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AFC + missing link?

Old 02-05-2004, 07:03 PM
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Default AFC + missing link?

Alright, I was talking to this kid he is in the process of building his JDM gsr motor and we were talking about fuel management. He has 2 booster pumps, FMU and 310cc injectors. He's planning on running 25psi. He saw my setup and decided he was going to get an AFC and 550 or above cc injectors to support the horsepower he's running (I know he won't be able to idle )

I told him that he didn't need to run the missing link with the AFC. I understand the concept of the AFC scaling back the map sensor voltage therefore you don't need check valves. But I couldn't give him a solid answer on why you couldn't run a missing link with the AFC. If somebody could explain this I'd appreciate it.
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Old 02-05-2004, 08:50 PM
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Default Re:AFC + missing link?

Because the Missing Link's sole purpose is to bleed off pressure to the MAP sensor so that it doesn't see boost. If you use this and an AFC, the AFC will think you're not seeing boost, and you'll be scaling back "NA" signal. It's a cluster ---- of hacks on on top of the the other.

Oh, and tell your friend he's a retard. If he's running more than 10psi, he needs to invest in a REAL engine management solution. With his built up hacks, h'ell never get it tuned, and will most likely blow it up *IF* he ever does reach his desired boost level.
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Old 02-05-2004, 11:11 PM
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Default Re:AFC + missing link?

Don't flame me because i'm not a ------- genius, but...

I was pretty sure that yes, the purpose of a missing link is to prevent the map sensor from seing boost (positive pressure) in the manifold and preventing a CEL, thus dreaded "limp mode".

And yeah, the SAFC will modify the map sensor signal to scale back the amount of fuel being sprayed by the injectors. But I think both these devices serve a purpose, and can be used together.

first, the SAFC is necessary to run larger injectors (of course) if you ever want your car to idle. Scale back fuel at idle and low rpms, increase fuel as the revs increase, yadda yadda yadda. But if you don't have the missing link in place, and you are at like 5000 rpm and 10 lbs of boost (roughly 168% sea level air pressure) you're going to throw a check engine light no matter what unless i'm mistaken.

this is because (i think) the SAFC can only modify the MAP signal +/-30%. even if you knock 30% off of the MAP sensor voltage you're still allowing your ECU to see positive manifold pressure, and bam-CEL.

of course this is all just on paper, right?
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Old 02-05-2004, 11:27 PM
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Default Re:AFC + missing link?

I understand its a clusterfuck of hacks but if you were to run the two at the same time couldn't you just change the AFC settings to compensate and scale to the 'NA' settings or doesn't it work like this?

BTW the kit is DEFINITELY not my friend. Thats why I didn't even bother to try to talk him into real fuel management. Personally, I can't wait for the very expensive fireworks show. Here's his full setup (you gonna like this): FMU, missing link, 310cc injectors, 255lph walboro, rpm activated 2nd fuel pump, JUN n/a road race ecu (yeah thats right) and now he's going to try to add the hack on top of that with some big *** injectors. How many psi will it take to destroy a fully built motor with this setup?
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Old 02-05-2004, 11:36 PM
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Default Re:AFC + missing link?

NO NO NO NO NO

OK, a stock MAP sensor can read about 9.5psi.

With a missing link, it won't see above 0, only vacuum.

The AFC needs to see the boost, otherwise it won't modify the MAP signal, and thus the fuel map.

The Booster pumps are for use with an FMU, which you would use with a missing link and stock, or small injectors.

The AFC would use the bigger injectors, but you would have to lower the fuel pressure to make it idle because the AFC can not run higher than 450s, it doesn't have the range.
Even so,if you try to run more than 9.5 psi with an AFC, the MAP sensor won't work right, and the ECU will stay put, and boom, dead motor.

To run 25psi (motor-->death) he'll need a 3Bar Map sensor, and some serious engine management with a ---- ton of tuning to make any use of it. Basically the kids a dumb ----, just nod and let him blow up his ----, then walk away...
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Old 02-06-2004, 12:19 AM
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Default Re:AFC + missing link?

Originally Posted by willahlborn
But if you don't have the missing link in place, and you are at like 5000 rpm and 10 lbs of boost (roughly 168% sea level air pressure) you're going to throw a check engine light no matter what unless i'm mistaken.

this is because (i think) the SAFC can only modify the MAP signal +/-30%.
Everything was great up to there.

I spike 12psi at 4400rpms all the time when I used the AFC hack. No problem, why? Because your idea is correct, however, you can scale to +/- 50%.

Thus, it is possible to run up to about 16psi before really freaking out your AFC. Not that this is recommended because you would have to lean passed stock injector flow rates, and compensate by massively increased fuel pressure. Not to forget that your MAP sensor will no longer register votlage after 9.5psi anyway so.. yada blah blah blah.
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Old 02-06-2004, 09:50 AM
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Default Re:AFC + missing link?

Originally Posted by jung4g
The AFC needs to see the boost, otherwise it won't modify the MAP signal, and thus the fuel map.
Ummmmmm I don't think so. The AFC only intercepts the map signal and modifies it before it gets to the ecu--it doesn't take the map signal reading into consideration before modifying it. It bases it's map correction on your tps sensor (low/high throttle) and what rpm you are in. I'm pretty sure that's it. Sure, it gives you a pretty readout of how much pressure is in your manifold but that's about it.

Granted, if you don't have a missing link in place, the map signal will be higher and therefore will need less correction. However, as I said before, once the signal from the AFC to the ecu goes into positive pressure territory, you're going to get a check engine light. If you have the missing link in place, you can tune your SAFC to compensate for a given amount of boost based on how much boost you are making at a given rpm. (just increase fuel modifier as the rpms increase.) The only downside to this is you can't use your SAFC as a boost guage (which nobody does anyway.)
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Old 02-06-2004, 11:03 AM
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Default Re:AFC + missing link?

Originally Posted by willahlborn
Granted, if you don't have a missing link in place, the map signal will be higher and therefore will need less correction.
..again, almost.

The problem with using the missing link and the AFC is the AFC *needs* to scale the MAP sensor back, otherwise, your larger injectors will be flowing entirely too much fuel. It needs to see an untouched MAP sensor signal for the percent of scaling to be correct.

A missing link will not let you see any boost. Therefore, if you were to scale with your AFC to try to correct with your larger injectors, you wouldn't use half of your fuel/timing maps. AFC and missing link won't work together.

Check out this hot little pbrush32.exe visual I threw together..



Why does the AFC and the missing link only reach half of the map? Because your still have to scale the larger injectors back otherwise, you're pusling 450 injectors with 240cc pulse widths, which will would be lucky to even start your car.
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Old 02-06-2004, 11:51 AM
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Default Re:AFC + missing link?

Originally Posted by TurboEF9
..again, almost.

The problem with using the missing link and the AFC is the AFC *needs* to scale the MAP sensor back, otherwise, your larger injectors will be flowing entirely too much fuel. It needs to see an untouched MAP sensor signal for the percent of scaling to be correct.
I'm glad we can at least have an intelligent argument on here.

Here is the example i'd like you to use (please don't take these percentages literally, this is just to illustrate my point)

This example will be with a turbo d15, stock wastegate at 8psi, dsm 450 injectors, a missing link, and an SAFC.

RPM: fuel correction: map input: boost:
1000 -40% 3v 1psi
2000 -39% 3v 3psi
3000 -37% 3v 8psi
4000 -37% 3v 8psi
5000 -36% 3v 8psi
6000 -36% 3v 8psi
7000 -36% 3v 8psi

again these numbers are not accurate; just to illustrate my point. Even with the map sensor reading no boost (limited to 3v by the missing link to prevent a CEL) the SAFC can still be adjusted by rpm to compensate for more fuel as revs, and therefore psi, increase. I guess this would only really work if you know exactly when and how your compressor reaches full boost.

EDIT:
the above example will basically be my setup in about a weeks time. I'll get it set up and running and then post some exact correction factors to prove that this works.
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Old 02-06-2004, 12:25 PM
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Default Re:AFC + missing link?

Originally Posted by willahlborn
Even with the map sensor reading no boost (limited to 3v by the missing link to prevent a CEL) the SAFC can still be adjusted by rpm to compensate for more fuel as revs, and therefore psi, increase.
Correct, up to the point of yes you can do it, but you won't get the fuel correct above the 3v. This is because you won't be seeing boost. The AFC needs to see boost when scaled that far back to use the rest of your fuel MAP.

This has already been proven using TurboEDIT's datalogger, NA with 450s, and V-AFC. Which would be a simulation of the missing link (if you think about it).

The datalogger's MAP Trace never exceeded about 2/3s of the maps available width. Why? Because there was no boost reach the rest of the map.
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