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Boost and knock issues? Or not.


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I finally got Freescan to work on my Esprit, and it showed a couple of issues that concern me.

 

First, as I suspected via the gauge, I am hitting the max boost (freescan = 2.24) very early.  AT 75% throttle and 4500 rpm I am already maxed out in freescan and on the factory boost gauge.  Is that right?  To date I've been backing off when I see the boost gauge pinned out as I don't want to pop the motor.  Are there fail-safes that will kick in if indeed the wastegate sticks or such?  Doesn't the ECU get involved?  I only hit the 2.24 for a bit before I chicken out.  Should I just be holding 'er down?

 

Second, I have knock counts of 3 or 4 on runs and in a few spots I had "knock retard" valves of 1.1 and 2.5 and even 4.5.  Is that typical?

 

My barometric was 0.75 as we are at 2200 feet altitude.  Motor 80C, MAT = 36C.  MAP=0.43 at idle.

 

1994 S4 with oversized chargecooler, secondary injectors and Walsh turbocharger.  But stock ECU I believe.

 

Those in the know, let me know.

 

 

 

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Sounds like the engine is overboosting.  The ECM is unable to control the boost pressure coming from the turbo.

 

Normally if the boost pressure is higher than required, the ECM will increase the pulse width going to the boost bleed valve.  This delivers a greater proportion of the manifold pressure to the wastegate causing it to open more.  Likewise the reverse occurs if the boost pressure is lower than required.  This feedback look is the primary mechanism by which the ECM controls the boost.

 

There are secondary measures the ECM will take when overboosting occurs in an attempt to reduce the additional stress on the engine, e.g. retarding the timing, descreasing the A/F ratio.  However, the effect of these is pretty minor.

 

The are a number of possible causes of overboosting:

1.  The wastegate flap is jammed open and not springing back to the closed position

2.  The bladder in the wastegate actuator is punctured and unable to open the wastegate flap when pressurised.

3.  The hose from the boost bleed value to the wastegate actuator has come loose or is cracked, split, punctured, etc.

 

I'd check these out to start with.

 

Cheers,

Neal

May: DON'T hit it with a hammer!

Clarkson: Why?

May: Cause it's the tool of a pikey.

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At your altitude, you will see higher boost levels on the dash gauge than the same car at sea level.

 

I live at 5300+ ft, and peg the gauge every time.  I also see 2.24 bar in Freescan.

 

The ECU does have some protection built in.  If the boost levels are too high it will try to open the waste gate.  Failing that, then it can cut spark and fuel.

 

Below 75% throttle, you should be limited to the mechanical waste gate limit of .65bar (mid way on the dash gauge).  You should check the waste gate adjustment, and set it per the manual!

 

Keep in mind that Freescan MAP sensor values are wrong, and you aren't actually seeing that much boost.

 

For knock, I often see 3 or 4 knock counts, on many different cars' data, so that is pretty normal.  Just as long as the knock counts don't keep increasing during the drive.

 

And that would be a John Welch turbo, if you are searching for info online.

Travis

Vulcan Grey 89SE

My Lotus Photo and Projects Album

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The ecu will start to control the wastegate at 37.5% throttle on the S4. With a non standard turbo the 'Wastegate Dutycycle' table and 'Wastegate Dutycycle Adjust for BP' function should be adjusted to avoid overboosting.

Also, as a minimum, the 3 volumetric efficiency tables and the EBPV VE multiplier table should be adjusted too.

Edited by sailorbob
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