Inconsistent boost with MBC

[AutoEuphoria]

I have a Turbosmart MBC. I am trying to set it for around 17.5psi, but I am having some major issues. I spent an hour last night fine tuning it so that I got exactly 17.5 psi (with a very short spike to 18.3) which tapered down to around 16psi in the high RPMs. AFRs looked good. I removed the boost cut defender while I wait for a separate boost gauge other than the DashHawk. So, I had it set perfectly...did a 3k RPM to 5500 RPM run in 4th, logged it, checked it...everything looked fine. I taped the MBC knob so that it wouldn't move accidentally. Well, tonight I put my foot down in 6th and was surprised to see the boost spike up to near 18.5+ psi and not come down. I immediately lifted so I wouldn't hit boost cut or damage anything. I ended up lowering the MBC another 1/4 turn and that seemed to take care of the issue, but still...why would it be fine one night and so far off the next? Would this be cause by a leak somewhere? I installed it as shown in this thread:

http://www.mazdas247.com/forum/showthread.php?t=123700407

 

[gimpo2]

is it colder outside?

 

[AutoEuphoria]

Maybe slightly. I was thinking that myself, but wasn't sure if it'd cause that big of a difference. It's just weird how I'll get different maximum boost pressures in 1st, 2nd, 3rd, 4th, and 6th.

 

[john blutarski]

The higher the gear the more load at the same rpm. I don't even have an MBC and I spiked 20 psi in 6th last night. It was also the coldest its been in a while here.

 

[AutoEuphoria]

AH, OK...that makes sense. So when I tune the MBC I should tune for the highest gear? I never knew that...thanks!

 

[AutoXRacer]

Also, how do you have it (MBC) installed?
If you are using the compressor nipple as a boost source, that will give you inconsistent boost levels due to the restrictor pill in the nipple.

 

[AutoEuphoria]

As I posted, I am running it per that thread above.

With my limited knowledge of the system, I will try to describe. The arrow on the MBC says "to wastegate actuator". I have a vacuum line running from the top nipple on the actuator (in stock form, this has a line to the stock boost control solenoid) to the arrow side of the MBC. I then have a line from the turbo inlet pipe to the side of the MBC that the arrow is not pointing to. I capped off the stock boost control solenoid nipples.

I think tomorrow when I'm under the car installing the DP I might re-route the line to the bottom of the actuator...this is where most directions say to connect it, correct? I bought the MBC used, and had no directions so I just went the way I saw in that thread.

 

[AutoXRacer]

Oh no, thats wrong!!!

You need to be using the bottom wastegate actuator nipple for the MBC wastegate side!!! The top one is a smaller nipple/restricted. In addition, you should not be using the turbo inlet nipple at all; this should be capped. Your "IN" to the MBC source should be "Tee-ed" at the BPV line or the compressor nipple; though that also has a restrictor in it. The best source is the BPV line.

So, to re-iterate; top actuator nipple capped, turbo inlet nipple capped, factory solenoid capped; both. You should only be using the bottom nipple on the actuator.

 

[AutoXRacer]

You should go to Mazdaspeed Forums . org and read my thread on the MS3 turbo section regarding the Turbosmart Boost "T". Its titled: Installed Turbosmart Boost T...MBC Suck!!!

 

[Speed3Rookie]

You should go to Mazdaspeed Forums . org and read my thread on the MS3 turbo section regarding the Turbosmart Boost "T". Its titled: Installed Turbosmart Boost T...MBC Suck!!!

Have you got rid of the spikes? Are you seeing consistancy throughout all gears? How much boost you seeing in 1st, 2nd, 3rd ect?

 

[AutoXRacer]

I only get spikes with the factory boost control...always have.

With the Turbosmart I peak around 19 and then slowly decrease to 13-14 or so by redline. I do have more consistency throughout the day though; maybe vary about 1 psi or so.

I haven't really been able to see the boost on 1st gear since its all wheel spin along with 2nd, but I at least get 15psi...I think it spikes at 16 PSI on 1st and 2nd gear. 3rd gear sits about 16-17, 4th-6th peak at 19-20 (quick peak at 20).

 

[AutoEuphoria]

OK, thanks! That should help a lot.

 

[Speed3Rookie]

I only get spikes with the factory boost control...always have.

With the Turbosmart I peak around 19 and then slowly decrease to 13-14 or so by redline. I do have more consistency throughout the day though; maybe vary about 1 psi or so.

I haven't really been able to see the boost on 1st gear since its all wheel spin along with 2nd, but I at least get 15psi...I think it spikes at 16 PSI on 1st and 2nd gear. 3rd gear sits about 16-17, 4th-6th peak at 19-20 (quick peak at 20).

Thanks for sharing.
I have yet to install mine so I guess I'll do so this weekend.

 

[oskinosmee]

Hey yes tune it to 4th and up cause the will be the only gears that will hold that for a while

 

[tru-boost]

thats just how MBC's work dude. it is a fixed rate bleeder not fancy electric pulse modulated device. your boost will vary based on temps and engine load. 6th gear has a ton of load and your boost will shyrocket if you just roll on it. just the nature of the beast. i am set at 18-19psi but if i mash it is 6th i see 25+psi !!

 

[Speed3Rookie]

thats just how MBC's work dude. it is a fixed rate bleeder not fancy electric pulse modulated device. your boost will vary based on temps and engine load. 6th gear has a ton of load and your boost will shyrocket if you just roll on it. just the nature of the beast. i am set at 18-19psi but if i mash it is 6th i see 25+psi !!

(omg)..... this I do not want.

 

[tru-boost]

then dont drive it like that. it isnt smart to load up the motor in an upper gear. if you need to go faster....downshift !! i dont advise going over 25-30% throttle in the upper gears from a low RPM.

 

[oskinosmee]

(omg)..... this I do not want.

Well dont mash it in 6th gear

 

[Speed3Rookie]

Ok.. I misunderstood what caused the high boost.
Thanks for clearing this up.

 

[Jarod7920]

thats just how MBC's work dude. it is a fixed rate bleeder not fancy electric pulse modulated device. your boost will vary based on temps and engine load. 6th gear has a ton of load and your boost will shyrocket if you just roll on it. just the nature of the beast. i am set at 18-19psi but if i mash it is 6th i see 25+psi !!

How do you see 25psi the car will cut at 22.5 lbs even if you have it clamped?