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Old 08-16-2013, 04:45 PM   #1
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Default Why is my boost so saggy?

Ok I've been fighting boost sag for awhile and figure it'd be time to consult the forum.

So I used to sag with my old set-up which was as follows:
MS2, no VVT control
Log manifold
2.5" straight through exhaust (only one muffler)
Ebay T3 turbo 50 trim compressor, 0.48 A/R turbine
Vac/boost source for the turbo was after the throttle plate

I never got to test this set-up with VVTuner because I blew up the turbo before I had it. But I would sag badly. I could hit boost cut which was 216kpa (16.5ish psi) but was at 8-10psi by redline.

So then I got my Turbonetics T3/To4e with 54 trim compressor wheel and 0.48 A/R turbine. I seemed to hold boost better but wasn't hitting full boost till 5000+ rpm's.

I then got VVTuner working. With everything else the same and DIY's base map I was spooling faster and holding boost perfectly.

Now I've got a new manifold (think S4 but upside down). This got me another 500rpm earlier spool but now I've got boost sag. I've since added a 3" exhaust (also just one muffler) and no change.

I've messed with MBC's, VVTuner, and finally I moved my wastegate source to the cold side of the intercooler.

No I've got a boost curve almost identical to my first setup (log + chinese turbo) I'm currently hitting 12.2psi by 4800rpm but quickly drop off to 8psi.

So my question. Do I suck at tuning that much that I can't hold pressure? Are my homemade parts so crappy they can't flow that much? Or am I just being hampered by a wastegate that can't hold that much pressure?

Right now I'm leaning towards the waste gate. It's just annoying how many things I've changed and that I'm still boost sagging.

Any idea's would be greatly appreciated,
Thanks,
Patrick
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Old 08-16-2013, 04:58 PM   #2
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Have you tried EBC? Ms2 EBC works pretty good afaik. I'm about to give the ms1 boost control a shot (god help me), but I think EBC could help.

Otherwise, what does the turbo run just off wastegate? I've heard a good rule of thumb is to try and run no more than 2x what the WG canister will do on its own. My t25 wastegate opens at like 5psi, and I'm not sure If I'll ever get it to hold more than 10.
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Old 08-16-2013, 05:09 PM   #3
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I'd check for boost leaks and exhaust leaks first. then the wga. then see if its the turbine.

post your msq and logs in the meantime
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Old 08-16-2013, 05:13 PM   #4
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No EBC yet but I'll have that in the near-ish future. I'm not sure what the max is. I have a MBC (ball and spring, also used a bleed type) in line but apparently it makes no difference. With the wastegate rod fully threaded on I hit boost cut (206kpa) ~15psi then I dialed it back a lot and have been working my way up again.
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Old 08-16-2013, 05:20 PM   #5
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for whatever its worth, I had a similar issue (not as drastic tho) with my godspeed 3076 on the previous car. changed the 7psi wg to the 14 and it got better, but still remained. Nothing I did improved it so I just bumped timing up top and that seemed to keep the power flat up there.

I blamed the tiny turbine
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Old 08-16-2013, 05:21 PM   #6
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Quote:
Originally Posted by 18psi View Post
I'd check for boost leaks and exhaust leaks first. then the wga. then see if its the turbine.

post your msq and logs in the meantime
Would an exhaust leak cause a pressure drop? I know I had an exhuast leak (pre-turbo) before and I would hit peak boost by like redline. I think I have that taken care of though. I shall post some logs.
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Old 08-16-2013, 05:31 PM   #7
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Logs. The one that says no EBC means MBC. Also you can see the difference between having the mbc and not having it (ball and spring style). Also the pulls were 3rd gear on the same piece of road.
Attached Files
File Type: msl vvt tighter wastegate mbc.msl (869.2 KB, 89 views)
File Type: msl 3inch vvt old wastegate can mbc.msl (1.23 MB, 66 views)
File Type: msl vvt no ebc tight wastegate.msl (770.3 KB, 65 views)
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Old 09-18-2013, 09:05 PM   #8
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I'm resurrecting this thread because having massive torque then nothing past 5500 rpm sucks. So I don't remember if I had fixed this before or after I started this thread but I found something interesting.

When I pulled my exhaust manifold it was clearly leaking exhaust. I had had it planed before I put it on but the machinist had a hard time with it because its funny shape. He recommended double gasketing but I did not do that. So running my hand across the flange there is a massive bump that pretty much runs down the middle. I tried to go find another gasket but everything was closed. So I slathered everything in gasket maker, put it together and let it sit over night. This finally got rid of the noise I was hearing in boost but didn't fix the sagging.

Is it possible that as pressure increases it pushes past the gasket and sealant and causing a pressure drop. Or, that as it heats the flange is warping enough to create a leak? I think it must be wastegate related because it always seems to go back to a certain pressure.
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Old 09-18-2013, 09:09 PM   #9
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there is no gasket in the world that will fix a warped flange.

machine both surfaces perfectly flat, reinstall and enjoy
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Old 09-18-2013, 09:19 PM   #10
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Quote:
Originally Posted by 18psi View Post
there is no gasket in the world that will fix a warped flange.

machine both surfaces perfectly flat, reinstall and enjoy

Lol I forgot to say I'll be taking the manifold to another shop to get it plained. I was just surprised that it stopped making a noise when I sealed it.

I will report back when it's done.

Thanks!
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Old 09-22-2013, 04:04 PM   #11
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Ground and sanded that modo till it shined. I'll take it out later just wastegate but I'm not optimistic. Also found I'm consuming lots of oil but I don't blow smoke, leak or anything. So I'm gonna try and find a leak down tester I can borrow.

Anyone have a spare wastegate actuator I could borrow?
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Old 09-22-2013, 10:21 PM   #12
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Didn't help. Anyone have a spare wastegate actuator I can borrow?
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