Best practical way to Twn Spark ignition 911 engine?

911hillclimber · Post by **911hillclimber** » Mon Sep 20, 2021 6:03 pm

As part of my engine re-build I am expecting to have to go twin spark.
Up to 8K rpm and 85 mm pistons and high dome pistons, but running big carbs.

Currently have RS 2.7 distributor, coil and condenser system, nothing else, stock pistons and 6500 red line.

That has set the scene, but I am not sure of the most economical way to 'fit' twin spark ign.
Not interested in RSR 12 lead distributor.
Prefer an ECU that will do twin sparks and readily programmed by a 'normal' rolling road technician.

To kick off in my mind I have an Emerald ecu, crank fired from a fan belt pully wheel and a coil pack.
The ecu side only needs a single map, this is not a road car, and a soft limiter.

Cams will be GE60, compression ratio 10.5:1, fuel 99 octane, engine base is a 3.2 if that has any bearing on the answer.

Any guidance is welcome.

Gary71 · Post by **Gary71** » Mon Sep 20, 2021 6:14 pm

My only practical experience is with the megasquirt ECU. It can do all you want, Tim is running my old ECU on his Targa (with EFI as well)

Tuning the ignition map is childs play with the laptop plugged in. Start with the stock 2D distributor curve and go from there.

Guess you might want a knock sensor on the head to really find how far you can push the advance.

911hillclimber · Post by **911hillclimber** » Mon Sep 20, 2021 7:04 pm

Good start!
Where would I fit the knock sensor though?
It will need a temperature sensor in one head as per the 3.2 stock ecu, but not sure if I simply get a pocket machined into another head, and where to get such a sensor from (Denso?)

Gary71 · Post by **Gary71** » Mon Sep 20, 2021 8:20 pm

The 993 has a knock sensor, so I’d copy (as close as you can) that location

911hillclimber · Post by **911hillclimber** » Mon Sep 20, 2021 8:32 pm

Excellent, that looks a good start when the engine is down to a long block.
Orderingpistons and rods and WEVO tomorrow...

hot66 · Post by **hot66** » Mon Sep 20, 2021 9:07 pm

I have clewett twin plug with 6 coil packs and crank trigger

Been running it for approx 10 years and it’s been great .. only issue I’ve had was one of the spark plug lead connectors failed

https://www.clewett.com/index.php?main_ ... 1_13_14_19

Mines basic .. so advance is simple matter of 3 knobs… each adds a bit more advance to the previous one . Pretty basic but works. I had my engine on an engine Dyno so the correct advance could be worked out

911hillclimber · Post by **911hillclimber** » Mon Sep 20, 2021 10:31 pm

Anything else needed James on top of that kit to complete the job?
A set of 911 plug caps I guess and the holding brackets?

hot66 · Post by **hot66** » Mon Sep 20, 2021 10:51 pm

Nothing else apart from the bits you state.. mike B made my mounting plates up.

I’ll be honest, I chose my kit as it was so much cheaper than going down the twin plug dizzy twin cdi route… as it is I’ve been really pleased with it

All the instructions and installation info can be downloaded from their website… would be worth doing and reading all that before you buy so you have a good idea of how the it works….. I

gridgway · Post by **gridgway** » Tue Sep 21, 2021 8:19 am

Just thinking about this and realised that I have a hole in my knowledge! Are the two plugs in a cylinder fired at the same time? If not how different are they?

Was also thinking if there is a good solution using the classic retrofit clever CDI box and a later 12plug distributor pair? 964/993

PeterK · Post by **PeterK** » Tue Sep 21, 2021 8:40 am

2 CDI boxes

911hillclimber · Post by **911hillclimber** » Tue Sep 21, 2021 9:05 am

As you all can read, I'm learning about this subject.
From what I gather the sparks fire to the plugs not at the same tie, but could/am wrong.

I want a simple solution that can be tuned by the lcal rolling road man who deals in all manner of cars, no numpty at all, but is 15 mins from me so good.
Not after period looks and space is not generous under the body of the Lola esp when considering somewhere quite dry!

The 964 twin dist unit was my first thought, but it is messy when you see the simple coil packs etc of modern kits to do the job, shown well by the Clewitt product. Had a look at Bill's 904 install on the weekend at Shelsley, and it is all very neat.
I think I'll have to seel my lovely Honda and the Peugeot BIMA bikes in the spring to get this lot by The Boss though she has been around motorsport for 42 years with me so knows there will be more to come.

Off to order my pistons and rods.

911GP · Post by **911GP** » Tue Sep 21, 2021 9:32 am

Have you considered a classic retrofit cdi box? They generate a twin spark without needing any modifications.

911hillclimber · Post by **911hillclimber** » Tue Sep 21, 2021 9:40 am

Yes, Jonney has put his unit to me before now, but I will look again in far more detail as this is now needed for the engine.
The Clewett package is all engineered to work together so simple to get right first time.
Open mind at present, lots of time to decide.

hot66 · Post by **hot66** » Tue Sep 21, 2021 11:59 am

gridgway wrote:Just thinking about this and realised that I have a hole in my knowledge! Are the two plugs in a cylinder fired at the same time? If not how different are they?

Was also thinking if there is a good solution using the classic retrofit clever CDI box and a later 12plug distributor pair? 964/993

yes they are .. have a read of the installation instructions here & things should become clear and it also explains they have a wasted spark as well in an opposing cylinder ... also look at how the coil packs & the spark plug wires are done ... you'll see each cyclinder has each spark plug run off an opposing coil pack
http://electromotive.com/wp-content/upl ... I-v1.7.pdf

911hillclimber · Post by **911hillclimber** » Thu Sep 23, 2021 12:35 pm

strikes me the Clewett system is the one for this engine, even got them all in stock!
Anyway, need to get the engine done and dusted first, so a later winter purchase.

DDK - Die Deutschen Klassiker