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Boost cut

chan

By chan,
in Engines & Forced Induction

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Downforce Grand Master

Downforce

Posted
Posted
I was just wondering would anyone know if the HKS EVC4 will get rid of boost cut?

What car is it for? Do you mean your 180km/h speed limiter ? or stock boost? There is not "boost cut" but you may have a fuel cut out when boosting too high with stock ECU. The stock boost is set at a certain level, if u replace the solenoid with an aftermarket electronic boost controller... u can boost to what ever u want. Just make sure you increase your intake air cooling capacity before boosting above 0.7 bar

:)

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Downforce Grand Master

Downforce

Posted
Posted
and just to clear it up.  Does that mean that if i have an electronic boost controller with a different boost solenoid i would not need to get a fuel cut defender?

It's not your solenoid which controls the fuel cut... it's your ECU. Adding an aftermarket solenoid, bypassing your stock solenoid, and having an electronic boost controller will allow you to increase your boost, but the ECU will still cut fuel once you go above a certain level of boost because it thinks your engine is receiving too much fuel, but in reality you're running more boost which is making more power and burning fuel quicker. You will need a fuel cut defender if you're having fuel cut out issues, but I wouldnt recommend using a FCD if you're boosting above 1.0 bar. Get a Power FC, and make sure you have adequate cooling capacity as you increase power.

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Sydneykid Grand Master

Sydneykid

Posted
Posted

Hi Chan, "Over boost protection" is what it is sometimes known as, but it is really "excessive airflow protection". The ecu makes this decision based on what the AFM is telling it. So boost really has nothing to do with it. The fix is an SAFC, voltage clamp, Boost Cut Defeater or better still replace the std ecu with a Power FC. Full fuel cut is the next step in engine protection programming in the ECU, but you get "retard" and "rich" first.

Here is a post I prepared earlier that may help

.................................................. .................................................. ...

Hi guys, a quick, simplistic explanation of how an SAFC works might help...

As the airflow into the engine increase, the AFM records this as increased voltage that the ECU sees. What an SAFC does is sit in between the AFM and the ECU and take the voltages from the AFM and either increase or decrease them depending on what you have programmed the SAFC to do. By increasing the voltage, this tricks the ecu into pumping in more fuel, you do this when the engine is running lean. By decreasing the voltage, this tricks the ecu into pumping in less fuel, you do this when the engine is running rich.

The next bit is hard to understand unless you remember that the standard ecu does not supply fuel in direct proportion to the afm voltage ie; 4 volts is not twice as much fuel as 2 volts. Engine rpm, boost and throttle position also help the ecu determine how much fuel to add.

It has been my experience that RB25's run a little lean down low in the rpm range (Nissan do this for fuel economy and emissions) and a lot rich up high (Nissan do this to protect the engine). So I have to increase the voltage (using the SAFC) up to around 3,000 rpm (part throttle especially) and decrease the voltage over 5,000 rpm (full throttle).

The real problem is in the 3,000 to 5,000 rpm range as the engine comes on boost. They really need lots of fuel very quickly in this area. This can mean that you need to increase the voltage (that the ecu sees) over 5.1 volts to get the right A/F ratios. The ecu then goes into engine protection mode, rich and retarded (sounds like a girl I knew once) :boobies2: . Past that rpm you can start leaning it out as it runs too rich, so the voltage needs to be turned down under 5.1 volts. The ecu sees this as the engine not needing protection mode any more, no more rich and retarded.

It is not unusual to get good performance up to 3,000 rpm, then sluggish from 3,000 rpm to 5,000 rpm and then good performance from 5,000 rpm and over. The SAFC may not help this, in fact as explained above, it can in fact make it worse if the tuner is not switched on to this stuff.

With bent afm voltages, the ecu (tricked by the SAFC) also fires the ignition to suite the airflow it THINKS the engine is getting. This is not a good thing as you generally end up with ignition that is too far advanced in some rpm ranges.

The poor tuner has to juggle the SAFC settings, so that the A/F ratios are OK, the ecu doesn't get into rich and retard (engine protection) mode and the ignition timing is not too far advanced so as to cause detonation. My experience (I am not a good tuner) has been that this is full of compromises, sometimes you just can't win and have to reduce the boost level a bit to get even a reasonable compromise.

Keep in mind that this explanation is very simplified to make it fit in a reasonable space, the rpm's used are rough guides only and every car is different.

Hope it helps (and makes some sense). :D

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