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Dump pipe problems

nizmokid

By nizmokid,
in Engines & Forced Induction

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Abo Bob Grand Master

Abo Bob

Posted
Posted

http://www.skylinesaustralia.com/forums/at...achmentid=16174

This is my dyno readout. Check out that flatspot. Boost is good and even at that point so I guess timing is being pulled. Can anyone confirm?

Is there anything I can do about this?

Sydneykid Grand Master

Sydneykid

Posted
Posted

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. 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).

Abo Bob Grand Master

Abo Bob

Posted
Posted

That was quite helpful thank you. Except for one thing... I think you just told me that I'm going to have that flat spot forever.

Perhaps, seeing as I use an AVC-R, I should drop the boost at that point then bring it back up later. Or maybe not now but when I get a fuel computer.

Oh hang on. Applying that to our problems... By upping the boost we are raising the voltage of the AFM, ECU pulls timing to protect... so.... back to my last para... I should maybe drop the boost a little between 5600 rpm and 6200 rpm, thus dropping the voltage the AFM sends to the ECU, which doesn't see any problem worth protecting the engine from, therefore I should pick up power through more advanced timing.

Does this sound right?

Sydneykid Grand Master

Sydneykid

Posted
Posted
That was quite helpful thank you.  Except for one thing... I think you just told me that I'm going to have that flat spot forever.

 

Perhaps, seeing as I use an AVC-R, I should drop the boost at that point then bring it back up later.  Or maybe not now but when I get a fuel computer.

 

Oh hang on.  Applying that to our problems... By upping the boost we are raising the voltage of the AFM, ECU pulls timing to protect... so.... back to my last para... I should maybe drop the boost a little between 5600 rpm and 6200 rpm, thus dropping the voltage the AFM sends to the ECU, which doesn't see any problem worth protecting the engine from, therefore I should pick up power through more advanced timing.

 

Does this sound right?

Yes, but keep in mind that a good tuner will work around this as best he can. You sneak up on the problem rpm range, watch the AFM voltage on the multimeter (before and after SAFC), move the boost up and down, advance and retard the CAS timing, a little more fuel correction on the SAFC, a little less correction etc etc. Eventually you end up with the best settings you can get, but it is still a COMPROMISE.

The reality is that all this fiddling takes much longer to do properly than doing a full tune on a Power FC. You keep having to go back and change things that you had already set. That's why I am not looking forward to doing it on the Stagea, I just know it is going to drive me crazy.

Stick with it though, it is worth it in the end.

NickR33 Grand Master

NickR33

Posted
Posted

I have had this problem too however I think the tune on my car is quite good (I didn't tune it). I had the flat spot too but usually on cold nights when the boost was turned up. The 2 things I did that seem to fix the problem (don't ask me how) are:

1. Changed from using optimax to bp ultimate (not as convenient for me but hey its better)

2. Keeping my ebc turned off in cold weather and only turning it on when the car is well warmed up (ie driving for at least half an hour allowing oil to get to temperature, not reading the stock temp gauge... It lies!!!)

I am not 100% on why these things have worked for me maybe someone could shed some light?

The Mafia Grand Master

The Mafia

Posted
Posted

I have EXACTLY the same problems guys, so your not alone.

Except, by turbo boost at 10 psi no matter what i do. I can't wind it back at all...which is weird..

I have a Custom Split Dump Pipe, intercooler, and full exhaust.

And one mother****nig stupid ECU...

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