can anyone tell me or guid me the theory behind spark advance?

 

There are two basically,
One is tuning to the limits of detonation.
The other is running the least amount of timing consistant with max performance.
Both have there places, the first one is generally for racing, and the second for the street.

 

in mechanical terms, the reason the spark plug is set off before the piston reaches top dead center, is because it is moving so fast, that it can be right at top dead center as the burning fuel/air mixture is reaching its highest pressure, which in turn will create more downward force on the piston.

 

but how do you know if detonation is from a to lean contition or to much spark advance?

 

Learnig to read plugs can tell you lots about whats going on. A WBO2 can also give you real time feedback on AFR. A FP gauge can also help with determining if a lack of fuel is a problem.

cheers, BW

 

So first get the BLMs good. Then turn up the spark advance to max, hook up a scan tool then back it out untill all of your spark retard is gone?

 

I'd make small 2-4 degree changes until you start getting knock retard, than back off a couple degrees in areas that corrospond to the knock. Better to approach detenation slowly, then to try and back away from it.

 

If you have to back of , lets say 4 degrees to get rid of knocks in a certain area. Do THEN have to go to the same area in the VE tabel and lean in up to compensate for the decrease in spark advace?

 

And i can se in my spark tabel that spark advace increase with rpm and then decrease with load, true?
And why does it decrease under load? Couldent the spark tabel increase with higher RPM and be constant under the hole load for that rpm?

 

I'll be changing to 730ECM soon............... so then we can help each other out.......

/N.

 

the reason spark advance "decreases" with load is because the engine "creates" more cylinder pressure when it has alot of drivetrain load on it

 

1) True.
2) I don't know if you have an SD system, but if you do, have a look at VE and you'll see it increases under load (MAP) , and that means more mixture per power stroke and that means higher cylinder pressures (as FstBrd6point3 said). Under high load, you've got the throttle wide open and the maximum pressure forcing maximum mixture in.

I think you can say the causes of detonation fall into two broad categories- pre-ignition and auto-ignition. In pre-ignition, something like a hot-spot sets off the charge before you want it to. That's not what you're concerned about with PROM tuning.

You're worried about autoignition caused by the cylinder pressure and/or temperature rising so fast that the mixture spontaneously combusts. That is, the spark ignites the mixture which burns out from the plug, but the temp/press increases to a degree that combustion happens elsewhere in the cylinder without a spark or the flame front reaching it. This releases a shock wave(s) that can generate huge temperatures and pressures and destroy your motor.
Detonation is thus uncontrolled combustion.

You can cause this problem by being too lean (increases the cylinder temperature) or too far advanced (gives enough time to build the pressure to autoignition point). The reason you have to run less advance at low revs is that there is more time to create the pressure rise required for autoignition.

That's how I think it works. If I'm wrong, I'm sure someone will quote Sir Harry Ricardo and disabuse me of my ignorance

John

 

ok isee. So the probleme I have with knocks at 40-60kpa and 1300-1600rpm could be fuel or spark? So if I add fuel in that area it could help cool down the heads? What is best to do, take down spark advance or richen it up?

 

Yep.
Read Grumpy's Tuning Tips. He regularly says something about trying to run as little timing as possible for the power you want, and he tries to run a tiny bit rich by preference. He'll chime in.

John

 

There are alot of things that just aren't cut and dry.
You have to experiment to find out what makes your engine happy. An engine will tell you what it likes, but you have to listen carefully.

In some cases, you'll get to where making a fuel change makes no difference to the way the car drives, and then the only thing left is timing. ie some cars have an off idle stumble, and guys just add accleration enrichment trying to get rid of it, or even change converters, when the problem is actually having too much timing. Again, gets back to running the least amount of timing consistant with best performance.

And remember, detonation takes time to develope, so you have to add some fuel or remove some timing at the point of detonation AND slightly before it.

 

here is the situation..

I hade a tuning company here in sweden tune my car with a wideband O2, both fuel and spark tabel.
This whas a exhange job, i fixed there wireless lan and they made a tune on my car. This company is good, its not that, Its just that I cant take the car back all the time when i whant to make a adjusment like a "normal" customer do.

After got the car back it couldent hold a idle when cold. And I got knock counts at 1600 1900 rpm at 50-75 Kpa load.
Abow 2000rpm the car is killer. Blms are ok and so is O2 v.

I also got of idle stumble.
I looked at the chip and saw that thay had tookaway 10 degrees at my idle kurve ( 900rpm 40-50 Kpa ).
I changed the main spark curve to a simple test mod. I set the timing to 30 instead of 22. What a diffrent, now the car idles fine and the of idle stumble is gone.
BUT now I have a strange timing curve due to my change in the idle area.
The company can take in my car again but after summer So I would really like to fix it before. ( and i learn )

Now I DONT whant to change the hole timing curve and fuel curve, because the car runs like hell abow 2000rpm.
Must I discard all the things the tuning company did?
Mayby I just can tune the below 2000rpm so the entire curve is good?

What I really need is som kind soul that can look at my bin files and mayby give me a tip or two? I cant offer any money ( i live in Sweden ) even if I whanted to. I cant give anything in return but my gratitude.
Or mayby you have a MAP TPI car with a hotcam in it so i can look at it?

Thanks