Well I'm back from round 2 of spring adjustments.

This is my problem:

I've began these 2 sessions with the MBC set at it's LOOSEST setting, which will allow the WG to open and hold boost where it sees fit (like the MBC isn't even there). My car is exactly the same as it was with no MBC. The car pulls well and holds boost in all gears at 5.5psi-6psi with NO creeping (ported WG).

BUT, as soon as I tighten down the MBC's spring, I actually begin seeing LESS boost...? Like it'll vary from 7psi to 2psi??? On average around 4psi. The MBC does nothing except have erratic and mostly LOWER boost, until it's set at it's TIGHTEST setting (finger tight)... Then it'll boost up to 8psi+ until I let off in fear of fuel cut... After I tried some fully tightened runs to see if it was a consistent high boost (I'm pretty sure it was), I loosened it about a thread or two (on the bolt), and I got the same inconsistent boost!!! What the heck?

What would happen if the vac lines were put in backwards? Or, maybe I just have a really weak spring that doesn't do anything until nearly fully compressed?

I heard that it takes very little 'twisting' on an MBC to cause the boost to raise or drop... Should I be more gentle in my adjustments?

 

what type of MBC is it?

 

this is the reference I used when making it:
http://www.geocities.com/chmwatson/FAQs/mbc.html

The only thing that might be different when comparing mine to his is, I don't know my spring rate.

I installed my RB FCD so that I don't have to worry about boost cut when doing my test runs.

I set the tightness of the MBC screw to fully tightened (finger tight). It would boost consistently, but it had very slow boost response. It won't start spooling until past 3200rpm's, then the boost will start creeping up as the RPM's climb, until hitting between 10psi and 12psi around 5k RPM's. It doesn't really pull hard though, because the turbo spools so slow... The max boost I hit was 12psi in 3rd gear (getting dangerous), but it would drop down to like 8psi or so as soon as it spiked...

I tried loosening the screw in very small increments, and although it would cause the spike's to be a little lower each time, it started getting inconsistent like I mentioned before, where the boost would go up to 7psi+ in 1st, but be like 4psi in 2nd. So, in frusteration, I loosened the screw fully to disable the MBC, and my car felt alot better... No boost spikes, just a solid 5.5psi-6psi right after 4k RPM's.

My boost response is really slow for some reason. Karack, I know you told me to check my twin scroll by removing it's vac. source (I did), but I noticed no difference before and after. Refresh my memory; does this mean that it's just always open? (meaning bad boost response). Or is it always shut? (meaning... bad boost response? lol)

I'm confused, any ideas anyone?

 

Did you drill the hole in the side of it?

 

No.

 

Do I need to?

 

7. Carefully drill a small (1/32") hole into the body of the MBC as shown by the arrow in the cut away aside. With the bleeder hole in this position the same end result is achieved as with the coupling, but it reduces the number of parts required to fabricate and install the MBC.

 

Whats probably happening is, you're probably hitting your target boost mark, letting off, and the wastegate isn't closing again (since it has nowhere to vent that trapped air once the ball slams closed again.

 

if you used that installation diagram, disregard the tee to the BOV, plug the opposite side to the cold side outlet on the turbo housing.

does the ball fit tightly in the housing? any binding of the screw when turning?

removing the vacuum from the twin scroll actuator just makes it stay open at all times actin similar to a S5 setup, assuming the flapper door arm hasn't siezed in the closed position but if it did i doubt you would even be able to boost to stock boost levels.

 

Really...

On the link it said that beyond 15psi+ it might cause inconsistent boost. I figured since I'd be running less than 10psi it wouldn't make a difference.


dDub: (or anyone really),

can you explain this last sentence; "... the same end result is achieved as with the coupling, but it reduces the number of parts required to fabricate and install the MBV"...? For some reason that doesn't make any sense to me...


SonicRaT: what do you mean by "you're probably hitting your target boost mark, *letting off*, and the wastegate isn't closing again".

In my test runs I didn't let off when the boost dropped, it just did naturally. (if that's what you're referring to).

Someone tell me what to do, lol.

 

Haha, either way, with that setup, the boost controller traps pressure behind the bearing, which won't release the wastegate, thus the wastegate remains open constantly and never closes. Drill the damn hole or I'll send dDub over to sodomize you! (not really, probably shouldn't have said that, he might get excited)

 

makes sense what sonic said, i would try that, likely will cure most if not all of the problem.

 

The ball is a nearly perfect fit. I don't think the screw is *binding*. It just tightens down easily (I can't really feel any resistance) until it stops... I assume at this point the spring is almost fully compressed.

Yeah, I just removed the vac. line from the 'rotating' nipple on the cold-side of the turbo--that stock, runs several inches towards the block into another nipple. I ran a 12" line from both of those nipples and into the MBC.

 

Fine then, lol. I'll drill the frikkin' hole! I'll be back to this thread with the results.

Thanks you all!

 

Just remember James, Hanna is coming home with 2 other girls tonight....

 

So, I wasn't able to find a 1/32" drill bit like the site suggested. The smallest size we could find anywhere was a 1/16" bit. This is what we used.

It worked!

After driving with the MBC fully loosened (to make sure everything was OK), I gave it probably 5 or so full rotations tighter to see what would happen. After several more adjustments, I got it so that it was consistently boosting 10psi.

I'd achieve about 8psi in 1st gear at around 4.5k RPM's until shifting at 5.5k RPM's.

In 2nd it would hit max boost of 10psi at around 4200 RPM's. It would hold 10psi until about 5.5k RPM's before dropping down to around 8psi until 6.2k RPM's (which for right now is about my casual redline)

Overall it definitely feels faster. It begins shaking at around 5.5k RPM's and continues until redline. This is why I'm a little anxious about taking it to redline.

Any comments?

 

Warped flywheel/clutch? Driveshaft out of balance (if it's speed related and not engine RPM related), if it's engine related check your tranny bushings & engine mounts and so forth.

Glad it worked.

 

Can the placing of that hole differ for different mbc's?I think the person who sold me the mbc made the hole in the wrong spot.I only hit 5-6psi at the most.

 

As long as it's a relief when you blow in the side that goes to the wastegate, it doesn't matter where it's drilled. When you blow on the ball, you shouldn't leak air at all.