Generally, advancing the timing must be done in conjunction with adding fuel for the reasons just mentioned. When going leaner, more retard is required to avoid det. However, retarding the ignition can lead to higher EGTs because the burning fuel gets shot out the exhaust. That's bad for exhaust valves.

I'm actually kinda surpised that there isn't more discussion of EGTs when tuning Evos. Some people argue that its more important to tune based off EGT and knock rather than using AFR and knock.

 

right so if I tune for 11.5 throughout the rev range I should be fine... would that be considered pretty conservative??

 

This is a question without a simple answer. It is definitely easy for members and Tuners to just throw out a given AFR and say optimize timing by advancing spark to Minimum Best Timing (MBT).

I have been tuning cars for roughly 7 years and although I do follow a strategy each time I tune a car, each tune comes out just a little bit different because no car is identical, even if it has exactly the same mods! And honestly that is what I find so rewarding with tuning, sometimes its like solving a little puzzle.

I tend to side with l2r99gst in tuning technique. I like to shoot for an AFR and hit it right before Peak Torque and hold it till about redline. You need the most amount of fuel at peak torque because this is where your engine load is the highest and also where knock is most prone to start and occur. Obviously this is not the only place it can occur though.

At crusing loads I shoot for somewhere between stoichiometric (14.7:1 .... its actually more like 14.6 now days with the additives in our gasoline) and 15.5:1. By cruising loads I mean between 20 and 50 KPa (kilo pascals). Between these loads and under them, although you will rarely see under 20 KPa except for decel, you can truly run the car much leaner than stoich, even to the point where the car is audibly pinging and almost not enought fuel to keep the car running without much worry about engine damage. The reason being is that there is no real engine load at these conditions. (Obviously timing needs to be set accordingly)

At atmospheric pressure ( right around 100 KPa depending on elevation) I shoot for 12.5:1 AFR. In general this is the Max load a Naturally Aspirated engine sees.

Research has shown that an 4-stroke gasoline combustion engine will make the most power between 12:1 and 13:1 AFR. But as all of you seem to know, we run richer than this on turbo cars. The reason we do this is for the thermal management of our engines. Adiabatic Flame Temperatures (think of it as cylinder temperatures) are the hottest at a stoichiometric AFR. On either side of stoich, whether it is richer or leaner, flame temperature is cooler.

From cruise loads to atmospheric pressure I taper from stoich to this 12.5:1 AFR.

As an engine comes onto boost I progressively richen the car to my target just before peak torque and hold it at this AFR depending on the characteristics of the particular engine.

*If the car hits peak torque and holds that torque through redline, I hold my target AFR and possibly richen it up a bit to redline. (Ex: bigger turbo in its efficiency range)

*If the car hits peak torque and immediately begins to drop and continues to do so, I will sometimes begin to lean the car back out a bit to redline. (Ex: small turbo out of its efficiency range)

Again, it really depends on the engines setup and characteristics.

It is important to remember that as RPM goes up, Load goes down. This happens on both NA and FI engines and is because as RPM does go up, there is less time for the cylinders to fill with air. By 10,000 Rpm the intake valves are open for an incredibly short period of time, its a wonder that the air actually has time to move! Thank God for pressure differentials!

One last thing.... and one of the reasons it is so hard to just say "Use This:". It is completely possible to make the same amount of torque (and hp, which is related to torque by an algebraic equation) at a given Load and RPM with a fairly wide range of AFR's. Timing does "create power" as some have said, but only to the extent of maximizing it for a given AFR. At a given Load and RPM, I can be on the dyno and make XXX lbft at an AFR of 11.7 and 17 degrees advance, then richen the AFR up to 10.2 and use 23 degrees advance and make the same amount of torque. (The mentioned numbers are just an example to get the point across)

So to say use this AFR is really pointless if you know what you are doing, and the AFR you choose is within the window where the car will make the most power, and you optimize your timing accordingly. Too much timing advance without enough fuel will cause irregular combustion (detonation) and if there is enough fuel, too much timing can cause pressures to be so high before TDC that the piston is actually having a hard time moving up. Too little timing (or to much retard) can cause EGT to become extremely high. Timing greatly effects EGT's.

Just thought I would share some info.

 

wow thats some nice info Kyle - thanks!

with cruise AFR's this is mostly handled by open loop which means we don't really have control over them...

 

Closed Loop instead of open loop is correct. The stock Ecu uses the O2 sensor to shoot for stoich (14.7:1) and does it very well, as long as your base open loop fuel map is not way too far out of wack! If it is your STFT (short term fuel trims are really high or low).

When tuning stand alone ecu's, there is typically an adjustable closed loop fuel map that you can decide to run which allows you to use a wideband O2 to feedback off of. This lets you can target AFR wherever you want.

 

FIY
MBT - Minimum (advance) Best Torque

 

It is also important to remember that as rpms go up and you make more power - temperature in cylinders rises. (so you can make all that power) that is why you need to RICHEN up the mixture not lean as i think you said.

 

cause egt's are metered in the manifold. they're only good for monitoring that they stay in the same range where you usually have them. If they are lower or higher then you will need to investigate. You cant tune off of that.

 

just a quick question what AFR's do a stock IX run - apparently its quite rich, but judging from this thread it must be SUPER rich stock...

 

It depends on who you talk to what they call it. I know several engineers that refer to it as either.

MBT - minimum best timing and minimum best torque refer to the same thing.

minimum best timing is the amount of advance required to produce the highest torque with the least amount of ignition advance, as seen on the dyno when mapping timing.

minimum best torque is made with the lease amount of ignition advance to create the same peak torque for a given rpm and load.

 

And the reason for this is...... because even though load is going down, you are displacing less air more frequently.

I am interested to know if you have ever been on an engine dyno with EGT sensors as well as cylinder temperature probes machined into the head/chamber. Cylinder temperature does not completely depend on RPM, but more on load, ignition advance and rpm, so it is therefor not 100% correct to define what you said as the trend.

As engine RPM goes up, VE goes down. This remains true with a FI motor with a constant boost level. Since with higher the RPM's typically comes less efficiency, meaning less load, it does not require the same amount of fuel.

For example:

Lets say this car is FI
2000 cc engine

@ 4000 rpm's it is 200% VE and requires X amount of fuel.

Then @ 8000 rpm's it is only 172% VE and requires X - a amount of fuel.

The load is reduced so less fuel is required. The reason that power is still being made is because power is related to displacement over time.

@ 4000 rpm's where the engine is 200% VE it displaces 4000cc = (2*2000) of air with 2 revolutions in .03 seconds

but @ 8000 rpm, it displaces (1.72*2000) = 3440 cc of air with 2 revolutions in 0.015 seconds. It displaces less air at 8000 rpm with 2 revolutions, but in half the time. So if we take how much air was displaced in the same amount of time, we see that 6880cc of air was displaced.

With this all said, it is easy to see that an engine that revs to 16000 rpm will be less efficient, but may still displace the same, if not more air in the same amount of time.

As we know, less load require less fuel, so the idea is if you are running 13.5:1 AFR at idle when the engine is 22% VE or 22 KPa, if you see that load (22 Kpa, which you wont) at 14000 rpm, it should run about the same amount of fuel.

I have heard that Formula 1 cars run about the same or even 10% leaner than Stoich. Most likely because they are always at very high RPM's.

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If I do lean the car back out again up top, it is not going from 11:1 to 12.2:1, it mabey more along to lines of 11:1 to 11.3:1. Obviously timing would need to be adjusted accordingly, and would achieve the same best torque.

Like I said, It depends on the characteristics of the engine whether or not it needs to be richened up or leaned out above peak torque. The only reason we typically richen it up is due to the necessity of thermal management, which I mentioned in my above post. Please reread.

 

why do you say 45KPa at idle?

is that because at sealevel standard air pressure is 100kpa (ish) and our engines are running a vacuum or 60kpa ish???

sorry for the retarded question but its quite interesting to me!

 

Oh, that was purely and example to get a point across. The evos depending on mods typically idle around 20 KPa. Let me edit it so it is more realistic to our engines.

 

but @ idle they run at a vacuum... like -9psi

 

multiply that by ~ 2 for the conversion and you get inches of mercury which would be ~ 20