I've got a question about my MCS. I was just wondering if there are Blow-off valves for superchargers like there is for turbos. If so what are my options?

 

Superchargers don't need blow-off valves so there are no options. The option you have for Superchargers is the pulley size to speed things up. The John Cooper Works kit and others include a 15% reduction of pulley size.

 

Turbo's need a blow-off valve 'cuz they're spun off exhuast gasses, aka no control.

Superchargers are driven off the crank, and turn proportional to the engine speed aka always controlled.

 

Although I'm not a techie, this is my understanding of the way the blower works at low rpm. Below 2800 rpm, there is a bypass that exhausts air that has been compressed before it enters the intake. That's one reason the MCS stumbles from a dead stop--it's a fairly heavy car (2800 lbs.?) powered by a small bore engine with only an 8:1 compression ratio and no enhanced combustion until 2800 rpm. Couple that with tall gearing, and you have stumble, until you learn to slip the clutch properly to compensate. Actually, this is a good thing from the standpoint of engine life, since there's no need for enhanced combustion when you're tooling down the avenue in 3rd at 2500 rpm waving at chicks.

After 2800 rpm the blower is fully engaged, and that's when the engine adds tons of horsepower, and it's also a reason for the yo-yo most of us get in 2nd gear when standing on it.

As I said, I'm not a techie, but a Mini technician posted this on the MINI2 forum a few months back.

Regards--Roger in New Orleans

 

FYI, a fully equipped MCS (with all factory options) weighs in at 2678 according to my 04 manual.

If there were some type of bypass prior to 2800 rpm, I would think you could hear it. Also, even though I have not taken apart the intake on my car, I have looked very closely at it and there does not seem to be any type of bypass valve to relieve pressure to the atmosphere. But after what you posted, I will take a closer look :smile: .

 

>>Although I'm not a techie, this is my understanding of the way the blower works at low rpm. Below 2800 rpm, there is a bypass that exhausts air that has been compressed before it enters the intake. That's one reason the MCS stumbles from a dead stop--it's a fairly heavy car (2800 lbs.?) powered by a small bore engine with only an 8:1 compression ratio and no enhanced combustion until 2800 rpm. Couple that with tall gearing, and you have stumble, until you learn to slip the clutch properly to compensate. Actually, this is a good thing from the standpoint of engine life, since there's no need for enhanced combustion when you're tooling down the avenue in 3rd at 2500 rpm waving at chicks.
>>
>> After 2800 rpm the blower is fully engaged, and that's when the engine adds tons of horsepower, and it's also a reason for the yo-yo most of us get in 2nd gear when standing on it.
>>
>>As I said, I'm not a techie, but a Mini technician posted this on the MINI2 forum a few months back.
>>
>>Regards--Roger in New Orleans
>>


The supercharger on the MCS is ALWAYS engaged, at low RPMs it just isn't spinning fast enough to provide much boost. Mercedes cars that are supercharged do use a clutch that disengages at low RPMs and low throttle inputs.
PS, love your town, just visited last Thurs-Sun. If you didn't see a DS/W MCS running around it's because we took the street car or walked most everywhere we went.

 

No, ole boy is right...there IS a bypass. It just doesn't vent to the atmosphere. It just allows the output of the blower to return to the intake stream. But since there is no separation of the air before the blower and after the blower, there is no boost. The diverter engages when boost is needed and separates the input from the output.

MJ

 

I didn't mean to imply that the blower was not hooked up continuously--it is, indeed. The bypass just is working until 2800
rpm, as confirmed above. If you think about it, though, it's logical. The engine has very little torque until revs start to build.
This pisses off lots of American drivers, who learned to drive on their Dad's Buick Monstrosity with a 12 litre V-36 engine,
and they're not used to the bog-down that small cars have while the drive train gets fully engaged and the engine is just beginning
to build revs.

This car is not a street racer. But get it on a track in the proper rev range with proper gearbox usage, and watch out! The best
performance you can get for about $22 grand.

By the way, you guys should know that there is a road course in the NOLA vicinity that rents out time by the hour to anybody w/
a competition license or some proof of performance driving instruction. So next time you come to The Big Easy, take one day off
of boozing to go drive your butt off.

Regards--Roger in New Orleans

 

Every car I've owned (all 9) except my '61 Corvair have been 4-bangers. The Cooper S is plenty torquey for me, and the 15% reduction pully helps with the torque down low. A FWD drive car with much more torque would be a real handful.

 

On an eaton-roots type blower you cannot have a bypass valve like a turbo car, but on a centrifugal it isn't a bad idea, because even when you let off the gas the centrifugal still spins because the supercharger pulley is attached to the SC via a one-way differential (or at least something similar in concept) which allows the SC to "freewheel" after you let off the gas. This air taht the compressor pumps after lifting off the throttle will have to be vented to atmosphere so that's why you sometimes see a BOV on an SC car.

 

Ramirz...you are right, the blower itself does not have a bypass valve or a blow-off valve built in.

However, the MINI and a LOT of applications that use Eaton roots blowers DO have a Bypass valve in the intake tract before the blower inlet, the blower spins constantly, but when there is little load on the engine the bypass is opened to eliminate the boost created by the supercharger, there-by letting the engine behave as if it was naturaly aspirated. This allows for lower HC emmissions under part throttle and cruising conditions.

I repeat... THERE IS A BYPASS MECHANISM IN THE MINI INTAKE.

MJ

 

MINIJunky, you bring an interesting point to the discussion.

Any way to programmatically turn off the bypass valve? Can it be bypassed electronically (i.e. unplugged)? Just curious?

 

Here are some quotes from the Eaton web site:

What are the benefits of the Eaton supercharger? 1) Patented technology to reduce noise, 2) Proven manufacturing capability, 3) Packaging flexibility, i.e. reduced package size, 4) Self-contained lubrication, i.e. no external oil connections to the engine, **5) Bypass system used for unloading supercharger during idle and light load, resulting in better fuel economy and quiet operation, **

Are Eaton superchargers noisy? The Eaton supercharger system incorporates a specially designed bypass valve, which is actuated by a vacuum motor near the throttle body, and recirculates the supercharger air flow when boost is not required. During typical driving conditions, the engine is under boost around 5% of the time, which means the remaining 95% of the time the engine is under vacuum, allowing for better fuel economy and a quieter ride.

This seems to prove out the presence of the bypass valve. The Eaton site also has som graphs of the hp draw vs RPM and temp rise. The superchargers are also rated to 14,000 rpm which of a stock pulley occurs at 6750 engine rpm.

 

I agree that there is a bypass somewhere regulating the super charger. I installed a boost gauge in my MCS and I am not a mechanic but i have found that the supercharger will not actually make boost unless your acceleration pedal is more than half way depressed.

By the way that supercharger makes an awful lot of boost from the factory. I am around 10-12 psi close to red line.

Convoy

 

technically 11.5PSI at 6950RPM, the M45 does with the factory 2.06:1 pulley. Also, the M45's redline is 17,200RPM, not 14k. Eaton does not publish actual redline data on their website.

--->dominic - you don't want to turn off the bypass valve. Doing so would kill your emissions and actually hurt your low-end torque at part-throttle. Honestly though, who drives their MINI under 2800RPM?

 

>>MINIJunky, you bring an interesting point to the discussion.
>>
>>Any way to programmatically turn off the bypass valve? Can it be bypassed electronically (i.e. unplugged)? Just curious?

If it's like the Miata SC, the bypass is mechanical, not electronic. Vacuum from the engine holds open a butterfly valve that allows boosted air to recirculate into the intake stream. The bypass is 'bypassed' when the accelerator is pushed X amount and engine vacuum goes to ~0.
You could eliminate the bypass by plugging the vacuum port to the bypass actuator, but as Ryephile says, there's no good reason to do this. The bypass was engineered to give the 'best of both worlds'.

Eric,
:indi: wannabe

 

>>--->dominic - you don't want to turn off the bypass valve. Doing so would kill your emissions and actually hurt your low-end torque at part-throttle. Honestly though, who drives their MINI under 2800RPM?

Interesting. I have much to learn about the ways of forced induction engines. Could this be a similar reason for turbo's having the "lag" ... so they don't actaully rob low-end torque?

 

Turbo 'lag' comes from the time it takes for the increased volume of exhaust gas to 'spin up' the turbo, thus producing boost. It's not a desired effect, but a cost of harnessing the 'free' power.
Not sure about Ryephiles' view that not using the bypass would reduce low end torque - it would create a sluggish idle, as the bypass reduces the SC drag on the engine (less resistance when it is off boost) but net torque shouldn't be affected much, if at all when accelerating. The SC isn't like an AC compressor, which is all drag and no push. With the bypass, however, you get low drag and idle and near instantaneous boost when you get on it.

the following site has good info on all sort of stuff
http://auto.howstuffworks.com/question122.htm

 

If there were no bypass valve, the supercharger would be stalled whenever you let off the gas, since it would be forced to spin down to the speed of the engine. The bypass valve allows the supercharger to freewheel faster than the engine, so when you get back on the gas boost comes on a bit quicker. You would probably only notice the benefit when you let off the gas and then get back on it after only a second or two.

 

amurray, what are you talking about? The supercharger is mechanically linked to the crankshaft. It cannot freewheel. As the engine RPMs drop, so does the blower speed at a fixed rate. As has been stated b4, the bypass mech. is used to lower emissions and provide for a smoother idle and better driveablilty.

MJ