CDI vs twin plug
 

Hi, pls pardon my noob question...

Am discussing 3.2 short stroke build alternatives for my SC, and I'm comparing two alternatives - one on 46mm pmo's with twin plug & 10.3 compression (Mahle kit), and the other on Motec single plug but CDI & 11:1 compression (LM nickers and JE pistons)

Both on ge60 or ge80 cams, SSI headers and Dansk exhaust (although am considering independent headers and Dansk exhaust)

Any thoughts on how well each would work?

Thanks!

The carb engine will be less expensive to build and easier to tune.

Twin plugging has to do with preventing or delaying "knock" or pre-ignition.
This depends on several things...but I have always considered it to be caused by combustion chamber design and compression.
In a Hemi style chamber (sudh as Porsche) the flame front after ignition has to pass over and around the pent-roof part of the piston.
If the flame front gets divided into two or more waves...when they come together on the far side of the chamber, they clash...or go bang if you like.
This is "knock" .... it can sound very faint like a small marble in a tin can...or can be one hell of a thump (indicating hole being created in the top of a piston) and followed by lots of oil smoke behind the car.
So...the base question...one or two plugs.
Compression ratios in the area of 10.5 /1 are the limit for one plug normally...but there are many exceptions.
After 10.5 ... you should have two plugs...and maybe even water injection to control the detonation...depending on RPMs.
My $.02
Bob

I was under the impression that modern EFI/Engine Management systems have the ability to detect pre-detonation conditions and adjust the ignition to avoid it. The more advanced MOTEC systems should be able to handle this.

http://forums.pelicanparts.com/porsche-911-technical-forum/730271-best-ecu-3-6-carbs-itb-efi-value-vs-capabilites.html

BTW...I love my 3.0 twin plug engine in my RSR clone. Same as your option 1 without the big bore P&C's.

http://forums.pelicanparts.com/porsche-911-technical-forum/246563-another-rsr-clone-project-dyno-day.html

Still running strong 9 years later.

But then you are living on the edge and not making use of your high compression. Better to just lower the compression if you are going to run extra advance to stop the kock.