[wedgepilot]

I fitted my Forge MBC at the weekend, in line between the wastegate and the APC solenoid. Being careful, I started with just half a turn from closed, so I was surprised that I got a fair bit of pinking under load. So I closed it up to just a quarter turn, and I still got some pinking, with the APC backing off the boost. (At a 1/4 turn open, I could barely blow through the MBC, BTW).

Any thoughts as to why this is? It's not like I'm tripping the overboost, it just seems that the fuel system can't deliver enough petrol to cope with even a slight increase in airflow. Is the system really that marginal?

Anyone have any thoughts on this? Could the ignition timing or advance somehow be 'out'? Anything else I haven't thought of?

TIA,
Alan

 

[ylee coyote]

you should be able to blow through the mbc with no problems
the screw opens a bleed to the out side which is there to bleed air from pressurising the wastegate actuator and so opening the wastegate
in effect you are increasing the pressure when it opens and hence the boost

If you restrict the flow to the wastegate then it will do similar things but in a dynamic way
A rapid rise of pressure will not flow through to the wastegate andit will open later so you will attain more boost
a slow increase will show no difference
Either way it should not bring on detonation
As you surmise there is not enough fuel flow so I would check out possible causes
Blocked fuel filter
fuel pump
kinked hose
Fuel pressure regulator goosed

hth

good luck

 

[wedgepilot]

I should have said that when I blew through the MBC, I blocked the connector that goes to the wastegate to see how much air came out the 'bleed' connector. With the MBC only open a 1/4 turn, hardly any pressure was being bled off.

So, fuelling looks like a potential problem. Before I start changing everything, is there a way to diagnose low fuel flow? I've heard of people disconnecting the fuel pipe, pointing it into a big jug, and measuring the fuel delivery over 60 seconds.

Anyone ever tried this, or have any other ideas?

Cheers,
Alan

 

[Andy M]

Hi

are you keeping an eye on the boost pressure after every time you adjust it?. MBC's don't need to release a lot of pressure for an increase in boost but it depends on what type of boost you're wishing to run. My last car was open 2 1/2 turns and only running 1 bar(14.4psi although this wasn't a saab) this depends on your wastegates spring and condition!


Andy

 

[wedgepilot]

There's a very slight increase in boost pressure, but it's still nowhere near the limit. I'm not looking for massive boost with this, mainly improved spool-up. A bit of extra boost would of course be a nice side-effect

The fuel filter is of unknown vintage, and new ones are cheap, so I'll try fitting one and see if that helps.