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!