Quote:
Originally Posted by curehead
I tried already searching for an explanation, but can't find any more details on how these actually work and what the difference is.
I understand the new S55 engine has two mono scroll Mitsubishi turbos while the N55 has a single twin scroll turbo. But how do the two turbos in the new one work? Do they run simultaneously or subsequently?
Thanks!
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The S55 will have a twin turbo system similar to the N54: 1 monoscroll turbo per 3 cylinders.
The N55 has a single turbo with twinscroll technology: 1 turbo with 2 channels for 6 cylinders.
Mono scroll has a single channel within its turbine housing. Twinscroll has 2 channels of different volumes within its turbine housing (not to be confused with divided turbine housing with 2 channels of the same volume). Having 2 channels within its turbine housing enables cylinders to pulse exhaust gases evenly onto the turbine blade to improve spool. Twinscroll technology enables quick spool per 1 channel, with optimal flow in the other channel (divided housing has channels of the same size, so both are for optimal flow). However, monoscroll turbine housings outflow twinscroll and divided housings. Below are pictures to help visualize what I'm trying to convey:
Monoscroll (basic):
Twinscroll (note the 2 channels have different volumes, 1 for velocity/spool, 1 for flow):
Divided housing (note the 2 channels have the same volume, both for flow):
To answer your question, no one really knows unless the tech specs are released. I can't imagine BMW M to NOT use N54 style setup (both actuated simultaneously) rather than a bi-turbo setup circa mk4 supras (1 small turbo and 1 large turbo where the smaller helps with spool and the larger helps with flow) since the N54 and Alpinas have proven to work just fine.
TLDR;
monoscroll = 1 channel in turbine housing
twinscroll = 2 channels in turbine housing, different sizes
divided housing = 2 channels in turbine housing, same size
S55 = Simultaneous, according to N54 and Alpinas? TBA