After getting my car's tune 95% perfect, I couldn't leave well enough alone and decided to redo my idle tuning from the ground up. I tweaked my TB until my IAC counts are perfect (drop down to ~14 steps on hot idle). I'm trying to dial in my IAC Effective Area now as I maxed out my TB set screw and ended up having to drill the TB blade to 15/64" so the table is way off.

I've been logging Dynamic Airflow (DYNAIR) and Desired Airflow (IACDES_B) and my Dynamic is about .5 g/s larger than Desired. I've been shifting my IAC Effective Area table down and down and down (to the right for you HP Tuner guys), and when I restart the car, they BOTH have increased/decreased by the same amount. For example, if the values were 1.50 and 1.00 g/s before, after the tune they're 1.75 and 1.25 g/s

At first I just thought that I hadn't adjusted it enough, but I've already shifted it 18 cells to no avail. Most people I know have only moved it 4-6. Am I doing something wrong or should I keep going?

Also, I remember someone here saying that the stock IAC Effective Area tables are basically worthless and that they build their own. How would one go about this? It seems like it would be an excruciating trial and error process, unless you can figure out some trigonometric function that models the TB blade.

 

nevermind, turns out I just had to keep shifting the table farther. It now starts at 50mm^2 instead of 12mm^2

Still, does anyone know how to go about building a new IAC Effective Area table? The only way I can think of doing it is logging Dynamic and Desired airflow from a complete cold start all the way up to hot, add subtract IAC steps according to the numbers, wait for the car to completely cool down and repeat until they're the same for the entire range. Seems like it would take a long long time to do.

 

Ok correct me if Im wrong here.

Effective area measures the area of open throttle available to the engine at certain iac counts. Its used so that it can provide the desired flow in g/sec by adjusting iac to "open" the throttle more.

With a new TB as long as its closed fully at zero TPS then the IAC effective area will remain stock as there is no flow through the blade, just through the IAC pindle, this is assuming the new TB uses a stock capacity idle air venturi etc.

If you need to open the blade a little because your IAC counts are huge and provide no room for cracker, follower and cold start idle functions then your IAC effective area is increased at all counts. As mentioned running at around 30 hot allows plenty of room for follower and cracker functions as well as extra meat for cold starts.

The only issue I see is knowing how much you have opened the blade prior to sorting out your IAC effective area.

Area of a circle is pi x r^2 so stock 74mm is 3.14 x 1369 = 4298 sq mm

We might need to reduce this a little because the area of the blade edge reduces flow a little say 74mm x 2mm = 148mm so 4150mm fully open. But at partially closed the blade face is in the way anyway so wont matter.

Im going to propose we use 45% TPS as equalling 45% of this area, 90% is 90% area, etc. This might not be 100% correct, but should be close (hopefully)

So assuming you turn your throttle to 3% open before you reset it, that means you have 129 sq mm as your zero IAC count baseline. We can immediately see that using a linear value is wrong as 129 is far too large, so TPS increases must increase sq mm by a non linear amount. I think Mr Calculus needs to be employed here. Im off to go searching for a formula to solve this problem.

Shifting it down until it clears up is a fairly valid way to do this. As mentioned the values themselves in the table wont change with a stock like IAC mechanism, just the zero baseline.

But I think this is what you would need to do to set your baseline mathematically. As mentioned as long as the IAC motor and venturi are stock you wont need to adjust the table. Hopefully aftermarket TB's are sensible enough to do this. Its just the baseline point that will need fixing based on your reset TPS position.

Comments on my methodology appreciated.

(Im off to look for a formula to solve the sq area in a cylinder with pivoting top. Hopefully the TB is not actually a funnel instead of a cylinder until after say 10% or so or this is going to be too hard as it will depend on porting etc, etc)

 

The IAC effective area doesn't include how much the throttle blade is cracked open or how bigger hole you have in the throttle blade. It is totaly unrelated.

It is PURELY the effective area (cross-sectional area) of the IAC bypass system versus how far open the IAC valve (stepper motor) is.

Look at the table, the maximum value is 120 sq.mm - that's about a 12mm dia. hole (1/2").

For the stock throttle body there should be no reason to adjust this table, unless you believe you can calibrate it's flow charateristics better than the manufacturer.

Adjusting your throttle blade or drilling the hole larger serves to make the dynamic range of the IAC useful. ie. if your cammed car needs to idle at 150 steps then you don't have as much headroom for cracker and follower as the car that idles at 30 steps. You have "used" 120 steps just to get the car idling. Changing the TB hole size doesn't affect the IAC effective area table at all.

Changing this table is like changing your trans force motor tables to modify the shift pressures or increasing the PE table to 11:1 so that you get 12.8:1 AFR, sure it works, but it's not really the right way. Generally, the real issue that is being compensated for is the RAF table.

Chris...

 

If it's purely the area in the IAC, why does the stock table have a positive value (12 mm^2 i think) at 0 IAC counts?

 

When you say IAC bypass system are you also taking into account TB size eg (90 mm tb) or is the IAC passages what the "Effective area" is referring to

 

To find how much open area you have would be something like:

area of throttle blade - (area of throttle blade - area of hole)*sin(TPS*0.9)

Although i don't see how that would be helpful in tuning.

 

According to the efilive description the desired airflow (raf) value is converted to a cross sectional area which is then converted to a number of iac counts.

I can "see" that cracking the throttle with the set screw will mean that lower iac counts mean a greater area because the throttle is cracked making the baseline more than zero.

Perhaps stock when you can assume throttle is closed the iac table is 100% right and shouldnt be touched. However with a partially opened throttle I can also see that opening iac lets in (baseline flow + iac count flow) with baseline flow being the amount its opened from drilling or set screw.

Maybe the stock table starts at 12 because of the area around the blade to body clearance? And/Or base IAC bypass flow.

gameover, I understand what you say, I take it the IAC has a base flow of 12sq-mm at zero counts? Does not RAF get managed by IAC? In which case are you saying the answer really would be a larger IAC mechanism for larger cams and therefore a more scaled IAC effective area? Else how else can you manage it if IAC is max'd out trying to sort out a large cam? RAF needs IAC movement to work, no IAC movement = no ability to get RAF correct. So TB has to be cracked or drilled to get IAC back into line with room to move.

If we get a proper formula we can see if it gives us a workable solution in either case for determining the baseline for IAC count steps.

 

I'd like to know this as well. My understanding was that the IAC motor serves as basically a computer controlled throttle body, fine tuning the airflow into the motor so it gets the proper amount. The stock TB has "X" amount of open surface area and can let in "Y" amount of air in with 0 IAC steps. The IAC Effective Area table tells the PCM, if you move the IAC motor "Z" steps, you can get "N" airflow. The problem is that after you expand the TB hole or open the set screw in, you're now letting in extra air that the PCM doesn't know about because it still thinks that at "Z" steps you're getting "N" airflow when in reality you're getting "N + some from larger hole". This is why you need to change your IAC Effective area.

It seemed to make sense because after I opened my set screw/drilled my blade, my Dynamic Airflow was much higher than Desired Airflow. As I moved my IAC Effective Area table the Dynamic moved down while the Desired stayed the same. Are you saying that the MAF and/or MAP handles the extra airflow through normal means and that RAF should be adjusted to bring Desired and Dynamic to the same level?

 

IMO if the IAC table calibration included the IAC area and the stock TB hole area then you should adjust the table if you drill the hole out or open the throttle blade up. If the table is just the for the IAC area then it does not need to be re-calibrated.

John

 

Agreed John.

From the software and results people are experiencing plus the table description I read it as a table to tell the PCM what the airflow through the "TB" (total) is at each IAC count, litterally. (ie) Plus any holes or cracks

I guess it depends on how you interpret it, 6 of one half a dozen of the other. Bottom line it needs adjusting for open throttle and a formula that tells you the additional sq mm opening for x% of throttle or dilled hole will show you where to place your IAC at zero count. JMO.

 

the 0's at the beginning of the table are due to the finite resolution of the stepper motor.

ie. at 14 sq.mm it's 1 step, at 16 sq.mm it's 10 steps etc.

There is no such thing as 0.5 steps or 0.25 steps so for anything below 14 sq.mm the only other choice is 0.

Chris...

 

I think you're missing my point, or i'm misunderstanding the range of the iac motor. At 0 steps the plunger is butted up against the metal of the throttlebody right? Meaning at 0 steps no air can get through by way of the iac. Meaning at 0 steps the effective area of the iac hole is 0 mm^2. Which would mean that the effective area table does include the area of the tb hole.

The only way your explanation makes sense to me is if the iac motor is not able to completely close the iac passageway.

 

I think your missing my point

At 0 steps the IAC is closed.

At 1 step the IAC is open 14 sq.mm

There is no in between.

The axis value is sq.mm, so if the PCM says i want 6 sq.mm, the IAC stays closed at 0 steps because it cannot give you 6 sq.mm. The first value it can give you is 14 sq.mm or 1 step.

 

I see what you're saying but i'm having a hard time buying it. Although i know you can read pcm code directly so i want to understand why you're saying it. Is there something in the pcm code that accounts for the area of the hole in the throttle body?

 

I understand both sides of this argument, I think. However I agree that the effective area table must include any airflow entering the engine independent of iac function in order for the ecm to control airflow correctly. As I understand it, this table tells the ecm what airflow is coming in at a given step in total. How could the ecm correctly control idle and transitions if all it new was the airflow through the iac. If that were the case we could all go drill and crack the tb and all would be well. But once that is done logging proves that the desired flow and actual flow are way off, and the car won't return to idle correctly, stalls, surges, all the good stuff.
In theory I can see both sides, but in practice, having to alter the iac table to correct tb alterations proves the point IMHO.
Back to the original question. The best way that I have found to build a new table is to log desired iac flow, desired iac postion, and actual air flow from the maf.
I set up a chart to tack all this and worked from there.
The first chart is with the iac table shifted, and it was way off. the second is after I build a new table from the ground up.

 

You don't understand this table correctly. It is almost the opposite of what you said. The PCM calculates what effective area it wants from the IAC, based on the IAC airflow numbers (completely seperate from the MAF/VE airflow numbers) then it looks at this table to work out how many steps to open the IAC.

simple. the idle airflow requirements of the engine have changed due to camshaft. Nobody said that it was a simple linear relationship. You need to recalibrate the RAF, cracker and follower and also ensure the dynamic range of the IAC allows you to reach high enough, ie. you may have to crack or drill the TB.

The single most common mistake people make is not allowing the idle trims to relearn after making changes hence they never get the idle transitions correct between follower, cracker etc.

Chris...

 

gameover - thanks for all of the great info you've provided over the past couple years. Please do not get frustrated and refrain from providing said info. We're all a little hardheaded....at least I am!!

 

no probs

This is not frustrating at all, in fact it's relaxing compared to writing powerpc assembly although i am starting to draw the line at saying the same thing 3 different ways per thread



Chris...

 

So does the first three columns of the VE table have any effect at idle with the MAF plugged in?