I've mounted a Mini Cooper M45 to my NB2.
Originally my plan was to run just a Bypass valve that is IM referenced, with a TB on the back of the supercharger.
That's how I've seen all the kits do it.
Then I started reading about how helpful dual TBs can be for throttle response and idle.
Its hard to be able to decipher what these old threads I'm finding are talking about exactly because they're all running a MAF with a power card.
My car is running SD on a Megasquirt, it seems like I can probably tune out the things that dual TBs fix.
The issue with running no bypass is funny things going on with the IC piping, and the supercharger heating up at idle.
This is a thread I've found that is all over the place, but sheds some light on whats going on.
https://forum.miata.net/vb/showthread.php?t=203287
Below is what seems to be the smartest thing to do, while keeping the TB at the IM. The Bypass valve keeps the supercharger happy while idling, and the BOV helps between shifts (and situations like that).
I don't quite understand it however. The supercharger is being bypassed. But, its still spinning? Is it just moving air through itself, and then back into itself through the bypass?
It seems like the BOV is almost working backwards to a turbo car. At idle, the supercharger is running, pressurizing the intake tubing (in theory). Thus, the BOV stays open. In boost, it stays shut. Acting just like the bypass valve would.


To me it seems like a BOV is just this "by pass" valve. It opens up at idle (IM vacuum), and provides the supercharger's outlet a way to go back to the inlet's pressure. Why run the complexity of two valves? Again it seems that the bypass valve is only there to help a MAF'd car meter its air correctly.

Maybe, no BOV? Is it completely unnecessary? Again, I'm having a hard time finding current, relevant data for what/where the bypass should be.

 

You are over thinking it.

you can run the supercharger throttled or not. You don’t need two throttle bodies.

You only need one bov/bpv your m45 might have a bypass in it already and you don’t need to do anything. Or if you want Wooshie sounds you can install a bov/bov and vent to atmosphere for all the Wooshie sounds. If you use an aftermarket bov/bpv make sure you get the appropriate spring in it.

the purpose for the valve in either case is to release pressure When the throttle blade is closed. post supercharger either to atmosphere or to pre supercharger.

i like to have the bov/bpv pre Intercooler and vented to atmosphere to vent out hot air out of the system.

 

Vast majority of M45/Mp62 threads on that other forum are primarily US-based Jackson kits -JR chose to install the TB on the SC inlet. The ONLY reason it’s done is because the unthrottled SC is considered a bit loud. M45s are tolerable. Larger blowers get quite a bit louder.

UK-originated Cooper kits kept the TB in the OEM location. M45 with TB on IM, unthrottled SC:
If you’re okay with that volume, then follow the UK kits and just keep the TB on the OEM location with a BOV between the SC and TB. Simpler with solid idle.

 

BTW these Mini chargers can be had for cheap, and I really like this particular kit from Salon.

 

My unthrottled MX3. Don’t skimp on BOV if you run this way, SC pushes a lot of air on high rpm off-pedal situations. I use a 38mm adjustable Turbosmart valve. Air temps are cool and car feels quite normal off boost. Yes, throttle response is immediate. I also think it would be a lot easier to upgrade to turbo later if I wanted to because I’m basically routed the same way besides exhaust

 

Oh yea and if you let your GF try your car do it in a wide open parking lot. I almost bought a storefront window yesterday. 😂 I really posted to remind you that in this case an atmospheric BOV will NOT work in a ‘piggyback’ condition. I am on Megasquirt, if you still use airflow meter you MUST recirculate as BOV stays open at idle and half throttle