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Twincharge, which way round & supercharger bypass or not?

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Hi all

In a bit of a conundrum with regards to which way round to run to run the turbo & supercharger

The engine is BMW's m57, 3.0 straight 6 diesel

I know if the turbo blows through the charger there a huge benefit of compound boost but this also increases the strais on the charger however instant throttle response & even more benefit from turbo when it comes on

If charger blows through the turbo I'll still get the benefit of the lower rpm charger but from what u understand it's even more critical for sizing the 2 as the turbo will then take over leaving the charger behind but this way theres no compound boost benefit

Has seen both ways done & at the moment my preference is this first way, any suggestions/should or should not/personal experience?

Also with the fact its diesel & having no throttle butterfly am I right in thinking I can eliminate the supercharger bypass valve which would normally control boost/open when throttle closed?

The overall boost reference would be taken from inlet manifold & controlled on turbo Watergate

Blower would be set to run 9-10psi so I cant see why I would need the bypass apart from to unload the charger for part throttle economy cruising

Thanks in advance

James

I went some way down the path of developing a twin charged setup for a 4AGE many years ago but abandoned it due to the complexity. Most of the functioning systems I've seen (there aren't many sadly) use a bypass valve so that once the turbo is on boost the air is bypassed around the supercharger. This is probably still how I'd approach it as the supercharger won't be as efficient as the turbocharger and you end up with a lot more heat and potentially a restriction to airflow. Running both in series would probably also require two intercoolers which is further weight and complexity.

The obvious question then is what do you use for a bypass valve and how should you control it. That is unfortunately the tricky part and I'm not too sure what off-the-shelf parts would be suitable. You may want to research how Norris Designs dealt with the twin charge setup on their EVO 4G63 kits - [url=https://www.youtube.com/watch?v=A48PTg4UKQM&feature=youtu.be]https://www.youtube.com/watch?v=A48PTg4UKQM&feature=youtu.be

Bypass would only be to unload belt drive once you had enough boost/fuel to power the turbo at desired cruise boost at reasonable afr.

I have seen a pikes peak truck that ran super into turbo but i really don't like that arrangement and it doesn't allow you to effectively compensate for ambient conditions if you have a target cylinder pressure/torque/power you wish to attain regardless of temp/humidity/altitude.

I do intend to "light" compound my car if I can package both turbo and supercharger, will likely need an electric oil scavenge to push the turbo down low at the firewall.

I suggest you make a spreadsheet with nominal pressure ratio of super and turbo feeding it to work out the ratio split you want to run, at target boost you want the turbo to be doing most of the work if you are keeping them in series and not bypassing the blower once target boost is reached, it will also smooth the transition to bypass if the blower ratio isn't massive relative to overall target but that will obviously dull response down low a bit if you don't work the blower as hard.

It is a performance and packaging compromise for most people, with a lesser supercharger PR you can probably get away with a larger diameter pulley for common/cheap blowers and use factory rib count belt retaining factory pulleys for crank etc if you are asking for a modest charger PR.

People have used exhausted wastegates successfully as bypass valves, without throttle considerations of SI engines it is a bit simpler but you probably still want to have a good think about spring setting and reference points, I imagine you would want a solenoid to your vac plenum to actuate the bypass by acting on the top port of an external type wastegate if you do want to bypass.

Have you checked the Gale Banks project, where he is working through the various problems and paramaters of super', turbo' and compound?

https://www.youtube.com/user/bankspowerinsider/featured - I think it was the monster truck build, might have been the 'killing a Duramax'?

Diesels work differently from petrol/gasoline - keep the supercharger operating at cruise.

Hey James, I’m working on a similar project myself I’m nowhere near finished but my opinion/take on the design and order.

With a Roots supercharger you are always building boost and taking power away from the crank. So on a road going car for daily driving i would go super into turbo. This gives you low end power then you can size up a bigger turbo without having to worry about the increased lag and give the opportunity to have move maximum power without sacrificing low rpm performance... and would assume a sucked on Supercharger would be less parasitic vs a compounded one as it's the resistance that is creating the loss.

If you use an electronic bypass you can basically "disconnect" the supercharger at any time IE not build positive pressure and limit parasitic losses, I'm going to be using an electronic throttle body off of a 2005 ford C-Max (this is sized fairly suitable for my application and can manually be set closed then opened with a PWM signal). So under standard driving conditions the super is bypassed and you run turbo only, when starting to pull away from a stand still you can start on super and as the pressure differential drops between the Super and the turbo the bypass can be opened.

I'm working with an old mechanical diesel so i have no standard means to control anything because there is no computer. So mine will be controlled by an Arduino microcontroller. End game i would have an APP sensor (you should have one already), a throttle switch (not needed but i have one) , 2 pressure sensors, and a pwm output to the bypass valve. I have already made a 2 stage progressive meth controller for my old common rail before so none this should be too difficult to design/program.

First build will use throttle switch, timer and pressure readings to run bypass but going forward would incorporate an app sensor.

So at no "throttle" it would open the bypass

put your foot down it would close the bypass the SC would build pressure and bypass once the boost at Turbo is at X and pressure at SC is lower than X. this would keep happening untill the supercharger had been on for x time say 10 seconds are you are driving in a stable state and the throttle switch has not been reset.

Once working will be upgraded to add accelerator pedal position sensor and position sensor (on the bypass valve) to turn supercharger back on when pedal pressed over X additional percent ie going for the overtake. Then only open say as much as needed to bypass for the turbo ie 50% so that it can close quicker ready for the next gear then after x time without changing gear go back to 100% bypass to maintain efficiency

In my case I would also need a 3 port waste gate to stop the super charger opening the waste gate prematurely

Hope this sounds logical, if you would like to hit me up on social media i wouldn't mind a bit of back and forth of ideas on this and control aspects etc.