Tricking your S-AFC Into reading boost (You WANT to read)

2588 Views 12 Replies 3 Participants Last post by williamb82, Oct 30, 2003

NoahFex

Discussion Starter · Oct 28, 2003

I originally posted this on ProbeTalk.com, and I think our cars already have a MAP sensor that gives off 0-5v (right?). If so, ignore the part on installing a MAP sensor.

For those of you running a S-AFC on a boosted vehicle, I have some info for you guys that could help tune your car. Imagine being able to use the S-AFC to tune your car based on RPM and boost pressure ALONE.

The S-AFC uses a throttle sensor to tune your vehicle. The throttle sensor puts off between 0v-5v and is a directly linear readout. That is 0 volts at closed throttle and 5 volts at full throttle. So all you need is a pressure sensor that reads 0 volts at the lowest pressure you'll see and 5 volts at a pressure a little above your maximum boost pressure. It needs to vary linearly in between those two points. The only part I could find is a Motorola MPX 4250 AP. It reads from 2.9 to 36.3 psia and gives a 0.2 to 4.9 volt output. It is actually designed to be used in turbocharged automotive MFI applications. (Around $20 from http://www.newark.com/)

Connect this to a vacuum line coming off of your intake manifold and wire it to your S-AFC with the TPS wire. Either connect power and ground to the same power and ground as your TPS, or find a 12v-5v step down voltage converter. (or any other 5v line on your car)

Start your car
Began tuning

Now the reason this is better is because RPM and Throttle position is not always directly relationaly to boost. Boost however, as you already know, needs to have linear A/F. i.e. 12.5:1 throughout entire boosting. Also, on 84 supras, the TPS is multi-staged and will not work with S-AFC except at WOT.

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NoahFex

Discussion Starter · #2 · Oct 28, 2003

Btw, about finding that MAP sensor. Try going to http://www.mot.com which is Motorola's website. Go to the "contact" portion, and fill out the form requesting that product.

When I was looking for detailed tech docs on the 68HC11, I came up empty handed on google.com, so I sent the form on a Sunday (closed), they were closed on Labor day (monday), and on tuesday a guy called me and gave me a URL to HUNDREDS of tech docs and info on how to obtain it. It's the same for any of their products.

Motorola treats every customer like they were buying 10,000 items. Cause they don't know your plans for that item, so they'll get you in touch with a reseller.

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williamb82

· #3 · Oct 28, 2003

this isnt a secret. good info and im sure alot of people dont know about it. and an fyi, our cars do not have a map sensor. however turbo mkiii's do. it reads up to 15psi. it is for the stock boost gauge. i personally want a 3 bar sensor.
william

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williamb82

· #4 · Oct 28, 2003

also, a gm map sensor will work fine and are redily available.
william

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ma61_turbo

· #5 · Oct 29, 2003

williamb82 said:
this isnt a secret. good info and im sure alot of people dont know about it. and an fyi, our cars do not have a map sensor. however turbo mkiii's do. it reads up to 15psi. it is for the stock boost gauge. i personally want a 3 bar sensor.
william

Turbo mk3's don't have a map sensor, MAF only. However, the JDM Group A version does use a MAP sensor.

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williamb82

· #6 · Oct 29, 2003

yes mkiii turbos DO have map sensors. it is for the stock boost gauge. it is on the drivers side shock tower. has a plug with 3 wires on it and a vacum port. i never said it was for the engine management, just said they come stock and they do. i have a couple of them.
william

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NoahFex

Discussion Starter · #7 · Oct 29, 2003

williamb82 said:
yes mkiii turbos DO have map sensors. it is for the stock boost gauge. it is on the drivers side shock tower. has a plug with 3 wires on it and a vacum port. i never said it was for the engine management, just said they come stock and they do. i have a couple of them.
william

Does it give off 0-5v?

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williamb82

· #8 · Oct 29, 2003

yes, and it reads vacum up to ~15psi even though the stock boost gauge only measures to 8.
william

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NoahFex

Discussion Starter · #9 · Oct 29, 2003

williamb82 said:
yes, and it reads vacum up to ~15psi even though the stock boost gauge only measures to 8.
william

Well the reason a MAP sensor off another car may not work is because it gives off voltage for vacuum.

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williamb82

· #10 · Oct 29, 2003

no, thats why it will work. if you add larger injectors it will allow you to lower the fuel at idle and off boost. the fuel needs to be adjust all across the board, not only under boost alone. espeacially on 5mgte cars running 295's
william

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NoahFex

Discussion Starter · #11 · Oct 29, 2003

williamb82 said:
no, thats why it will work. if you add larger injectors it will allow you to lower the fuel at idle and off boost. the fuel needs to be adjust all across the board, not only under boost alone. espeacially on 5mgte cars running 295's
william

Good point.

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ma61_turbo

· #12 · Oct 30, 2003

williamb82 said:
yes mkiii turbos DO have map sensors. it is for the stock boost gauge. it is on the drivers side shock tower. has a plug with 3 wires on it and a vacum port. i never said it was for the engine management, just said they come stock and they do. i have a couple of them.
william

Well, i stand corrected

I've never seen any reference to them ever before, oh well, at least I now know how it senses boost, I could never work that one out.

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williamb82

· #13 · Oct 30, 2003

btw, im in the us so when i said drivers side i meant the intake manifold side. didnt notice you were in oz. but yeah, its there. and for a 5mgte aplication it should work perfect cause most 5mgtes arent gonna go past 10psi, much less 15.
william

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