All you AFC hackers READ THIS!
08-01-2004, 09:13 PM
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All you AFC hackers READ THIS!
I came across this on Honda-tech, and found it very interesting. Found out some interesting info about the AFC hack I wasnt aware of. After reading this I decided I definitely will be going with uber/hondata as opposed to the vafc. I didnt mean to post it in the CRX forum, so any mod who wants to delete that feel free to.
Originally Posted by J.Davis
Accept these as facts, for speed density EFI anyway:
- MAP sensors indicate engine load. The more load, the more Manifold Absolute Pressure on the engine, the more fuel it requires. Under boost you need a whole lot of fuel, right?
- AFCs manipulate MAP signal to alter fuel. By lowering MAP voltage, you aim the ECU at a data location tuned for a lesser load, and you get a bigger set of injectors to act a lot like stock injectors. Making, say, the usual DSM 450cc behave like 240cc injectors for your off boost driving.
Let's have a look at a fuel map off of one of the freeware Honda ECU tuner applications to help visualize how this works. The numbers down the left hand side of the map, the Y-axis for you boring math types, are familiar old RPM values. The numbers across the top, on the X-axis are MAP input translated into mBar. For those of you unfamiliar with mBar, 1000 mBar is 1 Bar or 14.5 psi... 1055 mBar is 14.7 psi or atmospheric pressure. Looking at this B16 fuel map, you go from vacuum on the left to atmospheric pressure on the right.
I went ahead and filled in the blank spots below the stock mBar numbers with pink numbers to represent where on the fuel map an AFC sends you with it set to a -40% fuel cut. Pink is a very good color to use for this purpose, as this is a VTEC fuel map.
Now, guys, if you've choked all that down, before we look at the ignition map let me remind you of one single solitary fact in re speed density EFI:
- Ignition maps aren't like fuel map, bitches.
From this we conclude that if you transition into boost at 4000 rpms, you are at 39 degrees advance with the AFC hack where the stock crossover is at 24 degrees. Jacking your distributor forward 15 degrees before you hit boost is a great idea! Hey, let's buy an MSD BTM, and it'll dial that 15 degrees ar 0 psi boost right out, won't it?
Well, you want to be safe so you only run a low boost setup, 5 to 6 psi. Whatever the hell low boost has to do with safe, I keep thinking quality of tune and power output are your yardsticks to measure engine safety by... but what do I know? So you run 5 to 6 psi... ~1400 mBar. With the AFC hack you are at 31-32 degrees ignition advance, around 7 degrees advanced from where you would be without any ignition retard under boost with any other setup. Let's not even take into account the higher load your engine is under, and the heat and stresses involved. My, the sweet sound of detonation in the morning!
Now, heaven forfend you should have paid me a lick of attention when I advised people that running as much boost as the stock Honda MAP sensor is capable of reading is your route to safety with the AFC hack. Note how you end up within a degree or so of your otherwise normal ignition timing when you transition into boost. You can't help the transitory high ignition advance you pass through, but with how quickly turbos spool in most cases it doesn't matter; you spend the bulk of your time during the boost event at full boost, and if you have retarded your distributor 2-3 degrees you should have adequate ignition retard under boost.
- MAP sensors indicate engine load. The more load, the more Manifold Absolute Pressure on the engine, the more fuel it requires. Under boost you need a whole lot of fuel, right?
- AFCs manipulate MAP signal to alter fuel. By lowering MAP voltage, you aim the ECU at a data location tuned for a lesser load, and you get a bigger set of injectors to act a lot like stock injectors. Making, say, the usual DSM 450cc behave like 240cc injectors for your off boost driving.
Let's have a look at a fuel map off of one of the freeware Honda ECU tuner applications to help visualize how this works. The numbers down the left hand side of the map, the Y-axis for you boring math types, are familiar old RPM values. The numbers across the top, on the X-axis are MAP input translated into mBar. For those of you unfamiliar with mBar, 1000 mBar is 1 Bar or 14.5 psi... 1055 mBar is 14.7 psi or atmospheric pressure. Looking at this B16 fuel map, you go from vacuum on the left to atmospheric pressure on the right.
I went ahead and filled in the blank spots below the stock mBar numbers with pink numbers to represent where on the fuel map an AFC sends you with it set to a -40% fuel cut. Pink is a very good color to use for this purpose, as this is a VTEC fuel map.
Now, guys, if you've choked all that down, before we look at the ignition map let me remind you of one single solitary fact in re speed density EFI:
- Ignition maps aren't like fuel map, bitches.
From this we conclude that if you transition into boost at 4000 rpms, you are at 39 degrees advance with the AFC hack where the stock crossover is at 24 degrees. Jacking your distributor forward 15 degrees before you hit boost is a great idea! Hey, let's buy an MSD BTM, and it'll dial that 15 degrees ar 0 psi boost right out, won't it?
Well, you want to be safe so you only run a low boost setup, 5 to 6 psi. Whatever the hell low boost has to do with safe, I keep thinking quality of tune and power output are your yardsticks to measure engine safety by... but what do I know? So you run 5 to 6 psi... ~1400 mBar. With the AFC hack you are at 31-32 degrees ignition advance, around 7 degrees advanced from where you would be without any ignition retard under boost with any other setup. Let's not even take into account the higher load your engine is under, and the heat and stresses involved. My, the sweet sound of detonation in the morning!
Now, heaven forfend you should have paid me a lick of attention when I advised people that running as much boost as the stock Honda MAP sensor is capable of reading is your route to safety with the AFC hack. Note how you end up within a degree or so of your otherwise normal ignition timing when you transition into boost. You can't help the transitory high ignition advance you pass through, but with how quickly turbos spool in most cases it doesn't matter; you spend the bulk of your time during the boost event at full boost, and if you have retarded your distributor 2-3 degrees you should have adequate ignition retard under boost.
