thatsMR2u

iTrader: (19)

"I'm going to be working on getting some good base maps out in the next few days. Here is some info to chew on in the mean time.

Timing Basics:

Combustion is not an instant process, it happens very quick but if you slow the whole engine cycle down you'll see it's rather important when you start the combustion process. The piston is moving up on the compression stroke and right before it reaches the top the spark plug arcs and the combustion process is started. Depending on how early the spark starts before the piston reaches the top (TDC - top dead center) will also depend on how much cylinder pressure is produced (torque). When you look at the timing maps the #'s represent how many crank degrees before top dead center the spark is set off. You will notice that at lower RPM's the timing advance is less. Why? VE & engine speed


Volumetric Efficiency -
The engine is basically an air pump, air goes and it goes out. There are many things that control how the air goes through the engine and at low RPM's the air doesn't have much trouble getting in and out. As RPM's goes up the amount of air and how much time it has to get in and out in each cycle quickly diminishes. Peak volumetric efficiency is usually around the torque peak and as VE drops, so does torque. When the engine is at peak VE the cylinder is filled and pressurized with the most inlet charge (air & fuel)
Bear with me, I'm going to give simplify a few things here:


PROPANE, EXPLOSIONS & SUCH
Pretend you're cooking on your gas grill, the propane is burning nice and slowly under your hamburger and providing a controlled heat (combustion). Now take your propane tank off and throw it in bon-fire (don't! but pretend you just did), are you going to have a nice controlled burn? No! It's going to explode and you better be nowhere near it when it does.
Why did it explode?
1. Pressure - the propane inside the tank is under pressure, in fact it's pressurized enough to change phases, (from gas to liquid).
2. Heat - the heat of the bon-fire raises the pressure of the propane inside the cylinder to the point of bursting. It also provides ignition to the propane (fuel)
3. The combination of heated (excited molecules) propane, pressure, and ignition source make for one high energy and quick explosion.

Now pretend that propane tank is inside your cylinder. When the piston is moving up on the compression stroke, it's compressing the air/fuel mixture. When you compress (pressurize) something you are putting energy into it, in this case heat. To see this in action, quickly touch the line coming out of an air compressor, you'll see that it's very hot because the air is being pressurized.

So there you have one contributing component to your combustion process. It's very important to look back and take the volemetric efficiency back into account. If more air fills the cylinder, then there is more air to compress and the temperature gets higher, follow me here? As RPM's go up and VE drops, there is less air to compress and the temp. rise is lower. Very important concept here, please re-ready it or ask questions so you understand it. This compressed mixture will release energy and also burn at a certain rate depending on the amount of pressure and heat. The more pressure and heat, the faster the burn & more energy is released.


Spark timing & and the actual combustion process.
Your piston is almost near the top on your compression stroke and the spark goes off. There is not an instant explosion (unless you're detonating), but rather a controlled burn that continues as the piston crosses over the top and goes down during the power stroke. Most of the energy should be released during the power stroke, as the piston pushes down on the crank (quantified as MEP - Mean effective pressure). You don't want this energy released too early as it would be trying to push the piston back down (backwards!). You have to fire the spark plug at the correct time to extract the most amount of energy and apply it to the power stroke, this gives you the highest torque (quantifed as MBT - mean best torque). If you have high enough octane gas, this is no problem. Given that you have no detonation, you can also have too much ingition advance where you waste too much of the combustion process on the upstroke (compression cycle). Not only do you lose power but your cylinder pressures go way up, and this can cause other problems (headgaset issues, bent rods, broken pistons, beat-up bearings, ect). As you approach the ideal timing (best torque) you start to reach diminishing returns, this is one major clue to ignition timing.

Spark Timing & Octane
Ok, so give her more spark advance untill she quits making more power, easy right? Not so, because we're not all running on 120+ octane we have to deal with detonation & pre-ignition. To simplify things pretend that low octane fuel is an angry drunk, any little provocation will set him off and explode. While high octane fuel is one of those British Gaurds with the funny hat, he won't do anything but stand there keeping cool under pressure, ha! Unfotunately to make the best torque you have to provoke the angry drunk beyond his level of toleration and he'll explode on you....so you have to gently and slowly find out how much provocations (timing) you can give the angry drunk before he goes off. Detonation (angry drunk exploding) is violent and dangerous, it can crack ring lands, destroy bearings, blow headgaskets, ect. It's so violent in fact that the sudden explosion produces enough energy to vaporize parts of your aluminum combustion chamber and piston and wind up as deposits on your spark plug and exhaust port tract. Fresh detonation will actually look like tiny spheres of shiny aluminum on the tip of your spark plug (use a magnifying glass under sunlight).
Tuning on pump gas is difficult in that you have to approach this timing limit and leave some safety margin in the process. There are different levels of detonation and some engine will tolerate slight detonation for quite some time, while some will not...do you want to take the chance?
Now as RPM's rise you will have a drop in VE, which equates to less pressure/heat...thus you can increase igntion timing as rpm rises.
A good tuner will find this timing limit and map the entire engine operating range to make good power with safety in mind.

Engine Mods, Boost & Timing

What else affects timing? Let's say you run a larger more efficient turbo that heats the air up less and also has less backpressure so there is less hot nasty residual exhaust gas (end gas) left in the cylinder on the exhaust stroke. This usually means you can get away with a little more timing. Each modifcation is different and requires carefull thoght into the timing curve. I'm not saying that you need different timing for air filter changes or BOV changes, but if you change turbos or do some other major changes you could optimize your timing curve.

One last parting thought... as you raise the boost the cyilnder is filled with higher pressurized air, and also the more boost you run the hotter the air is. So as you raise your boost you have to reduce your ignition timing because the mixture is more sensitive to detonation and also burns quicker so you don't need as much timing.


This is a ton of information and a lot of it is simplified but it should give some basics and insight to igntion timing. If you have questions please ask away. I'm also probably forgetting a few important things (it's late and I'm tired) so if you see something I've missed let me know.

Regards,

Martin Musial"

Verbatim taken from another thread . https://www.evolutionm.net/forums/sh...7&page=1&pp=15

Mad_SB

iTrader: (8)

So, I guess my signgle biggest question is how to you konw when to start ramping up timing after peak torque? Obvliously the answer is to look at how the V.E. falls off, or rather the V.E. curve of the motor in it's current state of tune (modifications). Without this V.E. curve to work from how would you tell when to start adding in the advance?

I would assume that your lowest value of timing advance (least amount of advance) should occur at the point where the accumulated effect of V.E. and boost create the highest cylinder pressure? What I mean to say is if your peak V.E. and peak boost pressure occur at slightly different rpm the peak cylinder pressure could occur somewhere other than at peak V.E. given enough boost offset?

I doubt it would vary much but just making sure I understand the relationship.

Ludikraut

iTrader: (17)

This should be a sticky.

l8r)

NocturnalEVO

iTrader: (39)

awesome post!

colonelfox

iTrader: (41)

I agree, very good post.

Mad_SB

iTrader: (8)

Another question:

IIRC, peak flame speed occurs between 11.17 and 11.8 AFR. I would assume that at peak flame speed you would want to run a little less timing advance to produce peak torque due to the faster combustion than if say you were richer, is that correct?

AMS

iTrader: (3)

Yes, your thinking is correct. I'm not exactly sure if peak flame speed occurs at those A/F though.

Spooldyou

iTrader: (52)

lol, I love the referance to Drunks, Right on though.

freddiet

iTrader: (1)

When are the base maps arriving?