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Discussion Starter #1
so was reading a post on another forum and this guy was going on about how he has a boost controller for his JRSC that he got from JR.

my first thing was to flame the guy. but as i was typing i was thinking about it. and it made a lot of since. i knew what to look for. and idea that would work. and came across this.
928 Motorsports - Porsche® 928 Performance Parts - Universal Adjustable Boost Limiter Valve (BLV)

this is what i was thinking of. simple device. at a certain boost level will open and bleed of boost. and then close it down to get the boost from your pulley setup.

one setup for eco driving and one for crazy driving.

what is your guys take on this?
 

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The only boost controller you need with a roots blower is a bigger top pulley.
 

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Discussion Starter #3
ha ha ha. funny.

changing pulleys is a pain. this looks much better.
 

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Totally not needed. If you want to drive economical you have to be careful on the pedal, which means you wont build boost anyway. If you are not able to control boost (or stay out of it) with the pedal, you need to learn it, or you will never get good mpg anyway ;)

Put a smaller pulley on the snout if you are really worried about mileage. Lower speed on the blower gives you less drag and less wasted gas.

Edit: Brainfart. Bigger snout pulley = lower blower speed ofc :p
 

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Discussion Starter #5
true didnt think of it like that... its all about the right foot. 3.1" snout pulley small enough 12 psi with some housing porting. and custom cooling...
 

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Not funny at all. That is how it is done.

To make extra boost then bleed it off is pure waste of engine stress and fuel.
 

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The only boost controller you need with a roots blower is a bigger top pulley.
at 1st i rofl.. then started thinking when you say bigger pully you telling him to run lower boost? or do you mean run a smaller pulley so he could run higher boost? maybe im retarded and is completely lost in this thread?
 

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Well a boost controller other than pulley size can only reduce boost. It can't increase it, so a bigger pulley on the blower snout decreases blower speed or a smaller blower snout pulley increases blower speed and therefore boost.
 

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Discussion Starter #9
hahaha... think i should have post this in the joke section.
 

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no disrespect pat but i understand pulley sizes and the affect they can have.... i was just try to comperhend your post..... now i could understand if his sc push 18+ psi why he would might run a boost controller, but for wat hes got it is pointless
 

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A boost controller on a blower is still kinda pointless, unless it for some reason is impossible to change pulleys. Pumping air with the blower requires quite a bit of energy from the engine, and higher blower speed = higher iat's. So why go through all that, and then just bleed it out? Doesnt make sence.
 

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That would make sense on a centrifugal setup so you could overdrive it but keep peak boost down to a safe level, besides that, no.
 

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Overdriving the Roots and then bleeding off boost makes sense in some ways. the biggest of which is early boost. Even though it's a positive displacement machine, you can still increase the off idle and low RPM boost by overdriving it. Case in point: my M62 D series setup makes 11 psi at 2,000 rpm, by 3,000 I'm at 14.5, by 4,000 its above 17, etc... I also had some trouble with my power and torque falling off at 7,000 even though the boost was still climbing (at the 7,000 power peak it's 22psi, but it keeps climbing to 26psi at 9,000). If I could have somehow bled off the boost there, I might have made some more power by reducing the load on the crankshaft from the blower.

There was a guy a bunch of years ago that made a JRSC boost controller. It was an electronic device that read a MAP sensor and would control a R/C servo to open and close the bypass valve. I never got the opportunity to get my hands on one, but from all accounts it worked quite well.
 

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I cant see how the gains in boost at extremely low RPM (where its almost usless) would be worth the loss of efficiency
 

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A centrifugal is not positive displacement and is different that the air supplied increases exponentially with rpm whereas a roots builds up directly with rpm.

The engine airflow requirement also increases directly with rpm. Variations in boost at different rpm can occur due the the blower and the engine having different VE at different speeds which distorts this a little and it takes a few turns of the engine for the manifold volume to be pressurized as more air is pumped in than flows out.

I have never run a JRSC but I have run a lot of roots blowers. I never saw more than a few # difference in boost between idle and red line at WOT. The difference is greater with a worn blower.

I still see no point in fitting a blow off valve to a JRSC setup with an M45 on a D. If i had a doubling of boost over the normal powerband I would be having a close look at the rotor to housing clearance and the rotor phasing and I would take a close look at the discharge port to make sure it had not been over ported.

Boost can also build if the airflow in the head chokes or if cam timing goes whacky for some reason.
 
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