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Boost dropping off post throttle: Boost leak, ECU correction or ? - logs attached


Old 03-18-2017, 02:39 AM
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Default Boost dropping off post throttle: Boost leak, ECU correction or ? - logs attached

After some recent mods, I noticed by manifold boost pressure is not reaching target levels as the rpms climb. I found some very interesting info in this thread about the ECU effectively capping power. My car is reaching target boost levels pre throttle, but post throttle the pressure is bleeding off. I performed a boost leak test from turbo outlet to turbo outlet and everything was holding pressure up to 22.5 psi. The logs back this up showing pre throttle boost pressure at or above target levels, only after the throttle and at upper rpms does the post throttle pressure drop.
The turbos are capable of producing 20+ psi and the manifold is capable of containing it as witnessed by the log showing between 19 and 21 psi from ~3800 to ~4000 rpm as the pull starts.
2008 997 turbo cab 6MT
stock airbox
stock turbos
Cobb Stage 2 93 octane OTS tune
do88 inlets, ICs, and y-pipe
FVD headers and catted exhaust
Max IAT 77* F
0 knock retard

Full log here:

zoomed log here:

What say those wiser than I? Boost leak in the manifold? Weak/leaking diverter valves? ECU shenanigans?
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Old 03-18-2017, 04:16 AM
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I took a brief look at your logs..

Basically the ECU is calling/requesting boost at pressure levels less than 1.2 bars. The turbochargers are meeting the target levels. As you shift into 4th, 5th and 6th gear the turbochargers will struggle to meet higher requested boost levels. On this platform we don't have a constant pressure level requested thru the RPM rev range. The requested boost does vary between 1.05 to 1.2 bars..

Another point.. If the ECU load targets are met with timing.. There is ZERO reason why the ECU will overboost and supply pressure levels over the requested pressure.

Check your DV's and maybe another intake pressure test....

Last edited by Kevin; 03-18-2017 at 04:33 AM.
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Old 03-18-2017, 10:19 AM
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Post throttle body MAP value is modeled, not real. There is no MAP sensor in the intake mani like a 997.2TT. The only sensor is the pre-throttlebody one. What is funny is the deviation grows even larger if you pop a larger plenum in there, but at the end of the day this value is of little relevance.
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