Please tell me about ignition timing

[novocaine]

Ok I've been talking to people and reading articles and I am completely confused now.

With regards to 20v

Its reccommended that a 20v timing be set to anything from 12 degrees to 16 degrees with the standard being 10.
So basically at 1000 rpm the timing would be 10 and in the higher revs the dizzy would advance it right? to a certain maximum value.
Now, is the advancing of the timing done to lessen the "time" from 1000 to whatever revs sees the timing advanced to the maximum amount?

Does that change the amount that the the distributor advances? if say the dizzy advances 30 degrees and the max value is set to 40, and we manually advance timing to 16 degrees does the max value change to 46? and is it because of that higher degree of timing at higher revs that there is an increase in performance?

OR

is an increase in performance perceived because the dizzy only has to now advance from 16 to 40 and it is saving time on the last 6 degrees?

Have i completely lost the damn plot?

What is the "advance curve"? is it a measure of something or is it a phenomenon?

This guy i was speaking to told me that its best to get the advanced curve to a smaller number, what i understood by advance curve is that it is the amount of degrees from lowest to max degrees? and he told me that the way to do that was to modify the distributor...................


ok i have a few more questions but i would really appreciate some clarification on these since they might nulify the next.

[NoHachi]

First read this: http://auto.howstuffworks.com/ignition-system.htm

Try to get a good idea what it is that that an ignition system (or advance) is actually trying to achieve. To get the pressure from the explosion at exactly that moment where the piston is ready to perform work (see the vid on howstufworks) The reason why you need to do this earlier and earlier (in terms of crank degrees) is because the time it takes to ignite the mixture stays the same regardles of rpm. But at higher rpm the crank is moving through more degrees in the same XX milliseconds it takes to ignite the mixture.
So what ignition systems do is fire the spark at exactly the right time to provide maximum work. At idle this is called the base timing. As rpms increase, the crank moves through more degrees per second and firing needs to be advanced a few degrees. This is called the advance. The advance you are running plotted against the RPMs is called the advance curve.
There are two ways of getting an advance curve. The 4A-GE uses a system where the ECU fires the coil according to an advance curve stored in the ECU. It starts at basetiming at idle and sensing the rpms starts advancing the stored amount of advance.
Older cars used mechanical or vacuum advanced distrubutors. These had inbuilt curves that depended on mechanical or vacuum gadgetary to achieve advance given a certain rpm. If you wanted another ignition curve, you had to have these distributors "recurved".

[novocaine]

Hi NoHachi, thanks for that link, I just gave it a read. Ok so the advance curve is stored in the 4age ecu, so now my question is, what does manual advancing of the timing via distributor help if it's controlled via the ecu?

[robokill]

Ignition timing is set with the ECU in test mode. (meaning you combine to wires to show wether the ECU is in good working order)

Then you set the ignition timing to 10 BTDC When thats done you remove the wire for the ECU test mode. It should then read above 16 on idle. This is the main rule for the ignition.

The ignition is as far as I know in a 4age a electronic one.

[NoHachi]

novocaine Wrote:Hi NoHachi, thanks for that link, I just gave it a read. Ok so the advance curve is stored in the 4age ecu, so now my question is, what does manual advancing of the timing via distributor help if it's controlled via the ecu?

The ECU does not have a list of exact ignition moments. Instead ignition is calculated in the following manner:
Ignition crank angle = base timing angle + RPM dependent advance (from ECU ignition curve)

Hope that makes sense to you. If you shift your base timing by a couple of degrees, that shifts the entire ignition curve by the same amount of degrees.

Now if you have a manual advance or vacuum one..which are linear with RPM..that might or might not be a problem. Usually not.
With electronic ignition you get additional problems. You see, the time it takes from the sparkplug firing event to the explosion pressure peak is in fact not constant. It is dependent on cylinder pressure, mixture swish and swirl, air/fuel mix, temperature etc. That means that instead of having the theoretical linear RPM dependent curve, the actual curve is quite different. If you shift the base timing to improve a certain area of the curve..you shift all the little detailed hills and valleys in the curve too, possibly causing problems.

Thats where the caveat comes in..To be honest, you don't want to be messing with ECU base timing etc unless you have the instruments to check that your engine is still operating properly. That means a knocksensor and preferably four EGT gauges during tuning.

[edit: electronic ignition is a problem only because it already has such a good map..vacuum/mechanical is a lot cruder.]

[novocaine]

Thanks for all the replies guys, I think i understand now NoHachi.

I understand why i was told about the distributor mod. the dizzy advances the timing based on engine rotation speed using centrifugal weights, so modifying those accurately could have you leave your base timing as stock but still managing to have the max timing advanced by a certain number of degrees.

ie. if base timing is 10 and the timing after max advance is 40, setting the base timing to 16 degrees would give a max value of around 46+- and help high rev power, but with the drawback of losing bottom end power.
and to solve that the dizzy could be modified to have base timing of 10 but a max of 46...

is that right?

[NoHachi]

You are correct, only running more advance is not always better for high revs. This is different from car to car..Eg. miatas gain mid-band torque and lose a bit up top when you advance the timing from 12 to 14 or 16 degrees.

Anyway, if you have a 20V running stock 20V hardware (no 4k distributors and carbs stuff) then your advance comes from the ECU. NOT a vacuum advanced distributor. Same rules apply, but like I said, you might end up worse after the mod. Possibly even destroy your engine through detonation. Ignition timing is not really something for back yard mechanics.

EDIT: more easy reading: http://www.importtuner.com/tech/0612_imp...spark.html

[novocaine]

yep i know about the possible consequences, we also only have 95 octane fuel

There are a number of 20V's in sa running advanced timing and they do run really well. The most i've heard of with the stock set up is 16.

South Africa was lucky enough to get quite a number of toyota corollas with the 20V blacktop motor, but its only recently that guys have started to modify them and actually find out about these little things increase performance.

For instance the 63mm exhaust... The local exhaust guys thought it was too big but in fact it makes the car run quite a bit better.

I was trying to understand why the advanced timing would work, and to make sense of all the talk of the advanced curve by a certain guy, but i think its cleared up for the most part. Thanks again for the information.