Bad boost controller?

[X|Crescens]

I have just finished building my car for the time being and my modifications are as follows: Garrett T28 Turbo, Evo III o2 Housing, 3" Downpipe, 3" Exhaust, FMIC, 1g Throttle Body, 550cc Injectors, 255 walbro, SAFCII, Innovation Wideband, e85. I have some boost issue, I have a ball and spring boost controller, and it seems no matter how I set the thing, it wont stay at a constant boost. It seems to spike high when going through 3-4k rpms, then drops significantly. It will spike up to 22-25 psi, then after abour 4k rpms, it drops to about 12 and stays there. I want a constant 22 psi. Is this because of a bad boost controller, or mayeb because of my turbo size? Any input would be appreciated.

 

[No_Skillz]

A boost spike is pressure that will shoot for second then fall down to what you have your boost controller set at. This is mainly due to incorrect setup of the vacuum hoses.

What you are describing seems like it is set to about 22-25psi, which falls off as you climb rpm. First you should check for boost leaks. The t-28 is on the small side and is more sensitive to boost leaks. It will also be overspinning to try and compensate.

 

[jtmcinder]

Isn't an Evo3 O2 and 3" exhaust just begging for creep with a t28? Was the wastegate ported?

- Jtoby

 

[zippyshoe]

I have just finished building my car for the time being and my modifications are as follows: Garrett T28 Turbo, Evo III o2 Housing, 3" Downpipe, 3" Exhaust, FMIC, 1g Throttle Body, 550cc Injectors, 255 walbro, SAFCII, Innovation Wideband, e85. I have some boost issue, I have a ball and spring boost controller, and it seems no matter how I set the thing, it wont stay at a constant boost. It seems to spike high when going through 3-4k rpms, then drops significantly. It will spike up to 22-25 psi, then after abour 4k rpms, it drops to about 12 and stays there. I want a constant 22 psi. Is this because of a bad boost controller, or mayeb because of my turbo size? Any input would be appreciated.

Make sure that you do not have your vacuum lines reversed to your boost controller (one to your boost source and the other to the wastegate actuator.)

 

[X|Crescens]

The Evo III is ported, and the downpipe is not a true 3". It is 2 3/4" before the flex. Well the boost controller is "T'd" off with the vacuum on my Bov, is this not a good vacuum connection? I reversed the connections to see if I initially had the reversed to start with. Something even stranger happens now... In a roll from second, I'll WOT, and the boost will stay at a constant 10psi and creep a little to 11-12psi through the higher band. I shift to third and punch it and the boost instantly skyrockets to 25psi. Is there something else that could be the problem?

 

[jtmcinder]

When you say that the EvoIII is ported, does that include the wastegate hole? Porting the inlet, but not the wastegate, can make creep worse.

With that said, I'd be more inclined to borrow a known-good boost controller first.

Nothing wrong with tapping the line to the CBV. That's where mine is.

- Jtoby

 

[No_Skillz]

I shift to third and punch it and the boost instantly skyrockets to 25psi. Is there something else that could be the problem?

Does the boost stay at 25psi or drop off quickly? Tapping the BOV to intake manifold line has been associated with boost spike. You might want to try hooking up a line to somewhere closer to the boost source, like tapping off the compressor cover or on an IC pipe close to the turbo.

 

[kenamond]

I agree that you should run your MBC from the compressor outlet to the WGA. This is the feedback mechanism for controlling compressor outlet pressure, and the closer your feedback signal is to the compressor outlet, the faster and more accurate the response. If your compressor outlet has a nipple on it, run that to the MBC instead of the BOV line. If not, search the forum about adding a nipple to the outlet of the compressor housing or the compressor end of the outlet (IC or J) pipe.

If you still see problems, try running a vaccuum line straight from the compressor outlet to the WGA with no MBC in the loop. I don't know what the WGA actuation pressure is for that turbo, but it's probably between 8 and 10psi. Then do a test run. If it still shoots up to 20+psi, you may have a bad WGA or stuck/binding actuator arm or flapper or you may have creep.

You can test the WGA with a vaccuum/pressure tester. Pressurize at the WGA nipple and note what pressure is required to fully extend the actuator arm. If it is much higher that 10psi, check to see if something is binding:

Check that the WGA arm is not interfering with the exhaust manifold heat shield or something else.

To check if the lever on the turbine housing is binding, remove the WGA arm from the lever and try turning the lever by hand (don't do this on a hot motor, or you'll fry your fingers).

If nothing is binding, see about getting a new WGA.

If all of that checks out, you can check for creep by removing the WGA arm from the lever on the turbine housing and doing a test run that way. The flapper should be free to open, and you shouldn't build more than a few psi boost. If you do, you most likely have creep. If it doesn't go up past a few psi, you may still have too much restriction in the WG exit path: the flapper may only open a fraction of the way when the WGA arm is connected to it, but when you take the arm off, the flapper can open more than it would with full WGA extension. If you can jerry-rig a way to hold the lever at the position it would be in if the WGA arm were fully extended and do the test, that would be ideal, but that might be easier said than done.

Creep is caused by the inability of exhaust gas to adequately bypass the turbine wheel. The WG opens to permit a controlled fraction of the exhaust to bypass the turbine wheel, thus limiting the boost on the compressor side (just the right amount of exhaust goes through the turbine wheel, and the rest bypasses it). If the WG passage doesn't flow well enough, not enough gas gets bypassed, too much goes through the turbine wheel, and your boost "creeps" beyond the desired level. Fixes include porting, improved wastegate (external - requires special exhaust mani or modded exhaust mani - or aftermarket internal - requires removing stock WG flapper, a special O2 housing and maybe a special DP), or mods to safely run 20+psi. There are plenty of threads about all of these solutions if you deduce that creep is your problem.