My FD is currently running a BW SX300 SXE series turbo, with a Turblown tubular manifold with dual Turbosmart Comp 40 Gen 4 waste gates. Since I got it that way from the PO, today I got around to doing an on-car "bench test" of my waste gate functionality to characterize (1) At what pressure do they start to crack open?, (2) At what pressure are they fully open?, and finally (3) Do they operate smoothly as they should?

I did the test using my air compressor and a test rig I put together that has a 0 - 100psi pressure regulator, a 0-30 psi gauge (so I can get decent accuracy reading low pressures from 5~30psi), and some hose to plumb it into my boost control line.

Results were both gates passed the #3 test, operating smoothly and in unison together. The cracking pressure was 17psi and full open pressure was 25psi. This is higher than I'd like to go - so I figured I'll change out the springs to get my cracking pressure down to about 7~10psi. So off I went on the interwebz to get info from Turbosmart on replacement waste gate springs, repair parts and how-to information, where I found this tidbit of info in their FAQ's (quoted as-is), which paused me from buying new waste gate springs....

"Although you can perform a “bench test” of a wastegate to confirm that it is cycling properly and not seized, this bench test will not offer accurate results in regards to the amount of air required to open the wastegate. It is important to keep in mind during this test that when the wastegate is actually run on the vehicle, there will be additional exhaust backpressure helping to force the wastegate open. Because of this, a 14 PSI wastegate may require 20+ PSI or air pressure to open during a bench test. The best way to confirm the proper operation of a wastegate is to run it on a vehicle."

So my question to the more seasoned tuners is, given my bench test results, where should I expect my ACTUAL boost pressure to be when these suckers crack open to full open?

 

i think youre giving too much attention to this data set. on paper a lot of things are one way but in practice, they are what they are. so long as your boost pattern is consistent and doing what youre setting out to have it do then thats really the end of it.

to better help you reach whatever youre looking for, what prompted this testing? presumably its not doing what youre wanting it to do...

 

I am not going to pretend to know it all, but a 15 psi spring should be fully open at 15 psi. The waste gate could start to open at about 10-12 psi. If you are worried about controlling boost set up the boost controller. I would do an EBC. Most likely you knew this, but i was just taking a shot at it.

 

I'm just trying to fully characterize the hardware I inherited from the PO, and verify it's all good before I get too deep into tuning. Since the engine is new, and just about broken in now, so far I've only road tuned the N/A portions of the VE table, and dipped a little bit into low boost a bit (< 5psi boost), well below where the waste gate should have any effect. For the time being, I have the open/closed loop boost control features of my Link G4+ ECU turned off, and the waste gates are plumbed with a 3-port MAC valve so they will always see boost pressure. So in effect, boost is currently mechanically controlled by the gate springs. The PO told me the waste gates were running 12psi springs IIRC, so I wanted to verify that, hence the static test.

Ideally, I'd like to get my waste gates to be full open at an actual 9~11psi of boost pressure. That way I can use the boost control features of the Link G4+, along with a trim switch, to set a couple of boost profiles (i.e., low/medium/high)

 

That's what I thought, and why I did the static pressure test to characterize cracking & full open pressures. But if I am to believe the Turbosmart FAQ (who make these waste gates), the numbers you get from the static test I did are actually higher than the spring's rated opening pressure by some TBD amount, because exhaust pressure assists the spring in the opening the gate.

Plan is to go with a lower rated spring (say 9~11psi) which would be the lowest boost I can run, and use the either the open or closed loop boost control features provided by my Link G4+ ECU for higher boost levels.

 

You realise unless you make closures and also apply pressure to the underside of the valves and are only applying pressure to the diaphragm, depending on the area ratios, they may not open till near twice what they will on the car? They are probably on "10psi" rated springs and will probably operate around that pressure on the car.

Assuming at cracking conditions something in the order of 1:1 intake to exhaust pressure ratio, work out the presented valve and diaphragm areas and go from there. You will get close enough.

 

Well I do now, so my next step is to just open up the wastegate and find out what springs I actually have in there - the Turbosmart springs are color coded, so I can just look them up and find out what I got, and will then have some data points to correlate with the static pressure testing numbers I measured (i.e., 17psi static cracking pressure/25psi full open pressure = XXpsi rated spring).

Depending on what I got in there, I may just end up buying a set of the 7psi rated springs to swap in there anyway. That way I'll be able to tune a low stock-ish boost profile, and use my ECU boost control to tune higher boost level profiles. I can also re-do the static pressure test with the 7psi springs to see what static pressure they crack/full open at.

 

There is really no way to tell what boost pressure wastegate springs will start to open at and be fully open at.

They are poppet valves that open out instead of in so exhaust manifold pressure working on the bottom of the WG valve plus the boost pressure acting on the bottom of the diaphragm work to open the WG valve(s) together.

Manifold design (WG placement and flow priority), what turbo is used on what engine and whether you route boost to the top of the WG diaphragm all will affect the WG opening.

Just go by the manufacturer spring rating and expect them to blow open earlier on a rotary with a manifold that provides high priority WG close to the exhaust ports.

If you start too soft you can use boost pressure to the top of the WG diaphragm and a boost controller to mitigate exhaust manifold pressure pushing the WG valve open.

 

If folks are really really interested in knowing their WG position, then there is a cap option for the latest gen turbosmart gates.



The cap has a mag sensor installed that'll tell you the exact position of your gate at all times. It's a bit buried on the TS website, but it exists.

 

My manifold does prioritize the wastegates; it's a Turblown welded mani, with long-ish runners, that puts the turbo a bit forward & up high. The wastegates are located very close to the exhaust ports, at piping angles that support good flow to the gates. Since I'd rather err on the side of caution while tuning this thing, for now I'm just going to try the 7psi springs, and see what kind of boost I see on springs alone. Can always get higher boost and better control by using both sides of the WG diaphragm as suggested.

 

I've seen those WG sensor setups in the TS gen 5 gates, pretty sweet! If you REALLY want to spend crazy $$$, TS and I think Tial can sell you fully electronic E-gates where the gate itself is opened/closed electrically.

From the picture, it looks like you're planning to recirculate your wastegates back into the exhaust - I'm doing the same. What are you using to plumb the 2x wastegate dumps back into the down or mid-pipe?

 

Yeah, those e-gates are SLICK!!! If they came in 40mm I'd consider it though.

Here is how they came back in with the finished product.


I don't have any close up shots of the junction. They're dumped back in after the DP has transitioned to 4" though.

 

^Damn, that's SEXY! Should flow well too.