[ejenner]

Plug gap is important but when you're talking about the standard Saab 900 ignition setup it's worth considering the inadequacy of the stock ignition timing facility. You basically have two settings. One setting for off-boost and then a retarded setting for on-boost. It's better than not having any retard at all but because of the way it works the ignition timing is rarely (never) at the optimal setting for the engine conditions.

The three cheapest ways to get computer controlled ignition on the 900 are to go for a direct ignition swap (where you take the parts from the 9000) a megajolt http://picasso.org/mjlj/ or a megasquirt http://www.megasquirt.info/

The conversion to DI is probably the easiest way but you can't map the DI system yourself so it might not be the best conversion in the long-run. You also have to remove the engine to fit the crank sensor. However - I think I'd find that easier than messing about with megasquirt.

Also - your air/fuel ratio plays a big part in mantaining stable boost. If you're running a bit rich the fuel will drown the spark and cause instability. Have you got a wide-band lambda for measuring your air/fuel ratio? Quite likely to be a factor with the symptoms you describe. Imagine that the sparks are trying to fire but the fuel keeps squirting over them and drowning them out so they don't successfully earth and ignite the mixture.

 

[ejenner]

I guess you mean you don't want to spend time and money on it?

Any comments on the air/fuel ratio - could be causing your problem - have you measured it?

 

[ejenner]

Actually - I'm pretty sure it's the air/fuel ratio... with the symptoms your describing.

What colour were the plugs when you took them out after the spikey/jittery run?

 

[ejenner]

You wouldn't use the built-in sensor to measure the a/f ratio as that's a narrow-band sensor and does
not operate at much past idle engine speeds.


Something like this wide-band lambda kit could be used.

But when you put your car on a rolling-road they should be able to give you an a/f read-out. I guess with
all the extra fueling you've got the car is running pretty rich on boost with a wide-open-throttle.

The reason it runs better on 3/4 throttle is that the ECU isn't putting as much fuel in.

 

[ejenner]

Saab also recommends a plug gap of about 0.6-0.7mm so are you saying your plugs are gapped about half the size or are you specifiying a different measurement?

 

[ejenner]

Have you tried adjusting the ignition timing at all? Could be worth a shot. Just mark the original position so you can change it back to it's original setting.

It sounds to me like you want to keep it with the gap setting that works best and just leave it at that? But personally it would bother me if I knew there was only some small part of the setup keeping me away from a noticeable performance increase.

 

[ejenner]

You need a smaller gap because the conditions within the combustion chamber of a forced-induction
motor are sometimes too harsh to mantain a good spark. If you decrease the gap the spark won't have to
travel as far and the spark won't be blown out.

However... if you are using a direct-ignition coil-pack from the 9000 the sparks are so powerful you
can run with a gap of 0.9mm (for a big spark) and there's no chance of it failing to earth because
the spark is so much more powerful.

 

[ejenner]

Just found out how true this is.....

For me the eureka moment was just after I'd got my 99. After buying the car it had a bit of boost but after not too long this stopped and it was practically normally asipirated. To try and recover lost boost I tried a number of things. Eventually it prooved to be the APC not allowing boost for one reason or another but I found out about spark-plug gap during troubleshooting. The trick with single-coil dissy ignition is to ballance driveability and boost. If the gap is too big then the boost dissapears. If the gap is too small the engine runs quite rough but you get a big kick in the back when the boost cuts in. Somewhere in the middle is good.