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The mkiv Supra Owners Club

Building my own FCD?


Guest RudeGuy
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Guest RudeGuy

Hello!

 

What functions does a FCD have in a MKIV?

For most other engines you can build you own FCD verry cheap. Can I do this for my supra aswell?

 

/Fredrik from sweden...

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you can but tbh for around £70 it's really just worth buying one and having piece of mind that it does the right job and still offers some protection to the engine. Most cheap zenner mods etc are not really the way to go, why risk potentially £££thousands for less than £100 :)

 

this is the fcd you are looking for... http://www.thor-racing.co.uk/VFCC-00100006.asp

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seriously, don't do it.

 

if you open up the thor unit it has a chip or two in it, op amps probably or whatever, anyway point being they are not just there for fun, it's a much more advanced, safer fcd than lobbing a couple of resistors in will achieve, but you pay the money - and take the risks...

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will this work on a supra?

http://www.alltrac.net/mods/fuelcut.htm

 

Sounds like it might work, but the problem I can see is that this device is basically scaling the boost pressure signal down completely, so your ECU is being given a misreading of boost all through the range. Someone more knowledgeable will hopefully come along and explain how this might screw up your cars fuelling. I suspect that as the ECU is being misinformed on the amount of boost, it would not supply enough fuel, thus running your engine lean, which is definately not a good thing.

 

The Thor box works a bit smarter than that.. it allows the signal to pass un-modified, right up until just below the point of fuel cut, then it holds the signal right there.. by which time your injectors are running at 100% duty cycle anyway so concerns of running lean are erm.. invalid.

 

I'd hate to buy a Supra of someone who has skimped on the £70 for a proper FCD and used a 15p pot instead..

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I could be completely wrong of course. There are differences in the Dometic and Export Supras. Export uses MAP for fuel cut and for putting the fuel pump into 12v (full) mode only, so you can actually just disconnect the MAP, but you might then not be able to supply enough fuel to the engine.

Domestic (jap spec) cars use the MAF.

 

or it could be the other way around ;)

 

Read here http://www.mkiv.com/techarticles/ffcd/freefcd.htm

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AFAIA all fuel/boost cut defencers work in the same way the allow you to clamp the voltage of either the MAP sensor or the MFA to a point just below the threshold voltage of the ECUs cut point, so if the cut point is say 4.3 volts = say 0.8 BAR and you take the boost to 1.1 BAR = 4.8 volts, then the ECU is not supplying enough fuel for the increased boost/airflow as its clamped at the lower voltage, so ideally you really need some way of achieving extra fulling IE increased fuel pressure which works to a degree, or a way of fudging the injector signal SAFC etc!

Having said this it does not seem to bother some vehicles as some seem to overfuel quite a lot near the std fuel/boost cut voltage, so its fairly safe, but it does not apply to all!

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Yes the whole point of a good fcd is that the signal is untouched right up until where the ecu would introduce fuel cut and then it is clamped for an extra bit of headroom, go beyond that and the signal rises again so the ecu can do it's safety thing.

 

so, importantly:

 

The signal must be un-scaled before fuel cut

It must be clamped at a set level just below where standard ecu fuel cut occurrs

It must re-introduce fuel cut at a set limit beyond that.

 

You won't get all this from a simple zenner diode or resistor / pot setup.

 

Also, preferably the above should be adjustable, i.e. to adjustable limits.

 

The Thor VFCD does all this, many others do not...

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