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How to fit and adjust a MBC

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Old Aug 7th, 2010, 16:41   #1
Chris740R
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Default How to fit and adjust a MBC

This guide is intended to give the average car owner a step by step for fitting a manual boost controller (MBC) to their turbocharged Volvo. It’s probably the best bang for your buck modification you can make to your standard car. I hope it’s easy to understand. If you have any alterations or suggestions please chime in.

Firstly, I have to mention that if fitted and adjusted in a way that goes against what I have written below, your MBC can cause serious damage to your engine. We’re talking new engine time damage. Increasing the boost pressure beyond what is documented in this guide will lead to your engine operating outside the limits of the standard ECU.




Parts you will need:
• 3m of 4mm diameter silicon vacuum hose: http://www.partsforvolvosonline.com/...oducts_id=5319
3m is enough to fit the MBC, and run into the cabin for the gauge.

• Boost gauge – I’ve recently had this gauge recommended to me -http://www.autometer.com/cat_gaugede...id=3243&sid=12 The resolution of the scale is good for the boost levels we’ll be setting.

• Small cable ties – the silicon vacuum hose has a smaller outside diameter than the standard Volvo rubber hoses, therefore the standard retainer clips don’t work as well. It’s best to replace all of these with cable ties.

• Manual boost controller – The most important bit. Be careful here, you want to get a ‘ball bearing and spring pressure relief’ type valve. There are other types available, however general feedback tells me that they aren’t as good.
The one I have for sale is type you want, here’s a link:
http://www.volvoforums.org.uk/showthread.php?t=100784
There are also plenty more on ebay if you want to spend a bit more for bling.

And that’s it!

Installing MBC

If you’ve just been out for a drive, let the engine cool down, you’re going to have your hands in amongst all the hot bits so it’s worth waiting. Next step is to open the bonnet, and spend a few seconds finding the engine. Got it? Good, you’re clearly a professional mechanic.

To make things easy, stand so this is what you see:


I have highlighted the two connections you need to find. The input connector is green, and the output connector is blue. The path of the hose is shown in red.

The large grey right angle shaped bit of pipe that comes out the top of your turbocharger carries all the compressed air to your engine. When we talk about adjusting the boost pressure, it’s the pressure inside this pipe that we’re talking about. The MBC is connected to this pipe via the pressure tapping highlighted in green, this is where the pressure comes from to operate the MBC and in turn, the actuator (which I’ll discuss later).

Lean forwards a bit…

That’s better, now you can see the two pressure tapping connections. The blue connection is a little trickier to find because it’s hidden beneath the turbo. Some advanced body manoeuvring is required to see it.



Now, you might be scratching your head here thinking… something’s different. Well you’re right, it is, that’s because I already have a MBC fitted. For now, ignore the different hoses and concentrate on the two connections and the path of the hose you will see on your car, which is shown by the red line in the picture below.



You need to remove this hose. Chances are the rubber is baked onto the tapings, the best way to remove it is to use a Stanley knife to slice along the portion stretched over the tapping. Now it should pull off easily.

Now turn your attention to your MBC. If you have one of mine, it has an adjustment bolt, and a lock nut to lock it in place.



Blow through the inlet (green) side of the MBC, and adjust the bolt to the point just before the airflow begins to become restricted. Lightly tighten the locking nut.
This step will make it much easier later on when you come to calibrating it on the road.

Now cut two 17cm lengths of vacuum hose, and attach them to your MBC as shown below in the picture. Secure the hose with some cable ties.




Now connect the green end of the hose up to the green pressure tapping on the turbo, and the blue end of the hose to the blue pressure tapping on the actuator. Secure the hose with more cable ties.

Don’t go driving just yet. You still need to fit your boost gauge.

Installing Boost gauge

Look at your intake manifold. You should find this bit on the top and if you have a HPT model, you will also find the hose that I’m holding.



This hose goes into the cockpit of your car and connects to your stock boost gauge if you have one. If the hose isn’t there, you need to remove one of the blanking bolts and buy a ‘male 1/8” hose tail connector’ Machine mart and most plumbing shops will sell you one for about 70p.

This bit is very much up to you. On 740’s the stock boost gauge is removable. I removed mine and replaced it with an aftermarket one in the same space. I’ll write a guide for this if there is any interest.
People with 940’s aren’t so lucky as yours is permanently fixed in there. Choose a location for your new gauge, it should be somewhere easy to see, as you’ll be watching it like a hawk later on.
All that remains is for you to run a length of hose from the pressure tapping on your intake manifold along the path of the original hose, through one of the weatherproof holes in the bulkhead, into the cabin and into the back of your gauge.

And you’re done..

Now you’re remembering that paragraph at the start of this guide warning you about the monster you’re about to unleash. Please read the next bit and adhere to it before you do anything else with the car.

Adjusting your MBC

Volvo have more than one ECU type, some can handle more boost than others.
Initially, this portion of the guide said that a safe pressure setting for your MBC is 12psi, done and dusted. However subsequently I have learnt that some ECU’s can handle more boost than others. As a result of this, to be on the safe side, I’m going to say go to 10psi. If you want to go higher (I’m doing fine at 12psi with LH2.4 on a 1990 740) read into your particular ECU to see how well it copes at higher levels. Also consider adding a air fuel ratio gauge to be on the safe side.


During the following steps, listen out for any strange noises coming from your car. A rattling or ‘different’ sound coming from your exhaust is a sign of ping / detonation, which is caused by a lean fuel to air ratio. If you hear this at any point, lift off the accelerator. A healthy engine should not suffer this problem at 10psi. It is more often associated with poorly maintained engines. The audible ping sound is only heard when the ping effect is loud enough. Adjusting the MBC till the sound disappears does not necessarily mean that the engine is no longer pinging.

With the MBC set so that air can still freely pass through it (remember when I got you to blow through it) your car will behave as normal. It’s only when you start tightening that bolt that the pressure will increase. Find a suitable place to do some ‘hard acceleration’ you want a nice long straight section of dry tarmac, ideally with somewhere to turn around at each end.

It’s helpful to have a friend present at this point to watch the gauge for you. Accelerate off boost through first gear, the revs build too fast in first and the gauge needle is difficult to read. Once you’re in second, apply full power gradually whilst watching the needle. On the first pull, you should see the needle climb to roughly 4 PSI if you have a LPT car and 7 PSI if you have a HPT.

Stop the car, open the bonnet, loosen the lock nut and tighten the adjustment bolt a quarter of a turn. Retighten the lock nut, close the bonnet and line up for another pull.

Same as before save the hard acceleration for second gear. Watch the needle, it should climb a little higher than before and you should feel the car accelerating faster. Once you get a feel for how much pressure you get for each turn of the adjustment bolt, you can keep adjusting the bolt till the needle hits the magic 10psi. If the needle starts going beyond 10psi whilst you’re accelerating, lift off the accelerator pedal and unscrew the adjustment bolt to reduce the pressure. Once you’ve got it hitting 10psi with no overshoot, giggle a little (I did).

Over the next few days, keep an eye on the gauge, and if you feel the car accelerating abnormally fast or the needle going higher lift off the accelerator pedal. It’s unlikely, but there’s always a chance you’ve messed up somewhere in the installation or a hose has become blocked. It’s good practice to keep an eye on the gauge when you’re accelerating hard anyway.

If you’re asking where to go from here, scour the internet for information for your particular engine, and see how high people boost it safely, you might be able to get a few extra psi this way. You can play it safe and fit an AFR (air fuel ratio) gauge, which uses a sensor in the exhaust to measure the ratio of air to fuel. You can use this to keep a check on the amount of fuel being supplied.

I think that’s everything covered, for the price, this easily the most fun modification you can make to your standard car. The LPT guys are going to have a very big smile on their face.

Last edited by cumbrianmale; Aug 8th, 2010 at 13:24.
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