SC Boost Question

[KLMOTORSPORTS]

SC builders....at what RPM does your boost kick in? I am getting no boost even around 3-4K RPM and wondering what could be wrong. Checked the Bypass with a vacuum gauge and it holds steady vacuum and the arm moves free, so presume the bypass is working right.

My boost gauge (Autometer) is run off the vacuum line from the intake (on top of intake when looking down between SC and Valve Cover). I am running an aftermarket engine harness, but don’t believe that should have an effect on showing no boost....any ideas?

Thanks!

 

[Lonny]

I think on mine at any just about any RPM, it will show some positive pressure when I hit the accelerator pedal. It does go higher as the RPM goes up.

 

[ctuinstra]

Agreed. There is no lower limit to where boost kicks in. The bypass valve is the only thing that limits boost by bypassing it. You should be able unplug the vacuum line to the bypass valve to stop it from limiting boost.

If you are still not getting boost, it could be worn compressor vanes, slipping pulley, or simply failing to gauge the actual pressure. An OBDII scan gauge or other instrument should be able to read the MAP and/or boost. That would also give you an idea of the actual boost pressure.

 

[TheNuker]

Yep agreed, you should feel the power the second your in the gas petal. The boost gauge should shoot right up instantly.

Nuker-

 

[KLMOTORSPORTS]

Great, thanks all for the inputs...definitely gives me a good starting point for trouble shooting...appreciate it!

 

[DCMoney]

My bypass valve was sticking open and only building a little boost at higher RPM's.

 

[ctuinstra]

For what it's worth and probably information no one asked for, in HP Tuners there is a whole section where the you can adjust the boost parameters. You can't really make more, but you can adjust the limits of boost for certain conditions such as high IATs, etc. Most of these are simply controlling the opening of the bypass.

These are not the default settings. Many of them I have removed the limits. I'm so bad.

 

[Murcielago311]

Ya when you hit 318 mph, that's probably enough boost. lol

 

[Tinkles]

How are the vacuum lines installed on the bypass valve? When installing a M62 onto a Cavalier/Sunfire you have to connect the top port to the blower and the bottom vents to atmosphere.

Also the bypass valves are known to fail.

But yes, boost is instantaneous with a supercharger.

 

[ctuinstra]

I’m trying to figure out when and how the bypass is used and controlled by the ECM. I’m not having much luck with concrete information. Our “tuner” had us unplug it because of some “fluttering” and the top of the dyno pulls. But don’t like the idea and I recently plugged it back in.

I which I could monitor it using HP Tuners Scanner but there is no PID for it.

Is it only for limiting boost or is it used often at idle/cruise? If I was really so inclined, I would convert the solenoid signal into serial data and import during the scan.

 

[Tinkles]

By venting the bottom port to atmosphere you take all the computer control away from it and it just does it's thing.

On blower cars boost is limited/determined by pulley size and ratio(crank pulley size vs blower pulley size). Larger blower pulley and smaller crank pulley will decrease boost pressure while a smaller blower pulley and larger crank pulley will increase boost. The bypass valve is there to allow air to bypass the blower in unneeded situations such as idle or cruising with low engine load. As the pre-blower vacuum lessens the bypass valve should start to shut via pressure of a internal spring. Once it is shut then all of the air flows through the supercharger. Boost pressure will increase with RPM with a supercharger because it is directly connected to the engine's crankshaft.

 

[Ross]

When you shift, then the bypass is opened to prevent full boost under no-throttle.
@2:30 into this video:

... If I was really so inclined, I would convert the solenoid signal into serial data and import during the scan.

I am really interested in how to do this, as I have an arduino oil pressure gauge that I would like to import.

 

[ctuinstra]

By venting the bottom port to atmosphere you take all the computer control away from it and it just does it's thing.

On blower cars boost is limited/determined by pulley size and ratio(crank pulley size vs blower pulley size). Larger blower pulley and smaller crank pulley will decrease boost pressure while a smaller blower pulley and larger crank pulley will increase boost. The bypass valve is there to allow air to bypass the blower in unneeded situations such as idle or cruising with low engine load. As the pre-blower vacuum lessens the bypass valve should start to shut via pressure of a internal spring. Once it is shut then all of the air flows through the supercharger. Boost pressure will increase with RPM with a supercharger because it is directly connected to the engine's crankshaft.

I appreciate the information. I understand most of it and get the just of it. I’m trying to fill in some of the small holes in the complete understanding, the details.

So at idle/cruise there is a high vacuum post-throttle body where the vacuum line goes to the top of the bypass valve. So high vacuum opens up the bypass to allow air around the compressor. When it looses vacuum and goes towards pressure, it closes to help build pressure. So far this makes a lot of sense

Now on the other half, there is a vacuum line from the intake (post-compressor) that goes to a solenoid and then through to the bottom of the bypass diaphragm. I assume this can suck it back shut or pressure it open (the opposite of the top vacuum line). It’s controllered by the solenoid driven by the ECM. What is the function of this solenoid? Is it only to limit the boost based upon the tables I showed earlier?

 

[ccannx]

I believe that solenoid gives the ECU the ability to allow pressure from the intake after the supercharger to assist the other side of the diaphram inside of the bypass pushing up against the spring and inline with the vacuum pulling up on the spring. It opens a butterfly valve that goes straight to the intake manifold bypassing the supercharger. Mine was worn out and replacing it greatly improved my idle and jerky throttle problems.

 

[ctuinstra]

I believe that solenoid gives the ECU the ability to allow positive cranckcase pressure to assist the other side of the diaphram inside of the bypass pushing up against the spring and inline with the vacuum pulling up on the spring.

I think you mean intake pressure, the bottom vacuum line is attached to the intake just after the supercharger. I'm going to figure out how this works and when the solenoid it active.

 

[ccannx]

Yes will correct it, the vacuum hose comes off the intake after the supercharger.

 

[Ross]

I have considered temporarily adding a dash LED or 12V bulb to the solenoid.
So when the PCM decides to turn on the solenoid, the light turns on.

 

[ctuinstra]

I was thinking the same thing. Without the PRO version of the HP Tuners for the analog input, it's not real easy to scan that solenoid. I thought about using my old AEM with a bad O2 sensor and creating a voltage divider to get a 1-5V signal and using it's serial output. Actually that is a fantastic idea. I would really work without having to buy any expensive tools.

 

[ctuinstra]

Can anyone provide me with positive information on the correct plumbing of the bypass solenoid? I fully understand the bypass itself, but the solenoid and how my is plumbed is completely throwing me.

 

[Tinkles]

Now on the other half, there is a vacuum line from the intake (post-compressor) that goes to a solenoid and then through to the bottom of the bypass diaphragm. I assume this can suck it back shut or pressure it open (the opposite of the top vacuum line). It’s controllered by the solenoid driven by the ECM. What is the function of this solenoid? Is it only to limit the boost based upon the tables I showed earlier?

That I cannot say with any certainty because my S/C Ecotec experience is with a Cavalier not a Cobalt and when installing the same supercharger kit on Cavaliers you let the bottom of the bypass valve vent to atmosphere and there is no solenoid used. But it being a solenoid to limit boost seems odd to me since the M62 will not make enough power to hurt any stock Ecotec, so I don't see a need for it.