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Twincharge, which way round & supercharger bypass or not?

Diesel Tuning Fundamentals

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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.