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Fuel cut defender??

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Sydneykid Grand Master

Sydneykid

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Posted

Hi USRB25S13 , "Over boost protection" is what it is know 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

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

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 and decrease the voltage over 5,000 rpm.

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). 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.

So you 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).

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paulr33 Grand Master

paulr33

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the ecu will retard too much at 17psi. there will be too much airflow and the timing will get retarded heaps. i would seriously look at an getting an aftermarket ecu before even considering 17psi. youll get much more bang per $ if you had an aftermarket ecu and 15psi vs stock ecu and 17psi

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USRB25S13 Enthusiast

USRB25S13

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What kinda Timing adjustments can you make with the PFC? The main problem I have here in the US is finding one. I either have to take my chance on ebay or find a good source some where (wink wink) to sell me one. I may just get a standalone (Haltech or Tech III) for they may be cheaper/easier to get.

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Sydneykid Grand Master

Sydneykid

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Posted
Looks like a Power FC should be on the top of my future mods list.....Can you still have a boost cut with the power FC? Just for overboost protection.

Firstly you tune it properly for your boost level. You can of course tune it for more boost than you run via the boost table. In other words tune it "normally" with optimium A/F ratios and ignition timing. Or you can add extra fuel and/or retard the timing if the boost goes above your target. You can make this a "saftey net" (only a bit richer and a bit retarded) or "protection" (dump heaps of fuel and lots of retard). "Protection" is like the standard ECU. The standard ECU then has full on fuel cut off, Power FC doesn't have that for obvious reasons.

What kinda Timing adjustments can you make with the PFC?

Whatever you want, there is no limit to the ignition mapping as long as you stay within 20 X 20 load points. Plus you have correction tables for temperature and boost.

The main problem I have here in the US is finding one.

Try www.nengun.com they sell to anyone, anywhere at good prices. I have bought plenty of stuff from them, not one problem.

Hope that helps

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Sydneykid Grand Master

Sydneykid

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QUOTE ()
What abot putting s chip in, could that do it?

There are limits as to what you can do with the standard ECU, but yep it would work. But you have an R33, they are extremely tricky (ie; expensive) to chip.

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Sydneykid Grand Master

Sydneykid

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QUOTE ()
how much do u reckon im lookin at? after this im done no more money to the engine, unless it has to be done, and things always need to be done :)

Pretty much cheaper to buy and tune a Power FC and get it over and done with.

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