Building a 3,2 SC with 46 PMOs. Advice on twin plug needed
 

Hello all,

I am building a 911 SC with the following specs and I need advice on twin plugging it.

The engine is a 1979 SC 3,0 with new 3,2 Mahle pistons and cylinders. Compression is 10,3:1 and we are using new PMO 46mm carbs and 964 cams.
We also bought MSD 6AL ignition.
The car is going to be daily driver so we want something that runs reliably on pump gas.

We are thinking of going to twin plug setup due to higher compression. I am looking for 964 twin distributors. What do you guy think ? Any other setup that would work ?

Talk to Mark at Exotech (prschmn@comcast.net). I think he has a set of twin plug heads on the shelf ready to go.

I built a 3.3SS engine about a year ago. It also started as a 3.0 SC but got LN Engineering 100mm cylinders, Mahle 10.1:1 pistons, 964 cams, Carrera twin plug heads, an early SC large port air box and Electromotive XDi direct fire ignition. Exhaust is handled by a set of SSIs. Timing is set for 25 degrees max advance w/10 degrees initial advance. Using the XDi system allowed for the deletion of the distributor and the CDI unit. The engine starts easily, is easy to drive in traffic and has lots of mid range torque. Redline is set at 7,000 rpms. It is very strong from about 4,000 rpms and up, pulling well in all gears. I used a set of SC rods that had been reworked by Ollies that had been shot peened to relieve stress points, resized on both ends and balanced. The crank took stock bearings on both mains and rod journals. The Carrera heads have much larger intake and exhaust ports than the SC heads. The bottom line is that you will have an engine that will be easy in traffic and lots of fun on top end. Good luck!

I'd use more aggressive cams than 964 with some valve overlap. That change alone will net you far more than higher compression and twin plugging. I ran 46mm PMO's on a 78SC 3.0 with 66 solex cams, 8.3:1 comp and single plug and it was a screamer. No detonation on regular pump gas and ran cool. Only thing was I had to cut small valve relief pockets in the stock pistons.

I don't think the 964 distributor is a recommended approach. Either crank-fire or a JB Racing, Aaron Burns, etc. units seem to be the popular choice.

Todd

You can't use the stock 964 distributor without a controller of some kind. It has no mechanical or vacuum advance and you must have a method of controlling advance. A dual plug distributor with mechanical advance (all you need) is about $1,500.

The MSD 6AL-2 is digital, so he can program the advance if that is what he has .

Hello and thank you all for your responses.
Which on do you mean mreid ?

Who sells such a device ?

Hello Fred,

do you have a thread with that engine build up ?

Did you use carbs or FI ?

I need to stick with SC heads for the moment so I cant go to Carrera 3,2 heads. Is that so bad ?

The 79 SC heads that you have will work well they have the large ports.

Why not keep it single plug and lower the compression? Edit: Never mind, if you already have the Pistons then go for the twin plug.

Here is my build thread with a lot of commentary from experts:

http://forums.pelicanparts.com/911-engine-rebuilding-forum/524936-shanes-3-2-ss-recommendations-advice.html

Fred's build thread:

http://forums.pelicanparts.com/911-engine-rebuilding-forum/747632-3-3ss-rebuild-finally-underway.html

I just built a 3.2L with 46mm PMO's and 9.5:1 JE pistons. Single plugged with the Electromotive Xdi.

I really like the Xdi. You would be amazed at what a quick adjustment to initial timing does for the carbs - at ~ 10 deg initial, it's rather sluggish coming off the line, bumped to 14 deg and it's burning rubber.

Xdi can be done as dual plug. Its a bit pricy, but probably not too far off from other options.

Good luck,

Gordo

Gordo's thread:

http://forums.pelicanparts.com/911-engine-rebuilding-forum/785959-gordos-3-2l-engine-rebuild-ready-reassemble.html

Here's another, closer to what you are doing:

http://forums.pelicanparts.com/911-engine-rebuilding-forum/758301-3-2ss-twin-plug-underway.html

I'm running a 3.2SS with JE 10.3:1 pistons, 46mm PMO's and S cams. If it helps your build I highly recommend the S cams for a daily driver simply because it has so much torque. Very drivable from early in the rev range.

I'm in Australia and can get 98RON fuel anywhere which means I can stay single plug. I think if you have lower grade fuel (is it 90-91RON?) you need the twin plug for those pistons.

Budget for an AFR controller as well. Several variants out there, but it will also make a big difference in your car getting the AFR dialed in.


Todd

I'm curious about your P&C's... You said 3.2, are they 98mm? I'm asking because I didn't think those were available any longer. I always thought, if you could find them, the pistons were/are motorsport pistons with valve pockets which would allow for a better cam than the 964.?!

Your SC heads seem like a better choice for SSI's... I understand it's a good idea to match port size with primary size. SC heads match right up with SSI's. I doubt you'd see any improvement going with the later Carrera heads.

I'm building a similar engine but closer to Gordo2's with single plugs and lower compression. I like the simplicity for one and don't see a huge advantage to one point higher compression which would necessitate dual plugs but looks like you're going to need it running 10.5/1.

I really like the Xdi. You would be amazed at what a quick adjustment to initial timing does for the carbs - at ~ 10 deg initial, it's rather sluggish coming off the line, bumped to 14 deg and it's burning rubber.

Gordo2, I've been reading through your very descriptive build threads (thanks for that) and am interested in your timing settings as I'll have pretty much the same setup as yours except for the header sizing (yours seem a bit large for a street motor) ...so initial is now 14, up from 10. what is your max now that you've got things dialed in?