I was looking at a Hallman MBC today and noticed that the outlet nipple has a small bleeder hole in it. It actually lets a fair amount of blow-by to get through as long as the ball "valve" is open. I thought a MBC simply reduces the pressure to the wastegate by allowing the inlet boost pressure to do work and move a ball on a spring, reducing the pressure on the outlet. If someone was only looking to add a lb or two of boost over the WG spring (like me), it would seem that this bleeder hole would hinder that.
It may be a stupid question, but what is the bleeder hole for?

 

It does the opposite. It relieves a little pressure out of the line. You have to remember that manual boost controllers have to be hooked to the side port of an external wastegate. It's just like putting an MBC on the stock twins. It works the same way.

The less pressure in the line, the less pressure applied to the lower chamber of the external wastegate. Less pressure in the lower chamber will open the wastegate later. I wasn't very happy with an MBC on my external wastegate. I thought it was too inconsistent. I was always tinkering with it, especially as the weather changed. I had the same mentality as you, "I just want two or three more pounds than spring pressure." That's why I switched to the "free" ebc mod.

I haven't looked closely at the bleed hole on a Hallman (I had a home depot ball-and-spring MBC).

 

The bleed is there to prevent "line-lock", where the pressure applied to the actuator cannot bleed off when boost is reduced (off throttle, etc.). If it does not bleed off, the actuator will be held permanently open, preventing boost the next time you get on the throttle.

 

Exactly... it certainly seems as though it would be counter productive if you simply want a lb or two over the spring. Maybe I'll try the "Free" ebc mod...

Makes sense - thanks!

 

PM me or post in the original thread (which I admit is way too long and kind of confusing at this point) if you go that route and have any questions. it's just like hooking up any EBC really.

 

I have dual Hallman MBCs (one for precontrol, the other for wastegate) and have had rock solid consistency from them. If there is variation due to temperature, I can't detect it.

One of the very nice features of ball-and-spring MBCs is that boost builds very quickly. I get a consistent 14 psi at 3000 rpm with mine on stock twins (stabbing the throttle at ~2000 rpm; standard bolt-ons with metallic cat).

Tuning EBCs for rapid boost buildup has always seemed too much of a black art for my liking.

 

I used to have dual hallman's til this weekend. I have to say the dual hallmans were really awesome, but I got tired of fiddling around with the ***** and guessing the pressure for each run. I dropped in an AVC-R. I sure hope I don't regret it

 

it's certainly less intuitive than turning a **** or screw, but people used to say that about fuel injection vs carb. With an EBC on an external wastegate you get the advantage of fully pushing the wastegate shut instead of just interrupting the signal that pushes it open (MBC). I had an MBC on my old HKS external gate and I thought it was too flakey. It always required adjustment.

 

What kind of MBC did you have? Needle valve or ball-spring?

My ball-spring MBC's give rock-solid control and never have needed adjustment since the initial setup.

The needle valve MBC's (IMO, just an adjustable "pill"), on the other hand, still rely on the OE solenoids and ECU map to control and are, therefore, subject to changes in boost level with weather, etc., just as the pills were not consistent in their control.

 

^ ball and spring.

I think people's perceptions of how good their boost control is really depends on their expectations and how they are measuring it. When I had an MBC I would do log after log as the weather or intake temps changed and I could see the shape of the boost curve changing in different areas. Then I would calculate the % difference of boost at different rpms between the different logs from different ambient conditions or intake temps. From memory I would get fluctuations of +/- .75-1.0 psi peak boost with the ball-and-spring MBC (it was admittedly not a name brand). The behavior at the very top of the rpm range tended to fluctuate the most. This is where the MBC is more likely to let the boost drop off, because it has no self-adjusting logic like a good EBC.

My current Power FC boost control setup is fluctuating more like +/- .25psi , so something like a 60% tighter tolerance than with the MBC, if you want to think about it that way. These weren't all super scientific tests I did, but they were more controlled than eyeballing a boost gauge with an analog sweep like most people do. Analog boost gauges are somewhat buffered so you can't see fluctuations as much, and digital boost gauges have slow sample rates. It's hard to judge boost control through the rpm range without actual logs of a calibrated MAP sensor.

For most people I guess that's not a big difference in terms of actual psi. But if you are a numbers guy like me, if you want your 15+ year old car to perform with the consistency of a modern factory turbo vehicle, you may appreciate a good EBC setup. I have very, very high expectations for my engine management. I like to model my setups on modern OEM designs and tolerances. I like my modified cars to behave as if Mazda designed it to be the way it is, as if so much research went into it that anyone who drives it would think it's a relatively "normal" car. It's kind of like how a girl looks best when you can't even tell she's wearing makeup, even though she is.

 

^ thanks for the detailed answer.

As you say, I do not have any logging capability, so the exact accuracy is not known.

Thanks, again.

Dave

 

I've been logging for several years and have always been surprised at how consistent boost is when it reaches the selected value, particularly given the lack of feedback in the system. I'll post some logs later.

 

Here is a recent log. Ignore the high knock levels (they are normal for my car) and the richness between 3-4k rpm (I was doing some experimenting).

As can be seen, I get 0.9kg/cm2 (12.8psi) at 3k rpm and, apart from a slight drop off at at 3.8k rpm on the primary, it is ruler straight to 7.5k rpm (and will hold until 8k rpm consistently). I can get level maximum primary boost too if I reduce it lightly relative to the secondary (my theory is that diversion of primary boost through the precontrol gate is causing the slight drop off). I get equivalent control at 1kg/cm2 (14.2 psi). I can also get slightly earlier maximum boost if I am more aggressive with the throttle.

The ruler-like flatness and ease of adjustability is compelling here. I am not sure I have seen very many logs that should this level of control all the way up to 8k rpm.

 

^ I have had less luck with an external wastegate.