Almost two years after I start bitching about it, here's what I hope is a decent explaination. Pester me with Q's if I'm a tard and can't explain it properly. It's late, I'm tired, and I suck - I hope I've excused myself sufficiently in advance.

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.

I'd like to thank the following for inspiration in making this post: Del Slowest, insomnia, too much coffee, a complete and utter lack of respect for VTEC, and Speed Phreak's LS powered goodness:






Modified by J. Davis at 6:18 PM 2/26/2006

 

Thank you. I've read your previous posts about this and this summary helps out a lot.

 

very well explained

 

what formulas are used to convert in/HG vacuum into mbar, and also how to convert your AFC percentage into the scaled numbers we see in pink?

(i am OBD0 and turboedit doesnt use mbar )

 

Nice write-up Joseph...Will be looking forward to working with you on the duration of my build up.

 

Joseph = stunna

 

So basically the AFC hack sucks? Sorry, I'm trying to dissect the info. and that's the main point I could gather.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by J. Davis &raquo;</TD></TR><TR><TD CLASS="quote">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.
</TD></TR></TABLE>

Awesome post. This is a great clarification - I just recently changed from an FMU to the AFC hack (night & day diff. already), unfortunately (for now) my car's OBD2 so my tuning options are limited. Running 7 psi now, and have a boost controller on order, will be upping to 10psi as soon as it comes in especially since reading this post.

Noobish question re: retarding the dizzy - is that possible w/ OBD2? I was under the impression that the ECU would just put the timing back in. -B

 

J Davis:::

You really do love making/participating these afc posts huh???Anyway, good write up here!! However, you would not even want to know what I do with my afc, but all I can say is that its been working out good for a while now on my old *** ls motor....

 

I have a question for you guys running the afc hack. is it mandatory to run the 88-91 resistor box or i can use any honda resistor box. i have a box from a 94 honda accord can i use that along with the afc hack? and dsm injectors

 

I am runnin rc 440s with mine, so I dont need the resistor box....

 

damn...nice wrie up dude...

 

Thanks for the write up. I've been trying to explain this for awhile but i never can put it in words that ppl understand

 

btw- the pics aren't working anymore.

 

just want to make sure, so are you saying that it would be better to retard the timing as the dizzy opposed to using a MSD BTM?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by notoriousB &raquo;</TD></TR><TR><TD CLASS="quote">Awesome post. This is a great clarification - I just recently changed from an FMU to the AFC hack (night & day diff. already), unfortunately (for now) my car's OBD2 so my tuning options are limited. Running 7 psi now, and have a boost controller on order, will be upping to 10psi as soon as it comes in especially since reading this post.

Noobish question re: retarding the dizzy - is that possible w/ OBD2? I was under the impression that the ECU would just put the timing back in. -B</TD></TR></TABLE>

I want to know that too?

 

you can't retard at the dizzy on a 0bd2 motor. the ecu will know that something is up and change it back

 

uhm sure you can. Put a jumper at the ecu when you change it...it wont correct it then....

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by tvrsir &raquo;</TD></TR><TR><TD CLASS="quote">I have a question for you guys running the afc hack. is it mandatory to run the 88-91 resistor box or i can use any honda resistor box. i have a box from a 94 honda accord can i use that along with the afc hack? and dsm injectors</TD></TR></TABLE>

you can wire in 10 watt 10 ohm resistors to each yellow/black wire and not use a resisor box at all - works good for me.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by who00knows &raquo;</TD></TR><TR><TD CLASS="quote">uhm sure you can. Put a jumper at the ecu when you change it...it wont correct it then....</TD></TR></TABLE>

can you please clarify this? a jumper where on my OBD2 ecu?

 

good info for people not already aware... for posting a write up most should understand.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by notoriousB &raquo;</TD></TR><TR><TD CLASS="quote">

can you please clarify this? a jumper where on my OBD2 ecu?</TD></TR></TABLE>

Its a green plug by your ecu...might have a blue cover....you can use a paper clip if you dont have the honda jumper connector.

 

very helpful, but I could not see any pics

thanks again

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by LostCRX &raquo;</TD></TR><TR><TD CLASS="quote">just want to make sure, so are you saying that it would be better to retard the timing as the dizzy opposed to using a MSD BTM?</TD></TR></TABLE>

What he is asking...

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by who00knows &raquo;</TD></TR><TR><TD CLASS="quote">

Its a green plug by your ecu...might have a blue cover....you can use a paper clip if you dont have the honda jumper connector.</TD></TR></TABLE>

can't do that. thats only for obd1

 

JoJo, your pictures are screwed. I want to see the crazy fuel and timing map action!

Great post, and good idea. I never even thought to bring this up.

A while back I did a similar comparison, with similar results. Like you suggested, it manifests itself as knock. We're running a J&S Safeguard on my friend's turbo B18, and the thing would see knock and retard his timing to matter what I did with tuning and the distributor. Now that we are running proper engine management (Uberdata), the car runs amazingly better.