ECU outsmarted me - A3 Boost Mod not working

[P2B]

My 1Z A3 has PP520s but is otherwise stock. I wanted to increase boost to match the increased fueling, but chipping isn't in the budget right now so I decided to go with a bleed.

Just to be different, and because bleed valves are crude IMHO, I decided to do the bleed electrically, by installing a diode between the MAP sensor output and the ECU. The forward drop across a diode is 0.7V, which corresponds to 5 PSI in the transfer function for the A3 MAP sensor.

In other words, I modified my ECU such that the MAP sensor always reports 5 PSI less than actual to the ECU - so I expected maximum boost to increase by 5 PSI.

It doesn't work

Maximum boost (as reported by my LED boost gauge, which has it's own MAP sensor) is unchanged - still the stock 12.5 PSI.

Maximum requested boost (as reported by VAG-COM) is unchanged - still the stock 12.5 PSI, as expected.

Maximum actual boost (as reported by VAG-COM) has dropped from 12.5 PSI to 7.5 PSI - but at least it's not throwing any codes!

Obviously performance is unchanged, because I'm still running stock boost.

I assumed the ECU would simply control maximum boost to 12.5 PSI above nominal atmospheric pressure, but it seems be more sophisticated than that - it appears to read MAP output at idle, and use that as atmospheric pressure.

I now assume that, if I decide to pursue the idea, I'll have to design a circuit that only modifies the MAP output to the ECU when actual manifold pressure is more than 5 PSI above actual atmospheric pressure.

Anyone know more about this than I've learned through the experiment described above?

Simon

 

[Hasenwerk]

My 1Z A3 has PP520s but is otherwise stock. I wanted to increase boost to match the increased fueling, but chipping isn't in the budget right now so I decided to go with a bleed.

Just to be different, and because bleed valves are crude IMHO, I decided to do the bleed electrically, by installing a diode between the MAP sensor output and the ECU. The forward drop across a diode is 0.7V, which corresponds to 5 PSI in the transfer function for the A3 MAP sensor.

In other words, I modified my ECU such that the MAP sensor always reports 5 PSI less than actual to the ECU - so I expected maximum boost to increase by 5 PSI.

It doesn't work

Maximum boost (as reported by my LED boost gauge, which has it's own MAP sensor) is unchanged - still the stock 12.5 PSI.

Maximum requested boost (as reported by VAG-COM) is unchanged - still the stock 12.5 PSI, as expected.

Maximum actual boost (as reported by VAG-COM) has dropped from 12.5 PSI to 7.5 PSI - but at least it's not throwing any codes!

Obviously performance is unchanged, because I'm still running stock boost.

I assumed the ECU would simply control maximum boost to 12.5 PSI above nominal atmospheric pressure, but it seems be more sophisticated than that - it appears to read MAP output at idle, and use that as atmospheric pressure.

I now assume that, if I decide to pursue the idea, I'll have to design a circuit that only modifies the MAP output to the ECU when actual manifold pressure is more than 5 PSI above actual atmospheric pressure.

Anyone know more about this than I've learned through the experiment described above?

Simon

You must have done something wrong. If install a diode on the output of the MAP sensor you will get a voltage drop. 0.7V is a bit much for a mild "bump" and you are working with 0.5 to 4.5V to represent 0 to 2.5 bar. I have installed germanium diodes inline with the output of the ALH MAP sensors for a 0.3V drop which is brings the boost from 15 to 18 PSI. A silicone diode (0.7V) will givr you about 21 to 22 PSI which is too much for a K03 / VNT 15 turbo.

I have not tried doing this to the internals of an AHU ECU as of yet. The AHU that I was running in my Syncro TriStar was running a 0 to 10 PSI air pressure regulator t-ed into the boost line - open all the way gave me the desired constant 18 PSI that I was looking for.

 

[P2B]

You must have done something wrong.

I don't think so. Actual boost is exactly as it was before the mod according to the boost gauge and my butt, but is 5 PSI lower according to VAG-COM. That tells me I soldered the mod correctly, but it didn't fool the ECU.

If install a diode on the output of the MAP sensor you will get a voltage drop. 0.7V is a bit much for a mild "bump" and you are working with 0.5 to 4.5V to represent 0 to 2.5 bar.

The datasheet for a stock A3 2.5 bar sensor says the output is 3.52V @ 12.5PSI above nominal atmospheric pressure. I dropped that by 0.7V (silicon diode) to 2.82V, which corresponds to 7.4PSI above nominal according to the datasheet - so I expected a 5.1 PSI "bump".

I have installed germanium diodes inline with the output of the ALH MAP sensors for a 0.3V drop which is brings the boost from 15 to 18 PSI.

So this mod does work on an ALH. Nice to know.

I have not tried doing this to the internals of an AHU ECU as of yet.

Please do. I want to know if it works, like on the ALH, or fails, like on my 1Z.

Simon

 

[mojogoes]

As prehistoric as a bleed is it work really really well on the 1Z map/boost system.......

 

[TooRoundTDI]

Wouldnt you have to fool the MAF somehow also? The engine is still going to know how much air is going into it and maybe by adding the resistor the computer compensates because it sees the resistor at atmospheric (no boost) also.

 

[P2B]

Wouldnt you have to fool the MAF somehow also?

I don't think so.

I read all the old threads here about boost bleed on A3/B4 cars, and the consensus was the ECU appears to use a simple closed-loop algorithm to control boost, and doesn't notice if the MAF signal is inconsistent with the MAP signal - people who successfully implemented mechanical boost bleeds did not need to mess with the MAF signal. An electronic boost bleed should behave in the same way.

What surprised me is the ECU did not notice my modification or complain about it. It appears to accept the reduced MAP signal at idle as being the 'new normal' atmospheric pressure, and regulates boost from there.

In effect my modification tells the ECU the car is roughly 2 1/2 miles above sea level. It accepts that and controls boost accordingly, so on second thought I shouldn't be surprised.

I'll need to tell more sophisticated lies to the ECU before this will work

Simon

 

[Hasenwerk]

I don't think so.

I read all the old threads here about boost bleed on A3/B4 cars, and the consensus was the ECU appears to use a simple closed-loop algorithm to control boost, and doesn't notice if the MAF signal is inconsistent with the MAP signal - people who successfully implemented mechanical boost bleeds did not need to mess with the MAF signal. An electronic boost bleed should behave in the same way.

What surprised me is the ECU did not notice my modification or complain about it. It appears to accept the reduced MAP signal at idle as being the 'new normal' atmospheric pressure, and regulates boost from there.

In effect my modification tells the ECU the car is roughly 2 1/2 miles above sea level. It accepts that and controls boost accordingly, so on second thought I shouldn't be surprised.

I'll need to tell more sophisticated lies to the ECU before this will work

Simon

My Syncro TriStar used to have an AHU with 0.216mm nozzles and a boost line bleed to make it a constant 1.2 bar of pressure (18 PSI) and a blocked off EGR system. I never had a check engine light and that engine definately outpulled the 115 PD that is in there now. I think a simple bleed is all that is really needed to make more pressure on the 1Z and AHU engines.

 

[Andrei Rinea]

Just to be different, and because bleed valves are crude IMHO, I decided to do the bleed electrically, by installing a diode between the MAP sensor output and the ECU. The forward drop across a diode is 0.7V, which corresponds to 5 PSI in the transfer function for the A3 MAP sensor.

In other words, I modified my ECU such that the MAP sensor always reports 5 PSI less than actual to the ECU - so I expected maximum boost to increase by 5 PSI.

Did you open the ECU and install the diode there between the MAP sensor inside the ECU and the ECU board itself?

 

[P2B]

Did you open the ECU and install the diode there between the MAP sensor inside the ECU and the ECU board itself?

Yes, exactly.

Simon

 

[diesel des]

Put a bleed in forget a diode in a old a3 ecu. Bleed is also variable.....