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View Full Version : how does an safc adjust map voltage?


evil_2
11-11-2019, 01:11 AM
how does the safc adjust map voltage so i can run higher boost levels?

i mean , a map clamp sets your fullboost map volts with a turn of a screw...how does the safc know what to adjust it to?

lunchbox660
11-11-2019, 01:41 AM
its the exact same principle as a MAP clamp. all they are is voltage regulators. the simple MAP clamps use a screw that has a slider on it. as the slider increases and decreases it distance between the contacts inside it increases or decreases the voltage that can pass thru accordingly. the SAFC is the exact same thing but uses digital stuff to achieve the exact same goal. think of it kind of like the volume button on a stereo. volume buttons/knobs are in fact the EXACT same thing as an SAFC/MAP clamp. it is called a potentiometer. so with the buttons on the SAFC, you increase and decrease it does the same thing as when you increase or decrease to volume on your stereo, it just allows more or less votlage to pass thru. very simple concept, really.

matter of fact, if someone wanted to make a digital MAP clamp, they could do it the same way as the one with the screw and it would work. all you woud need is a digital control for it that runs off of 5 volts. it wouldnt work the same as an SAFC cus it would not have RPM specific tables, but it still would do the same thing as both.

evil_2
11-11-2019, 01:59 AM
where are the voltage adjustments on the safc?
i had an safc in my 400hp rx7 but it had a mass air meter so i never messed with map voltage

lunchbox660
11-11-2019, 02:13 AM
MAF, MAP, it doesnt matter what kind of sensor the car uses. some cars even use both, and some cars use even more than that. the main goal of the SAFC or MAP clamp is to intercept the votlage from the sensor and input a regulated voltage to the car's computer. thats why i was telling you when you mentioned you were gettin an SAFC that it really isnt any different or necesarily better than a MAP clamp. i know there are guys who would beg to differ, but i have PLENTY of dyno sheets that will prove that you can achieve just as flat of an AFR line with a MAP clamp as you can with an SAFC.

evil_2
11-11-2019, 09:45 AM
MAF, MAP, it doesnt matter what kind of sensor the car uses. some cars even use both, and some cars use even more than that. the main goal of the SAFC or MAP clamp is to intercept the votlage from the sensor and input a regulated voltage to the car's computer. thats why i was telling you when you mentioned you were gettin an SAFC that it really isnt any different or necesarily better than a MAP clamp. i know there are guys who would beg to differ, but i have PLENTY of dyno sheets that will prove that you can achieve just as flat of an AFR line with a MAP clamp as you can with an SAFC.


well i guess that the safcis auto adjusting according to boost nad thats why when you wire it you use the map sesor as the reference vs the TPS

blackbird
11-11-2019, 04:53 PM
The MAP clamp is a simple electrical device and most all of them are a variation in design consisting of a Zener diode(s) and potentiometer (which is a variable resistor and the part that you adjust). When the voltage coming off the MAP sensor goes above a certain voltage the MAP clamp "switches on" and dumps that excess voltage to a ground wire and maintains a constant maximum voltage output. So as the boost and therefore MAP sensor voltage continue to increase all that extra voltage is getting dumped to the ground wire and the PCM isn't seeing anything above the cutoff point.

The S-AFC and essentially any of the current crop of piggybacks all work on essentially the same principle how they alter the MAP voltage that goes to the PCM, but electrically they are in a different ballpark from a MAP clamp. The piggyback has a microprocessor and can be thought of as miniature computer that can read an input (from the MAP sensor) and then produce an output (creating an altered MAP voltage signal to send to the PCM). The second part is important. Instead of just electrically cutting off the voltage at one point and shorting that excess voltage to a ground wire like in a MAP clamp, the piggyback's input from the MAP sensor and output that goes to the PCM are completely separate. It reads an input then creates an output depending on how you have it set up and adjusted.

In other words let's say you install an S-AFC II in your car but you set it to zero correction where it's not making any changes to the MAP signal. It reads an input from the MAP sensor and then creates and outputs the same voltage to the PCM. Because of how it reads the input and then actively creates an output you have the flexibility to alter settings for how and when it does that in software using the buttons and knobs on the front panel control interface. It can then increase or decease the output voltage going to the PCM (and that voltage is used by the PCM to determine how much boost and air there is and in turn how much fuel is required). If the car is running great but is rich in only one area you can use the controls on the S-AFC to adjust the software inside the unit so it takes out a percentage of the MAP voltage only in that one area.

The MAP clamp is a passive device and far less capable but also far less expensive and sometime works "good enough". With a MAP clamp the MAP sensor voltage (i.e. boost pressure signal) going to the PCM can go up or down normally until it goes over that trigger point where it gets cut off and any additional voltage will not reach the PCM. For an example let's say it is set to cut off at the equivalent to 15 psi of boost. If the boost continues to go up the PCM will still only "see" the same 15 psi max reading and won't add any more fuel.

When combined with how the boost curve on a stock turbo drops towards redline you get a little more adjustability of the A/F over a broader range on a stock turbo SRT-4 than you would on other forced induction cars. The stock/Stage PCM's are programmed to take this boost drop into consideration. They know it’s going to happen but also know that you still need extra fuel up top. Just because boost is dropping north of 5k rpm's the PCM still has to add more fuel up top. In our previous example of a MAP clamp set to 15 psi, if the boost on the top end started to drop below that 15 psi or not, the PCM is already expecting it and is altering the fuel mixture accordingly where as a different car with a big turbo that has a linear increase in boost all the way to redline would just keep getting leaner and leaner with a MAP clamp. In other words you can clamp lower on an SRT-4 than you could on other cars.

Since the piggyback is essentially an adjustable digital computer you can tell it to pull a percentage of the MAP voltage out or even add MAP voltage back in to richen up the mixture (up to certain limits). How the piggyback will know when to add or remove the fuel (by changing the MAP signal that it outputs) will depending on the settings and how it's hooked up. There are a few different ways you can wire a piggyback up to make them work. The least preferable is to have them use the rpm's and throttle position to calculate how much to alter the MAP voltage going to the PCM. In that configuration the piggyback will know that when throttle goes over a certain percentage/voltage (i.e. going to WOT) and that between 3500-3700 rpm's it should take out 7% off the MAP voltage, then from 3700-4200 take out 9%, at 4200-4600 take out 7%, and so on. In theory it should work okay but on a car that is driven in the real world out on the street in different gears and under different conditions will have different loads and might be at 17 psi in the 4200-4600 rpm range one time and then be at 15 psi in the same rpm range another time, so the fueling requirements will be different and can't be accurately based off throttle and rpm's alone. This makes it inconsistent and impossible to tune perfectly.

The other method involves wiring up the MAP sensor's output into the piggyback in place of the throttle position. This is considered setting the piggyback up to operate as "load based". Instead of using rpm and throttle position to determine how much to change the MAP signal going to the PCM it will use rpm and the MAP voltage itself to calculate how much fuel to add or remove. When talking about "load" in this context you can think of load as being the fueling requirements needed to make a certain amount of power. The powertrain computer works off a speed-density system that is load based. The PCM knows physically how much air the cylinders can take in and can determine exactly how much air there is to burn (and the fueling required) while the engine is running in vacuum or boost by using the MAP sensor and rpm's as well as the other sensors. Because of this the MAP sensor is sometimes referred to as a 'load sensor' because of its importance to the PCM in being able to calculate the available air in the combustion chamber and subsequently the fueling requirements.

By setting the piggyback up to also use the MAP sensor as an input in its calculations instead of throttle position (which would not tell the piggyback how much air or boost is actually going into the engine) you can then have it add/remove fuel more precisely and accurately for the condition you are actually running. The piggyback just like the PCM will know how much air is being forced into the engine by the turbo vice using just the throttle position, which is not as accurate as already mentioned (since the throttle regulates flow and not pressure). The PCM can then make changes to fueling depending on what rpm you're are at and the amount of air going into the engine.

If it is rpm/throttle based you might have a scenario where it is calculating "between 3500-4000 rpm's if the throttle position indicates WOT remove xx% fuel". But you might be at 12 psi boost or maybe 16 psi of boost and the piggyback will still take away the same amount of fuel. When you set it up as load based going off rpm/MAP you would have something like "between 3500-4000 rpm's and the MAP voltage (i.e. boost) is between 3.5-3.7 volts remove xx% fuel" or if "between 3500-4000 rpm's if the MAP voltage is now between 4.0-3.2 volts remove a different xx% fuel", etc.

cookie32
11-11-2019, 05:30 PM
dayum Blackbird, what don't you know.

I was debating on whether to get the dasmopar clamp, or the apexi s-afc, and now there is no question in my mind, definitely the apexi. I'm going to put your brain on speed dial............:thumb:

evil_2
11-12-2019, 02:08 AM
blackbird..you rock man...


i do have my safc referencing the map sensor instead of the TPS....

lunchbox660
11-12-2019, 02:28 AM
psh...there goes eric raining on my parade with his techno-babble again...lol :waves: hi eric! :waves: but yea, its true, sure the SAFC does it better than the MAP clamp, but do you want to spend $5 or $350 to acheive that end result, plus the impending doom of having to wire up an SAFC (evil, i know you already did yours, but this is for other who are reading the thread)

basically what im saying is that if you need the technology and have the money, an SAFC/DTEC/EManage are nice tools to have and would be a bonus. but me personally, i dont mind settling for this result here for only fiev bucks:

http://img.photobucket.com/albums/v647/lunchbox660/dyno%20sheets/dynosheets0001.jpg

thats on 20 psi of boost with an AGP WGA, a 3" turboback, MAP clamp, and EVERYTHING ELSE is stock. even the intake. the red line represents the baseline run before the MAP clamp. the blue line indicates the MAP clamp once it was tuned. there was a nice gain in HP and TQ all the way across the board, and look at that SUPER SWEET AFR line on the bottom in blue baby!

i can give you the part numbers and directions to build your own MAP clamps or you can look at the HOW-Tos.....or you can spend the green on an SAFC, still only get the same amuont of horsepower and TQ, ans spend a whole friggin weekend wiring it all in....

evil_2
11-12-2019, 02:45 AM
i paid 95 bux shipped for my safc.....only 50 bux more than a mapclamp...

lunchbox660
11-12-2019, 11:39 AM
werd. like i said, i know you got a good deal on yours, but they arent ALL like that. i made my map clamp out of parts from radio shack for under $5, so youre still $90 more expensive than mine...lol

but you had to solder WAY more than 3 wires tho! lol

evil_2
11-12-2019, 01:09 PM
werd. like i said, i know you got a good deal on yours, but they arent ALL like that. i made my map clamp out of parts from radio shack for under $5, so youre still $90 more expensive than mine...lol

but you had to solder WAY more than 3 wires tho! lol

yeah, true....

i know if nomotivgirl would have sold me a good mapclamp instead of that peice of shit she sent me then id probably have one on my car.

lunchbox660
11-12-2019, 02:41 PM
i coulda sold you one for 10 bucks shipped...

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