Hello all. I compound turbo'ed my wife's B6 A4 1.8T a while ago. We've daily driven it now for 2 years just on 18psi and big turbo on wastegate (6psi).

I left it alone and now would like to start up again. I've considered a few different approaches to boost control and wanted some input from this forum.

I have a 2nd N75 (for 2nd WG) and MAP with integrated IAT (between turbos) all wired into ECU. ECU can control 2nd N75, but it's only open loop right now adjustable by unisettings (adaptation channel). ECU has access to 2nd MAP and IAT as well.

I could completely duplicate existing PID control functions or just run thru existing functions twice with 2nd set of maps.

I could set low pressure (LP=big turbo) target boost independent of high pressure (HP=small, stock turbo) target boost or have a pressure ratio vs RPM map of one turbo and extrapolate target boost for both turbos.

I'm concerned both kfdmix maps (1 for each turbo) will change based on what boost the other turbo is at, requiring an infinite number of kfdmix variations.

Just thinking out load.

A few notes.

1. There needs to be only one boost target.
2. You must be using some sort of bypass for the small turbo, right? That is your switch point.
3. To control the big turbo you can just write your own code. I see no point in running ME7's (broken) algorithm twice.

I have source Keil files for a standalone boost controller within the ME7 ECU that I wrote.
You will need to adapt it to the ECU and adapt the switchover, but you could use it. It is full PID with a single pre-control map.

I'd love to feel what this car drives like. Compressor outlets Y'd together? Intake the same through a single Maf?

I love this project, can't advise on tuning strategies but I will follow this! I think k03 bypass control (exhaust) is a big factor in how much power this compound will make, so in that case there is not much pressure control needed after the t3/t4 is spooled? I mean you should be looking for flow, not for boost with this concept I would say.. My first thought is to open up all gates as soon as you have spooled the big turbo 

Why don't you just run a manual boost controller? Apparently you get faster spool times

i would add a bypassvalve over k03 compressor.
Will be restrictive in topend.
And use big wastegate over k03 turbine.
using stock k03 wastegate will restrict airflow and cause overspeed.

As long as the K03 can spin fast enough to generate ANY positive pressure, it will not be a restriction on the intake side. Exhaust side is a different story. However, this setup focused on simplicity, not peak power. Installing it is not much more than replacing downpipe and turbo inlet pipe.

Consider running in colder weather. Colder = more dense intake. Is there a point where ambient temp is so cold and air is so dense that K03 becomes a restiction instead of still pressurizing intake? No. Compressor takes pressue in front and multiplies it, regardless of front pressure. Instead of cold weather providing more dense pre-compressor air, the big turbo does. Same idea as elevation changes.

For the record, what is considered more that just K03 power?

If I want percise control of both turbos to maximize their efficiencies, I need two boost targets.

prj you've said twice I only need one, both times without explanation, but that's expected of you.


The big problem with this setup is the small K03 exhaust housing and wastegate. I haven't turned it up enough yet to see the problem, but I'm sure it will come up.

If I can see big turbo 22-24psi boost up top AND small turbo 20-22psi boost way down low, I'd consider this a success.

That's ~5-6psi from K03 + 16-19psi from T3/T4 up top. <- neither turbo breaking a sweat.

manual vs ecu boost control- what's the fun in having an ECU at my disposal if I don't use it as I want


The limited testing I've done has K03 still controlling final boost up to 18psi while big turbo is on wg at 6psi. I gave BT wg 10% duty with no other changes and K03 duty goes to 0% while slightly over boosting. I don't remeber exactly, but BT boost was maybe 12-14psi while K03 boost was 6ish psi at 0%.  I'll go back and look at logs.

K03 spools faster than stock.

Thanks,
Rey

So, what is 'noticeably more power than a single K03'? I guessing it's a little more than 'any more power than just the K03'. On a B6 A4 quattro that is. I'm having a hard time qauntifying this statements.

What are compressor maps used for again? overall flow or pressure ratios where flow is derived from a specified pre-compressor pressure?

Ideally small turbo will 'multiply' big turbo's boost, though slightly. So, there will be no full switch-over. Maybe you're thinking about sequential turbos?

Thanks,
Rey