The problem with that statement...is assumption.

Some do run with no reference, some deliberately remove the reference.

And if you had say a 2psi spring....just what would "all the boost" be ?

It would be determined from exhaust pressure acting on the face of the valve in the gate, which would take minimal effort to open. So "all the boost" would still be quite low especially if EGBP was high relative to intake pressure.


To be fair, there is a lot of arguing and bickering in this thread that really isnt called for and it should be cleaned up and simplified for the original question.

Bottom line is simple.

Apply pressure to bottom chamber against spring, and the valve will open at a pressure very close to rated spring pressure.

Apply pressure to top chamber and this will try and blow it closed again basically assisting spring pressure.

Of course when the valve in the gate is closed and sealing, you also have the exhaust pressure acting on the valve trying to push it open against any of the above pressures trying to keep it closed. Larger valve, more force applied to the valve from that pressure.
However once it is open, that dynamic will change.

Some will use a very light spring and rely more so on exhaust pressure in the manifold to blow the valve open with no control on the chamber...some then add pressure to the top chamber to blow it shut again to increase boost.

Where the pressure for the chamber comes from to control is entirely up to you. Some use boost, some use exhaust pressure and some use a totally independent high pressure air source. Really doesnt matter.

And as said before, there are multiple ways to configure the system based on all of the above.


So until the OP outlines exactly how they intend to configure their setup there is no simple answer other than one, either, or all of the above control options.

 

Area of the WG valve plays a roll too. Assuming the same WG spring is used, a 2:1 BP ratio acting on a 60mm valve face will have a different cracking pressure than on a 38mm valve face.

Same deal with back pressure. Assuming the same dia. WG is used, a 10lb spring on a 2.5:1 BP ratio setup will have a different cracking pressure than on a 1:1 BP ratio setup.

Alot more to factor in than just spring pressure.

 

Well said Stevieturbo and Forcefed.

 

Bringing this back up. I was considering doing some changes like a cam and maybe intake, but wondering if backpressure is creating a bottleneck that would make any other mods worthless. On an 18 psi pull at only about 5800 rpms, I'm showing 46 psi of backpressure. Which calculates to about a 2.5:1 ratio My question is would a cam change still be worthwhile or worthless? Currently running the LS6 cam, thinking of something a little bigger like the Triple 12 etc. Or, perhaps a larger turbine. But I'm in love with the response of this turbo. Hard to pass up some of the gains that some of these cams are giving.

"Dome Pressure" = backpressure

 

I'd see as high as 63-64psi and was still making more power on my 1.10 T4 S475 at 25lbs on a 5.3. I wouldn't sweat the back pressure with the baby overlap cams. I'm sure the trip 12 would pick up over the LS6 with your back pressure.

If you went with a large wheel T6 S475 you could install a more aggressive cam (LJMS stage2 or something) and it would pick up a ton I bet. The NA power would be bumped way up so you wouldn't notice the lag as much. Also if you keep the compressor smallish (75-76mm) it helps spool times alot.

For comparison I still see a little over 2:1 with the big 87/96 exh wheel and 1.25 T4 S476 around the same boost as you. (370" motor 224/224 @ 113 cam .600 lift)

 

Seems like with it oscillating like that on the graph you really have to take the average, really probably only around 35psi back pressure. I had those S200 turbos on my 6.0 with tiny .48 housings and saw around 30psi back pressure at 16psi boost I think, that setup had to be the same or worse on the exhaust side.

Its possible a slightly bigger cam might lower your back pressure even, kind of like how a bigger cam can lower your DCR. Ask Jarrett about it too, he had that same exact turbo on his truck right?!

 

You would need to filter or dampen the signal, the exhaust manifolds can be a pretty violent place to get a stable reading.

 

How large of line and where is it tapped at?

 

If I dampen the readings it will end up around 35psi, which doesn't seem accurate when I'm getting the same 45-46 psi pulses consistently. Admittedly they are very jagged as is.

I think its 1/8" copper tapped into the drivers exh manifold as it was easy to get there.

 

Undamped you are getting incorrect readings and fluctuations. Simple as that

 

If I use max smoothing it knocks the peak reading down my about 10psi. Is this a more realistic #, 35psi BP at 18psi intake? That's about 2:1.

 

2.5:1 is huge, even 2:1 is a lot, unless it really is a fast spooling responsive setup.
Although it does seem singles suffer a lot more than twins with high back pressure.

Even an undamped MAP signal will have fluctuations and the exhaust side is a more extreme environment to take a reading from.

 

What is the sample rate on the system & sensor?

I would expect to see a visual extrapolation of the the exhaust pulse pressure waves that would correlate to cylinder firing if the sensor & sampling rate are fast enough to keep up.

 

most drive pressure sensor set ups i've seen have several coiled loops. i'm not sure if thats to keep the heat off the sensor or the dampen the exhaust pulses.

anyone care to enlighten me?

 

More than anything its a vibration loop. Lets the line flex and not crack as easily.

 

It's both for heat and vibration. If you simply move it to the b turbine housing it will clean up your signal, larger line will dampen it aswell

 

Rotary where you tapped does it matter if it's a divided housing or not?

 

That would depend whether the entire manifold system to the turbo is also fully divided.

 

As mentioned, I have always seen the cleanest readings in the turbine scroll. I actually like to tap in the scroll directly across form the first exposure point of the turbine wheel.

You may never see a clean log where you have it tapped even dampened currently as you are likely seeing some mixture of exhaust pulsing as well.