All you AFC hackers READ THIS!
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SC34 powered LS-T
Joined: Jul 2003
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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.
Member
Joined: Jun 2002
Posts: 141
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From: Lafayette, CO
Too bad that's a bunch of BS... I hate it when people spread misinformation. I'm going to do something different here and talk actual real world experience, rather than just make shit up and pass it off as fact.
Here's what I posted on Acura-Integra.com about this very same subject:
In my experience, the AFC setup really isn't all that bad, if you do it right. I got a real good deal on a VAFC from a friend, so I planned on using it for a short period of time (I was anxious to boost!), and then getting to a standalone. Being that I regularly road race this car, my needs are considerably higher than most, but I found that this setup really does work great for me, so I've had less motivation to upgrade to a standalone.
My setup is a B18B @ 7 psi with a VAFC, 450cc injectors, Walbro 255 lph pump, tuned to 12.5:1 A/F and base ignition timing retarded 2 degrees. I've been running it that way for the past 10 months and have put this car through more hard use in that time than most cars will see in their entire lifetime. Zero engine problems and it is still running very strong. I don't even know how to begin explaining all of the abuse that this engine has seen, but it suffices to say that this setup has proven itself many times over. Yes, it's a hack, but that doesn't mean it can't work well. Afterall, Hondata or Uberdata is a hack too... just a more direct one.
The timing issue really isn't that big of a deal... You're right that the timing is getting advanced. How much really depends on what ECU you are running, boost levels, and what the AFC is set to at every RPM. Your example assumes the AFC is set to -43% at all RPMs and throttle levels, which is unrealistic if you have 450cc injectors. That would be very lean and it also exaggerates the difference in timing. Real settings for 450cc injectors are typically around -38% (idle) to -35% (boost) on the wide throttle map.
-36% would be a realistic average for the AFC adjustment under boost. By your example, that would mean that there is really only a 2-3 degree advance in the timing under boost. You can easily compensate for that by retarding the base timing on the distributor. This obviously isn't the best ignition setup, since I have little control over it, but it happens to work out in my favor. It's really not the end of the world if you have a little ignition advance under boost anyway. I've never heard any pinging or knocking, and I'm sure that if it were a real problem, my engine would have blown up long ago. I've taken it to the road course 9 times with this setup and many of them were on rather hot days, so if it were going to have a problem it would have.
A good standalone really is the ideal solution, but the AFC is inexpensive and happens to work very well. My only complaint is that it bucks a little bit when you're crusing at like 35mph and let off of the throttle, but it's only a small annoyance.
Here's what I posted on Acura-Integra.com about this very same subject:
In my experience, the AFC setup really isn't all that bad, if you do it right. I got a real good deal on a VAFC from a friend, so I planned on using it for a short period of time (I was anxious to boost!), and then getting to a standalone. Being that I regularly road race this car, my needs are considerably higher than most, but I found that this setup really does work great for me, so I've had less motivation to upgrade to a standalone.
My setup is a B18B @ 7 psi with a VAFC, 450cc injectors, Walbro 255 lph pump, tuned to 12.5:1 A/F and base ignition timing retarded 2 degrees. I've been running it that way for the past 10 months and have put this car through more hard use in that time than most cars will see in their entire lifetime. Zero engine problems and it is still running very strong. I don't even know how to begin explaining all of the abuse that this engine has seen, but it suffices to say that this setup has proven itself many times over. Yes, it's a hack, but that doesn't mean it can't work well. Afterall, Hondata or Uberdata is a hack too... just a more direct one.
The timing issue really isn't that big of a deal... You're right that the timing is getting advanced. How much really depends on what ECU you are running, boost levels, and what the AFC is set to at every RPM. Your example assumes the AFC is set to -43% at all RPMs and throttle levels, which is unrealistic if you have 450cc injectors. That would be very lean and it also exaggerates the difference in timing. Real settings for 450cc injectors are typically around -38% (idle) to -35% (boost) on the wide throttle map.
-36% would be a realistic average for the AFC adjustment under boost. By your example, that would mean that there is really only a 2-3 degree advance in the timing under boost. You can easily compensate for that by retarding the base timing on the distributor. This obviously isn't the best ignition setup, since I have little control over it, but it happens to work out in my favor. It's really not the end of the world if you have a little ignition advance under boost anyway. I've never heard any pinging or knocking, and I'm sure that if it were a real problem, my engine would have blown up long ago. I've taken it to the road course 9 times with this setup and many of them were on rather hot days, so if it were going to have a problem it would have.
A good standalone really is the ideal solution, but the AFC is inexpensive and happens to work very well. My only complaint is that it bucks a little bit when you're crusing at like 35mph and let off of the throttle, but it's only a small annoyance.
Thread Starter
SC34 powered LS-T
Joined: Jul 2003
Posts: 438
Likes: 0
Too bad that's a bunch of BS... I hate it when people spread misinformation. I'm going to do something different here and talk actual real world experience, rather than just make shit up and pass it off as fact.
Member
Joined: Jun 2002
Posts: 141
Likes: 0
From: Lafayette, CO
That's a really weak argument for your case, and if you can't support it then you shouldn't be spreading this crap around. People can make up all the arguments against it that they want, but the fact remains that I have proven, time and time again, that it's reliable under the the extreme abuse of road course racing. Hard use on the street or going down the drag strip are a day in the park compared to what I regularly put this car through.
The ignition timing example is exaggerated, plain and simple... you would NEVER set the AFC that lean unless you have bigger than 450cc injectors or increased the fuel pressure. He claims it's only -40%, but the numbers say -43%... either way, that's too lean (even at idle). The ignition timing will also vary depending on the ECU, but a little ignition advance under boost really isn't as bad as people think it is anyway. It's all about combustion chamber heat and pressure... too much causes pre-ignition and detonation, as well as other problems. FI creates a lot of that heat and pressure, and spark advance adds some more, but you can do things to cool it... you really have to for FI anyway. Running colder spark plugs is one of the ways that I deal with this, and there are others. However, I would not recommend running race fuel unless there is a lot of boost involved; if you have problems with premium pump gas from a decent brand, then you need to fix something.
The reason that a lot of people have problems is because they don't do anything about the combustion chamber heat, never tune the A/F properly, and/or totally screw up the narrow throttle map and throttle point settings. The narrow throttle map should be about -38 or -39 at all rpms, and the throttle points should be something like 20% and 50%. Many people run it leaner than that and use 76% and 77% for the points, which is absolutely horrible.
The ignition timing example is exaggerated, plain and simple... you would NEVER set the AFC that lean unless you have bigger than 450cc injectors or increased the fuel pressure. He claims it's only -40%, but the numbers say -43%... either way, that's too lean (even at idle). The ignition timing will also vary depending on the ECU, but a little ignition advance under boost really isn't as bad as people think it is anyway. It's all about combustion chamber heat and pressure... too much causes pre-ignition and detonation, as well as other problems. FI creates a lot of that heat and pressure, and spark advance adds some more, but you can do things to cool it... you really have to for FI anyway. Running colder spark plugs is one of the ways that I deal with this, and there are others. However, I would not recommend running race fuel unless there is a lot of boost involved; if you have problems with premium pump gas from a decent brand, then you need to fix something.
The reason that a lot of people have problems is because they don't do anything about the combustion chamber heat, never tune the A/F properly, and/or totally screw up the narrow throttle map and throttle point settings. The narrow throttle map should be about -38 or -39 at all rpms, and the throttle points should be something like 20% and 50%. Many people run it leaner than that and use 76% and 77% for the points, which is absolutely horrible.
Thread Starter
SC34 powered LS-T
Joined: Jul 2003
Posts: 438
Likes: 0
That's a really weak argument for your case, and if you can't support it then you shouldn't be spreading this crap around.
