View Full Version : boost gauge/mbc wrong? on 1st gen 3sgte

10-06-2009, 12:00 AM
Ok, here's the situation
Have an aftermarket Autometer boost gauge installed, vacuum line runs to one of the blocked off spots on the throttle body.

I also have a MBC installed and the TVIS disabled.

Heres a couple of issues I've noticed

if I'm just over the 0 mmHg line on the factory gauge, my boost gauge reads nothing (Just boost, shoulda bought one that does vaccum/boost methinks)

The other bigger issue is, my turbosmart MBC, seems to have about 6 different settings that all result in me ending up with ~10.5 PSI of boost in all gears. The higher up I get on those specific 6 settings ,the more often I'll randomly get an overboost CEL and fuel cut.

Am I confused or shouldn't it be something like
Setting 1 - 10 PSI
Setting 2 - 11 psi
Setting 3 - 12 PSI


Instead I can put it on setting 6, and after say, 20 minutes of driving hard, then I'll start hitting Fuel cut.

Also, this MBC has many, many more settings than 6, but I've found this range of 6 clicks where it works as desired.

10-06-2009, 12:11 AM
TVIS honestly should not be disabled. There are no negatives to keeping it well into the 400hp range.

Your boost issue, is that with an MBC you set it at one setting and that is supposed to be it in all gears.
From the factory, boost is supposed to be limited in gears 1 and 2.
Gears 3,4,5 will provide the engine "full load" and you are supposed to get max boost at WOT.
Do you have a DP or open exhaust at all?
If so, this could be a cause of boost spike at full boost in the lower gears (3,4,5)

Get back to us. :)

10-06-2009, 02:10 AM
Err sorry, TVIS is still there, it's the TVSV I disabled.

I still have my stock cat, exhaust is 3" with a 4" Muffler.

My MBC has an option, where it'll either spike the boost to get the desired PSI, or it'll slowly ramp it up to the required PSI. I may want to try the latter setting instead, and see how it behaves then.

10-06-2009, 05:36 AM
Sounds like you just found your issue. ;) :D

10-06-2009, 05:50 AM
You cant use any of the ports on the throttle body. The boost gauge must be hooked up to a port going directly to the intake manifold, after the throttle body.

When setting boost, you have to do it in 3rd or 4th gear, on a full highway pull. Setting it in lower gears, will cause you to hit fuel cut in higher ones.

10-06-2009, 11:01 AM
For reference, I tapped into the A/C idle up vacuum source for my AVCR.

If you'd like, I can post a picture tomorrow.

10-07-2009, 04:40 PM
I found a port that was blanked out on the back-right hand side of the manifold. My car doesn't have A/C so maybe this is the same port used for the AC Idle up?

10-07-2009, 08:57 PM
Photo of your setup?

10-12-2009, 03:39 AM
Ok, So my MBC had a spring & ball inside of it. I've removed those, and the turbo seems to function MUCH better. better pull, etc. through the gears and I was doing 12-13 PSI in 5th and I didn't hit fuel cut. Before I'd hit fuel cut when my boost gauge read 10.5PSI with the ball and spring installed.

I'm not sure if the slower ramp up makes life easier on the old ECU or what, but it works a lot better now.

10-13-2009, 06:23 PM
Are you saying you removed the ball & spring from your ball & spring boost controller? That means you no longer have a boost controller no? You might get a little more boost due to the bleed hole which should be in the MBC unit... Anyways without the ball & spring, the adjustment knob does absolutely nothing now.

Your issue could be the stock MAP sensor not reading quite right.

10-14-2009, 12:53 AM
This turbosmart MBC has 2 modes of operation documented in the instructions, one with the ball and spring which results in a very quick ramp up beyond the requested PSI, then down to the desired PSI.

With the ball/spring removed, you get a gentle curve up to the required level of boost, then it flattens out.

Unless I find a MAP sensor going for like $10, I don't really plan on investing any more money in this engine, and plan on putting it towards my swap.

10-14-2009, 07:05 PM
Does the unit have a bleed hole drilled in the side anywhere? If not, that could be a cause of the boost spike.

I still don't understand this specific MBC - I've never seen one that has two different ways to control boost. Do you have a link that I could see? I googled and found a few different designs.

To get past the fuel cut issue, you can just unplug the hose to the MAP sensor. Engine does not use this signal for the maps (uses the AFM). Just keep an eye on the boost gauge.

10-14-2009, 07:48 PM
ball/spring controllers HAVE to have a bleed hole yeah.

But its a Turbosmart, that unit is well known and it works fine.