Missing Boost

luigi90210 · 08-09-2013, 02:49 PM

Quote:

Originally Posted by Chuck33079

He is supercharged. You want it open at idle. You're correct for turbo cars.

could you explain why?

i have always been under the impression that centrifugal superchargers are basically belt driven turbos, so wouldnt anything that applies to a turbo setup apply to a supercharger setup(except for obvious stuff like boost controllers and other turbo specific parts)

Sh0velMan · 08-09-2013, 03:05 PM

Quote:

Originally Posted by luigi90210

could you explain why?

i have always been under the impression that centrifugal superchargers are basically belt driven turbos, so wouldnt anything that applies to a turbo setup apply to a supercharger setup(except for obvious stuff like boost controllers and other turbo specific parts)

Because the supercharger would restrict airflow and affect idle quality etc.

The turbo can just spin freely in the relative vacuum created by being off boost, so it isn't an issue, the supercharger effectively functions as an air restrictor in a feedback loop... limit air, engine slows down, engine slows down, supercharger spins slower, air is more limited, repeat.

There are ways to compensate around it, but the simplest is to leave the BOV open.

And I refuse to use a different acronym for BOV simply to differentiate between recirc and non-recirc. :P

Mike@GTM · 08-09-2013, 03:14 PM

Quote:

Originally Posted by luigi90210

could you explain why?

i have always been under the impression that centrifugal superchargers are basically belt driven turbos, so wouldnt anything that applies to a turbo setup apply to a supercharger setup(except for obvious stuff like boost controllers and other turbo specific parts)

Since the supercharger is driven by the belt, the rpm of the supercharger is dictated by engine rpm regardless of throttle position. Therefore, at small throttle angles, the supercharger is trying to push air into the engine against the throttle. Since the bypass valve operates on the differential pressure between the intake manifold and the pressure in the pipe, it will be open when the manifold is seeing vacuum while the pipe is pressurized. This condition will occur at idle and at low throttle angles. Once the throttle is opened enough, the manifold will see pressure and so will the pipe. In that case, the bypass valve will close since the differential pressure will not be high enough to overcome the spring inside the bypass valve.

A turbocharger's rpm, however, is dictated by exhaust flow. At low rpm and/or low throttle angles, there is insufficient exhaust gas flow to spin the turbocharger fast enough to develop boost. Therefore, the pressure in the charge pipe will not be significantly higher than the pressure in the intake manifold and the bypass valve will remain closed.

I hope that clears things up for you.

Sh0velMan · 08-09-2013, 03:30 PM

Quote:

Originally Posted by Mike@GTM

Since the supercharger is driven by the belt, the rpm of the supercharger is dictated by engine rpm regardless of throttle position. Therefore, at small throttle angles, the supercharger is trying to push air into the engine against the throttle. Since the bypass valve operates on the differential pressure between the intake manifold and the pressure in the pipe, it will be open when the manifold is seeing vacuum while the pipe is pressurized. This condition will occur at idle and at low throttle angles. Once the throttle is opened enough, the manifold will see pressure and so will the pipe. In that case, the bypass valve will close since the differential pressure will not be high enough to overcome the spring inside the bypass valve.

A turbocharger's rpm, however, is dictated by exhaust flow. At low rpm and/or low throttle angles, there is insufficient exhaust gas flow to spin the turbocharger fast enough to develop boost. Therefore, the pressure in the charge pipe will not be significantly higher than the pressure in the intake manifold and the bypass valve will remain closed.

I hope that clears things up for you.

I had it backwards, S/C trying to increase boost pressure even at idle, not the other way around.

Thanks Mike!

luigi90210 · 08-09-2013, 03:43 PM

Quote:

Originally Posted by Sh0velMan

And I refuse to use a different acronym for BOV simply to differentiate between recirc and non-recirc. :P

lol, you're not alone with that one XD

i use to do the same all the time but i kept getting corrected by friends so i started using BPV and BOV

Quote:

Originally Posted by Mike@GTM

Since the supercharger is driven by the belt, the rpm of the supercharger is dictated by engine rpm regardless of throttle position. Therefore, at small throttle angles, the supercharger is trying to push air into the engine against the throttle. Since the bypass valve operates on the differential pressure between the intake manifold and the pressure in the pipe, it will be open when the manifold is seeing vacuum while the pipe is pressurized. This condition will occur at idle and at low throttle angles. Once the throttle is opened enough, the manifold will see pressure and so will the pipe. In that case, the bypass valve will close since the differential pressure will not be high enough to overcome the spring inside the bypass valve.

A turbocharger's rpm, however, is dictated by exhaust flow. At low rpm and/or low throttle angles, there is insufficient exhaust gas flow to spin the turbocharger fast enough to develop boost. Therefore, the pressure in the charge pipe will not be significantly higher than the pressure in the intake manifold and the bypass valve will remain closed.

I hope that clears things up for you.

ok im still a bit confused about it but i get the basic idea, so even though the valve is the same internally, it will function differently on a supercharger setup vs a turbo setup?

could you in theory have a BPV designed for a turbo to work with a supercharger assuming everything fits? or would you need a different valve?

Sh0velMan · 08-09-2013, 03:47 PM

Quote:

Originally Posted by luigi90210

lol, you're not alone with that one XD

i use to do the same all the time but i kept getting corrected by friends so i started using BPV and BOV

ok im still a bit confused about it but i get the basic idea, so even though the valve is the same internally, it will function differently on a supercharger setup vs a turbo setup?

could you in theory have a BPV designed for a turbo to work with a supercharger assuming everything fits? or would you need a different valve?

It's the same. The key you're missing is after the boost as been released on a turbo setup, generally there is no more boost being generated and the valve closes. The valve is "tuned" so that it requires a certain pressure differential between the manifold and the cold pipe in order for it to open. This is controlled by adjusting spring pressure inside the valve.

With the supercharger, boost is continuing to be generated at idle, so that differential is such that it holds the valve open slightly to release that pressure back to the pre-S/C pipe.

Get it?

luigi90210 · 08-09-2013, 05:14 PM

Quote:

Originally Posted by Sh0velMan

It's the same. The key you're missing is after the boost as been released on a turbo setup, generally there is no more boost being generated and the valve closes. The valve is "tuned" so that it requires a certain pressure differential between the manifold and the cold pipe in order for it to open. This is controlled by adjusting spring pressure inside the valve.

With the supercharger, boost is continuing to be generated at idle, so that differential is such that it holds the valve open slightly to release that pressure back to the pre-S/C pipe.

Get it?

OK I think that really cleared things up, since the supercharger is always creating boost, you want the valve open at idle because if it isn't, you will run into problems.

pokeyl · 08-10-2013, 04:36 PM

The valve

other, thanks Mike

pokeyl · 08-23-2013, 11:42 AM

New valve is on, BUT, still missing boost. As soon as the traction fluid is warm the boost goes away.

If I WOT as soon as the oil temp is 160 I get 3 PSI at 3K

15 minutes later I get ZERO boost at 3K

outside temp is 75F

I looks like I may need a new supercharger. I have found other rotrex failures over at the BMW sight.

A lot of talk about overspeeding the SC causing the problem, and the user I bought mine from had the smallest pulley GMT makes on it with the stage 1 intake. He may have fried the unit and then sold it to me. As I did my install in the winter, the fluid was cool enough to keep the boost up.

I am working with Sam and Mike to come up with a solution.

08-10-2013, 04:36 PM	#8 (permalink)
pokeyl Track Member Join Date: Nov 2010 Location: DFS Posts: 900 Drives: ??? Rep Power: 55	The valve other, thanks Mike __________________ It's not my fault I'm old, I was born a very long time ago!

08-23-2013, 11:42 AM	#9 (permalink)
pokeyl Track Member Join Date: Nov 2010 Location: DFS Posts: 900 Drives: ??? Rep Power: 55	New valve is on, BUT, still missing boost. As soon as the traction fluid is warm the boost goes away. If I WOT as soon as the oil temp is 160 I get 3 PSI at 3K 15 minutes later I get ZERO boost at 3K outside temp is 75F I looks like I may need a new supercharger. I have found other rotrex failures over at the BMW sight. A lot of talk about overspeeding the SC causing the problem, and the user I bought mine from had the smallest pulley GMT makes on it with the stage 1 intake. He may have fried the unit and then sold it to me. As I did my install in the winter, the fluid was cool enough to keep the boost up. I am working with Sam and Mike to come up with a solution. __________________ It's not my fault I'm old, I was born a very long time ago!

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