Boost Falling off up top

[VR4coop]

My car is running well except Boost is falling off up top after about 5500 it falls from 25psi down to 20, not quickly but steadily up to 7k. I check my 38mm tial wastegate spring and I think it should be fine. It is the green one which is rated at 13psi, I heard you double this for what it is will hold for boost, so 26. I also added some shims to my Forge BOV which the spring in that was rated from 15-23 and I put two shims in it. So that should be fine.

I have done a few boost leak tests and nothing major has come up. I just had the TB rebuilt(www.throttlebodys.com)and compression is good across all cyl 175 psi ( Wiesco 9:1).

I am going to do another thorough boost leak test i guess. Do you think even a small leak could cause boost to fall like that??

The turbo is a Slowboy, ya i know, GT12, has a 55lb wheel. A few ppl have hinted it to not flowing enough to keep up. I find this hard to believe, but could it be the case?


Thanks guys
Clint

 

[CutlassJim]

*Copied from original post.*

Your probably just hitting the limits of the turbo causing your boost to fall off. I've seen 2.0's with cams make a EvoIII drop to 20lbs at 7-7.5kRPM. Your motor is just sucking in too much air and the turbo can't put enough out to keep the pressure up. Kind of like a small fuel pump with big injectors, if you need a comparison.

Give us more info on your set-up. If your running a 2.3 or 2.4 this "situation" is even worse. I consider this a good thing as you know when you should upgrade your turbo. Too many people run HUGE turbo's when they really shouldn't be.

 

[VR4coop]

Things to note.

2.0 with 9:1's
Stock head and cams
SlowBoy GT12 56 Trim turbo Turbo Specs
Extrude Honed stock manifold
Evo 2 exhaust manifold
Water/meth injection
26 psi

All supporting other mods

 

[slugsgomoo]

have you tried free boosting it to see if it holds higher (unplug wastegate).

Honestly i wouldn't be suprised if that exhaust housing is choking it a bit up top. /ubbthreads/images//graemlins/dunno.gif

 

[Dialcaliper]

What oxygen sensor housing are you running, and what turbo/cat-back, intake, intercooler and piping? (all other supporting mods is a bit vague in this situation.)

The exhaust manifold is probably fine. I believe the Evo 2 is the same as a 2G - is the outlet into the turbo ported? The other potential bottleneck could be the stock cams.

Edit: I looked up your "member's ride"

Do you have a MBC with that TiAL wastegate, or are you just running a spring? You might try a stiffer base spring, and then reset the boost controller. With all the other hardware you have.

I'm going to guess that it's either the wastegate opening too easily at high boost, or more likely, running stock cams with a moderately aggressive setup.

The other possibilities are a problem with a restriction between the turbo and throttle body causing pressure drop, or else your cams. Specifically, your exhaust cam is not flowing enough air to push the turbo at high RPM. 264's or 264/272 would probably be a decent choice if you don't want to jump straight to 272's

Come to think of it, I'd definitely say cams at that boost level.

 

[Nate]

If you're using an MBC that requires the little brass bleeder after the MBC before the WG, make sure that bleeder piece is still in there.

 

[VR4coop]

I am running a mbc with the wastegate. It is nothing special, just your typical $10 one. I have been told cams a few times, I will look into this over the winter. The car is pretty much done for the year.

You really think that O2 housing is choking it up??


Let me tell you though, She still goes like HELL!!!!!


Thanks guys

 

[VR4coop]

Quote:
If you're using an MBC that requires the little brass bleeder after the MBC before the WG, make sure that bleeder piece is still in there.




not sure what you mean.

 

[Nate]

If you have an external wg dumping to the atmosphere, then the O2 housing isn't causing that problem.

As for the bleeder thing, some of the older Hallman MBCs had a little brass double male fitting that you placed in the vacuum line that went between the WG and the outlet of the MBC, and it had a little hole drilled into it to bleed off boost so that the WG wouldn't be held open once the level of boost fell. If it's not in there, it can hold the WG open.

 

[VR4coop]

It's a no name brand MBC. I am going to change it out with a know working one. I am leaning toward the problem being a stock head. But I will try the cheap stuff first. haha

 

[Dialcaliper]

No, not the O2 sensor housing - I wrote that before I looked up your car in the member's rides. I was wondering if you were still bolting up a small housing with the dump pipe or something.

I'd probably try upgrading the MBC first, then a stiffer wastegate spring/readjustment, and if that doesn't fix it, think about cams. What intercooler are you using?


Quote:
I am running a mbc with the wastegate. It is nothing special, just your typical $10 one. I have been told cams a few times, I will look into this over the winter. The car is pretty much done for the year.

You really think that O2 housing is choking it up??


Let me tell you though, She still goes like HELL!!!!!


Thanks guys

 

[VR4coop]

The intercooler I bought used off a friend. I know he ran about 24psi through with no problems.
It is 24x12x3 core.

Configured like this:

 

[atc250r]

I would be suspicious of the $10 MBC. I went to a Profec last year and it was money VERY well spent. The turbo spools quicker and hits harder than it did with my $50 Hallman MBC.

John

 

[belize1334]

If your MBC doesn't have a bleed or a checkvalve it won't hold boost as RPMS climb. The reason for this is that in many situations you need the wastegate to close slightly as the RPMs climb in order to hold boost. W/o a checkvalve or bleeder the pressure can't escape from the wg actuator so the wastegate doesn't "know" that boost is falling off and it just stays open. Replace your mbc or install a checkvalve and see if that helps.

 

[VR4coop]

My quick drawing of what i think you guys mean by a check valve.

Does this seem right?

 

[belize1334]

The checkvalve allows pressure to bleed from the wg back to the source if the pressure at the source starts to fall off. It only goes one way though so it won't allow pressure to bypass the mbc. Many people do this with a bleed valve that just vents to atmosphere. It's basically a small leak in the mbc and accomplishes the same task, though the checkvalve works better IMHO. Either way, you need a way for the wg to get rid of that excess pressure (trapped between the wg and the one-way mbc) if the source pressure starts to fall.

 

[Galantvr41062]

I would get a better MBC, it is not the head, Brent Rau ran a 7.3x @ 19x.x mph on a stock head. The cams and exhaust housing are a week spot in the setup, but not the problem. If you miss match a turbo, to big of a compressor wheel and to little of an exhaust wheel or A/R on the housing you can loose boost that way, you end up with to little exhaust energy to drive the compressor. Or if exhaust manifold pressure gets to high you can hit a wall on boost or loose boost up top.

~John

 

[grocery_getter]

A lot of cheap/homebuild MBC out there wont have the tiny bleed hole drilled into the output nipple. So easy yet it's the small sh*t that makes the difference.

 

[VR4coop]

Thank you guys for all the help, I will let you now how it goes when I get it resolved

 

[VR4coop]

Well, I said I would post when I got it resolved.

Today, Yes a year and a half later I got it resolved.

A old school greddy profec B was part of the solution along with a big and small green spring in my Tial 38mm.

Put the greddy in last summer and car would only boost to 23 psi. Put in a helper small green spring (Spring chart) in the Tial last weekend. Car runs as at least 30 psi ( gauge only goes to 30 right now).

So my setup now is. Greddy profec B (old school), the Tial 38mm with a with both a large green (13psi) and a small green (7.25psi)

Hope this helps someone so they don't scratch their head for a year and half like me.


Thanks
Clint