Boost sag / boost tails off
 

Hi guys,

I'm suffering from a case of boost sag. I suspected the car wasn't holding boost to the redline, and I've gone and confirmed it with a data log.

The problem is boost builds steadily to 16psi at 4200 rpm then drops to 14psi at 7000 rpm. This was a 4th gear pull. :vash:

Setup is a Garrett T28, Ball and Spring MBC, signal taken from after the intercooler as per the sticky and the car previously held boost way better than now. FM2 cast manifold, BEGI SGSSDP, Idle valve has a one way check valve and mini air filter on it.

The actuator is a new 7psi unit from Garrett, and prior to this it seems to hold boost a lot better.

Is my turbo on its way out? Should I be tightening the rod on the wastegate actuator? Is the actuator simply too weak to run over double its original rating? Anything else I should be checking?

Try tightening the rod

Then my next question is ... Will tightening the rod directly increase the amount of boost I'll be running? My guess is that the MBC should still hold the same amount of boost, but would like to confirm.

Technically it should not affect peak boost, butttttttttt: It most likely will, so just to be safe turn down the mbc a little then tighten the rod and log the changes.

FWIW looking at your log the sag really isn't that bad, worst case you can just taper up the timing past peak torque towards redline to make up the couple lost hp

leak somewhere?

That's possible too. Check for leaks and also check to make sure your bov is set up correctly (if its adjustable). Tightening my forge helped my car hold boost up top when I had this issue

A leak isn't beyond the realms of possibility, but I'm showing no other signs of one. Idle is still around 900 rpm as usual etc.

All the pipes are flared and tightened down as much as possible but it's not impossible I guess. I'll set the boost cut then give the rod a few turns then see how I get on.

The only difficulty is that if this solves the problem, it's surely just a matter of time before it stretches and I need to do it again.

Also regarding the BOV, it's possible the spings I've put in have relaxed a little in the past few months and need a helper shim.

The joys of turbo motoring huh?

Actually the more I think about this, the more I think you're right.:idea: The car dumps a lot better than it did when I first installed the spring in the BOV. I no longer have any compressor surge, which was a small problem before when the car held boost.

Maybe it's just slackened off ever so slightly and it's now *just* too weak to hold it closed at 15-16psi. I've got the spring tuning kit from Forge so I'll try and experiment with a stronger spring.

Unlikely this is your problem, but mine used to lose boost at high RPM, and it turned out to be the alternator. Slip rings were worn, above 5000 RPM it couldn't even make 30 amps, so the system voltage sagged to the point (like 10.5 volts) that the EBC stopped working and I was down to mechanical boost.

--Ian

he's using an mbc

I installed a stiffer spring into the BOV today and even put 3 shims in there to the extent where the valve was struggling to dump. Despite this, the datalog shows the exact same problem; making 16psi on a 4th gear roll and the boost tailing to 14psi by redline.

Next step is tightening the actuator rod to add some pre-load. I guess that the spring in there is simply too weak to withstand 16psi and is the flapper is gradually being let open despite not being told to open by the MBC.

Make sense?

Update:

Tightened rod almost all the way in and went for a drive. Went over 20psi so obviously backed off. It looks like I'll have to wind down the MBC to compensate for the pre-load on the actuator.

So you learn something new every day :0)

I bet boost hit harder (built faster) when you added that preload...

Now I'm really confused.

I added the pre-load to the actuator and found I overshot my target boost level which is 15-16 psi. I was quickly making over 20psi of boost which was obviously not ideal if I want to keep the engine in one piece.

Since then I've unwound the MBC until a level I was happy with (15.9 peak in Megalogviewer) BUT now the boost drops down worse than before to late 12's.

WTF?? :vash:

So to summarise:

* Changed to a stiffer spring in BOV - No effect
* Added preload to the actuator rod but left MBC alone - boost went too high
* Kept the preload on actuator but unwound MBC - boost now tails off worse than before.

Now I'm really confused. Any opinions on what I should try next?

I'm beginning to think I should have ordered a 14psi actuator in the first place.

How long and what ID is the line that runs from the MBC to the top of the actuator?

The line is not very long. I have the MBC zip tied to the water outlet at the front of the engine (in between the engine and radiator). At a guess that makes it around 15-20inches long and I think it's 8mm hose. What makes you say that?

Just curious, as your wastegate and MBC are only things controlling boost (assuming no other leaks). Basically you are balancing the pressure/forces on the wastegate actuator, between the spring force (preload on the actuator) and boost pressure (source from your MBC to the backside of the actuator). When trying to hold boost at high RPM, its the source from the MBC trying to hold the actuator shut, which is why you saw more taper when you added more preload and backed off the MBC. There may be a sweet spot in between the two settings you tried, the hard part is setting it up so you dont over boost in transient conditions (ie spool up).

Boost taper when using a MBC with a boost target much higher than the waste-gate spring is pretty common, sometimes there is only so much that can be done with the way a MBC operates in keeping a wastegate shut. The DSM guys have been messing around with it forever, but usually at much higher pressures. Switching over to an EBC solved the issue for me on my old car

Also, you are almost aways going to see some taper at higher RPMs. The engine is simply moving much more air, so you dont "build" as much boost. That's not to say you are moving any less air into the engine, it just means there is less restriction after the turbo