Turbos: to boost or not to boost...that is the question.

[S4TAN]

Question for the experts out there:

Throughout the gear range is there a specific rpm at which the turbos should start producing boost? I assume that the ECU controls when boost is produced via the N75 actuating the wastegate.. (and i guess the ECU requests boost based on the rpm and other factors/measurements registered on it's various sensors - e.g. engine load? MAF? MAP? etc)...but would like to know if there is a specific rpm in each gear which is the optimal rpm for the turbos to generate boost (and i'm talking about stock K03's here with a custom MRC re-map)

Anyone know?

TIA

[MarkB]

As far as I know boost coming in is a purely mechanical thing. Spool up time is not controlled by the ECU

[jfeb]

I have heard 1800 rpm - which "sounds" about right to me as when I have my gear-stick gator and sound deadening off recently I could hear the turbos spool up at about this point (regardless of gear)

[Alps]

I have heard 1800 rpm - which "sounds" about right to me as when I have my gear-stick gator and sound deadening off recently I could hear the turbos spool up at about this point (regardless of gear)

... and by the time the RPM get to 3500 the turbo's should be in full effect?

Is this gap what is known as turbo lag?

[MarkB]

Yep

[Dr-Al]

It will depend on throttle position as well as revs.

1800 rpm in over-run will push much less volume through the turbos than 1800 rpm at WOT.

This is because, during over-run, there is little or no fuel going in, so it's mainly air passing through. Also, the cylinders are drawing from a manifold at ~0.4 Bar absolute pressure because of the closed throttle.

Once the throttle opens, the pressure in the manifold goes up to 1 Bar absolute (even if the turbos are not boosting) so volume of air passing through the hot sides of the turbos increases and they start to spool.

Also bear in mind that, in over-run, the DVs are open and the wastegates are closed so although all of the hot-side gas passes through the turbine, this does not build boost until the DVs close. The turbunes are essentially spinning free. Once the throttle opens, the manifold pressure goes up, the DVs close and the charge pressure builds.

[Dr-Al]

I have heard 1800 rpm - which "sounds" about right to me as when I have my gear-stick gator and sound deadening off recently I could hear the turbos spool up at about this point (regardless of gear)

... and by the time the RPM get to 3500 the turbo's should be in full effect?

Is this gap what is known as turbo lag?

If I floor it from ~1500 rpm, I get maximum boost at about 2200-3000 rpm. After that it slowly tails off.

[Dr-Al]

Is this gap what is known as turbo lag?

You also get lag when driving within the boost range if you have had your foot off the accelerator, then push it back down. In this case, it is not caused by the turbos spooling up (they are already spinning). It is caused by the time taken to fill up the intercoolers and tubing to the required pressure once the DVs close.

Obviously, keeping the volume of tubing and intercoolers to a minimum will reduce this type of lag.

[S4TAN]

Yo - the Doc is back! So how does the "ECU specified boost" thingy work.....what i mean by that is the ECU must be controlling something to specify boost in the first place - i thought this was controlled by the N75 solenoid controlling the opening of the wastegates....?

The reason i asked this question in the first place is that i saw an interesting thread on AW concerning symptoms of healthy and unhealthy turbos - it basically shows a graph plot of spec. boost against RPM for healthy turbos overlaid with a plot of spec. boost against RPM for unhealthy turbos; with the unhealthy turbos the boost is produced later in the rev range....

The thread is interesting as it can indicate symptoms of failing turbos before you get the audible warnings such as dentist drill noises/siren noises etc.....a useful diagnostic tool methinks (possibly) for vagcom users; plot your spec. boost against RPM from your logs and compare to the reference graph to see if you may have turbo trouble ahead....

[MarkB]

I ploted this sort of thing a while ago. My spec'ed boost goes up, the turbo's follow but then drop and then rise again to spec.

Don't think it's a turbo issue but it might be an N75 playing tricks

[MarkB]

Here's my original post and plots....

http://www.rs246.com/index.php?name=PNp ... st+problem

[S4TAN]

Don't think it's a turbo issue but it might be an N75 playing tricks

You haven't been a naughty boy and tweaked it have you...?

[MarkB]

not yet I haven't, maybe I need to

[S4TAN]

maybe I need to

Go on....tweak it, you know you want to....

P.S. Anti-clockwise.......BWAHAHAHAHAHahahahaha......

[flashyg]

3800 on ko4s

That is to 1.5 bar
G