weird boost problem

[Vitwagen]

Hi All, help if you can please, I'm stumped!

I'm running an AAZ, 2.5" downpipe & exhaust, Mild Gov mod & boost pin (proper LDA on this pump) and Garrett T2 turbo.

I'm aiming to get 15PSI but whenever I get to around 3k rpm, the boost drops to 10 psi. Solid 15-16psi, then straight to 10psi. If I keep revving, it'll creep back up to around 12psi.

I've one of these ball & spring boost controllers fitted: ebay link

Any ideas welcome, I'm stuck!

Cheers.

Steve

[8v-of-fury]

Your waste gate is opening.. ?

What do you have the MBC set too?

[Vitwagen]

Without the boost controller? Dunno, the wastegate always opened at about 8-9psi. I'll disconnect it and see.

I've set the MBC to 15-16psi, and it'll see that until it hits 3K, then it plummets to 10psi.

 
 

[8v-of-fury]

That is the MBC doing what it is supposed to. How do you think it works? It makes it to 15-16psi and then the MBC pops and sends the 15-16psi to the 9-10psi waste gate. Waste gate opens and there you have it.

Did you figure when you made it to 15-16psi that your MBC would somehow stop injecting fuel? Thus remaining at 15-16psi?

Do not disconnect your waste gate. That will make the boost controller completely and utterly useless. You put the controller on there so you can control boost, now instead of making 9-10psi you make 15-16 psi. If you are not happy then you need to adjust the MBC to make more boost, or less fuel so it will only make 14psi and wont pop your MBC set to 15-16psi.

[745 turbogreasel]

You need to engineer a small  leak downstream of the MBC, once it opens, it pressurizes the actuator to 15 PSI, then holds that pressure there keeping your WG open.

[Vangruver]

I had a similar problem as well with my MBC. I had initially tried to increase the boost to a max 20psi to see where it would sustain consistency at. it's all about the tweaking and testing. finally fiddled with it enough to maintain a solid 16 in the end.

What gear are you using to test the changes? As I was told that a solid 3rd gear pull usually is best.

[R.O.R-2.0]

You need to engineer a small  leak downstream of the MBC, once it opens, it pressurizes the actuator to 15 PSI, then holds that pressure there keeping your WG open.

THIS ^ ^ ^

my leak is built into my boost controller..

[8v-of-fury]

You need to engineer a small  leak downstream of the MBC, once it opens, it pressurizes the actuator to 15 PSI, then holds that pressure there keeping your WG open.

THIS ^ ^ ^

my leak is built into my boost controller..

Same here.

[Vitwagen]

My leak is also built into the MBC, and the MBC functions perfectly up to 3000rpm.

It happens in all gears, and has exactly the same habit of dropping to 10psi, it does this at WOT.

Foot to the floor, revs hit 1.5k and I get 15 psi. keep the foot to the floor, and when it hits 3k, it drops straight to 10 psi.

I removed the MBC today, and the 10 PSI is indeed the standard actuator pressure.

[R.O.R-2.0]

sounds as tho you have a factory overboost protector on your manifold, and dont have it blocked off..

they open at about 15psi, and dont let you boost any higher than 10psi once open..

[gldgti]

problem could actually be that the wastegate valve is opening from exhaust pressure. Regarless what the actuator is doing, if the stock boost setting is low enough, exhaust pressure on the valve will push it open if the spring is soft enough.

If the wastegate has an adjustable preload (like by lengthening/shortening a rod, or by moving the actuator can a bit) then you can probably fix this by adding some preload to the actuator.

that way the exhaust pressure wont be able to push the valve open.

[745 turbogreasel]

What gear are you using to test the changes? As I was told that a solid 3rd gear pull usually is best.

I think that's crap, mine boosts the same in every gear, except the higher ones I can't build over 3 PSI engine braking....or maybe I have a shortage of places I want to hold my descent speed to 110+

[Vangruver]

What gear are you using to test the changes? As I was told that a solid 3rd gear pull usually is best.

I think that's crap, mine boosts the same in every gear, except the higher ones I can't build over 3 PSI engine braking....or maybe I have a shortage of places I want to hold my descent speed to 110+

I never said i live by that rule. I was just suggested to do my boost tuning in 3rd gear.

[rodpaslow]

This may be a stupid question, but I have a universal wastegate i use with my vnt, how much exhaust leakage will cause a drop in boost?  I imagine that will all depend on turbine/ hot side size, back pressure, revs, etc.

I have a similar problem the wastegate spring is for 7 psi and I think the exhaust pressure is pushing it open as I can only get about 8 psi without any boost control hooked up.  I'm going to add a spring and see if that helps.

[vanbcguy]

Re: the eBay boost controller... I had the exact same symptoms when I had mine connected backwards.  I'm assuming you have already disabled the manifold BOV as it would also do pretty much the same thing.  Can you confirm that the BOV is blocked off, and that you've got the MBC connected in the right direction?