Slight loss of peak boost

[Dippy]

Driving back from the dealer who did my MOT, I noticed that I could not reach the same peak boost that I could before. I used to be able to get to 1.25-1.3 bar on WOT quite easily. I even recorded a 1.35 peak once.

However on the way back the best I got was 1.16 bar (although I didn't get a chance to really push it).

On the way to work this morning I had a couple of very good runs and it peaked at 1.25 bar.

Now although this is hardly a huge drop, I know my car and it is different. I do not feel any loss of performance (mid-range torque is just as good as it was) and there are no DTCs.

So I'm left wondering if either:
1) It's down to a much higher ambient air temperature (I got to work at about 8.30am, and went back out to check for DTC at about 10.30, and noticed that the air intake temp on idle was still as high as 60 degrees!)
2) I have DV problems

Any ideas?

[JonnyX]

I would say it's because of the ambient air temperature. I believe it's used in the
load calculations and affects timing and boost. Try logging group 3 and see if
you MAF readings are also lower.

[sitas3]

I carried out some datalogging on block 003 last night on my car and the values are indeed slightly down, but still respectable (well for my S3 anyway!). Boost looks to be about the same though.

Dippy, I have a spare pair of hyperboost DV's if you want to try them to see if your current setup is causing a few probs.

Are there any abnormal fluctuations in the actual boost values?

[tartan_rob]

Also think about air pressure, its quite high just (hence the nice weather) as such your ECU may be compensating accordingly.

[tartan_rob]

Also think about air pressure, its quite high just (hence the nice weather) as such your ECU may be compensating accordingly.

[JonnyX]

Also think about air pressure, its quite high just (hence the nice weather) as such your ECU may be compensating accordingly.

Thats a joke, right?

[tartan_rob]

What? weather or air pressure?
Weather is a joke - yes.
Air pressure = no. The ECU uses this in its calculations. Idea being in a low pressure environment, such as driving over a high hill in low pressure the turbo's don't destroy themselves trying to get max boost pressure achieved.

[gavsteryoung]

So thats what the Altitude item if for when looking on the VAGCOM. It all becomes clear!

[moktoya]

I think it is down to ambient temperature.... My boost gauge has a peak record function so I have been keeping an eye on my peak boost. Through the first 5 months this yr the peak registered was 1.41bar... Summer has rolled around and we are mostly at low 30's temps during the day, and my peak boost in the past month was 1.51bar. (Running conditions were very similar as all were flat out runs performed early in the morning.) I have been told that the ECU is simply making the turbos work harder to churn out the same (or at least similar) output during hot days, hence the higher boost, which i think makes perfect sense. All this is not very technical i know, but thot it'd be interesting to share.....(and i know my boost levels are catastrophically high for 03's, but that's how Mr. Mayer makes his money i guess)

So Dippy I assume your lower boost scenario happened on a relatively cooler day compared to your high boost scenario. Correct?

[tartan_rob]

Moktoya - how do you get K03 boost levels that high?

[Dippy]

Thanks for the responses. Last night on the way home I got a peak of 1.3 bar, so it looks like its OK. However I will log my MAF to check.

My scenario is actually that the lower peak was on a hotter day. However I'm sure if it's as simple as that I would have noticed it on previous summers.

I also suspect my DVs. I'm going to check them at the weekend, however I've nearly decided that I'd prefer to get new ones (they are the old design Forge valves), and am veering towards getting Forge serviceable diaphragm valves. (Thanks for the offer anyway Sita).

[moktoya]

MTM stage 1, that's it. Also a Milltek Turbo back. A mate runs the same software with MTM turboback, also peaks at 1.5 under hot weather.

[KayGee]

What gauge are you using with peak hold? electrical or mechanical?

[KayGee]

Altitude/atmosperic pressure does make a difference to performance. Due to wastegate being sealed at @ 1 bar or @ atmospheric pressure. if you are in a region with lower atmosperic pressure than this you lose the difference as wastegate will open that much lower. ECU doesn't compensate for this.

It does compensate for ambient temp and IAT. You should see less boost in cold weather than hot which is the opposite of what dippy was reporting.

It was probably due to the IATs if the car had been on a test drive at the MOT station and was seeing hot IATs. VAGcom logs against requested/actual and MAF readings should show any problems with DVs or MAF.

[UKS4APR1]

Just my twopeneth!
Agree! Hot weather gives me a peak boost around the 1.4 bar mark & in the sub-zero winter I was getting peaks as low as 1.0 bar (chipped obviously)
This makes me wonder about the guys claiming how much better their cars perform in the cool air, it's all in the mind with S4's as the boost is reduced when there is lovely dense/cool air?
The intercoolers will be more efficient I suppose.
These cars are far more sophisticted than the old "Cossies" you know!