Acceleration enrichment
After all this tuning the car should be running quite well. Now it's time to make it bark under throttle changes. Crisp throttle response is why you ditched the CIS in the first place. But, ITBs can be tricky to get the AE set right. You already correctly tuned the steady state conditions with AE off. Now it's time to turn it back on.
Acceleration Enrichment is what drives the throttle response. How well does an engine rev when you heel-toe a downshift? How does the car feel when you floor it to change lanes or make a legal pass? How does the car slow when you come off the throttle into a corner? These changing states are what define part of a 911’s driving character. When I talk about changing states I refer to moving the gas pedal (up or down) and having the engine respond quickly to that pedal movement. Think of a race car with that light switch throttle response. Yeah, we all want that and EFI can give you that if setup correctly. Let’s cover some basics.
Most AE Algorithms can control based on the rate of change of either the manifold pressure (MAP) or the throttle position (TPS). Some ECUs allow you to do a combination of MAP or TPS based AE and even allow you to change the percentage weight one vs the other. BTW, For those math guys this is a first derivative calculation.
What types of AE are there? There are 3 basic models that are used; time-based, Accelerator pump and wall-wetting. I am not going to explain each. I find that they can each be beneficial to different builds and configurations. Do your homework, experiment and find the right one for your application. Oh, And read the manual to determine how they work.
How do we trigger AE? I mentioned that AE is based on gas pedal movement since you are requesting the engine to do something different. This can be measured by throttle position or a change in engine pressure because of movement in throttle position. Think of this as 2 ways of measuring the same thing.
There is a lot of background information that says TPS AE is way better to tune because the gas pedal based change is much faster than the pressure based change when considering MAP. The truth is… It depends. On a common plenum single throttle the TPS AE tuning will be faster. This is due to the increased volume of the plenum and the time lag for the MAP to actually change when the throttle is moved. With Most ITB setups, the throttle is very close to the cylinder head intake tract. In this case they can be near equal. In some systems, like an MFI throttle body, where the throttle is effectively sitting on top of the cylinder head intake tract, the MAP based pressure change can be faster than the TPS. The point is, you will need to determine the best AE protocol for your particular installation.
There are accel and decel parameters to control a throttle blip. So, you have to separate accel (foot going down) from decel (foot coming up). They each do different things in the software. The acceleration part (throttle opening) is usually the more important to get right. Decel will control how responsive the engine is coming off throttle and how you perceive the start of engine braking.
I typically tune a TPS curve and a MAP curve separately and then use the slider (blend %) to try in 25% increments until the car drives smoothly. There are times when MAP will respond faster than TPS signal. Either one can be used. Below is an example of the setup from a Megasquirt AE turning table.
Ref Pic 11
One of the challenges with ITB AE tuning is getting rid of the bog - specifically off-idle. When you are working on AE keep in mind that there is an adder for accel and subtractor for decel. Often you can move your foot fast enough with a throttle blip that the engine will first squirt a small amount and then pull a large amount when throttle closes. These will compete against themselves and cause some poor responding throttle. To fix you have to separate what is throttle opening from what is throttle closing.
So, A throttle blip should be open throttle and hold open for a few seconds. If you are watching the time based AE panel there is a scrolling graph at the bottom. Watch the TPS_dot or MAP_dot traces at the top. A bouncing ball will move up the graph to indicate how fast the rate is changing. Then glance at the AFR trace on the bottom (usually yellow). open throttle and add fuel until the AFR dips just rich. This should eliminate the stumble. Then add more fuel in the graph until it is crisp. Usually, you have to add accell time duration (increase milliseconds) not just the amount of fuel squirted. Test in the car by driving at light load and then floor it and hold it down with different opening speeds. There should be a snap feeling (no stumble or bog) and a smooth transition to increased load in the car.
What I usually do is to drive the car with the AE dialog open. Something like 3rd gear and going 2K rpm. Here you will first tune TPS AE and then MAP AE. The idea is that you will “accel” the car at different rates. You should trigger AE and then see a bouncing ball go up the curve. You have to try adding fuel (ms adder, Y-axis) and also increasing the time of the injection squirt. There is both an accel time and a time taper. You will open the throttle and hold it open. Then watch the AFR trace line (blue) after you have opened the throttle. It might go very slightly lean but should not have a spike. Add some fuel (ms) until you see the initial spike diminish (delay). Then add time until it stays relatively flat. At this point the engine should have a smooth transition to a higher load before it starts to pick up RPM. Once you get it to stay flat you can use faster throttle openings to move up the curve.
The decel fuel cut is done in the car. Hold throttle open driving and then snap closed. In MS the fuel decel amount is a percentage of the actual fuel delivery. In the example picture above this is set to 60. Meaning I cut fuel to 60% was the throttle is closing. This should help you get the throttle response. You will have to spend the time to experiment on your particular engine to get this right. And I will tell you that AE is the hardest to get right.