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Spark plug gap has a large effect on boost

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23K views 26 replies 11 participants last post by  dustinnaylor94  
#1 ·
I have just fitted a JSP chip to my 1988 900 Turbo 16, to take advantage of the water injection, SaabPerformanceParts larger injectors & advance mech etc. Excellent choice as the results are immediate and I can now take tuning a step further.
The biggest thing I have noticed is the effect the spark plug gap (BCP7ES) has on the turbo boost. At .030 gap the car takes off like a startled cat but the boost starts wavering at higher revs. At this gap setting you are actually better to only put your foot down 3/4 throttle as beyond this the boost does not feel right.
At .38 gap the boost is rock steady but no where near as exciting (sort of nice to drive but not quite the same thrill!).....I have just reset the gap to .034 to see what the intermediate effect is.
What I want to know is this. Is this effect the result of my ignition system coming up against it's limits? So should I be going to a better coil to provide a better spark? Or would it be better to just back off the gap to the point where it feels best?
 
#2 ·
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.
 
#3 ·
Thanks, I'm not really wanting to re-invent the car but I think I can get it to run a good compromise performance level. I have fitted the SaabPerformanceParts AID distributor diaphragm, I will perhaps need to tweak it a bit further to give more retard (anyone got any tips for doing that?). Then perhaps something just to beef up the spark a bit, then I think that will do really.
Not wanting to race the car or anything, just have plenty of grunt for overtaking :cool: ....
 
#6 ·
This car is a UK version so has Bosch LH 2.2 with no catalytic converter and no A/F sensor. Have spent lots of time and money on it so far anyway :p , but do not want to go overboard for what I use it for which is mostly long highway journeys - I just enjoy the extra torque already provided by the mods so far:
* Aquamist water injection
* SPP Blue series injectors
* JSP ECU chip
* SPP Red Series APC computer
* SPP AID Diaphragm

I would actually be very happy with it how it is at this stage but can always be tempted by modifications that do not take the car too far away from stock systems and can be fitted by me while still being able to hop into it each week and enjoy it!
 
#8 ·
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.

Image



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.
 
#10 ·
I am fairly confident that the JSP ECU chip has taken care of the fuelling problems (rich at idle, lean at full noise) that the engine had with just the standard ECU and the blue series injectors. Jak made the chip for my car specifically and it seems to be a big improvement so far. I know a/f tests on the road would be the ideal but I am so close to having the car perform as I like without doing them that I have been relying on checking the plugs visually and getting to know what lean and rich "feel" like - sort of a Kiwi No8 fencing wire (Bert Monroe?) approach :roll:
 
#11 ·
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.
 
#12 ·
No I haven't tried changing the ignition timing from standard 16 BTDC. I guess I would try retarding that if I were wanting to experiment. As for the plug gap, I have always gapped them standard but Jak suggested a wider gap to suit his new ECU chip. Thats when I noticed the big difference in boost characteristics.
 
#13 ·
I have also in the past considered the MSD ignition, fairly easy installation and MSD have given me the following part numbers to suit the car:
* 6462 6BTM Ignition module
* 8207 Blaster SS Coil
* 8910 Tach adaptor

Perahps I will look intot this over the next while.
 
#14 ·
nigelgrn said:
No I haven't tried changing the ignition timing from standard 16 BTDC..
Click to expand...
It's SOP to set the timing by applying your max boost pressure to the timing capsule. Then set the timing for what you'd like it to be at max boost ( 9Ëš btdc for me@ 22lbs of boost and 30# injectors and a extra 36# at 3/4 throttle running regapped bosch 4418 plugs ) The system is rock steady for me. I do how ever use custom wires, with the standard rotor.

Here are some 4418 multi-electrode plug pictures of before and after.
http://www.saabcentral.com/forums/showthread.php?t=42409&highlight=4418
 
#16 ·
I like that, the 'scientific method'. Alter one variable at a time.

By minimum I mean, I have the side electrodes ( cathodes ) touching the central insulator. The resultant gap is about .028" by sight. It does work under full boost with out a lot of knock, or blowing out the spark.
The spark travels longitudinally and is un obscured.
BTW:
This affect is also used by evenrude outboard's new 2cycle
high performance engines. They have stripes on the outside porcelain
insulators which coincide with the cathode. These stripes must be in a certain orientation for the engine to perform optimally. The devil is in the details.
 
#17 ·
been looking up that plug you mention (4418) here in NZ, and I am interested to know how you found the listing for that. Bosch's latest catalogue only lists the FR6DCX plug which is a stock plug I think. The Super 4 FR56/4PK is listed according to my local supplier but I am a bit confused about all the numbers :cry: ....
 
#18 ·
ejenner said:
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?
Click to expand...
yeah... what? That gap looks HUGE to me... I've always run .024, the tighter gap seems to work very well for me. Factory range is .024 - .028... maybe he typed a "3" when he meant to type a "2" :confused:
 
#19 ·
I run mine at 0.9mm. Works fine. Remember it's not only about the spark not beeing blown out when on boost, it's also something about driveability when off-boost. Larger gaps here provides larger spark, which ignites the mixture better .
Daniel.
 
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#20 ·
Hi!

Hello!

I was just checking my spark plugs and I had a realy big big gap.

In my saab manual they say that the gap should be between 0.06 and 0.07
I found that really tooooo small, there was just tiny space between. so now it is about at 0.020 and you can at least see a samll gap.

So what is the difference if the gap is smaller or bigger? does it play any difference?
And what should it be set at for a non-turbo?

thanks and help!
mati
 
S
#21 ·
I just changed my plugs last weekend (ngk....7es) The bentley stipulates .024-.028 inches. My previous plugs were set to about .040. (Probobly set that way from NGK) The car runs much smoother now on high boost but the old plugs were a bit fouled.

According to Saab-Daniel I should re-gap them to be larger?

Is there any guildlines to determine which plugs and gap its right for a given application?

The saab tech I spoke with recommended running a "colder" plug for higher boost applications. I am not sure what ngk offers though.
 
#23 ·
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.

Image
 
#24 ·
Just found out how true this is.....

Was on a two week holiday in the interior of BC, Canada. Was fighting a problem with some moisture in the fuel and would not start after 8 days of sitting in -20 C temps. Had car towed into a warm shop and explained to the mechanic that it just needed to be warmed up and it should start. He got it going after putting in new plugs even though the plugs were only two months old. On the drive home to the coast car ran badly under boost and would blow out the spark after 5 psi of boost. Today I checked the gap on the newly installed plugs and found they were at .040. Dropped them down to spec .028 and the car runs fine again.

My previous 16vt engine run .050 gap with standalone and MSD 6A and Blaster Coil.
 
#25 ·
99sven said:
Just found out how true this is.....
Click to expand...


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.
 
#26 ·
nigelgrn said:
This car is a UK version so has Bosch LH 2.2 with no catalytic converter and no A/F sensor. Have spent lots of time and money on it so far anyway :p , but do not want to go overboard for what I use it for which is mostly long highway journeys - I just enjoy the extra torque already provided by the mods so far:
* Aquamist water injection
* SPP Blue series injectors
* JSP ECU chip
* SPP Red Series APC computer
* SPP AID Diaphragm

I would actually be very happy with it how it is at this stage but can always be tempted by modifications that do not take the car too far away from stock systems and can be fitted by me while still being able to hop into it each week and enjoy it!
Click to expand...
Off your current topic but adding a FMIC is an easy mod that is easily reversible and that will give you stronger and more consistent boost.
 
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