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Originally Posted by sleepydsm
Concerning 91-94 awd 5 spd:
Can someone explain to me why exactly a stock ECU sucks once you get to the point of upgraded turbo and supporting mods? If I remember right it has something to do timing or airflow?
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The problem has to do with a few things. The stock 1G ECU is programmed to support 450cc fuel injectors. One problem is that the stock ECU runs on the extra rich side stock from the factory for safety and then when it sees additional airflow it over compensates even more in the fuel department. Second, the timing maps in the 1G and 2G max out at an airflow level that isn't all that high. Sometimes that is a good thing for tuning(on a higher HP race gas car), sometimes it's a problem and sometimes it makes no difference. Third, people often run bigger injectors than they need. Or the injector they need for high boost and race gas is too much for pump gas. The fourth and biggest problem is that tuning devices like an SAFC, MAFT, E-Manage, work strictly off of fooling the ECU by modifying the airflow signal.
Here is a typical 1G stock ECU timing map:
The X-axis is RPM. 5.5 = 5500 RPMs.
The Y-axis is airflow. The numbers in this chart on the Y-axis don't mean anything.
A car making somewhere in the 280-320HP range will hit the "high" row of the timing map with an unaltered airflow signal. So it will try to run 22 degrees of timing advance at 5500 RPMs. This is often fine for a stock 7.8:1 compression 1G motor at 15-18psi.
The high row is for 2.06 grams/rev of airflow. When your airflow exceeds that, it just keeps running the same timing found in that highest row. So, if you are making a lot of power and using 3.00 gm/rev of air, it would still try to run 22 degrees of timing at 5500 RPMs with an unaltered airflow signal. On pump gas, this might be too much timing for the boost required to get that kind of airflow. On race gas, it might be perfectly fine. So, going off the top of the map can be a problem is some circumstances, but it isn't the main problem.
On the Evo, the stock fuel and timing maps support a fair amount more airflow than the 1G/2G ECUs, but they run a lot more boost stock than a 1G/2G. You will see timing in the negatives in the higher airflow parts of the stock Evo timing map. The Evo motor in general requires very little timing though compared to the older 4G63s.
The real problem shows itself when you run bigger injectors, on pump gas. So you have a bigger turbo, you turn up the boost, and bigger injectors. Let say you have 660cc injectors. They will flow around 32% more fuel than the stock 450cc injectors. So you go into your piggyback style fuel controller and remove 32%. Under boost with your bigger turbo, the ECU sees extra airflow overcompensates by adding too much fuel and say you need to take out another 13% fuel to get it to a reasonable A/F ratio.
So now you are taking 45% out using your SAFC or whatever at full boost. Say for example, you are flowing exactly 2.06 gm/rev and your timing values should have been pulled from the top high airflow timing row, but since your SAFC, MAFT, E-Manage, etc is interupting the airflow signal from the MAS and reporting 45% less airflow signal to the ECU, the ECU sees 1.13 gm/rev and pulls from row 7.
So now instead of it running 22 degrees of timing at 5500 RPMs, it is trying to run 28 degrees. Even worse, at 6500 RPMs, it is trying to run 33 degrees instead of 20 degrees. The engine ends up knocking like crazy and the car becomes untuneable. You can do things like retard the base timing using the CAS on the 1Gs, but you end up losing power everywhere else besides at full boost and you can only retard the base timing so far before the car won't even idle.
The excessive timing problem is much worse on a stock 1G ECU car that is running higher than stock compression. In the 1G timing map, you see 20-24 degrees in the highest airflow sections of the map. In the stock 2G timing maps, the highest timing value in the high airflow row is only 16 degrees. So, when you stick 8.5:1 to 9.0:1 compression pistons in a 1G and the ECU is trying to run all kinds of timing for a 7.8:1 compression motor, you end up with even more knock.
On race gas when you run high boost, the timing problem isn't as severe. If your airflow is way up, say 3 gm/rev and you remove 45%, then you still end up close to the highest airflow row in the timing map. Also, race gas can safely handle more timing than pump gas.
When you get a basic EPROM chip for your injector size, you can get rid of a lot of the correction that needs to be done in the piggyback. And that way you aren't getting too far out of wack in the timing maps. You can also have the commanded A/F ratios leaned out a bit in the chip and/or have exteneded timing/fuel maps put in to make it handle higher boost better.
With a custom tuned EPROM chip or DSMLink, there is no fooling of the ECU going on. The ECU sees the full amount of airflow and you tell it how much fuel to add and what timing to run. I run a custom chip in my girlfriend's 1G, she has 2G 8.5:1 compression pistons in the motor, very hacked 1G MAS, some of my old 720cc injectors, HKS 272s, and a very old inefficient Mutt turbo. All of the tuning is done in the chip. We only run 12 degrees of timing under full boost on pump gas or else it detonates.
sleepydsm: Your MAFT unit should be capable of finer tuning than the old 5 knob AFC, with the options it has besides the base injector size change. You still might have to go leaner with the MAFT or AFC because of how rich the stock ECU likes to run under boost. The way the MAFT compensates for injectors isn't like having an EPROM ECU with a chip in it to compensate, you might run into "too much timing" problems because both the MAFT anf AFC are just tricking the ECU by lowering the airflow signal it recieves.
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