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| 01-23-2012, 06:42 PM | #1 |
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Major General
 
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Interesting news from COBB
Thought you guys might like this info. In case no one knows the car is Dzenno's car with FBO + RB + Meth.
HOT: N54 Fuel Limitation found in the DME operating system!...RESOLVED! First some background: As many of us know the N54 tuning has always mentioned fueling as a bottleneck and its been discussed many times on this and other boards. At first injectors were to blame, then HPFP, then recently the LPFP but some also alluded to fuel lines, etc. As piggyback tunes are/were more the more predominant choice of tune since the N54 came out we basically know today that there are definite limits as far as what you can do with fueling with a piggyback. This isn't much of a concern on stock turbo setups as those have proven to run reliably for many hundreds of thousands of miles on many N54s around the world. The issue starts to really become apparent as power is pushed past what the stock turbos can provide. Piggybacks use 2 things to control fuel: 1) fuel pressure (aka open loop, where based on certain load params fuel pressure is raised to hit richer AFR target) 2) front o2 wideband sensor voltage biasing (telling the engine its running leaner than it is so it injects more fuel through closed loop control). Piggybacks cannot change the following directly: 1) Fuel injector timing/pulse width/duty cycle 2) DME load based "logic" for fuel control -> STOCK N54 FUEL LIMIT IS BURIED IN HERE! The fuel limits we were seeing on piggybacks are neither fuel pumps, nor injectors, or fuel lines. The issue lies right in the DME's operating system logic. I'm just a messenger obviously but I'll try to explain what was just discovered by Rob and the boys at COBB in N54 DME fuel control logic. First a bit of background on RB Turbos and COBB: I've been running this flash for a while now with the JB4 stacked on top for additional boost when meth is flowing. The issue I encountered was that AFR would start to lean towards 13AFR towards redline on their most aggressive stage2+fmic map. Adding more boost on top with the JB4 would make this even more apparent. The way I dealt with it was to up fuel pressure using JB4's fuel pressure feature which would take care of it to a point but really it wasn't ideal as with considerably upped boost AFR really didn't look as great as when flash was working on its own. This puzzled Rob@cobb as the fuel handling is 100% closed loop on this car and the DME "should" be correcting for this, obviously provided that the fuel system wasn't hardware limited in some way. What they realized is that the DME has either a feature, a bug or a deliberate limitation imposed right in the operating system code to do with MAF calculations. Here's what Rob@Cobb had to say about this finding while testing with RB turbos: Originally Posted by Rob@Cobb I think I found a bug, feature, or very odd deliberate limit. I'm betting it's not the last one, but I digress. Fuel is staying where it should be when running the stage map I have been running. Several of the internal calculations use MAF, a calculated value. Two different calculated MAF values are compared together to create a calculated pressure that, way down the road, hits the fuel calculations. In short one of the values was being clipped above 290ish g/s. I fixed the clipping and all is well now. Basically the stock DME will not allow for more fuel past a certain point as it is not programmed to do EVEN when the tables are setup correctly, unless the logic for MAF calculation is fixed. With all that said, I got a new map from Rob to try on the car to see if the AFR stays on target without upping fuel pressure using the JB4..and it DID! Flat 12AFR across.. This current race map runs higher boost than stage2+ (20psi down to 16-17psi at 6k rpm 15.5-ish at redline), stage 2 timing curve (6deg-10.5deg timing up top). On the Cobb Mustang dyno this was good for 440whp/465wtq on pump gas and 80/20 meth using the Aquamist HFS-3 kit and dual 1mm nozzles. If you take 8% (very conservative) correction to a typical Dynojet this is about 475whp/502wtq, at just 16-17psi at 6krpm where peak power is made so lots more room left. I was/am impressed at the numbers with such timing and boost curve. This is a huge win for N54 tuning in general and definitely a huge step in the right direction in terms of fueling the N54 for some BIG power numbers. Now that we know AFR is staying on target without drama its time to start cranking boost up and squeeze RBs by the ballzzz Cobb says that RB turbos provide for a cooler charge than stock turbos which makes sense given larger turbine and compressor and that more power is on tap...we're planning more boost, more timing will be added only for high octane as well... N54+Cobb 1, DME limitation 0 Great job to Rob and all the guys at Cobb! |
| 01-23-2012, 07:23 PM | #7 |
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Colonel
 
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Good to see the DME crackers mining away. It's only a matter of time.
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.-=[ Kenny ]=-. 1999 BMW M Coupe 10.775 @ 134.35 mph w/1.600 60' (Best 136.07 mph) 25th August 2004. +2010 X5 35D+
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| 01-23-2012, 07:25 PM | #8 | ||
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Quote:
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| 01-24-2012, 04:05 PM | #15 |
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Hey Guys,
It is great to see that Cobb are making inroads on their fuel limitations, but the information regarding piggybacks in that article is incorrect. The piggybacks never hit a mass air limit. This is because they tell the ECU it has less mass air then what the engine is actually running. As a result they never get mass air clipped as the mass air never gets high enough to be clipped. I am sure that this is indeed an issue for flash tunes, but the limit they found is not a limit that causes any issue for the piggy back tunes. Cheers, Adrian |
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