S280 Weird Track Behavior

[cidsamuth]

Okay . . . so 2015 FiST, track dedicated. S280, ITG intake, Stratified 4-port, cPe charge cooler, Mishimoto radiator, etc. At my request to the tuner, boost is kept to 28 lbs, and that's were it sits nicely during the little bit it gets street driven.

On the track, it starts there just fine. But, over a 25 minute session, it slowly and consistently begins dropping boost, before it sits at 22-23 lbs. This happens every single time. It's consistent, and the drop is slow. Next time out after everything has cooled off, it's back to 28 lbs again.

I've taken two 25 minute logs at the same track but at different ambients. At 70 degrees ambient, charge temps stayed under 90 and coolant temps under 200. At 95 degrees ambient, charge temps got to 120 and coolant to 208. With both, the drop from 28 to 23ish lbs happened at the same rate, so it does not seem to be based on either of those temps.

Wastegate Duty Cycle Actual % is between 68-71% at WOT, high RPM, and max boost . . . regardless to whether that max boost is 28lbs, 23 lbs, or something in between.

I can find no obvious boost leaks. Even if it was a leak, it would have to only manifest itself when hot, and the amount it leaks would have to create a super consistent rate of drop and final floor for the boost. So, I don't *think* that's the problem.

Any ideas? My tuner has been great, and he continues to help me diagnose in case it's a tune issue. But, so far, we have both been stumped.

 

[agangel]

If it was me I would email another tunner and ask them, this seems like something in the logic of the tune. My tunner made my boost target variable depending on barometric pressure. So even at sea level I've been getting boost pressure from 23-25psi. Maybe Wastegate Duty Cycle Actual % needs to be more variable if boost is to stay at a solid 28psi? Wish I knew more. Interested in hearing what others have to say.

 

[cidsamuth]

Thanks. I have emailed another tuner. But, they can only speculate, and the speculation isn’t anything we didn’t already try. I’m hesitant to waste another tuner’s time, and I really don’t want to start over with another tuner if I can avoid it.

 

[agangel]

First I would probably put the stock airbox on and make sure it's not some weird heatsoke problem. Then I would check every bolt on exhaust manifold/turbo/downpipe. Then make sure boost solenoid is good or even put in a new one. Then I would get a new wastegate as the current one could be sticking when hot. Or last could be a bad turbo where the seals weaken when heated and cause low boost.