My S3 experiences so far with driveability & diagnosis


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The story so far ...

I have been playing about with my S3 recently ... I've had it 3 weeks and it has the GTRS eliminator with FMIC, decats and Milltek and it just felt a bit laggy. Very quick above 3.5-4k revs but laggy below. I had to stir the gears up a bit to get the best out of it - and I guess this is just part of learning the power delivery characteristics of a new car. The car also appeared to be faster on part throttle than WOT especially when it came on boost.

I wanted to explore the laggy nature of the car and also understand the boost control mechanism and a little about the mapping process - my previous experience being Japanese turbos and a less sophisticated ECU boost control systems. I used to make and sell relief valve type MBC's so explored these a great deal but tend to prefer EBC such as the Profec A or Blitz DSBC.

Random drive in another S3 ...

A couple of Saturday ago I was at the Audi dealer buying the ambient light LED's and bumped into a local S3 8L owner - Carl (maybe he is a membe on here). He took me for a spin in his car and from the drivers seat it felt very peaky at low revs and rapid right off the bat. I understood it used a stock K04 turbo, stock intercoolers, induction and exhaust with a stage 1 map. I could also tell he was revving the car through the gears but all-in-all it was pretty impressive. It made me realise my driving characteristics and car were very different and so it was something I had to get used to perhaps, certainly in the way I drove my car and how it behaved. It was great to experience another S3 and chew the fat a bit.

All this got me thinking more and gave me the motivation to do some more research and understand my particular S3 and tune more ...

What was the car setup ...

The car was orginally tuned by star performance to 330hp in 2006 but quickly de-tuned (a few days later) due to it's aggresive nature. A recent dyno run in 2011 establihsed it was running around 210hp and so was upped to 265hp but I am not sure whether the current map is the de-tuned star performance one or an intermediate one.
I confirmed the N75 was fully working (electrical tests) but simply electrically plugged into the loom but disconnected plumbing wise.
The N249 was fully operational and connected to a Forge 007P diverter.
A forge Unos had been fitted as the MBC.
I boost tested the wastegate actuator and this was set around 1 bar, maybe 0.9.

The above setup seems fairly traditional from what I can see apart from the N249 still being operational.

What were some of my symptoms ...

Before I fitted a boost gauge (which I would always have in any other car especially with an EBC, but Audi S3's seem not to have them i think due to static boost settings) I was experiencing lag between 3.5 - 4k RPM and sometimes a drop in power and dumping of boost. I also thought I was getting compressor surge at some point as this normally sounds like a dump valve flutter. No codes threw up on my OBD2 reader (which I have) and I tried the things I'd researched like disconnecting the MAF (which rasied a code) and trying actuator boost only etc. I also fully tested and plumbed in the N75 fully to compare behaviour and feel. I also did all the boost leak and hose tests that I had read about - which tend to be fairly common Turbo issue checks anyway. Despite there being no codes and leaks, and after eliminating compressor surge, I suspected an overboost and some form of limp mode and saw some articles supporting the symptoms of dumping and throttle backing off but seeing no codes surprised me. The behaviours were ...

1. With MBC and N75 disabled - loss in power, laggy below 3.5 - 4k. Strong power delivery afterwards.
2. Actuator boost only - sluggish power delivery coming in stronger 3.5 - 4k.
3. With N75 enabled and MBC disabled - strong smooth power delivery, still laggy 3.5 - 4k, no power loss, but not as strong above 4k as configuration 1.

I strongly suspected overboost as I knew from the previous owner the car had been previously detuned from it's original 330hp to a more driveable figure and had a dyno plot for 265hp which was set at basically the max boost the detuned map would take.

Next steps ...

I bought a VAG-COM cable and downloaded VCDS-lite and found only 1 code, an intermittent overboost code - which doesn't show up on the OBD2 reader. I also fitted a boost gauge, I always intended to do this anyway but wanted to try and self-diagnose the issue blind. The boost gauge basically revealed the following for the above 3 scenarious ...

1. Boost was coming on from low RPM but was around 0.5 bar at 2k RPM rising to maximum boost around 4k. Maximum being over 1.5 bar as my gauge only goes to that at present. I could feel a drop in power as the boost surge kicked in and as it dropped the power returned - hence a flat spot due to a boost spike or overboost. I really need to establish the maximum boost at this setting which was 265hp on the dyno.
2. This was around 0.9 to 1 bar which explained the behaviour.
3. This spiked to around 1.4 bar and dropped back to 1.2 bar and thus not appearing to hit an overboost condition.

So where does that leave me ...

Well I accept the GTRS is a bit laggy and have found a few articles now which confirm this. To get the best performance I need to rev the engine a bit more into the 4k plus range - and the thing flies !!

Due to the boost spike and difficulty in getting the MBC set low enough to avoid the overboost I have decided to run in configuration 3 with the N75 connected and MBC disconnected. This feels very smooth and can hold 1.2 bar with a small spike. The lag is similar to that of scenario 1 and so is acceptable and the part throttle boost holding is superb - none of the on-off you get with an MBC or EBC (this was also an issue with Jap cars I found). For now I am happy with the OEM setup though as the car is so smooth to drive.

I have a Profec A and Blitz DSBC so may try these and see if I can explore the peak boost the map is coded for another option is the APEXI AVCR which is more tuneable in the rev range. I also have a set of 440 injectors and 'may' have the car remapped accordingly to release the beast but am now enjoying the car and feeling better for what I have learnt, largely from this site.

The above may help new owners who go on a mission of discovery like I did :)

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My other car is a MINI!!!!
Seeing as you know what boost it's running via the n75 now, next step I'd be taking is logging actual boost vs requested (block 115 I think), that'll tell you what the map is asking for and how near to that figure you are.

Id also log fuelling to make sure it's safe (block 31 I think) and airflow to get a rough idea of its current power output.

Te previous 330bhp map years ago, was that with standard injectors? Something sounds odd, as the stock injectors arnt capable of 330bhp.

Random question: it's not a noggy blue import is it?


Registered User
Badger5 - yes I want to get into all that logging business and compare requested v actual etc. Software engineering was my original training so it all floats my boat so to speak. I have been looking at what OBD software is available or whether to register my VCDS-lite as I don't think the unregistered version does logging ? Is there another way i.e. EasyOBD looked fairly good and with a free trial ? At a stretch I might consider the full VAG-COM kit if that's the best. The liquid gauge is also on my radar too as an 'in car' gadget.

Prawn - as per above, logs it will be as that seems to be so common with the VAG cars and great that the tools and expertise is available with you guys and the forums etc. It was originally mapped with 440 injectors and direct-port-tuning, I have the receipt for the original work done and dyno plots from Star so it all seems to stack up - I also briefly spoke to Jim who seemed to remember the car. I have some 440's to go back in if I want but think standard are in at the moment, partly why I don't want to run the car near to any of its limits until I know for certain - Unit20 are nearby to me in Neston so may take it there for them to have a look. Oh and BTW it's not the car you describe - good or bad.

I also presume so far that by mapping the 'requested' boost figures then whether you have an EBC/MBC/N75/N249 the ECU will simply try and match the 'actual' boost with 'requested' based on its other sensors and actuators. Personally I am liking the ME7.5 but it is pretty sophisticated (read controlling) so can see why people 'eliminate' some of the control systems LOL

And thanks for your replies and ideas :)



My other car is a MINI!!!!
An, fair enough, reason I ask, there used to be a member called ChriS3 who had a noggy blue s3 with a GTRS eliminator and all the kit you mention, mapped to 330 by star around 2006! Had a blue interior I think, and a variety of wheels (black rs4's and silver MTM's)

It sounded very similar that's all :)

If you're able to log airflow, divide the figure by 0.8 for a rough bhp estimate. If equations are your thing you can also use rpm from the logs with your calculated bhp to plot a torque estimate :)


Registered User
Thanks and interesting as I was gonna mention ChrisS3 as having a similar spec as mine which was done in November 2006 I think - I have read a lot of his posts in my research ... Been keeping an eye on your excellent build thread too Prawn :)

It seems to be, as with most things, you can't have everything and have to know what you want before you start out spending shedload of cash. If you want big reliable power with a relatively modest CC engine you are gonna have to install a higher flow turbo, introducing an element of lag and then in an attempt to raise boost quickly implement an aggressive boost control strategy and overboost risk. Not ideal for me as I really like the luxury and driveability of the S3 and especially with the N75 part throttle/boost control which is a rare for me on a Turbo car which I like. So for now I can live with stirring up the gears and revving the car a bit to release the power and still have it driveable. I am pleased, certainly with hindsight, that I bought a modified S3 with this kit on it rather than have to make the choice as to which road to go down and then pay for it all !