Quote:
Originally Posted by Karl Owens
I have found the Alpha N makes the car more consistent. The MAP sensor senses barometric pressure when the key is on and the engine is not running and makes adjustments to the fuel map accordingly, also at WOT there really isn't a difference between Alpha N and Speed Density unless you ave a really restrictive throttle body.
|
Alpha-N will make your fueling more consistent, no changes, basically, an electronic carburetor. If you are using an Air Temp Sensor and a MAP sensor (or baro sensor in this case), then the ECU will make some corrections based on temp and pressure. The issue with this is that basically, every super stocker (with the exception of the new combos that come with a 4 bbl, 1000+ cfm throttle body) will actually begin to pull vacuum at high RPMs and with Alpha-N, there is no compensation for this. As Ed pointed out, in Speed Density mode, when the manifold pressure moves down into vacuum, it will not only move down a row or two in the table, but the ECU takes into account the fact that the air has become less dense in the manifold (because it is in vacuum) and fuels the engine accordingly. You are right that Alpha-N will make your fueling more consistent, but why do you want your fuel to stay the same when the air is always changing. When the system fuels according to the atmospheric conditions (inside the manifold), then your performance is consistent. The goal here is to make the car only speed up or slow down because of weather changes. That way, you can use your weather station to predict the change in performance. If your fueling stays static and the air changes, then your state of tune (actual A/F ratio) will change with the weather so you have to get your laptop out and chase the tune.
I have a perfect example of a test I saw that proved this theory. I had a customer, running in Speed Density, Open Loop, who iced his manifold to pick up some ET. This car has a near perfect calibration and the actual A/F lays right on the Target A/F even though it is not using any O2 correction. When he iced the manifold, we did not think about the fact that the Air Temp Sensor was in front of the throttle body. The thing was so lean that it died as soon as he left. This was because the air temperature actually entering the engine was much colder than the sensor was reading. We took another car with the same combination, same exact tuning strategy, but we put the Air Temp Sensor in the manifold. We iced the manifold and this time, the car picked up .080 and the actual A/F laid right on top of the target A/F again (just like it is supposed to). I pointed out to him that when we looked at a datalog from a run with no ice compared to a run with ice, everything was the same except for on the run with ice, the air temp was lower and the pulse width was higher. He asked how the ECU knew to add the perfect amount of fuel for the air temp change, I told him that the math in this thing just works! This is not some formula that we made up, this is based on the physical laws of the density of air. If this had been an Alpha-N tune, we would have had to guess how much fuel to add for the air temp. Using Alpha-N is like having a spreadsheet with 100 rows of numbers, and instead of using the auto sum function to add them together, you decide to add them with pencil and paper because you just don't trust the computer! Many guys use it with success, and more power to them. I just see a lot of benefits to Speed Density. It does take a little more work to correctly tune in Speed Density, but once you do, I am convinced it is more consistent and does not require re-tuning for different tracks.