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carrpet

930.66 Ignition timing for Classic Retrofit CDI+ installation

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1 hour ago, carrpet said:

 However some of the vacuum advance stuff at starting is thermo time switch related is it not ?

 I understand that my 930 has a way of changing the ignition timing for cold starting. 

Hi Peter, 

A Euro 930 does change ignition timing to help starting (cold or hot) but it has no connection with the thermotime switch, which is a part of CIS, or temperature. It does this with an idle retard which takes advance out only at idle (no throttle). 

My 930 has timing at idle of 2 BTDC (hoses on) and idle retard (15 degrees)  working. As soon as you come off idle, but before centrifugal comes into play, timing moves to 17 BTDC. This would be your static timing and is where your mechanical curve begins. I have 12 degrees of mechanical advance, all in by 3,500rpm. This gives 29 BTDC hoses off (ie no vacuum advance in play). 

1 hour ago, carrpet said:

 Could an inoperative or partially operating vacuum advance contribute to a cold starting problem ?

Yes - if the idle retard wasn’t working then my 930/66 would be timed at 17 BTDC when starting which I imagine might cause a problem. 930/68s, earlier 930s and SCs have different curves and functions. 

My 930 has about 16 degrees of vacuum advance which give total timing of 45 BTDC on a trailing throttle in the cruise. When on boost advance is taken out rather than being added in. 

This is my distributor- the left hose is vacuum advance/boost retard and the right idle retard. 

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44 minutes ago, PeterK said:

2 - lock the distributor (effectively removing the centrifugal advance element) and fully program an ignition curve into the CDI+

You can lock the distributor in 3 ways, either disabling the mechanical advance, vacuum advance/retard or both. In the way you describe Jonny’s box does the first and if this is right you can’t fully programme an ignition curve, only the mechanical bit. IMHO you need to be able to programme vacuum advance/retard as well to optimise your curve.

44 minutes ago, PeterK said:

I'll be locking my distributor and programming a standard ignition curve, then visiting a rolling road to have the curve optimised (once my resto is finished).

The aim is to stay just the right side of detonation in all conditions to optimise burn. This constantly changes with temperature, boost, rpm, load and fuelling. To get near this you need a way of sensing the point of detonation and the ability to pull/add timing and vary fuelling. This is EFI but a largely standard 930 can’t do this so I’m not sure that you’ll get much of a gain on a rolling road. How would you know if it’s detonating? Even if you did at a point in time that might not stay the same as other parameters change. I think the best you can do is push the envelope just a little in the knowledge that Porsche kept it super conservative. You need your fuelling to by top top though if doing this. 

You could of course add  knock sensors - the MSD box can take an input - but you’re still dealing with CIS. Better to EFI if you are chasing performance. 

David 

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^ I think we are missing something here.  Most people are thinking in terms of increasing the max advance at high RPM to get more power.   That’s great for a race car on a fast track. 
 

With a road car, increasing the advance in the low and mid ranges will make the car more driveable and pick up better.    The major advantage of the electronic control is that the advance can come back down again at high RPM.  Weights can’t do that. 
 

One of the workshops in US had good results on a Turbo (a Gemballa Avalanche!) adding more advance down low made the turbo spool up sooner. 

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