First let me say this.....I do not want to start WW III with this question, since the last time I was involved in a similar thread topic it got ugly fast and I really do not want a repeat.

I recently installed a boost/vacuum gauge and have noticed this. The Mini produces more vacuum than boost. This is where the confusion kicks in. I come from the diesel engine world and they produce boost constantly except when you take your foot off it. I can not say close the throttle since they do not have a throttle body.

Now what I have noticed is the Mini produces vacuum most of the time this value being anywhere from 5-20 in. hg. Changing in value with engine load and it is not until I really open the throttle do I see any boost with a max reading of 12 psi.

My question is why? The SC is always turning just like my turbo in the diesel so why does it not always produce boost? Does this have to do with having a throttle body? Or is my install incorrect?

Thanks for reading my post and questions.

 

You are seeing the effects of the bypass valve. You don't really get full boost levels unless you are at full throttle. The bypass valve exists to help improve fuel economy by reducing the load on the supercharger (and thus fuel consumption) under part-throttle conditions.

Scott
90SM

 

My Mercedes E55 with an aftermarket supercharger is the same way. I have 20 psi vaccum and only 6psi boost. This is the correct way for positive displacement supercharger. I know tubo's work differently, my roomates diseal has zero vaccuum according to the gauge but hits 30psi of boost. Your Mini is working perfect, no worries.

 

I'm going to correct you here, you have 6 psi of boost and 20 in hg of vaccum, there is a difference.

since the mini has a gasoline engine, it has a throttle body. lets just say your crusing on the road at 60mp at about 5k rpm. the throttle plate will be open quite a bit. now jump off the gas, the throttle plate smacks closed. now, instead of pulling air past the throttle plate, the air is cut off, and the motor is crating a vaccum by trying to pull air into the motor with the throttle plate closed. dont confuse this with true vaccum, which it is not. at an idle the car is doing the same thing, but the bypass valve is open, since thhere is a vaccum, the spring cant hold it closed, and the throttle plate is also mostly closed, so it will also draw a vaccum, but ot nearly as much as on decel.

help any?

 

Why whatever are you talking about

But thanks for proving my point

To the point though 90STX pretty much nailed it. Although on a "normal" roots blown motor(let's say a 6:71 on a small block Chevy) you will see similar condotions.
The bypass isn't the only reason for this vacuum/boost condition you're seeing.

 

20 in of hg is not the same as 20psi of boost. they are not the same unit of measurement. like apples and oranges.

 

Diesel vehicles dont produce vaccum, there is no throttle body.

thats why they have to have hydroboost brake assist or a vaccum pump on the motor.

 

90STX, thanks for your explanation. That makes sense, to get a better understanding where does the by-passed air go. When I shut down the engine I can hear what sounds like a pressure relief valve open and at the same time watch the boost-vacuum gauge go to zero.

obehave, I do not know what to say if you can believe that. CrAAp you mean I just proved your point, how the hell did that happen?

 

It's good to know that after returning from a 3 month NAM hiatus, you're still up to your antics.

 

Diesel and gas engines are two distinctively different animals, mostly at the combustion chamber. A gas engine must maintain a certain air/fuel ratio across all operating ranges. As such, in order for a gas engine to operate at idle, the supplied air must be restricted to match the given amount of fuel. That's where the throttle comes in. By closing up to reduce airflow, the air/fuel ratio is maintained close to its 14.7:1 ideal ratio. To increase engine power, both the air and fuel must remain mostly in lock step with each other (albeit becoming a bit richer at max output) for maximum power, efficiency and lowest emissions.

A diesel engine relies on the heating of the intake air at compression to ignite a fuel charge. No spark plugs! A diesel engine at idle takes in a full, unrestricted, cylinder-filling charge of air, this is compressed, raising it's temperature over 500 degrees before the fuel is injected directly into the chamber. The fuel, once hitting the superheated and compressed air, ignites and creates the pressure in the cylinder to drive the piston through the power stroke. The unrestricted air intake system does not allow a diesel engine to make any vacuum during it's operation. In fact, any sort of intake restriction severely undermines a diesel engine's efficiency. By contrast, the more air you can stuff into a diesel engine raises it's efficiency and power output. This is why most of today's diesel engines are turbocharged. Because of the operational means of a diesel engine, it's able to utilized a huge variance of air/fuel ratios, especially at idle and low power situations. Also, not having a throttle vastly decreases air pumping losses at low and mid power usage. This is one of the reasons for a diesel's fuel efficiency.

The supercharged MCS engine, even though the blower is constantly turning, has both the throttle body as well as the bypass valve to regulate boost. Operating below atmospheric pressure range (in vacuum) the engine behaves like any other gas engine. It's when you start making boost that the magic happens.

 

Stop!! If you keep this up I will have to take a cold shower.

Thanks for the reply Greatbear. So at idle or normal cruising RPMs the SC produced air is too much for the ideal mixture. Correct? In turn, if the air was not by-passed than the engine would stall or run rough? Correct? If the fuel was brought up to the level for the air the engine does not have the displacement for that much fuel/air? I am still on track or getting off base now?

When the MCS engine is running and producing a vacuum (by-passing the SC air) where does it go? And how is it actually being by-passed?

 

Actually, if the bypass valve were not there, things would still run pretty much normally, albeit with some efficiency tradeoffs. The blower sits between the throttle body and the engine. Closing the throttle will still restrict airflow to the engine. The inefficiency happens because you are first restricting air to the blower, which is overdriven to move (pump) more air than the engine would move (pump) on it's own. So, in addition to restricting engine airflow, you are creating an additional drag on the blower. The bypass valve opens a path around the blower at idle and part throttle operation. This action prevents any pressure differential from existing across the blower. In effect, the blower is sitting in free air, reducing the drag that would come from having a differential pressure across it. When you approach near zero vacuum, the valve closes allowing the blower to work it's magic.

In other words, the bypass valve is not necessary for proper engine operation, it just aids in efficiency by eliminating drag during the period of time where the engine is spending most of it's time operating (no one is making boost at idle or part throttle). If you are pushing enough air in a gas engine to make 'boost', you would need to follow it up with the proper amount of fuel to burn properly. If you cut back on fuel, you will run extremely lean, which causes high emissions, grossly low efficiency and ultimately engine damage. This happens because in a typical gas engine, you introduce the air and fuel together as a package into the cylinder, compress the charge then light it with a spark at the proper time. If that package does not follow the proper 'recipe' by having too much or too little fuel, you will not make optimum power and you can damage your engine, since that 'package' has to remain stable before and during combustion.

Diesel engines work entirely different, and as such can be tuned differently. In effect, you want to fill the cylinder completely with air during the intake stroke, even at idle. The air becomes heated during compression, much hotter than would a similar gas engine because the ratio is much higher. Note that so far the engine is dealing only with air, not an air/fuel payload package as a gas engine. The now superheated air at the peak of the compression stroke is hot enough to ignite fuel without any other supplemental means, like a spark. It is at this time that the fuel is introduced into the cylinder. The instant the smallest amount of fuel enters the cylinder, it begins to burn. By varying the amount of fuel going in at this time you are able to regulate how much energy the resulting power stroke will contain. A tiny amount of fuel results in an idling engine. A larger amount will make the engine make power to increase it's RPMs and move the vehicle forward. Interestingly, compared to a gas engine, the more air you make available (to a point) to a diesel engine during it's operation, the more air is available to either burn the existing small amount of, say, part-throttle (a misnomer, since a diesel has no throttle!) fuel for greater efficiency during cruising, or have the capacity to burn a larger amount of fuel for making more power than would be possible without any boost (foot on the floor).

Make sense?

 

Yes it does.

I am good with jet and diesel engines but for some reason this super charge thing was clear as mud. I understand the how the pump (sc) works but the vacuum readings on the gauge threw me off.

Thanks for taking the time to answer my question.

Rick

 

Hey!! Crap happens

 

Once an ahole always an ahole




Uh.....I mean me...just to be clear