://forums.pelicanparts.com/911-engine-rebuilding-forum/619651-bathtube-combustion-chambers.html

http://forums.pelicanparts.com/uploa...1494444022.jpg
http://forums.pelicanparts.com/uploa...1494444022.jpg

Thanks Burn Bros

Well, I used to do some fiberglass repair and body work on Preston Henn's Swap Shop 962 chassis number 962104 (it was the fourth one made) back in the mid eightees when it was racing in the Florida IMSA races and it was not a 4 valve DOHC motor.
It was an air cooled single overhead cam two valve per cylinder 962 motor with Motronics.
The 4 valve 962 motors came later. If I remember correctly they had water cooled DOHC 4 valve heads and air cooled cylinders.

1. Cams
2. Headers (not those)
3. Twin plug

A lot of development is being done in that area right now.

TIG should mitigate warpage. But, seems like you'd need to bring everything up to hot to relieve the stresses of welding once done building up material?

Or you could go with an Xtreme head casting and carve your own combustion chamber....;)

That would be sweet!

I think you're right Tippy. The link Mike Billings posted shows that it can be done. I am seriously thinking about doing this, especially since I have a victim for this project:

http://forums.pelicanparts.com/uploa...1494459897.jpg

Less material has to be adddd for us, probably a lot like the 956 head Ollie mentioned, since we need a CR in the nrighborhood of 8.

Keep in mind that when welding up a combustion chamber, no matter how careful you are and how much you preheat the heads, you will need to replace the valve seats, as they will loosen up and the heads will need to be resurfaced. Not a big deal, but be aware of the extra cost.

Funny, just saw this thread, and a few hours ago I happened to have Waynes 'Modifying 911 Engines' book on my desk and happened to open to a section on Twin Plugging. He says you won't get much benefit in performance unless your compression ratio is greater than 10:1. There are other benefits however.

We have found that rapid multi-spark has a similar effect, provided the second spark is suitably timed to follow the first (the sweet spot being around 400us for CDI+). We believe that after the first spark, unburnt turbulent mixture moves into the path of the second spark where it is ignited.

It certainly shows on the combustion analyser and the AFR shows leaner. Like you say, the effect is more noticeable on the early 911s with domed pistons. Typically, a '69S will see gains of 6 ft lbs across the board.

Will be watching if you do so!

Great insight!