Manual Boost Controller

[sambb]

Fair enough. Like I said, get down to repco and buy a 1/8th thread smallest size nipple and maybe the next one up. Make that your MBC inlet nipple or you could run the mbc outlet nipple on the inlet side and put the repco one on the back end. Plumb it in front of/in series with the N75 again and give it another go. Moral of the story it will run waayyy better if the N75 is still active. Its just a matter of playing with MBC inlet sizes until you get sufficient flow through it so that you can adjust from where stock boost was upward. With it adjusted close to fully open, just keep enlarging the mbc inlet till you get to that point. Keep in mind you can't keep enlarging the inlet size if it gets to the point where its bigger than the mbc outlet size. At this point you need to enlarge the outlet and then start again on the inlet side.

[sambb]

Something else - the suby's were set up with pre and post solenoid restrictions to get the desired boost characteristics. Some had restrictor pills hidden within the tubing on the solenoid inlets, outlets to the actuator and also the bleed line back into the TIP. Some even had combinations of all three. Once you get the correct inlet restriction to your N75 and you are satisfied its all fluffy, you can ditch the MBC and just have a neat restrictor hidden inside the tube that replicates it.

[CupraGTi]

Sam,

thanks mate. I understand the theory behind what you are saying... I think... basically as the MBC is at the moment, the nipples are too large and letting too much air through to start with?? which upsets the n75?? so play with the nipples (LOL) until there is a sweet spot that you can adjust from with the n75 intact??

Why does it run better with the n75 in the loop??

[sambb]

I would just put it back to how you had it in post#5, but increase MBC inlet size. You weren't getting enough air through the MBC inlet which is why the MBC had no adjustment and why your boost went too high (The MBC wasn't passing enough air onto the N75 so it had no air to work with an couldn't move the wastegate flap).

Its better with the N75 in the loop for increasing throttle and high load situations. eg. I found that if you are trying to get onto the loud pedal as early as possible exiting a corner, the 'MBC only' was over inclined to go to full boost and way too early despite what my foot was telling it, rather than letting you modulate the throttle. Also when loads are high eg 3rd or 4th gear trundling up a long gentle hill at legal speeds and you decide you want to overtake, the boost can ramp very quickly - much quicker and earlier than 1st or 2nd gear.
With an 'mbc only' it'll go straight to max boost in a blink and the fuelling and/or timing pull probably won't be able to keep up since the altered boost ramp up will be outstripping the timing map you are on. Just because it wants to spool there doesn't mean it should. In those instances the ECU will be looking at your foot position, MAF and rpm etc and be determining load and then from that what maps and what safe boost level you should really be at which equals driveability and and no detonation. In contrast an'MBC only' system will more often than not choose the same max boost level regardless of load and the situation. Keeping the N75 in the loop (assuming you only increase the boost slightly) will still mean that the ECU keeps a final say on boost so to speak. Sometimes when the MBC just says yee ha! lets go for it but all the conditions for all out boost arent met, the N75 will still be able to put a lid on it. eg. If the MBC is letting 15psi of end boost worth of air through, but in that moment the ECU is just going "nup" and telling the N75 to pulse away at a rate that would normally only allow 9psi becasue loads are too high, then the N75 wins. 9psi is all you'll get. This is just an analogy of course but you get the drift.

[h100vw]

With the n75 out of circuit, the ecu has no control of what's going on. Hoping the knock sensors will protect the engine is a big risk. Imo. Far better not to promote the possibility in the first place. Rapidly ramping boost is also what can bend the rods. Using the mbc with the n75 would be much ore preferable. If you're desperate to use it.

[CupraGTi]

Sam and Gav,

Thanks guys, I understand what you are saying. Sam, I need to INCREASE the I.D of the MBC inlet GRADUALLY until ENOUGH air gets to the n75 and I find a sweet spot to increase a COUPLE of psi???

Gav, I need to keep the n75 in the picture as it is THE COMPUTERS version of an ELECTRONIC BOOST CONTROLLLER?

I must say, I am not getting any 'rapid ramping of boost'... its not going to 'full boost at part throttle'... or anything like that... but I will take your advice.

I'm a leopard tank driver... trying to make my leopard go as fast as an M1 Abrams... forgive me please.

[sambb]

If I read it right and when you had the MBC plumbed in series pre- N75 (and the N75 plugged in), you were overboosting, then yes, you need to increase the I.D of the MBC inlet.
As I said earlier, I would just buy a 1/8th thread nipple from repco of whatever size fits your N75 signal hose. Put this on the MBC outlet. That way you know you are not restricting too much on that side of things. Then take the nipple that is currently in the MBC outlet and put it on the inlet of the MBC. Its a bit bigger than the one (check yours, mine is bigger) in there now and you might be lucky and it'll give you the adjustment range you need. Basically if the MBC is fully open/max flow and you are overboosting then you need to increase nipple sizes. In contrast if you are screwing the MBC tap in/constricting flow but your boost isn't going higher than stock then your nipple sizes are too big. An MBC will only adjust in the range you need it too if its tailored to the air flow that exists there.
When you get that bit right, have the MBC in its fully open position and go for a drive and (if the MBC is flowing enough now) your boost should be close to stock and the car operating on its N75 as it was before. Then you can start to close the tap until you see the slight boost rise you are after.
Yeah the N75 is just the VAG factory designation of its boost control solenoid/valve. You will just be giving it less air to process so it'll be a bit slower and later in getting the wastegate open which is what will give you your boost increase.
Like others have said I'd be very careful doing this. I've used all sorts of bleeds to reshape and lower boost curves but never to increase boost curves. Seriously 2 psi max. Don't get greedy or it'll rattle itself to bits.

[BL33SU]

iv used an MBC and EBC for my 1.8t, MBC is rubbish.... spend the bit extra and buy a EBC with the solenoid... the car will drive like it has the N75 no bad throttle etc..

[CupraGTi]

Sam and everyone interested...

Thanks mate, I've done exactly as you suggested, and got a good result straight away. I put the outlet from the MBC on the inlet side which opened it up significantly, and bought a 1/4 hose size nipple to put on the rear end. Straight away gave me 11psi with the tap wound to the full - position. For some reason, due to replacing the nipples, the tap no longer has little clicks for reference, so I have gradually opened it up to 13psi and used the allen lug to lock the tap in place. Its running much the same, but I feel better having the n75 in the loop.

Thanks sam for your advice, and Gavin for his words of caution.

As I have previously stated, I do not believe I am getting the most out of the extra boost I have stolen. Its just an extra bit of air and fuel, but no timing changes have been made, and in so hasn't utilized the extra boost to the motor's advantage. A custom tune is a better option than this, but I wanted to see what I could do with what I had in my hands. I would not suggest doing this unless you have done a few things to get your motor breathing better in the first place.

When I get a bigger turbo and want more fuel as well as timing changes, there will be no option BUT to get a custom tune. What I am doing is cheating, but I do not advise people to do it, unless they have a fair understanding of how a turbocharged motor works. I basically fooled the computer, but I MAY end up being the bigger fool for screwing with things....



Kris

[CupraGTi]

Well I spoke too soon, it isn't perfect... 1st 2nd and 3rd to an extent are holding different boost levels... then when in 4th and fifth, its making and holding 13psi for a little bit then at about 3300 rpm it suddenly ramps up and overboosts...

I think this is due to a number of reasons... but mainly because the n75 is pretty touchy by the looks of things... that and I've totally changed the flow dynamics through the MBC.

I could possibly go crazy trying to get the perfect flow rate through the MBC...

In all ways it drives the same when on 13psi regardless of whether the n75 is in the loop or not... I'm going to go back to bypassing the n75 for now. an extra 3psi isn't going to do any damage IMHO.