I have a 3.3 965 that's been converted to EFI using Autronic SMC.
I am using SC cams and B&B style headers. Injectors are RC Engineering 550 with fuel pressure at 5.5bar. Max boost is 0.9 bar.

There is a fuel cut on overrun to prevent over rich condition that causes backfire on overrun which works fine for overrun rpm above 2,000rpm. Once it gets below that, the fuel cut seems to be over-ridden and fuel comes back on and causes backfire on overrun between 2,000-1,500rpm. The vacuum pressure seems to the determining factor for this strange override - fuel comes back on below 500mm/Hg vacuum no matter what is set on the Autronic (at the moment fuel set to come back at 1,400rpm but gets ignored)

I tried re-installing the overrun bypass diaphram valve from the stock 965 engine but it makes no difference so I removed it again.

Any suggestions on how to cure the overrun backfire?

 

I use the Link ECU and software.

The Link software allows you to set the over-run by RPM and engine coolant temp. This allows you to cut the fuel at a higher RPM at a cool engine temp so you don't kill the higher idle for warm up. It also allows you to retard the timing when you come back on throttle from a closed throttle position.

I ended up turning the overrun off because a lean situation causes back-fire. I preferred to keep the injectors fueling the cylinders to keep them cooler. I now trim the fuel delivery by adjusting the pulse-width at the bottom of the fuel table (low MAP). My AFR for decel is now around 14 instead of off-the-chart high (more than 16.0:1). This has eliminated backfiring and provides my motor a smooth transition from decel to part-throttle.

Maybe this will give you some ideas on your system.

 

Hello ,
are you getting popping in the exhaust or a backfire through the intake on the overrun ?

 

My popping is in the exhaust and it is from over rich condition when the fuel cut comes back on prematurely ie. does not follow programming.

I have programmed fuel cut on overrun from 3000-1400rpm. Popping only occurs from 2000-1500rpm when apparently the fuel comes back on despite my programmin for it to come back on from <1400rpm

Before programming the fuel cut, I had popping from 3000rpm-1500rpm

I will try richer AFR to see if there's any effect although it seems to go against the overrich situation that I am facing now.

I guess if it were due to overrich situation, I will try to see if I can program the idle valve to open on overrun between 2000-1400 to replicate the function of the original overrun bypass valve which has now been removed (the stock overrun bypass valve is not programmable so I dont know at what vacuum it kicks in)

BTW, is the exhaust popping in anyway harmful to anything other than the ego when cruising to a halt at traffic lights? Had quite alot of stares as it really does sound like gunshots

 

Use the overrun shutdown commence and recovery. Set the commence to 2000-2200 rpm and the recovery to 1400-1800.

You can also try tuning the 30% or less load sites to 12.5-12.7 AFR and set the overrun ignition timing to a low number.

 

On overrun, the engine will be drawing the most vacuum of any of the engine running conditions. Most MAP sensors will read to about 20kPa and on occasion I have seen some of the 911 engines draw more than 20kPa (lower number) which is beyond what the sensor can read and it will default to the error condition setting (if you have programmed it in). On the overrun you want to make sure that there is little to no fuel going through the engine, if you don't and you come off the throttle after being on boost, you will have flames shooting out the exhaust. Running a rich mixture will certainly cause this to occur. The overrun will be the point in the map where the engine runs its leanest, so it is also where you will have the most ignition timing (40 degrees or so) since you need the ignition timing to light the lean mixture to ensure the cylinder has burned all of the fuel and is clean and ready for the new charge air when you step back on the fuel. I would check with the data logging to see what the vacuum is during the overrrun.

 

Prior to any fine tuning (using a base map) I had popping starting from 3000rpm down to 1500rpm.

Overrun shutdown com is now set at 3000rpm and rec at 1400rpm. No popping on overrun from 3000rpm to 2000rpm, but there is still popping from 2000rpm to 1500rpm.

Havin't logged on autronics but have a boost gauge with vacumm which reads 55kPa at stationary idle. On overrun it is beyond the scale of my vacuum gauge which maxes at 80kPa. As rpm drops so does the vacuum and popping starts at 2000rpm down when vacuum falls below about 66kPa

This weekend I will try playing around with ignition timing and load numbers on overrun.

I haven't connected up the idle control valve yet (so cold start is bad for about 15 seconds where I have to depress throttle a little to hold rpm at 1000 afterwhich engine holds idle well). I am wondering if I can somehow trigger the idle control valve (which bypasses throttle butterfly to allow air into the manifolds) to open on overrun to cure the suspected overrich condition.

No flames on letting off the throttle after full boost. That has been tuned out earlier

Can there be potential damage from all this popping? It;s loud and like gunshots. Need to know if safe to track the car this weekend

 

Geoffrey,
You are spot on on this, as usual. ,cheers, Sonny.

 

Totally agree.
Actually on vacuum loadsites, use as many load bins as you can, it's more important to have high resolution at pre-boost than on-boost area.
Most start-up maps starting at 20 or even 30 kPa, but you should reconfigure load sites starting from 5kPa (and set fuel cut under 5 kPa).

 

Good points about the resolution of the load sites. I see much the opposite with startup maps starting at 0kPa. I rarely see the engines run lower than 20kPa so I use that as my lowest value. I have seen them go lower, but the resolution there isn't going to be helpful since the difference between 20kPa or even 25kPa and 10kPa will be the same engine running condition with the same fueling requirement since it is overrun and the fuel is off anyway if the overrun feature is activated. The GM MAP sensors don't read lower than 18kPa and the one in the Autronic unit is a 2.5bar absolute (1.5bar of boost) piece that I don't think is nearly as good as the GM one.

 

So why is my vacuum reading so different from the ones mentioned by you guys? Is there an absolute and a relative pressure scale?

Geoffrey, I tried what you suggested:
1) Fuel Cut at 2400rpm, Fuel Rec at 1400rpm
2) Overrun Ign at 40 down to 2000rpm; 32 at 1750rpm and 25 at 1500rpm
3) Overrun TPS load at 2%

I still get popping though it seems a little less severe. Less of widely spaced loud pops and more gurgling (ie. improvem,ent). Gurgling also stops at about 1600rpm when before it stops at 1400rpm.

 

Actually I use VEMS management with it's integrated 2.5bar MAP (4 Bar also as an option) and we got vacuum readings as low as 10 kPa on Audi 3.6 V8 PT engine (on a startup map starting from 20 kPa the cursor was actually off the map on 3D window ) Checked the MAP sensor offset to be sure, and it was fine.

 

OK I think I figured out why my vacuum numbers are so different from you guys. You guys are talking absolute pressure whereas my meter shows pressure difference from 1 atmosphere. Since 1 bar=100kpa, hence::

1) my car idles at 45kpa
2) Overrun at high rpm vacuum is higher than 20kpa ie. smaller number (off my meter's scale at -80kpa from 1 bar)
3) Vacuum reduces with rpm and at 2000rpm it reads 34kpa when popping begins

My installer called autronics who advised that at vacuum less than 30kpa (ie higher number) overrun fuel cut is defeated regardless of what overrun rec rpm is set. This is to prevent stalling??

So my problem seems to be that even though I set Overrun Rec at 1400rpm, at about 2000rpm the vacuum is less than 30kpa (higher number), fuel comes back on regardless of my setting.

As my car is currently idling at 950rpm, I think I can reduce throttle butterfly opening at idle (0% load) to lower idle rpm as well as increase vacuum on overrun (due to smaller throttle opening) such that the 30kpa level is breached hopefully only below 1600rpm where I dont have anymore popping. Does this make sense to you?

 

Flew my tuner in and overrun popping solved. Solution was a combination of what you guys mentioned above especially more resolution at the lower revs