DSMTalk Forums: Mitsubishi Eclipse, Plymouth Laser, and Eagle Talon Forum banner

1 - 8 of 8 Posts

·
Registered
Joined
·
1,366 Posts
Discussion Starter #1
I have a Turbo XS HPBC. The thing leaks like a bitch at as little as 5 psi (during press tests) at 15...look out, why does it do this? I thought the ball and spring will HOLD boost press untill preset boost is met ( 15lbs in my case). It stops leaking if I tighten the fine adj screw all the way clockwise (something the manual says NOT to do when driving). So, what it seems to look like to me is, one BIG boost leak. I simply do not understand why they leak like this. Do other MBCs leak like this as well? If not, I'm concerned about turbo lag (and other problems) caused by the MBC bleeding off boost press before the wastegate even opens. Even worse, if any of you have it T-ed into any other press sources (Bov) you are in more trouble bcouse the FPR or bov will not get the proper press signal, due to the MBC bleeding off press. Someone exsplain this to me:confused:
BTW , the thing seems to work and perform fine, yet a press test says otherwise:eek:
 

·
Registered
Joined
·
263 Posts
I have an EXtreme MBC. It does the same thing. There is a small hole that the air come out of.
 

·
Registered
Joined
·
195 Posts
No, the small leak is to prevent boost spikes, its a nessecity, no worries. If you had no hole, the boost would get on both sides of the ball and then hold it closed, but if air bleeds off after the ball it wont start closing....hard to explain...

Sean
 

·
Registered
Joined
·
612 Posts
I've wondered this before too...if it leaks during a boost leak test, it would be leaking while I'm spooling right? Like Rodman said I thought the whole point of a ball & spring MBC was to hold the boost with the ball and then released at a certain pressure. If that's the case, then it shouldn't leak any air at all.

I'm beginning to wonder if theres any difference between MBC's and simple fish tank bleeders if it leaks out during boosting.
 

·
Registered
Joined
·
412 Posts
There is a real reason that has been discussed before. The whole is so that when you let off boost and the ball goes back down and shuts boost from going into the controller it has to have a way to relieve the pressure left between the boost controller and the WG. If not the wastegate would not close and then you would get some serious lag when you go into the next gear. Here is an example.

YOu are in 2nd gear and boost to 15 lbs the MBC opens lets the 15 lbs of boost go through it which in turn activates the WG soleniod and the WG flapper opens. You go to shift into 3rd let off the gas there is still 15 lbs in the line between the MBC and the WG the ball closes on the MBC and now there is still 15 lbs of pressure there which would keep the WG open you shift and step on it the WG is open major lag. With the hole when you shift and the ball closes on the MBC the air pressure is let out the tiny hoen the MBC allowing the WG to fully close you shift and boost build up fast like it should. Hope this helps.

:scholar:
 

·
Registered
Joined
·
612 Posts
I understand why theres a bleeder hole in the line to the wastegate, but what I wonder about is if it leaks while you are building boost, isn't that kinda defeating the whole purpose of the idea behind ball & spring MBC's? You can get the same results by using a fish tank bleeder if all it does is leaking out X psi to fool the wastegate.
 

·
Honorary Administrator
Joined
·
4,869 Posts
I understand why theres a bleeder hole in the line to the wastegate, but what I wonder about is if it leaks while you are building boost, isn't that kinda defeating the whole purpose of the idea behind ball & spring MBC's? You can get the same results by using a fish tank bleeder if all it does is leaking out X psi to fool the wastegate.
The vent hole is not there to act like a fish tank valve! It is not a bleeder. The vent hole doesn't see boost pressure until AFTER the valve opens. The only way it would hurt spoolup is if your MBC spring is too soft, causing the MBC to partially open before full boost. All MBCs will open a little bit before full boost, but the better ones have springs that are closely matched to the application, so they don't crack open too early. The whole point of a ball/spring MBC is to keep the wastegate fully closed for as long as possible. A soft spring defeats that purpose - the vent hole has nothing to do with it.

migamb is correct in his explaination of why the vent hole exists. If there were no vent hole, once you reached full boost, pressure would be equal on both sides of the MBC (inlet and outlet) and the MBC would close. Now since there is no vent hole, you have full boost trapped between the MBC outlet and wastegate actuator (remember, a ball/spring MBC is a one-way valve). Since the actuator is still seeing full boost, the wastegate will never close.

The vent hole allows pressure to bleed off from the outlet hose after the MBC closes. Yes the vent hole is a small boost leak while you are at full boost, and the only thing you can do about it is go to an electronic controller so you don't have a vent hole anymore. However, the vent hole leaks a very minute amount of air and should be of little concern. Now if you're hearing noise from the vent hole during a 5psi intake leak test, I'd say your MBC spring is too soft. Most people just disconnect their MBC during a leak test and plug the line, since the vent hole is a false alarm.
 
1 - 8 of 8 Posts
Top