One more bout with SAFC...let's get to the bottom of the myth...

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One more bout with SAFC...let's get to the bottom of the myth...

Postby KATwo40 » Wed May 24, 2006 12:07 pm

Recently I had a discussion with Nissanfanatic on this board about the SAFC2 causing the referenced area of the fuel and timing tables to be incorrect, resulting in the wrong timing for any given load/rpm point.

Just a couple days ago, a thought hit me, and I think I might owe Nissanfanatic an apology for saying he's so wrong about this subject, as he was dead set on the idea of it not changing the timing.

For two years now, all I've heard about the SAFC and SAFC2 is that, when using larger-than-stock injectors, the units will pitch the ECU into the wrong areas of the correction maps, resulting in timing values that are advanced more than they would be during normal operation.

To set a baseline for the discussion let's establish the setup:

MAF system SAFC2 30% larger injectors

Ok, the SAFC intercepts the MAF voltage to the ECU and translates according to the adjustments made (i.e. -30% correction reduces the MAF voltage signal to approximate an airflow of -30%, resulting in -30% pulsewidth).

Before the ECU even looks at the fuel and timing correction tables, the MAF voltage is processed through the VQ (Volumetric Quantization) scale, along with the K value, to determine the correct TP (theoretical pulsewidth) to achieve a desired baseline AFR for any given MAF output.

Since the lookup tables are a cross-reference of throttle position and RPM, how in the world could the MAF signal even become a factor in the tables? The MAF voltage, from the information I have gathered, is only used to determine the base TP, against which the lookup tables are used.

So, in short, I don't believe that altering MAF output voltage has anything to do with the timing lookup map. I believe that altering the MAF voltage is another way of simply changing the baseline pulsewidth, as opposed to changing the K factor, via software.

Ok...open for critique and/or correction.


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Postby Checkered-Member » Wed May 24, 2006 1:05 pm

In other words, don't use a SAFC as it is a band-aid type of fuel management.

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Re: (Checkered-Member)

Postby KATwo40 » Wed May 24, 2006 1:13 pm

Checkered-Member wrote:In other words, don't use a SAFC as it is a band-aid type of fuel management.
That's not at all what I'm saying. I'm offering up the suggestion that the SAFC does NOT affect timing.

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Postby S14tat » Wed May 24, 2006 1:42 pm

yeah me and nissanfanatic had a chat about this too. i once thought that the SAFC would alter the timing curve too, but from years of observing that it doesn't, and nissanfanatic actually tellin me that it doesn't makes me believe that it doesn't as well ( duh ). lol

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Postby volfan98 » Wed May 24, 2006 2:07 pm

All I know is that ever since I put in my S15 injectors and SAFC (not turbo yet), my car will detonate like crazy if I don't use premium gas. Didn't ever do that before. Makes me think that the timing is advanced.

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Postby KATwo40 » Wed May 24, 2006 2:53 pm

Eh...maybe you're tuned too lean?

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Re: (KATwo40)

Postby j-z » Wed May 24, 2006 4:40 pm

well thats good to know. ill be using an safc with 550s on my setup. i also have an itc (ignition timing control) that ill be using, so i think i have a pretty good setup for 300 whp while retaining reliability and driveability.

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Re: One more bout with SAFC...let's get to the bottom of the myth... (KATwo40)

Postby Edub1 » Wed May 24, 2006 4:54 pm

Good question. I think there are a few assumptions that might not be correct.

MAF output is a factor of K. If you change MAFs, you need to further alter K to reflect this change. But this shouldn't put you in the wrong area of the map.

However, the map axise are not necessarily throttle position V. RPM, but load V. RPM. Load, might very well be influenced by MAF output in more ways than one might think. Now there is a suggestion that there are two timing maps based on SCV position.

Bottom line, untill we know a lot more about how the ECU handles all the info, it isn't a good idea to make such a dramatic change. Timing or no, it's got to be confusing the ECU.

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Re: One more bout with SAFC...let's get to the bottom of the myth... (Edub1)

Postby deviousKA » Wed May 24, 2006 6:05 pm

The VQ table is not associated with rpm or throttle position.

The VQ is an airflow percentage lookup based solely on maf voltage vs. percent of maximum cfm (or m3/min, m3/h, l/s, l/m, or any other volume rate of flow). This table is arranged in 64 points beginning at 0.08v and ending at 5.12v, in 0.08v increments of maf voltage output.

Values plotted at each increment are percent of airflow applied as a 16 bit number, 0-65535 decimal or 0000-FFFFhex (0 = 0% , 65535/FFFFhex = 100%), which is based off of the maximum airflow of the maf diameter as just mentioned, regardless of total hotwire capability.

Each oem maf has an element that is usually matched to the diameter so that at maximum cfm the output is near 5.12v, although this is not exactly necessary, and custom combinations, such as a 'hacked' maf, can be calibrated appropriately with full understanding and equipment (flowbench and daq). If the maximum airflow of the diameter is met before the hotwire reaches its maximum voltage of 5.12v, this will be represented as a 100% flow (65535/FFFF) much earlier in the 64 point voltage scale. If the maximum airflow of the diameter is met after the hotwire has reached its 5.12v maximum, the plotted percent of airflow will not meet a maximum 100% (65535/FFFF) before the end of the voltage scale.

In other words, the VQ does not represent actual flow numbers as a whole and that is why two completely different maf diameters (and maximum flow capabilities) will have "similar" looking values in respect to how they are used by this particular table. The main difference in airflow capability is taken care of with other global trim values (K number).

The maf voltage passes through this VQ table to obtain a percentage of flow before that percentage is used in the much larger TP calculation. The TP calculation encompasses airflow percentage, rpm, K number (global trim), temperature, acceleration of throttle position, and also o2 voltage in some applications. Airflow percentage, rpm, and K number are the main factors that are of most importance, and for the sake of this discussion overshadow the others mentioned.

The TP, or theoretical pulsewidth, is a fuel injection pulsewidth that has been calculated to provide the engine with a 14.7 A/F ratio at that particular rpm and airflow state. With all settings appropriate this pulsewidth will provide a stoich A/F ratio without any additional correction.

Now, this is where the fuel and ignition tables come into play. We all know that a stoich A/F ratio is not appropriate for all engine operating conditions, and further correction must be applied.

In the case of the fuel maps, the TP is dual purpose. Not only is this TP value applied some final correction via the fuel map value to provide the desired A/F ratio, but it is also referenced to the X axis scale (TP scale) to plot it against RPM (Y axis) to place it on this fuel correction map for a final fuel injector pulsewidth. If all values of the fuel map were set to '0', the engine would always run at stoich.

Now, to finally answer the question about the ignition timing map. Remember how the TP is referenced to an X axis scale on the fuel map in order to place it? Well, it is used in this same way for the ignition map, but instead of it being "dual-purpose", it is used only as the X axis reference. The values located on this ignition map are total timing advance degrees btdc, and after the ecu has located each value using the X axis (tp) and Y axis (rpm), the timing advance value is applied. Simple huh?

So, by altering the maf voltage via piggyback (safc, emanage) so that a different airflow percentage (VQ) is used to provide different fueling characteristics via the TP calculation, the timing map X axis reference is also directly affected. The ecu will locate/plot diffently on the ignition map, providing a different total timing advance btdc. By decreasing airflow voltage so that injector pulsewidth is less (like correcting for larger injectors), more timing advance will result due to the taper-off-as-load-increases nature of the ignition map.

In laymens terms, the ecu thinks there is less engine load, while this is usually quite the opposite (turbo app).

There is no myth to bust. This is why those who know how a piggy back works usually dont use them, and discourage others from using them as well.

Also, for those interested in more detail of the VQ and other ecu information, check out eccs.hybridka.com . There was a post where someone wanted to run nissan efi on a turbocharged lawnmower and I went into a little more detail on the specifics of the VQ.


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Re: One more bout with SAFC...let's get to the bottom of the myth... (deviousKA)

Postby j-z » Wed May 24, 2006 7:02 pm

god damn devious! how the hell do you know all of that?? LOL dang man. so can you put it into laymens terms on how much the afc advances timing for how much fuel you correct. for instance correcting for 550s is usually around -40% +or- a few all across. how much timing would be advanced in that situation? does it advance directly for how much you correct? or it depends on other variables like some of the things yo were mentioning? most of what you said went way over my head, but i tried to comprehend as much as i could. so can you correct this timing advancing by retarding with a timing adj device such as the ITC like i mentioned i have? i know it wouldnt be nothing like having a tuned ecu or close to a standalone. well i know you could retard the timing to make it more feasible for a boosted application, but just how well would it work. please give me some laymens insight as i know youve got it. thanks

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Re: One more bout with SAFC...let's get to the bottom of the myth... (j-z)

Postby deviousKA » Wed May 24, 2006 7:30 pm

I just like figuring out how things tick I guess, and I like to tune, Ive also learned a lot from nissan efi experts around the world.

It would be hard for me to put my finger on just how much the airflow voltage is changed by a piggy back to run any given injector. If you were to run some logs somehow and get a short list of the rpm's and maf voltage (after it leaves piggyback), I would be able to tell you how the timing advance characteristics have changed.

And yes, you could correct the timing (at least areas of main importance)by adjusting with an alternate device, or even backing off the distributor adjustment. I dont know much about the ITC really, does it intercept the ignition signal after it leaves ecu before it goes to the ignitor/power transistor?

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Re: One more bout with SAFC...let's get to the bottom of the myth... (deviousKA)

Postby j-z » Wed May 24, 2006 7:49 pm

the ITC intercepts the reference signal from the camshaft position sensor. you can +-15 degrees at 5 diff rev points on the band. but let me ask you this, with the afc is it always advancing no matter where at on the revs, just depending on fuel correction? say like at 1krpm with -45 corrected, will the afc be adding timing there? i just dont understand how it will be advancing the timing. i guess you explained in depth too much for me LOL. tnks for helping and let me know what you think.

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Re: One more bout with SAFC...let's get to the bottom of the myth... (j-z)

Postby deviousKA » Wed May 24, 2006 8:02 pm

So I would assume this ITC is geared mainly towards nissan's and mitsubishi's? That would work, but any changes you do to the TDC sync of the CAS would also change some idle quality in regards to the sequential injection.

And yes, the safc dropping the maf voltage will be cause the ecu to run more timing at all rpms. Timing advance that would normally be in a decel only area (up to 46 degrees on a ka24de) may be running at mid throttle/mid rpm, and timing intended for mid throttle/load would be running at full load.

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Re: One more bout with SAFC...let's get to the bottom of the myth... (deviousKA)

Postby j-z » Wed May 24, 2006 8:59 pm

the itc is made by apexi. its like the old school afc with the dial knobs on it. i know that toyotas and subarus use it as well. so youre talking about the idle being rough if i alter the reference signal? i can just leave the timing at the idle range stock and still have smooth idle correct? just retard whenever i start to spool and retard even more when in full boost according to where all that happens in the rev band. dang so the afc adds ALOT of timing huh. i can retard up to 15 degrees. maybe retard on the itc around 8 degrees as im spooling, and 12 degrees when in boost. guess ill just have to run her and find out. thanks again, and your last paragraph really cleared things up for me on how much and when the afc actually does advance timing.

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Re: One more bout with SAFC...let's get to the bottom of the myth... (j-z)

Postby Edub1 » Thu May 25, 2006 2:54 am

J-Z, the advance Devious is explaining is found on an ignition "map." This is like a spread sheet with load as the x axis and RPM as the y axis.

The decreased MAF signal causes the ECU to look at the wrong cells for ignition data - it thinks it has a lower load.

It's not like a hard wired, perfectly linear advance that can be turned back like it was turned forward - it's accessing a target cell value. It can actually be much worse as the timing changes more radically, especially in open loop. This is one of the reasons EFI cars get so much better milage and performance. The old systems were limited by their linear nature.

Now if you fuel correct your ECU and then use a piggy back for timing, fine tune you have circumvented the problem. Of course not everyone likes this method either.


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Re: One more bout with SAFC...let's get to the bottom of the myth... (Edub1)

Postby j-z » Thu May 25, 2006 6:08 am

thanks for some further insight edub. now i understand why it advances the timing. i just needed it in more laymens terms. so the timing changes depend on load and rpm. well thats how it is stock, but like you said the decreased maf signal (from tuning for bigger injectors) puts it at a lower point on the ignition spread. NOW that i get. concerning the itc, what do you mean by circumventing the problem?

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Postby TheOne » Thu May 25, 2006 6:51 am

how about....we get somebody with a consult port(to read/maybe log timing) while tuning with the SAFC?

wouldn't you be able to log timing at any given rpm with the consult? it should be able to read timing right out of the ecu, along with rpm and pulse width.

nice explanation devious, i kinda got how the safc works into the ecu.

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Postby Florida240sx » Thu May 25, 2006 12:34 pm

Everyoen keeps thinking to correct for larger injectors. What ahppens say fo instance when I was at 5psi on my stock injectors and I had the safc on +35% Therefore the engine would be thinking I'm under more load. If you get your injectors to the hp goal of them maxing out. You could then have minimal correction where timing wouldn't matter. Am I correct?

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Re: (Florida240sx)

Postby Jookmasta » Thu May 25, 2006 12:55 pm

Florida240sx wrote:Everyoen keeps thinking to correct for larger injectors. What ahppens say fo instance when I was at 5psi on my stock injectors and I had the safc on +35% Therefore the engine would be thinking I'm under more load. If you get your injectors to the hp goal of them maxing out. You could then have minimal correction where timing wouldn't matter. Am I correct?
the line where u say that "if you get your injectors to the hp goal of them maxing out" are u referring to when the pulse is maxed out or when ur safc setting is at +50% across the board for when your in boost? if you are referring to the numbers on the safc, well then timing would still matter as the ecu will go into some different timing tables which then obviously would be bad if your running an NA ecu and 20 degrees base timing.................(but i assume know you know this)

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Re: (Florida240sx)

Postby Edub1 » Fri May 26, 2006 5:40 am

The ECU looks at MAF voltage among other things to determine how long to keep the injector open. This is "TP" or "pulse width."

If you use injectors that are 50% bigger and do not reduce the TP and other values accordingly, you end up with 50% more fuel.

A piggy back corrects for larger injectors by altering the MAF signal, thus fooling the ECU into thinking that less air is entering the motor (less load). It therefore cuts back fuel.

The problem is that the ECU also uses load to determine where on the timing map to look for timing info. Because the signal is cut by 50% it will look to the wrong place.

For my car, I am tuning the ECU for fuel, so I won't have this problem but using a BTM for timing as I'm not willing to spend the time dyno tuning for timing.

The Emanage can be used in conjunction with a fuel corrected tune so this doesn't happen.

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Re: One more bout with SAFC...let's get to the bottom of the myth... (deviousKA)

Postby WDRacing » Fri May 26, 2006 10:18 am

Devious, Yet another great post man. A little over my head at some points, but very informative none the less. When I get back in 4 months we'll need to talk offline. I'll need to tap into your resources... WD

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Postby Florida240sx » Fri May 26, 2006 11:44 am

I was thinkign of if you have +% anywhere on your safc settings then your ecu would think your engine is under more load than it is.You can injectors to fit your goal hp where on your safc you have only +/- 10% in WOT therfore timign is very little affected.Which you cna thus retard off base which you shoudl anywayz to be safe. Am I thinking on the wrong track???

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Re: (Florida240sx)

Postby Edub1 » Fri May 26, 2006 3:04 pm

Man, it's a little hard to understand what you are saying. Espanol?

I think you are asking if you can add fuel with the SAFC to get more fuel from the stock injectors. I'd imagine the SAFC will add some fuel - never owned one. But, the stock injectors are only 270cc. Turbos need at least 370cc - 550cc injectors.

Just re-read your posts. That would work up to about 170 HP and then your injectors would be maxed out. In this case, timing would not advance.

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Postby Chezedik » Sat May 27, 2006 7:24 am

Okay, so when you go WOT with high load readings is when you would get the max advance, right? So if you are WOT without that, wouldn't it mean less timing advance? Also, I would think that emanage with it's various harnesses would be able to solve this problem, am I right?

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Postby KATwo40 » Sat May 27, 2006 8:33 am

WOT = more retard.

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Re: (KATwo40)

Postby deviousKA » Sat May 27, 2006 9:21 am

Here is an oem ka24de timing map, in green. Red comparison map is a 'someone else's' NA map if I recall, just had this picture handy.



On a related note, I have just recently finished converting the ka24e sohc ecu over to MAP-sensor based timing control.

This system does not suffer from timing map scaling issues related to adjusting the TP for fuel. Timing map scales always stay the same when changing injectors or maf, and they are direct relation to pressure (kpa, psi, etc.), just like a speed-density type timing map (RPM/MAP XY axis). The axis is fully scalable up or down to unlimited pressures so long as the sensor has no more than a 0-5.1v range.

Basically it takes full control of the original ecus 16x16 timing map system while still using all of the oem hardware and direct implementation. Its like a megajolt lite, but instead of controlling ford ignition it controls nissan, and its built into the sohc ecu.

It still uses original MAF and TP based system for all fueling needs. The MAP sensor uses a spare ecu input (disabled egr temp. a/d circuit), any 0-5v map sensor can be used so long as the pressure/voltage relationship is known. Maps can be viewed and maptraced in real time with a laptop, and there will also be some palm pilot boost gauge software.

You could run these ka24e ecus on ka24de's just as well. All of the information and code will be freely available.

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Re: (deviousKA)

Postby j-z » Sat May 27, 2006 3:27 pm

damn devious LOL, youre a fuking wiz! so you run the map sensor in addition to the maf? the only reason you have the map is to control timing? instead of it being maf based which will put you anywhere on the map depending on load and rpm. dude you should really get into doing full tunes for people. id be willing to give you my money for a tuned boost application ecu. so with just the map addition based ecu you made, its just for controlling timing better than with the maf setup? so does it automatically adjust the timing with the map, or do you have to make your own plots? and how do you get the E ecu to work with the DE?

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Postby Florida240sx » Sun May 28, 2006 5:41 am

I'm not talking about using stock injectors.SAFC affects timing. If you have to take away fuel it makes more engine thinks it's under less load, thus timing advance.So shouldn't it work backwards if you have say 440cc maxed out. Which isn't safe. Across your whole HI fuel map you have to add 5-10% Therefore your ecu would be thinking your engine is under more load than what it is currently at.

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Re: (Florida240sx)

Postby Edub1 » Mon May 29, 2006 2:51 am

Not quite. Fooling the ECU into seeing increased load would cause it to enter the high load (WOT) regions of the map prematurely. This region does have less advance as you reach max load. Pushing over max load would do nothing.

If you put in 440cc injectors and do not change the injector's pulse width, you will have 63% more fuel and your car will not run. An SAFC corrects this problem by decreasing load and thus shortening the pulse width.

Are you asking what will happen if you increase injector size and load? You would have even more fuel and the car will not run. Unchanged with 440cc injectors gives 163% fuel plus what you add.

I'm not sure what you think you would accomplish. Perhaps it would help if you read a bit on the basics of engine management.

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Postby Florida240sx » Mon May 29, 2006 7:56 am

I'm talking about the WOT runs, not the idle and cruise.


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