Normal Boost Peaks on an ALH

[catlin_cava]

Whats the MOST Boost I should spike on the gauge under load at Full Throttle??? I burried the boost gauge passed 30PSI a few times so far

[R.O.R-2.0]

thats about what my car would do..

40 and your gonna start lifting heads.

[Vincent Waldon]

Perhaps, but 30 psi spikes are not normal for an ALH with a properly functioning vane control...and VNT-15s have a less-than-stellar track record of handling big boost spikes over the long haul.

Clogged vanes or a corroded actuator are the usual causes...the actuator is available as a replacement part,

[the caveman]

when checking with measuring block, you are supposed to see just over 2000 mBar, so some where just below 30 psi. Once above that you will light up a code. As Mr. Vincent says, those model VNT's have a habit of either the actuator [vacuum can] or the vanes seizing up

[the caveman]

Make that 2200 mBar -so 30 lbs.

[Vincent Waldon]

And that's absolute pressure...to think of it in boost terms you need to subtract barametric pressure at your location.

Normal operating boost for an ALH is around 15 psi, with spikes 17-20 briefly.

My Jetta with the new turbo controls boost perfectly with only a couple psi overshoot.  My other Jetta with the 10 year old turbo overboosts by around 5 psi.

[the caveman]

And that's absolute pressure...to think of it in boost terms you need to subtract barametric pressure at your location.

Normal operating boost for an ALH is around 15 psi, with spikes 17-20 briefly.

My Jetta with the new turbo controls boost perfectly with only a couple psi overshoot.  My other Jetta with the 10 year old turbo overboosts by around 5 psi.

Of course you are right. But then my question is [ as i personally have little experience with a boost gauge]: will a gauge show real or absolute pressure ?

[cyrus #1]

All of the gauges I have seen read gauge pressure.  ie: They read 0 psi at rest and about 15 psi under full load.

[Vincent Waldon]

Of course you are right. But then my question is [ as i personally have little experience with a boost gauge]: will a gauge show real or absolute pressure ?

Nah... there's no "of course"... I just know that this tends to confuse the hell out of folks when they are learning the TDI side of the universe and start messing around with what the computer is seeing.  "OMG I'm supposed to see 15 psi but according to VCDS I'm peaking at 30!!!"   VCDS can be confusing enough as it is.   

The car's ECU is concerned with absolute pressure since it's trying to figure out the right amount of fuel to burn for the amount of O2 available... so the sensors are absolute,  as is VCDS's readout.

The driver, on the other hand, is usually more interested in what he's asking from the turbo above today's resting or atmospheric pressure... so "boost" is a generally relative reading.

Analog boost gauges tend to use a differential pressure sensor so that they adjust to the local barometric pressure.  Some electronic boost gauges do the same, others use an absolute sensor but calibrate themselves to local barometric pressure when you first turn 'em on.

To confuse things a bit further, if you were a pilot you'd also be more worried about air/fuel and so as I understand it turboed airplanes have absolute boost or pressure gauges rather than relative boost gauges.

[R.O.R-2.0]

ok, so then my turbo, even being on the wrong engine, and not being controlled, is still within perfect operating range.. AWESOME!

[Vincent Waldon]

Perfect operating range is a nice even 15 psi with a very brief couple of psi overshoot under extreme circumstances ... at the top of this thread you seem to say you're doing 30+ psi.

Then there's this thread:

i run 25 psi all the time..

its seen spikes of 40,

So, in the interests of clearing up the confusion... which is it?

[catlin_cava]

I killed something its been gutless all weekend but boost seamed normal all weekend..till on the way home I was running 130 the whole time I solwed down for the tolls. Pulling out she spiked 30psi and held then limp mode 0 boost

[R.O.R-2.0]

Perfect operating range is a nice even 15 psi with a very brief couple of psi overshoot under extreme circumstances ... at the top of this thread you seem to say you're doing 30+ psi.

Then there's this thread:

i run 25 psi all the time..

its seen spikes of 40,

So, in the interests of clearing up the confusion... which is it?

it sees 22-25 psi fairly often.. most of the time i keep it closer to 15 psi tho..

on my last engine, with horribly retarded timing, and a more cranked up fuel pump, it would make 40 psi pretty easy.

i didnt literally mean that every time i boost up, im doing 25 psi. in the big picture, i bet it sees 20+ psi less than 10% of the time..

unless im passing like oh, a dozen cars at a time, it usually just gets to 15 psi or so. it really takes alot to get those last 10 psi out of the turbo.. now, on this engine, it wont make 40 psi, it barely makes anything over 25..