Best practical way to Twn Spark ignition 911 engine?

[neilbardsley]

Do you need to twin spark this engine? Can someone port the heads for you. I don't know much about the shape of the 911 heads but, a 10.5 compression, can you get away with a stronger spark. My understanding is that one of the main advantages of twin spark is that you can reduce the advance making denotation less likely? Is this so important for a 40 effort? Does the less advance also increase efficiency because more power/push is applied nearer to the piston at its top point?

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[911hillclimber]

From my limited understanding Neil, the twin spark is an advantage particularly when the pistons start getting large in diameter.
The 3.2 has 95mm dia pistons and standard, one plug/cylinder.
The 3.6 964 had twin sparks from the factory.

The plugs are in the top(ish) of the hemi head dome but to one side as the valves are close together at the top of the dome. The second plug fires after the first and the combustion initiated by the first meets the charge from the other plug so the combustion is double sided.

The 10.5:1 pistons had a high crown roughly splitting the dome at TDC into 2 halves. so instead of the one plug firing the charge and the flame going across the one sode of the crown and down the other, the 2nd plug fires on the other side of the crown.

At 8,000 rpm it is quite busy in that chamber and 2 plugs help keep the explosion as effective as possible.

With 4 valves/head the plug could sit right at the centre of the chamber, sitting there with the 4 valves surrounding it. This is very effective, but I would think modern petrol performance engines with big pistons still have one plug even with big pistons, though I stand to be corrected.

Anyway, that is my understanding.

[PeterK]

... but do remember that most 'experience' is from the US, where pump gas is like bat pi$$. Our 98 octane fuel pushes the need for twin-plug a lot higher up the chain. I have 98mm 10.5 pistons and am informed there is no need to twin plug (but time will tell). I am using one of Jonny's CDI+ units to give me two spark pulses each time, but. from the same spark plug

[Lightweight_911]

Compression ration in my Giulietta Sprint's (single plug) engine is 11.2:1 & it will rev easily to 8k rpm - possibly safe to 9k but I've never been brave enough to try !!

.

[PMNorris]

I have classic retrofit. See my setup on my resto thread. It works really well

https://www.ddk-online.com/phpBB2/viewt ... 1&start=75

[911hillclimber]

But...
How do you define works really well?
The engine could run to 8k on single plug, but might return more performance on twin plug.
Without a rolling road tune you will never know.

For £1400 the Clewett will do the lot and no distributor.
Not sure if the other unit is a direct comparison.

Factory competition cars ran twin plug on 2.8 and prob smaller engines.

The rational is not clear.

[hot66]

From memory Neil Bainbridge said I could run up to 10.2 in my S on single plug … what cam you use can effect this though. I chose twin plug for added security if traveling Europe etc and could only find poor fuel. Obviously you need to tune the motor to get the most out of twin plug . Think we ran less advance , can’t remember now as it was 11 years ago ?

[neilbardsley]

This was the point I was struggling to make

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[911hillclimber]

Remember my engine is a frantic hillclimb only lump.
Always 99 octane, always given a big Bol&/o king every run, sometime the engine too hot or too cold but no mercy.
Reliability not at the top of the list.

[Jonny Hart]

For outright power at WOT, CDI cannot be beaten on our large hemi head engines. The move to inductive ignition (C3.2 onwards) was largely driven by economy, inductive being better suited to lean burn engines.

The reason the 964 went to a twin plug is because it had to make up for the slower flame produced by the inductive system. It did this by burning the mixture from both sides of the chamber.

With a CDI system, the spark energy is delivered in approximately one fifth of the time of inductive. Think of it as inductive being a slow push away, CDI is a short sharp punch.

Of course, having a twin plug CDI based ignition is having your cake and eating it.

Neil Harvey (a respected builder of Porsche racing engines) wrote this article which is worth a read:

https://performancedevelopments.com/rep ... iignition/

[911hillclimber]

Thank you Jonny, I'll have a read of that tonight.
As a summary..

How will this engine perform if I have on of your cdi units, keep the RS distributor I have, a matching coil, the GE cams, 10.5 ratio and the ability to rev to 8k so to max the cams out and one plug?

You know the Clewett system that uses (I think) modern coil packs etc and crank fire, so no moving parts?

[PMNorris]

But...
How do you define works really well?
The engine could run to 8k on single plug, but might return more performance on twin plug.
Without a rolling road tune you will never know.

For £1400 the Clewett will do the lot and no distributor.
Not sure if the other unit is a direct comparison.

Factory competition cars ran twin plug on 2.8 and prob smaller engines.

The rational is not clear.

Its a lot smoother. Pretty hard to be very specific, but it just feels a lot better throughout the rev range. Neil Bainbridge did my twin plugging. The rest of the engine wast built by BoxerMotoren in Germany.

[911hillclimber]

That link from Jonny is worth the read, explains things well.
Twin sparks are absent, so I'm not that much further forward, but better informed!

I'm still not convinced I can get away with one plug/CDI as opposed to 2 plugs CDI.

There is about £1000 difference I think between the Clewett and Jonny's designs, most of the cost is in the head machining.
Anyone on here had this done and care to approximate the cost?
Be irritating to build this engine without the heads machined for twin plugs then find I need them.

[brembo]

[url=https://www.flickr.com/gp/92639320 ... .jpg[/img]SAM_4309 by brembo26, on Flickr[/url]

[url=https://www.flickr.com/gp/92639320 ... .jpg[/img]SAM_4308 by brembo26, on Flickr[/url]

Mike Bainbridge fitted Jonny's set up on my 2.8 twin plug engine,which sorted out a lot of the problems he had getting engine running properly.
Must say,I'm very happy with the result.

[BILLY BEAN]

I have a conventional distributor twin plug set up on a 2.7 litre. Two CDI boxes are part of the set up and a Hall effect in the distributor rather than breaker points. The compression ratio is slightly north of 10.5:1. The pistons have a raised crown and pockets to clear the inlet and exhaust valves. The raised crown effectively bisects the compression space at TDC. This I understand provides little passage for the flame front on ignition and without twin plugs would result in unburnt fuel and lower performance. Lower octain fuel makes this situation worse. For the record the engine will rev to 8,000 on quite soft cams (GE40) and produces 271 BHP at the flywheel at 7500rpm.