I’m trying to understand the boost gauge in the dash of the Z06. I thought a supercharger made boost all the time. Why does the gauge show vacuum? Or appear to? Thanks!

 

There is a bypass valve in operation that prevents excess S/C pressure at idle and part throttle conditions.

The valve closes fully and directs all S/C pressure to the intake at high RPM and high load conditions based on vacuum.

 

Thanks Andy!

 

You're welcome!

More info here also:
https://www.corvetteforum.com/forums...st-at-all.html

 

Sigh

this explanation is what a recirculation valve does, just like in a turbo setup, when you are at full boost and suddenly get right off the throttle. The boost has to have somewhere to go
so a recirc valve bypasses it back around since it was already metered air.
This has less than nothing to do with why you see vacuum on your guage at low throttle inputs and idle.

You are seeing vacuum because your system is working perfectly.
A supercharger absolutely does NOT "build boost all the time". If that were true you'd literally have a runaway engine that would ramp up past redline and blow up 2 seconds after startup.
Same as a runaway diesel.
Your supercharger is throttled, same as any gas engine. When you are at part throttle or idle you are heavily throttling it so it simply can't get the air to build boost (this is a good thing)
And just like how any gas engine is modulated, a supercharged one is no different. When you step on it you open that valve and NOW your supercharger has all the air it wants and
will immediately ramp up to full boost from vacuum.
The statement that they build boost all the time is false. They build boost instantly because it's a positive displacement compressor thus needs essentially just one revolution to be at full boost once the throttle is opened.

 

a modern street supercharger does not build boost all the time. even an old 14-71 roots blower doesn't truly build boost all the time. its RPM related mainly along with throttle. throttle blades barely open equals very little airflow. you need air to compress to "make boost" there is a lot involved in it and the way they act. twin screw, centrifugal, roots if it has a bypass valve or not etc.

But yes you absolutely should be seeing vacuum at idle, cruising etc if you don't see boost at heavy throttle input then you have a problem. if you saw boost all the time you would also get to see every single gas station you come near.

 

While we are at school, can someone explain in simple terms why opening the exhaust up lowers boost? I am a turbo guy, this is my first blower car.

 

the only way it could lower boost on the guage is if it increased the volumetric efficiency of the motor on the exhaust side by increasing scavenging to let you get in just a tad more fresh charge
All other things being equal (throttle position, rpm, ambient air temp, etc) would technically reduce gauge pressure a bit while maintaining the same actual flow since it's positive displacement and throttled as it has to be.

basically you are just replacing a little spent exhaust with fresh charge utilzing the same output from the compressor

Kind of why an NA motor needs a solid 4-5k rpm to hit peak torque (not because of the exhaust but because of the improved volumetric efficiency at higher rpm)

 

Any ideas why the dash gauge would not be reading properly but the car seems to run fine? There are no codes thrown or check engine light on.

 

what is not reading properly on the dash guage?

 

The gauge on the dash will do a sweep on start up like it is doing a start up check but then it will not move while under boost or pull vacuum at idle. It just sits at about 5 on the vacuum side.

 

 

oh ok, yeah that is a guage problem if it runs fine because it would run like a nightmare if it only had 5psi vacuum at idle
no idea how this guage gets its reading but somebody here probably knows

 

Wasn't dumbed down enough for me haha, I still don't understand.

I don't even know if the statement I said is true? I'm not talking about reading the gauge. I am saying on the dyno reading boost through HPtuners. I was told that if you take a bone stock Z06 and put a larger exhaust on it, you will lose boost pressure. So, if stock boost is 12psi and you only upgrade or open up the exhaust, you might drop to 11psi but somehow make more power? If this is true, I can't comprehend it.

 

It's kind of like a 2 cycle where that gooseneck in the exhaust "tunes" it to send a pulse back to the exhaust valve and purge more spent gas allowing more fresh charge in.

But in this case a better exhaust (it would have to be way better btw) will let the cylinder purge more.
Keep in mind there is always some exhaust left in the cylinder, so imagine a crappy exhaust with high back pressure leaves 10% of that cylinder left with exhaust still in it.
Now the supercharger, or even an NA engine, can only actually fill 90% of the cylinder. So all things being equal it's compressing "x" volume of fresh air to "y" pressure to fill 90% of a cylinder.
Then image getting that remaining 10% out and now the same supercharger is still compressing the same volume of fresh air, but now all of a sudden instead of filling a 0.9 liter cylinder (making up numbers here)
it's filling a 1.0 liter cylinder because you got it completely empty now rather than having that 10% of worthless exhaust left in there.
So the total volume of air that the supercharger can compress is a fixed amount, but now you "effectively" increased your displacement of the engine simply by getting more exhaust out to replace with fresh charge (increased volumetric efficiency)
Ultimately the boost reading in the plenum feeding the intakes would be slightly lower because each cylinder is now taking in a little more of it.

Essentially it's the same idea as if somebody modded their motor and punched out the cylinder by .050 over or something to increase displacement but didn't alter the supercharger in any way.
Now that same supercharger is feeding a larger motor so it won't be able to build the same boost as it did with a smaller one.
Likewise if somebody put this supercharger on top of a 4.0 V8 or something it would boost out of control high and probably blow it up.

 

Ding ding ding, you hit the nail on the head! Thank you, that makes complete sense to me now. In my head, I kept telling myself if all the valves are closed during the compression stroke, how the hell does the exhaust effect boost pressure. You explained it perfectly, thanks.