Boost Gauge

[Profpriv]

Im asking TC if he still has a nipple attached to the in let manifold on the HPT 940 that he is breaking....

[Profpriv]

I just thought the MBC had to vent some air,although I cant see why when
the turbo doesnt now.



Chorleybo- its worth boosting your turbo now while you have such interesting,excellent help.

[Jim760]

thats a top post there chris! clearer than any haynes manual could be
prof- does your car not have a boost gauge built into the rev counter? im wondering if you could just use the line thats in your car already for that.. or would the reading form that line be inacurate??

[Profpriv]

Hi Jim760.


They didnt bother to fit a gauge on the Low Pressure Turbos. Probably because no-one would be that interested, other than the fact that a turbo had been fitted.The HPT's do,but they are more interesting....


Shaun

[gpl1968]

Originally Posted by Profpriv

I just thought the MBC had to vent some air,although I cant see why when
the turbo doesnt now.



Chorleybo- its worth boosting your turbo now while you have such interesting,excellent help.

Yes, there needs to be a way of allowing the wastegate actuator to return to it's resting position when the engine is off boost. Normally there will be a pin-hole in the body of the mbc. It only needs to be 0.5mm approx.
If your mbc doen't allow air to vent from the output side of the valve, you need to provide an outlet of some sort.

Just to recap the logic behind the mbc operation:

Boost pressure below pressure threshold set by mbc > mbc closed,wga closed (wga=wastegate actuator).
Boost pressure rises above mbc threshold pressure > mbc opens, wga opens and boost is limited to pressure set by mbc.
Boost pressure falls below mbc threshold pressure > mbc closes, excess pressure within the wga vents to atmosphere through the vent hole or similar and wga returns to it's closed position.

Without the vent on the output side of the mbc, the wastegate actuator (which is basically a diaphragm pushing against a spring) will not close the wastegate, effectively stopping the turbocharger from working after the mbc has opened the first time.
The slight pressure loss from the vent, when the mbc is open, is negated by the large pressure differential between the mbc threshold pressure and the wastegate actuator operating pressure.

I hope ths makes sense.

Gavin

[Profpriv]

Now I really am confused.

As I understand it, I have Gavin saying i need a small vent hole, and Chris 740R,saying its not necessary.


I guess you both have experiences of the same thing, but . Do I need to drill one?I definitely cant see any hole on the mbc.I have bought the red one which is widely available on Ebay.

Many thanks


Shaun

[1stRaven]

What Gavin and Chris have written depends on the type of MBC you have. Most of the MBC's wont have a hole.

There is an easy way of testing your mbc. If you blow into one of the mbc inlets, you will find that one will not allow air past until you fit a certain pressue (very hard to do with Human lungs) and the other side, you can blow without very little resistance at all.

The way you fit the mbc is the inlet with resistance takes the feed from the turbo compressor and the other inlet goes to the actuator input. When the throttle is released, the pressure can release back through the mbc, into the turbo compressor housing and out through the cbv/bov.

[Profpriv]

Wow, what a fantastic pice of kit, and |I've only just taken it out of the box.


Included for the £25.99 with free p&p from Ebay is a length of clearish hose,a manifold adaptor,a plastic tee piece,and of course the gauge itself.


Ive ordered separately a Tim gauge pod which should arrive soon.

[gpl1968]

Originally Posted by 1stRaven

What Gavin and Chris have written depends on the type of MBC you have. Most of the MBC's wont have a hole.

There is an easy way of testing your mbc. If you blow into one of the mbc inlets, you will find that one will not allow air past until you fit a certain pressue (very hard to do with Human lungs) and the other side, you can blow without very little resistance at all.

The way you fit the mbc is the inlet with resistance takes the feed from the turbo compressor and the other inlet goes to the actuator input. When the throttle is released, the pressure can release back through the mbc, into the turbo compressor housing and out through the cbv/bov.

Surely that would only be true for a 'bleed valve' type mbc.
With a 'ball & spring' type mbc, it is effectively a pressure relief valve, only opening above a set pressure. It also works as a non-return valve when the boost pressure drops below it's threshold. For the return air to pass back into the turbine housing, there would be an open circuit, rendering the mbc useless, unless the return line incorporated a one way valve.
My first mbc had a pinhole on the 'actuator' side of the valve.
My current mbc is more sophisticated and only vents (again on the 'actuator' side) when the valve is closed, so not wasting boost pressure.

[gpl1968]

Shaun, is this what you have bought?




If so, I would speculate that this is a 'bleed valve' type mbc.
With this type the valve continuously vents some of the boost pressure to atmosphere, so as the boost pressure rises, the actuator reads a lower than actual pressure and opens at a higher boost level.