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Author Topic: General opinion and question about overboost and rail pressure deviation  (Read 623 times)
henpaiu
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Hi everybody,

I have bought a S3 8P about a year ago, been running great, seemed like a stage 1 tune on it (unknown tuner) considering what the previous owner told me. Car was stock with regards to hardware(only cats removed by previous owner), I added the bigger intake from RAM couple months back. Now I decided to do some logging today and see how the trims, and the boost levels are and I can see that the car is overboosting to about 2700 mbar @ 3000RPM. Fuel trims seem to be single digits except when this overboost happens. Also, timing is quite low from what I've read around the forum (it goes down to 0, -1.5 even) so the car is most likely pulling some timing. An aquaintance of mine took a look at the bin file and estimated based on what he saw that he values in there are for a stage2+ tune but I guarantee that the car is not pulling like a stage 2+ car( did a run with a S4 3.0T 333hp stock and even though that car is heavier they ran the same up until 120mph.)

Now the questions:
1. I've been wanting to add a WMI system on the car for a couple months now as the temps around here are quite high in the summer time, not really tune around WMI but just benefit from the added cooling and octane boost overall. Do you guys think that based on the WMI install alone(no software change) the car will get back up to good timing values resulting in a increase in power?
2. Do you think based on the rail pressure deviation that the car will benefit by just calibrating the radius stroke and displacement for an upgraded HPFP, again resulting a increase in power?
3. Why does it overboost that much and is this current tune safe? (this safety thing I must say isn't necessary because I pounded on the car for a year winter and summer, up the mountain and at sea level, and it never let me down once but I just want to know your opinions)

I attached the unmodified log down below but I must say that I missed some logging blocks like requested and actual lambda, IAT temps, and injection timing. I can add those a little bit later once I finish my day job.

All your responses are greatly appreciated and I thank you for taking time and looking through my logs  Grin
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EanDem
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« Reply #1 on: September 23, 2024, 07:19:19 AM »

2,7 is higher stock boost - so seems something already messed with CAL. Timing topic is more complicated - but depending on fuel numbers you have can be right ones. S4 and S3 different platforms - so comparison is heavily aspirational.

WMI may prevent knock - but whole usage strategy has to blended with CAL.
HPI wise - why not MPI?
Over boost is bit blind statement - are you sure it is not boost requested in tune?

Cheeers!
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aef
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« Reply #2 on: September 23, 2024, 09:41:31 AM »

it may overboosts because of the missing cats.
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prj
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« Reply #3 on: September 23, 2024, 05:43:37 PM »

1. If you want to tune anything, use real logging. Yes, not free, but one time purchase and can use forever.
2. WMI - good idea to add a lot, not a little. E.g. Aquamist with 2x 0.9mm nozzles should be good for you. If you don't know how to implement mapswitching in the ECU based on rear o2 input, then you can use the relay to cut the boost solenoid when the WMI has a failure condition. That way you don't blow your stuff up and you can still drive the car.
3. Overboost is probably because of bad tune. But I didn't look at your log. If the throttle is closing, then the boost before the throttle will be sky high, but it's actually not reaching the engine. However 100% the ECU is tuned because stock request is like 1.1 bar IIRC and 1.7 bar is instant limp mode.
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henpaiu
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« Reply #4 on: September 24, 2024, 04:30:27 AM »

Thank you all for your information, I logged the car again still with vag-com because I do not posses any other tool at the moment and I noticed something really weird.
I checked timing retard on all 4 cylinders and the car is pulling anywhere from -6 to -4 degrees of timing which I think is really bad (I need your opinion on this), also, rail pressure really dropped this time in the mid area all the way to 77 BAR!!!! and fuel trims reaching around positive 20-22% which got me thinking that there might be some issue related with the low pressure fuel pump.

So I logged the LPFP after and I noticed that the pressure drops as the RPM increase all the way from around 5 bar @ 2000 RPM to 2.2-2.3 bar @ 6900 RPM.
I know from the bin file that the requested fuel pump pressure should never dip below 3.0 bar so now I wonder considering all of the above, in what kind of trouble am I?

Yesterday I stated that everything seems to be safe as I pounded heavily on the car with almost no issues but now I am starting to fear that something bad might happen.
I changed the spark plugs and coils today, also replaced the cam follower (it was fine almost no wear), to my shame, the fuel filter has never been replaced since who knows when(I never changed it, could it be clogged and cause a loss of pressure as well?). What do you guys think? Could it be a dying LPFP or something else?

Also @prj can you please suggest some better logging tools for the MED9.1?

PS: Logs attached + spark plug picture
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prj
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« Reply #5 on: September 24, 2024, 06:52:25 AM »

I recommend to flash the car to stock and start again.
Also LPFP should not drop at all like that, but it's not causing high pressure to drop.
It is clear the tune has disabled all safeguards. No limp mode on high boost, no limp mode on underpressure etc.

For the current aftermarket industry standard in logging check my signature.

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EanDem
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« Reply #6 on: September 24, 2024, 10:00:53 AM »

It looks like you trying solve bucket of issues in wrong way. As already suggested - flash stock and log car to check hardware is working correctly. For any issue found there is way faster to find cure if any.

By the way - take look to brass fitting going from HPFP to MPI rail - in stock shape it has tiny hole where drilling to 4/5mm will solve your issue with pressure.

Cheers!
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