Fuel Scaling
After some initial tuning you will often see one or both of two things. If the fuel global multiplier is too large, or your injectors are too large, the idle VE numbers will be really small. So small that you will see big swings in AFR just by single digit changes to VE values. If the fuel global number is too small, the VE values will max out. These are digital calculations and can only go to 255. For you turbo guys you may see both at the same time depending on how much power you are pushing under boost.
Below is an example datalog that has had a fuel delivery analysis completed. The cells highlighted in RED were too rich and the fuel has been reduced. The cells highlighted in BLUE were too lean and fuel was increased. Notice in the upper right hand of the table the fuel value is 253. In this case, the fuel has maxed out on the table. This must be scaled back to allow further tuning. Yes, this engine is thirsty and wants more go-juice. How to we make that happen?
Ref Pic 12
Remember I commented before that the fuel map should look like your torque curve on a dyno. Even if you have not taken the car to a dyno I am sure everyone here has seen a power readout trace from a dyno. Visualize this in your head. Peak fuel requirements should match peak torque and they should occur roughly at the same rpm range. . In this example case 4800 rpm should be right about the peak engine torque for this particular 3.2 engine. After our tuning process the max VE value is 253. The challenge is that this will max out at 255 (8-bit in digital numbers). And this means you can’t really add more fuel if you need to. This means you have to scale the fuel table.
Open the tune file in TunerStudio. Then open “basic/load settings> Engine and Sequential settings” and then open “fuel settings > VE Table 1”. To scale you have to mathematically increase “req_fuel” value and decrease the entire fuel table. I would start with 5% change.
So, req_fuel 3.5(starting value) x 1.05(5% increase) = 3.675 so we will input a new value of 3.7. Next we go to the VE Table and select all the bins in the table. Then use the multiply function [ (X) on top header] and multiply by 0.95 for a 5% reduction. Now the maximum table value is 240 and there is room to add more fuel up top (remember the max is 255). It is possible that you have to scale the fuel table several times as you reach a max fuel limit while tuning. When you can complete a WOT pull hitting your target AFRs you do not need to further scale the VE table as long as you are on scale.
Now, I always tune an engine so that the maximum VE value is above 220. This is because I get maximum resolution in the idle range. The ECU nor the engine care if this mathematical calculator matches any real VE values. Now, I will say that some ECUs do care. AEM get’s unhappy if you try to manipulate curves like this. But, there are other things I do not like about AEM and that’s why it's not my favorite. I digress and will not discuss further.
Maximizing Resolution.
This is a topic I get asked about often. My ITB car doesn’t make much vacuum at idle and It's like a light switch from low to high throttle. How do you fix that? You have to apply some non-linear scaling to the load axis. This is because ITBs have increased swept throttle area compared to a single plenum. They also have resonance tuning effects due to equal length runners. Ultimately, this gives an efficiency increase at higher load values that can not be matched in a common plenum, single throttle. Wicked good that’s why we choose them
This is what a plot of MAP vs TPS looks like on an ITB engine. Notice that iit is quite curved. If your bin division on the load axis is divided into even steps you will miss all the fine throttle change resolution. This is true for speed density or alpha-n. We can see that this car makes a minimum vacuum reading at 42kPa. The cams in this particular engine have a lot of overlap and reduce overall vacuum at closed throttle. This engine is also very efficient at moving air so it will see atmospheric maximums above approximately 50% throttle. So, we have to setup tables to operate in those ranges.
Ref Pic 13
Speed density will have a lot of MAP value change with very little throttle opening. Usually the MAP will rise very quickly to 85-90kPa with only 1-5% throttle opening. Then you have the last 90% throttle for the final 10kPa. So, you can have something like 20-kPa jumps in low rows on the load table, Then 5 or 10 kpa jumps in the middle, and 1-2kPa jumps as you cross 90kpa. An example of non-linear scaling is below. Basically, you want more rows where your engine spends the most time; don’t waste table rows down at 10kpa when your car will never see them. As an example, the engine above should have a minimum row for MAP based load at 40kPa.
The example below shows the effect of scaling an ITB MAP based load detection. Notice the Y axis has tight increment spacing of the rows above 82 kPa. This particular engine needed more resolution to control the fuel delivery as resonant tuning drives fuel requirements.
Ref Pic 14
Alpha-N is the same but opposite direction. You want 1-2%TPS jumps up to 10 and then increasing step size as you approach 60. 70 to 100% TPS can be one step in resolution.
Ref Pic 15
Spending some time to optimize here will be required for you to fine tune the part throttle driveability, especially with ITBs. Don’t be afraid to make these type of changes. The world is non-linear. Your EFI tables need to be too.
