Here is some info i found on the MKv website
Thought i would post it here
Happy reading
Here's the explanation of the fuel cuts as described by Keith@APR
Well, here's the situation with FSI's and recalibrations.
Typically, VAG turbo engines, 1.8T, 2.7T etc. have plenty of fuel available to realize the full potential of the oem turbocharger. This is not the case with the 2.0T FSI.
The FSI uses a rail pump in addition to the intank pump to deliver fuel AND pressure to the fuel rail. This rail pump is cam driven and moves a piston up and down to force the fuel into the rail. Deisel engines have been using this method of fuel delivery for many years. The rail pump generates pressure behind the fuel injectors and when the ecu tells the injectors to pulse or open to inject fuel into the combustion chamber a small amount of pressue and fuel is lost from the rail and the rail pump must then force more fuel into the rail to bring the fuel volume and pressure back up to spec. Simple.
Basically, because the rail pump is driven by the cam shaft, the rate at which the rail pump is able to rebuild pressure and volume in the rail is determined by how fast the cam shaft is spinning or forcing the piston in the rail pump up and down. So the limitation in fueling for the FSI is greater at lower rpm's and in the midrange than in the higher rpm range where the cam shaft is spinning faster.
Now you could say, well at higher rpm you are also using more fuel so why isn't the
problem occurring there as well? The answer is because the rate at which the fuel pump delivers fuel increases beyond the engine's ability to consume fuel with the oem turbo at higher rpm's. Basically, the turbo provides too much power in the low and mid range than in the upper range as matched to the rail pump. The turbo runs out of breath still in the higher rpm's so fuel demand is less. Peak hp numbers are made up top as a product of torque over time where the rpm's are greater. More rpm's, more torque over time accumatively, less immediate demand on the fuel delivery.
In the low and midrange, the fuel demand is all in torque or immediate requirement over less time. The rail pump simply can't get to a great enough speed to deliver the fuel that is demanded at peak torque rpm's. This is a very brief and momentary demand in relation to a sustained continual demand that hp requires in the high rpm's.
So what happens is when the ecu is calling for more fuel pressure a very temporary situation can occur where the pump is not spinning fast enough to provide the requested fuel pressure in the low and mid rpm's. This results in the ecu enabling a safety feature that cuts spark and fuel temporarily to allow the pressure in the rail to build back up. This is the fuel cut some are experiencing. This is an electronically controlled safety measure that in no way means you are running lean or any threat to damage of any of your engine components may occur. The ecu is merely asking for more time for the rail pressure to build back up.
Now knowing this, how do calibrators still extrapolate the full potential of the turbocharger in the low and mid rpm's?
There are a couple of different ways and philosophies to accomplish what everyone wants, the most power possible.
APR's engineering team chooses to follow the recommendation of Bosch, Audi and VW engineers of maintaining the oem specified air fuel ratio of 10.5:1. The reason for this requirement is that direct injection engines run a higher compression ratio over standard injection engines due to the benefits of FSI which results in higher exhaust gas temperatures. Turbocharged engines have always resulted in higher egt's that wear the head, exhaust valves, exhaust manifold and turbocharger itself more rapidly than a normally aspirated engine will. Engineers have combated these higher temperatures with sodium filled valves, high nickel content manifolds and inconel turbo housings, etc. The best strategy for controlling high egt's however is richer afr's to cool everything down in lieu of expensive parts that oems can't typically provide at a reasonable price. So unless you plan on changing out your valves, exhaust manifold, turbocharger and even the headgasket, you can only maintain safety to your components by maintaining the OEM specified afr's.
What this means to APR clients is that APR is doing everything within the reccomended specifications of oem teir 1 suppliers and VAG/Bosch engineering reports to ensure no deterioration of life occurs to any of your engine components. This results in greater demand of the rail pump and to maximize the potential of the turbocharger at low and mid rpm's, fuel pressure must be maintained and this is much more difficult at the oem recommended afr spec as opposed to merely leaning it out and watching the egt's climb but no fuel cuts will occur.
Other companies have chosen to lean out the air fuel ratios to as much as 12.5:1 to avoid the fuel cuts. This results in egt's that are well above Borg Warner's specification of 970deg cel max preturbine egt's. This spec was put in place by BW for VAG engineers to ensure no loss of life to the turbocharger. During BW's testing and development of the turbo's found on VAG FSI engines, egt's outside of their spec was proven to show accelerated wear and even total failure well before the normal lifespan of the turbo as per its design requirements from VAG.
EGT's are so important to FSI engines that Bosch, at great expense, integrated an egt model into the ME 9 ecu to calculate preturbine egt's. If the egt's climb out of spec as per the ecu's model, the ecu then dumps fuel to cool everything down for a period of time until the egt's come back to spec. This is known as the hardware protection map to most engineers familiar with Bosch EMS.
APR engineers find themselves walking a fine line between maximum power output and safety. APR WILL NOT lean out afr's that result in egt's outside of the specification. APR WILL NOT provide an irresponsible product to the marketplace.
This philosophy results in a difficult position in the marketplace as our competition chooses to ignore these basic engineering principles and recklessly provides products that are known to result in loss of life of your engine. They don't hit fuel cut because they are running leaner mixtures that result in less fuel demand and therefore less demand on the rail pump at the low and mid rpm's. EGT's however, are not safe at these afr's. APR recalibrations will always maintain safe egt's and if they do climb out of spec the hardware protection map will kick on and bring everything back very quickly. Our competition circumvents or disables hardware protection and allows the egt's to continue to climb.
This is not merely speculation or arguable. It is proven fact. To satisfy yourself, please data log lamba, egt's and fuel pressure. This is especially concerning for track enthusiasts that find themselves hot lapping during driving events or participating competitively. The street enthusiast may be able to get away with higher egt's for longer periods of time as typically you are at wot in relatively short bursts where on a road course you are wot almost the entire time. All client situations must be considered and please know that with APR calibration you will enjoy safe operation in any circumstance.
Now that we understand why the fuel cuts occur you may ask well, what is APR doing about satifying the concern as fuel cut is not fun.
We have discovered opportunities in the ecu to increase fuel pressure from 110 bar to 130 bar. This results in higher overall fuel pressure so the demands in the low and midrange can be satisfied by having more pressure so when the pressure dips due to the demand it will not dip as far and fuel cut will be avoided. This is a relatively new discovery that is currently being integrated into all of our calibrations. In the USA we have been offering a "beta" version of our stage 2+ programming that incorporates the additional fuel pressure and fuel cuts are greatly diminished and only occur very rarely with significantly more power delivery in the low and midrange. The testing is complete and we are now working to provide new files with absolutely no fuel cuts but with almost all of the power available to be had by the oem turbo.
Further development has resulted in a new higher capacity rail pump that will be the ultimate end all be all to the fuel cut
problem. The addition of the APR rail pump results in 15-20 more hp and 20-25 more lb. ft. of torque over the beta file in the mid and low rpm's and the oem turbo is fully optimized.
"