RollsCoal

I have been searching for information on boost control devices for the 12 valve Cummins. Everyone seems to say just get a boost elbow and slap it on. I realize these are tested and work, but I don't understand how they could work well ALL the time. For example, when pulling a long grade in the mountains.

It looks to me like a boost elbow is nothing more than a fitting with a small orifice to restrict the FLOW of air to the wastegate actuator. It does not appear that it would restrict pressure at steady state conditions to the actuator, only flow. So it seems that for transient conditions such as short bursts of acceleration it would work well, limiting the flow and thus the pressure for that short period of acceleration. However, for pulling long grades at high boost pressures, it seems the boost elbow would eventually allow the pressure to equalize on either side of it, and the wimpy spring in the factory actuator would compress, opening the wastegate and you would lose maximum boost pressure.

So, I wondered does anyone uses a pressure regulator such as the ones found on a small air compressor?

Then I wondered if anyone uses a pressure relief valve which would stay closed at low boost pressures, but at high boost pressures pop open to atmosphere, and relieve pressure to the wastegate. I realize this would be a loss of pressure, but from such a small line would it matter?

Finally, what are your opinions on an improved wastegate actuator with stronger spring? I would really like for my wastegate to continue to function, relieving drive pressure when needed, but allow building 30 - 35 psi on demand.

I even considered using a boost elbow or needle valve, along with a pressure relief valve on the wastegate side of the boost elbow. This way I could set the needle valve if needed to restrict flow and prevent too much air loss when the relief valve opened. The pressure relief valve would vent to atmosphere during the long pulls up a mountain at high boost pressures, thus preventing premature opening of the wastegate. (ensure pressure did not equalize on either side of the needle valve)

Wow, this was a long post. Looking forward to your thoughts on this. Thanks.

cdennyb

My son builds turbo systems from scratch for fast street bikes like the Hyabusa, etc. He finds with a higher flow the demand for a bigger opening on the wastegate is increased. It kinda reminds you of exponential increase formula stuff.
He can run a 7# spring in his 38mm wastegate and his bike will boost to over 12 psi. Ask yourself how this possible. Well, initially the spring does in fact stay closed but as boost increases the amount of flow doubles as it opens. If you have any experience with porting heads you know what I mean. Venting the first 10% of the exhaust accounts for the most flow diversion and pressure drop. Opening further is a law of diminishing returns in a sort of way.
I have installed a pressure relief valve on the line going to the wastegate on my gen 3 and it holds the pressure until say, 26 lbs, then cracks open effectively dropping the psi and diverting the flow to the wastegate actuator. The red spotted **** is the atmospheric vent. Opening this more will vent more flow. It's necessary as the spring driven ball in there would trap the boost pressure at the actuator effectively keeping the wastegat valve open all the time. The other adjustment **** will increase you boost pressure to the engine, it will boost to whatever your turbo will put out, so be careful with aftermarket turbos.

https://www.dieseltruckresource.com/...3&d=1200532352

https://www.dieseltruckresource.com/...1&d=1200532352

As the pressure increases, nothing more happens, the flow as equalized because the path for the exhaust gas to the actuator is a dead end street. P1 = P2 and if you put a small orfice in the actuator side, venting the pressure from there you can 'soften' the wastgate actuation process but eventually the turbo will flow far more volume than the tiny orfice can handle and once again P1=P2. If the flow thru the orfice is equal or greater than the turbo can produce you will stop the wastegate from opening further but it's now all the way open or enough to limit the psi to whatever the spring has allowed.
I'm able to only prevent premature venting of the exhaust gasses by keeping the wastegate closed until said pressure setting is reached then it works like stock. I've been able to see boost pressures of 45-48 psi from my stock turbo but I read that is a little hard on them so I limit mine to no more than 35 psi. It's handy for starting off with a load and will 'burp' the turbo intake if you come off the throttle too quick like in changing gears on something with a manual trans.
An asymetrical wastegate is available but for our trucks I don't know who might make them.
Playing around with venting the upstream side will be interesting as it will soften the pressure limiting of the wastegate. Have fun.

Tate

Boost elbows have a bleed off to atmosphere orifice already, so it already does what you are trying to do. Thats how they work without dropping boost pressure. Think of the volume in the wastegate line vs. the pressure that the turbo creates. If it didn't have the bleed off orifice, it would equalize the pressure on the other side of the boost elbow almost immediately.