Fuel Cut Defence.

[Nick Mann]

I am looking at removing my HKS FCD, and using the fuel cut defender built in to the MAP ECU2. So I am trying to wrap my head around the function of said HKS FCD to make sure I wire in the MAP ECU2 properly.

The problem lies in working out what the HKS FCD is doing.

It is wired:
Ignition
Ground
KVF in
KVF out
and one other wire is spliced into, which is pin 8 on connector 3. This is described in
The Vees wiring diagrams as:

Engine speed
Meter & gauge
Constant pulse generator
Ignition failure sensor

whereas in a post by GalantOnly in enigmas thread it is listed as only

58 W Ignition failture sensor

So, what is that wire?
How does the FCD work?

Ignoring the white wire on connector three, it appears that the FCD must limit the KVF getting to the ecu. But surely that means it is impossible to tune the engine using a modified KVF value at those points which the signal is being altered by the FCD? You increase the fuelling on the MAP ECU2, which achieves its goal by telling the cars ECU it has more air arriving. So the KVF value goes up at that point. But then the FCD limits the KVF signal, so no matter what you do you'll never fuel enough?

The MAP ECU2 is set up to limit the voltage seen by the ECU to prevent fuel cut occurring. But to my simple mind a voltage limiter will not help limit a frequency. And I'm not sure I want to, because it will upset the tuning as above. Won't it? If the ignition failure switch is the voltage that the ecu sees for fuel cut, however, then maybe it's all okay anyway?

So maybe the white wire does do something else.... But what????

[Gly]

the FCD blocks or limits the KVF on at the upper most frequency the point at which you hit fuel cut,

it does not alter any signal below that frequency,

so tuning is fine, as you would never tune to hit fuel cut.

if you have ever hit fuel cut with wideband EGO monitering you would notice ther car is run VERY rich,
at the point you hit fuel cu the car thinks it has maxed out what it can do safely, so cuts fuel to potect the motor,

what you are trying to do with the map ecu is lower the amout of fuel to get better power (lower the KVF to get less fuel),
so in turn you do not get fuel cut,

also if you put in larger injectors, the frequencys would need to a alot lower than factory to get the same amount of fuel so you need not worry about fuel cut then either

[Gly]

oh and ignition failure sensor is the little black thing on top of the front rocker cover,

and picks up miss fires, nothing to do with KVF/KVM

[Nutter_John]

On the MAP2 ecu and standard injectors you still get fuel cut around 0.9 bar , you just need to reduce duty cycle through the 3500->4500 zone to limit the chances .

By going to bigger injectors you move the point at which fuel cut would occur so for instance if fuel cut was to happen at 0.9 bar and a freqency of 1200hz then becasue the injectors flow 395 more fuel then your frequency would be around 732 hz

As far as I know the MAP2 only has voltage clamping FCD rather than freq based clamping .

to be honest Nick i would be tempted to leave the HKS FCD in place and back it off to stock levels , as in the future to you may want to use the fcd again to save going to bigger injectors

[Nick Mann]

so tuning is fine, as you would never tune to hit fuel cut.

That only applies if you don't intend to increase the amount of air going in, surely?

As far as I know the MAP2 only has voltage clamping FCD rather than freq based clamping .

I was afraid someone would say that....

I have no intention of backing it off. I was just trying to wrap my head around the way it works. So, theoretically, if I want to get more fuel in an area prone to fuel cut, I can't do it? Because the only way to get more fuel is to tell the ECU. So where the FCD is capping the frequencies, there is a maximum fuel level regardless of the settings in the MAP ECU2.

Lets hope I don't need that much fuel then!

[Nutter_John]

yeah thats correct Nick , but remember on our cars fuel cut is not at a set frequency or boost , it is to do the the load cell it is in

and yes you can never exceed the level the fcd is set at so even if the map2 ius running with more hz the fcd will not allow the stock ecu to see it

As your running the map2 you maybe able to back off the fcd a little so instaed of clamping at 1200 it clamps at 1300hz ,as your map will be leaner on average than the stock one so should stay clear of fuel cut .

But I know from sepaking with the guru of map2's that the main issue is around the 3500-4500 rpm band as this is where max torque sits and your in a high load cell the chance of hitting FC is much higher

best option in all honesty is the one i took which is the bigger injectors to move you well outside the zone

[Eurospec]

The HKS FCD works by clamping the Hz signal. Lets say fuel cut occurs at 1000hz. Then the HKS FCD will see 1000 come in, and send out 999.

Problems occur when 1200 is going in, and only 999 is coming out. In such circumstances then its gonna run lean.

Map2 has the facility to run a KVF FCD, its the KVF clamp. If the KVF value in the fuel tables exceed the value in KVF clamp in the ecu config table then the KVF clamp value will be sent. The other FCD stuff is for hotwire, so ignore all that.

However, nutter is basically on the money. If you have your afrs where you want them (ie you cant go safely any leaner) and you are still getting fuel cut, then you do need bigger injectors. (And you are going mega fast!) Using the fcd in such circumstances is only gonna make it run leaner than you want.

Personally i never use the kvf clamp, i just use the fuel tables. If i have the afr where i want it and its still cutting, then i back off the boost a bit. If ts rich, then i lean out the table and that will get you round the cut. This only applies however if you can safely go leaner at the boost level its occuring at.

Cheers,

Ben.

[Nick Mann]

Thanks Nutless and Ben!

That's pretty much the conclusion I had arrived at myself.... Lets see how far I can go without bigger injectors then! I do have some, but I don't want to start modifying the fuel rail unless I have to.