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Boost Gauge Question


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My wife drives the GT 500 most of the time, to and from school, etc. So, we're cruising down the road the other day and she says, "Why doesn't the boost gauge show pressure all the time?" I explain that it's not boosting all the time, then she asks, "But isn't it turning all the time?" Yeah, but..... I realized I didn't know how to explain that. If the supercharger (a big air compressor) is turning all the time, at what point does it go in to "boost" and what is it doing before that point? It's like explaining why a 2.92 is a tall gear and a 3.08 is a low gear--I know, but I can't describe it. Can you?

 

The other point is, she's been on it hard enough to see the needle move!

 

Thanks!

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My wife drives the GT 500 most of the time, to and from school, etc. So, we're cruising down the road the other day and she says, "Why doesn't the boost gauge show pressure all the time?" I explain that it's not boosting all the time, then she asks, "But isn't it turning all the time?" Yeah, but..... I realized I didn't know how to explain that. If the supercharger (a big air compressor) is turning all the time, at what point does it go in to "boost" and what is it doing before that point? It's like explaining why a 2.92 is a tall gear and a 3.08 is a low gear--I know, but I can't describe it. Can you?

 

The other point is, she's been on it hard enough to see the needle move!

 

Thanks!

There's a vacuum actuated bypass valve in the SC that is open essentially bypassing the air around the compressor to the intake. When boost is needed, the valve closes and all the air is fed by way of the compressor. That's my simple understanding of it. I didn't know wives ask those kind of questions.

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Like you said, the supercharger is connected to the crankshaft and hence is always turning, but it is not always compressing. There is some mechanism like a wastegate that is open during times of low power demand and hence no boost. The wastegate is controlled by gas pedal position, so when you accelerate hard (gas pedal in last third of travel or so) the wastegate closes and boost builds instantly.

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My wife drives the GT 500 most of the time, to and from school, etc. So, we're cruising down the road the other day and she says, "Why doesn't the boost gauge show pressure all the time?" I explain that it's not boosting all the time, then she asks, "But isn't it turning all the time?" Yeah, but..... I realized I didn't know how to explain that. If the supercharger (a big air compressor) is turning all the time, at what point does it go in to "boost" and what is it doing before that point? It's like explaining why a 2.92 is a tall gear and a 3.08 is a low gear--I know, but I can't describe it. Can you?

 

The other point is, she's been on it hard enough to see the needle move!

 

Thanks!

 

My understanding is that the bypass valve is not the anwser to your question, I only say that because I have had several blown cars without a bypass valve that all respond the same way, my siimple explanation is that the blower must overcome atmospheric pressure in order to show boost on the guage, bypass is a good thing, but I understood it to only open when say you lift at full throttle, it gives the compressed air a path to vent to as oppossed to pushing open the throttle body, until you overcome atmospheric pressure boost will not register on the guage, the guage measures pressure above 14.7 psi (1 bar), if your guage swung both ways (that will get a few comments) you would actually see vacuum when you lift throttle on the guage....

 

The bypass valve is also what I believe is held open by the computer when the cars are new (no boost) until something on the order of 50 continious miles or 25 starts...then it closes and allows boost insuring at least minimum break in under no boost conditions...

 

Just glad to see she has the guts to move the needle!!

 

 

FP

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My understanding is that the bypass valve is not the anwser to your question, I only say that because I have had several blown cars without a bypass valve that all respond the same way, my siimple explanation is that the blower must overcome atmospheric pressure in order to show boost on the guage, bypass is a good thing, but I understood it to only open when say you lift at full throttle, it gives the compressed air a path to vent to as oppossed to pushing open the throttle body, until you overcome atmospheric pressure boost will not register on the guage, the guage measures pressure above 14.7 psi (1 bar), if your guage swung both ways (that will get a few comments) you would actually see vacuum when you lift throttle on the guage....

 

The bypass valve is also what I believe is held open by the computer when the cars are new (no boost) until something on the order of 50 continious miles or 25 starts...then it closes and allows boost insuring at least minimum break in under no boost conditions...

 

Just glad to see she has the guts to move the needle!!

 

 

FP

True, but I'm not sure you can overcome 1 bar with the bypass valve in the open position.

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True, but I'm not sure you can overcome 1 bar with the bypass valve in the open position.

 

Agree with you 6sp, open valve = no boosty, my point is that closing the bypass valve is not what creates the boost, you can drive with it closed and not show any on the guage till have enough demand to overcome atm press. The air to the engine always goes thru the supercharger, when the bypass is open it does not go around the blower to the intake, it is actually gets recirulated back into the system and used again (second hand air).

 

FP

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First, the bypass vavle is not computer controlled. It is controlled by the difference in pressure across the throttle body. When the throttle is closed there is more pressure ahead of the throttle than behind it, this differential holds the bypass open against a spring. Open the throttle and at some point the differential is not sufficient to overcome the spring and the bypass snaps closed. This point that it closes is actually adjustable to some extent.

 

The reason the compressor does not make boost when the throttle is closed is because the throttle body is ahead of the compressor. With the throttle closed or at very light throttle, the rotors are essentially spinning in a vacume. There is no air to compress. Thats not quite true, there is just less air. Less enough that the compressor ratio does not build the discharge above atmospheric.

 

OK, that should sufficiently confuse the topic. :baby:

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Here is my take because the engine is sucking air than there is negative air pressure in the SC then once you spin the SC more the design of the SC produces more pressure than the vacuum created by the motor.

More RPM means more pressure from the SC so you are driving down the road at 55 and the motor is only turning 1500 RPM the SC is only turning so fast that the motor is still sucking more air than the SC is producing once the RPM's go up than the design of the SC is made so it will suck in more air than then motor is able to do on its own making boost and with the added air the fuel injectors shoot in more gas to keep the fuel to air mixture at the right level so the motor won't run lean.

The wast gate is for when you let off the gas the computer tells the fuel injectors that the motor needs less fuel because the throttle is closed. At this time if you still inject massive air into the motor then you would create a lean condition. The air in the SC has to go somewhere because the motor cant use it so the waste gate opens releasing the air.

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You are confusing the subject Step on the go pedal and boost goes up when the throttle bodie

opens and allows the SC to get more air & more rpm's means more air needed means more boost

take big foot off go pedal and throttle closes and shuts air flow down==boost goes away

simple. (only in my little bitty mine) :lurk::shift:

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First, the bypass vavle is not computer controlled. It is controlled by the difference in pressure across the throttle body. When the throttle is closed there is more pressure ahead of the throttle than behind it, this differential holds the bypass open against a spring. Open the throttle and at some point the differential is not sufficient to overcome the spring and the bypass snaps closed. This point that it closes is actually adjustable to some extent.

 

The reason the compressor does not make boost when the throttle is closed is because the throttle body is ahead of the compressor. With the throttle closed or at very light throttle, the rotors are essentially spinning in a vacume. There is no air to compress. Thats not quite true, there is just less air. Less enough that the compressor ratio does not build the discharge above atmospheric.

 

OK, that should sufficiently confuse the topic. :baby:

JC,

I agree most bypass valves are not computer controlled, I assumed it had to be on the shelby in order for the car to not allow boost when it is new, do you know how this is acheived if it is not done in the bypass valve? Just curious how they make that happen for the first 50 miles or so :headscratch: (dont like it when they take away my boost).

 

FP

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The super charger displaces the same amount of air per revolution irrespective of RPM. The throttle body is a valve that controls the amount of air allowed to enter the engine at any given moment. Under part/no throttle the super charger is still trying to pump the same amount of air. Open the trottle all the way, the restriction is gone and it will pump the maximum amount air it can pump. Thats why at low speed when you dump it you get max boost.

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This is from FRPP TVS installation instructions. Not sure it helps.

 

INFORMATION ABOUT THE SUPERCHARGER BYPASS OPERATION

 

 

There is a great deal of misinformation about the function of supercharger bypass systems. The

supercharger is a positive-displacement pump; that is, so long as it is rotating, it is always pumping air.

During low demand or high vacuum operation (i.e. idle, deceleration, and light throttle cruise), the

pumping action is undesirable as it creates unwanted heat and noise. The bypass circuit, when open,

prevents any pressure buildup across the supercharger and allows air to circulate through the rotors,

allowing the supercharger to “idle” freely during these conditions. This results in reduced noise, and by

reducing heat buildup in the intake, significantly improves street and strip performance. As throttle

demand increases, the bypass circuit is closed, resulting in full performance from the supercharger. The

bypass circuit is never used to limit or control boost during full-throttle operation and defeating or altering

the bypass function will not result in improved performance in any condition, and will result in poor

drivability.

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This is from FRPP TVS installation instructions. Not sure it helps.

 

INFORMATION ABOUT THE SUPERCHARGER BYPASS OPERATION

 

 

There is a great deal of misinformation about the function of supercharger bypass systems. The

supercharger is a positive-displacement pump; that is, so long as it is rotating, it is always pumping air.

During low demand or high vacuum operation (i.e. idle, deceleration, and light throttle cruise), the

pumping action is undesirable as it creates unwanted heat and noise. The bypass circuit, when open,

prevents any pressure buildup across the supercharger and allows air to circulate through the rotors,

allowing the supercharger to “idle” freely during these conditions. This results in reduced noise, and by

reducing heat buildup in the intake, significantly improves street and strip performance. As throttle

demand increases, the bypass circuit is closed, resulting in full performance from the supercharger. The

bypass circuit is never used to limit or control boost during full-throttle operation and defeating or altering

the bypass function will not result in improved performance in any condition, and will result in poor

drivability.

Thought I already said that in one sentence. :beerchug:

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At low rpm or low load the engien is using all the air created the air is still being pulled, Under high load higher rpm so much air is being sucked in and backing up the air gets compressed ... I think i could be wrong

 

You have an interesting stragedy to answering questions on the forum...

 

I prefer the dont answer if you dont know approach for myself...but reading your response made me laugh my butt off! :hysterical: Keep those completely ludicris responses coming! :lurk:

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JC,

I agree most bypass valves are not computer controlled, I assumed it had to be on the shelby in order for the car to not allow boost when it is new, do you know how this is acheived if it is not done in the bypass valve? Just curious how they make that happen for the first 50 miles or so :headscratch: (dont like it when they take away my boost).

 

FP

 

Yes, the throttle is computer controlled. During the first 50 miles the computer will not open the throttle sufficiently to make boost.

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Ah, well, ok, this has all been very informative. Now I know how she felt the night we were watching Knight Rider and she asked what was the difference between our car and KITT. Twenty minutes later she finally interrupted me and said, I just want to be able to tell the kindergarteners whethere it's like KITT or not! Oh, you wanted the short answer, sorry.

 

I think I'll just tell her a valve opens to allow the compressed air to blow off until you really put your foot into it and then it closes and brings up the boost. How's that?

 

Thanks! This has been a hoot!

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