Stalling under hard decel - Ford MAF / 1000cc injectors

[Z from Japan]

Here is the setup: R32 GT-R, T72 turbo, 1000cc injectors, high lift 264/272 cams, 100mm throttle, 4" IC piping, big FMIC, Ford MAF...


Okay so I got sick of the HKS VPC + GCC and installed the Ford Lightning MAF sensor into a 4" pipe before the turbo (T72). Getting a starting K value to work with was simple enough and away we went. Car ran very smooth other than some big variances on the A/F ratio - I'm thinking that the VQ map in Nistune might not be a good match for this since it is in a 4" pipe.

Anyways, the car dies after a hard decel. I can hear the woosh of air reverting back through the turbo and my A/F guage goes extremely rich right before the car stalls. Is there any way to tune around this? Maybe with the fuel cut / recovery?

I really don't want to run a BOV, or move the MAF to another location.

So, is there a way to get the car to cut fuel when I let off the gas?

May I be at the point where I need to go to a standalone ecu.....????

[Eric]

you will probably need a ROM with custom/modified code to iron this problem out (completely), but a better and more simple option (IMO) would be to get rid of the MAF and switch back to MAP again...by using a more modern version of the VPC, eg. a MAP-ECU or MAFT-pro

Does it happen below specific rpm's ?
If so, you could try to change the table at &H7F40 (CUT) and maybe the table at &H7F50 (RECOVER)
These are the tables that handle the fuel cut/recovery with closed throttle.

the tables are scaled coolant temperature vs. rpm.

eg. when the engine is at 80C, the throttle is closed and the rpms are > 2000RPM (CUT-table): fuel will be cut completely.
As soon as the rpms fall between 2000 RPM and 1400 RPM (RECOVER-table) fuel delivery is started to be recovered again..
When rpms get below 1400 RPM, fuel delivery is fully recovered and back to normal values again.

you could try to play around with the values in the CUT-table, to see if this helps.


Hope this helps.

-Eric

[skylinegtrhr]

Sorry for little O.T. but anyone try this together with NIStune?

http://www.rhdjapan.com/hks-eids-pro-el ... ssan-14247

[Z from Japan]

My Wideband is on the fritz so I can't try this out until I get another O2 sensor but..... could we crank up the values in the areas affected by the throttle? That way, when you let off the throttle on a hard run, the shutting of the throttle would help bring down the excess fuel on decel.

[FHCRSky]

Only way I can see to solve the problem (even though you don't want to do this) is run the MAF as a blow through setup (on Ford Lightening weld on fitting that's mounted on intake piping between throttlebody, intercooler). Might be more tricky to tune, but should solve the problem of having MAF infront of turbo and can run engine without BOV.

Here's some upgraded Ford Lightening MAF's, blow through weld on fittings for Ford Ligthening MAF (one of these links was posted by someone else on this forum) :-

Higher hp MAF's (up to 825rwhp) -

http://www.sctflash.com/BigAir.php

MAF adapters for blow through MAF sensor -

http://www.vmptuning.com/store/index.ph ... nt=31&pg=1

Pipe with blow through MAF -

http://www.atomicperformance.com/browse ... Meter.html

From what I understand, reversion would happen when turbo slows down during gearchanges or backing off throttle quickly (due to lack of BOV) and air trapped in turbo's compressor blade + air reversing direction when throttlebody closes would head towards MAF infront of turbo and MAF makes ECU run engine rich on deacceleration. The idle stabiliser mentioned smoothes out the signal from MAF that looks like signal from a CAS on a oscillioscope when reversion happens. Basically it stays where it should be on ECU side. Electronic way of solving the problem, but not the cause.

Another way to eliminate or reduce reversion, would be to fit a Synapse BOV that is a pull type piston BOV (doesn't leak under boost) and reacts quicker due to not using a slow reacting diaphram. Synapse BOV stops turbo slowing down too much via removing the air that reverses direction when throttlebody closes and eliminates or reduces reversion that is caused by turbo slowing down alot.

This videoclip from Synapse shows what I'm talking about at 3:50+ -

http://www.youtube.com/watch?v=M2--IIMSa9A

Note how the engine recovers quickly when BOV works properly and when it doesn't work properly (not venting the air that's reversing direction when throttlebody closes), engine struggles to recover.

[Bernardd]

I had a similar problem with 720cc inj and a z31 ecu/lightning maf. To correct it I lowered the max tp scale below the fuel recovery rpms to allow the car to run as needed at that load level at a stoich afr as this is a very low load area. Viewing the fuel map showed that the on decel the ecu was seeing nearly full load at throttle release. You will notice the maptrace shoot over to the lower right of the fuel map and follow the bottom two rows back to the idle area. Those bottom two rows need to be leaned out drastically to help out with afr's going too rich.

[BlueRB240]

Whats the max TP you guys are hitting? I can only seem to do TP of 45 @ 16 or 18psi.