Boost drop when hitting certain RPM

[alexandermf]

So this problem started right after I did my auto to auto swap. It happens almost all the time when I'm 9+ lbs boost, doesn't do it when I'm running low/wastegate. Didn't happen with last tranny but this tranny was in great condition and low miles so I think it could be something else as well.

Problem:
When going into power mode/downshifting from wot my boost will hold FINE until my rpm needle comes up on 5k. Then it'll just straight drop to around 2 ish lbs. While the boost needle drops the tranny seems to just wanna slip and afr is low 10s. I can still press the gas and get on it or use the 1,2,3 gears and it won't happen as long as I don't let it get to 5k.

Someone suggested my mbc might be dirty from oil and not compressing right at that rpm which makes the the boost drop like that. Its a hallman and only 20k on it maybe... Any other suggestions before I rip open my hallman to clean it? No codes from ecu or tcu and my car runs fine besides that. It'd be nice to figure this out so I can actually shift through gears properly with 9+ lbs of boost. Its quite annoying

[Legacy777]

If you aren't on full boost will the rpms go past 5k?

What happens if you dial down the boost 1 psi at a time via your hallman, does it get the same results? At what boost level does this issue go away? You can try cleaning out the MBC, but I think the issue may be related to air flow and maxing out your MAF sensor. What boost gauge are you using to read your boost level? What turbo are you running and what other mods do you have?

[alexandermf]

If I "manually" control it, It still boost drops as soon as rpm approaches 5k. Td04, hallman mbc, prosport halo series gauge. Yes josh I've dialed it in slowly with the mbc. Issue goes away if boost is set under 9lbs. Ive run fine before at 12lbs with same setup and same gauge. My car does like to eat MAFs though...I've gone through a couple before, but only for way off afr readings. I have it set now at 12lbs and runs fine as long as I don't push it past 5k....

[dankberries]

Is your factory waste gate solenoid still hooked up?

[Legacy777]

You said your car likes eating MAF's, do you have the stock intake setup or something different?

[alexandermf]

The factory one has been capped since I got the hallman roughly a year ago, josh my intake is stock box-->maf-->coffee cup

[alexandermf]

I've had to change the maf out twice, but the symptons were my car leaning way out at idle and almost wanting to die, never this issue. Idk why its 5k rpm has to deal with it...mind boggling to me. I literally can see my boost level at 13 running without issue as long as I shift it/or it shifts itself before 5k

[Legacy777]

Can you rev the engine above 5k when parked or not under boost?

[alexandermf]

Yes, it just hits rev limiter and bounces like it would normally. This problem only occurs when driving and over 9-10lbs boost roughly. Could have anything to do with tcu or tranny?

[Alphius]

This sounds like a physical problem, not related to ECU/TCU/transmission.

Is your silicone elbow collapsing under load?

[alexandermf]

Alphius, that actually was an idea floating around in my head, would it only be my silicon elbow from intake to pre-turbo or could it be something post turbo like silicon going to ypipe, etc? Ill check my silicon pieces Monday when I'm off, but if it was collapsing from boost load at 5k, how can I run 12lbs at 4k perfectly fine vs running 10-11lbs at 5k and boost loss

[Alphius]

Post-turbo piping is pressurized so it won't collapse. This doesn't sound like a boost leak.

As far as why it doesn't happen at 4k RPM even at higher boost, think of the air you are pulling into the engine:
12psi at 4k is 284cfm
10psi at 5k is 328cfm

So as you can see the turbo is pulling much more air from the intake tract at 5k RPM even at a lower boost level. This causes a slight vacuum in the pre-turbo intake, and if the vacuum is severe enough it can collapse the weak silicon piping, especially if the silicone is going around a bend (lowers torsional strength)

I would bet on your silicon elbow collapsing right before the turbo.

[alexandermf]

Makes perfect sense! Ill look into it tomorrow morning and report back

[alexandermf]

Spent last hour taking apart my entire silicon parts and inspecting them, no holes or rips didn't see any spot where they could be collapsing under high load. ONLY thing out of ordinary I found is the coolant hose going from wrx tank to turbo has a tiny leak/hole from my silicon hose going to the turbo. Could this be the problem? Its do miniscule that it coildnt have been leaking coolant and my silicon hose woulda been stuffed up against it. Here's a pic

[alexandermf]

Ended up pulling this coolant hose entirely off as it was fossilized rock hard lol. Ill get a new one on here soon. In the meantime still inspecting parts with no luck

[Alphius]

Coolant line would not change how the engine runs or cause this issue, the worst it will do is leak coolant.
The silicone elbow could still be collapsing under vacuum caused from airflow, that won't leave any visible signs at all.

There are a couple things you can try:
Make sure whatever intake pipe goes into the elbow pre-turbo is seated as deep as possible within the elbow.
Buy a stronger elbow from somewhere like siliconeintakes.com

One thing I've always wondered about with the collapsing intake problem is if you could use an appropriate spring on the ID of the bend to prevent it from collapsing? For instance, like the fancy collapse-proof radiator hoses that have the support spring inside them.

[alexandermf]

All my silicon is from siliconintakes, their warehouse is ten mins away also ok I resat and tightened everything so ill report back. Its just weird cuz this has been my setup for over a year and its been fine, and this all of sudden

[Alphius]

Silicone does get weaker with time and heat.

Here's my logic on this issue:
There's nothing in the ECU's control of the turbo or engine that can cause it to drop to 2lbs boost and still be running OK. Therefore, your issue has to be a problem causing reduced airflow. An intake obstruction under high airflow numbers is all that makes sense, and the only place in the intake stream for that to occur is at the bend into the turbo.

[alexandermf]

Well boost problem is still there. For now ill just run low boost, about to engine/tranny swap it so hopefully that'll just get rid of whatever it may be

[Alphius]

Dang, I was hoping it would be one of those problems that just disappears when you touch everything.
I hope the swap cures your problems!

[Alphius]

BTW, here's a thread where wtdash had the same issue.

http://bbs.legacycentral.org/viewtopic.php?f=2&t=39470

[alexandermf]

Wow crazy, same exact symptoms! Guess ill have to go down to siliconintakes and look at their metal piping fabrications, ill report back in a week or so, going elk hunting for the next week and taking the legacy with lol

[Legacy777]

Have someone watch the intake as you get it 100% throttle in neutral and see if the the intake elbow collapses slightly or does anything. If you have a go pro camera or something similar you could set it up in the engine bay to record what is happening to the elbow.

[alexandermf]

Well the problem was fixed, elbow was collapsing after swapping a buddies pre turbo intake and the problem disappeared. Went to silicon intakes and got a better fit elbow and some better reinforcement and all is well now! Thnx for all the help

[Alphius]

I'm glad we helped you solve the problem. Good fix.