K04 with their 6psi waste gate pre-load leave it up to ECU to extract anything more out of them via N75 valve activity.

GT turbos, like Tial 770s, are recommended (by Tial) to set have wastegates set to 20psi.

What are pros/cons of such high cracking pressure?

The way I understand it, until the turbo reaches 20psi you don't have any control over boost moderation besides using your gas pedal. But is that enough control or would it make more sense to have them set to something less than 20psi to re-gain control over spool?

I am interested in opinions of people who actually tuned such cars and their drivability.

In low cracking pressure scenario and running on n75, how efficient is N75 running at 100% duty cycle? Is it capable to shed all the pressure off the wastegate lines to prevent wastegates from opening?

It is working fine with k04s as far as I can tell so I wonder if properly tuned car can run GTs with much lower cracking pressure than prescribed 20psi without any issues but with far more control part throttle...

I think that preloading at 20 psi is a band-aid for tuners that don't know how to tune. Running GTs at over 21-22psi (in case of 20psi cracking pressure) still gives a very fast car even if your tuner sucks
 

I know most here tune on the N75, but are there any GT cars on here going solely off of an MBC? 

And if you go with a lower cracking pressure spring, the springs can only effectively control to approx twice the wastegate spring pressure (in external gates).

But how would you know "where you have to be"? I think that the only way to figure out if n75 is at 100% and is effectively preventing waste gates from opening would be to monitor wastegate pressure lines to see what they show at 100% duty cycle.

In my case it would actually be fairly easy as I have zeitronix and VEI pressure senders already installed in crawl space so routing one more vac hose and inserting a T between n75 and wastegates really easy.

Thanks for that idea!

People are running k04s and bigger turbos at pretty high pressures on stock n75 with normal cracking pressure and are ok so I doubt GTs would change how the system behaves much.

One of the disadvantages of having high cracking pressures is that you will end up running relatively high boost levels (as the boost level is not controlled by the driver nor the ECU) on partly closed throttles which is a waste of energy. Why having a turbo charger trying to force air into a "closed throttle"?

This will also affect the driving dynamics in a negative manner as you might reach full boost well before 100% throttle, let's say already at 50%, which means that the area between 50% and 100% throttle position will be "dynamically dead", i.e. pushing the accelerator pedal from 50% to 100% won't yield any performance increase.

This behaviour can of course be set if you really want to in any "normal" setup by just adjusting KFPED and a few other settings, but who wants to decrease the dynamic behaviour of the accelerator pedal to that extent?

Wait a minute... There are actually a few guys who thinks this is a jolly good idea and tries to market something called a "sprint booster" by just tampering with the pedal position sensor in the car to make it react more aggressively... Lovely. :-)
"Tuning the car without making it faster"... Seems like a good way to do things? ;-)

^This.  I dont have any experience with N75 control of GT or TiAL internal wastegates, but the consensus was that higher cracking pressure was needed because they bleed when set at the usual K04 cracking pressure.  I'd like to see first hand why they don't like operating at 8-10 lbs with sufficient N75 DC to keep boost pressure on the opposing side.  I wonder if the WG actuators or the angle they're driving the WG (leverage) is an issue.  Hoping I'll be able to reflect on this personally in due time...

*edit
I wonder if clearanced or clipped hot sides on the GT or TiAL's would be beneficial to try to keep the TIP down and minimize the WG flutter with lower cracking pressure?

this thread actually touches on some very interesting issues.  on my k04 setup, i've switched to hallman mbc in parallel with the n75.  the results are fantastic in my opinion.  of course the logical next question, and also where the tie-in to this thread occurs, is what occurs if the hallman mbc is switched to a greddy or hks adaptive boost controller in parallel to the n75?  i think the best solution to your wg preload is somewhere in the answer to understanding the dynamics of a real adaptive boost controller...      coming from the fc3s rx7 crowd, i can tell you that there are many stories about how astonished owners are when they switch to an adaptive boost controller.  for starters, the thing learns and adapts to boost patterns much faster than retuning or modifying ecu / mbc settings.  anyways, just some food for thought...

Might need a linear potentiometer on the gate to know for sure. Setting cracking pressure on the bench does not take into account the exhaust manifold pressure trying to blow the gate open.

Have a buddy that was having probs making boost with a ft 1.8. Its uni so he doesnt mess with tuning. He tried every combination of n75/mbc known to man, and would either underboost or overboost. He finally ended up running mbc in parallel and putting a hole in the line that goes to the mbc. Took a few tries but he got the hole the right size to get where he wanted to be.

Seems to me it would be better to limit the control pressure up front as opposed to just bleeding it off on the backside. My guess is he never set the cracking pressure and the n75 alone just didnt have enough oomph to cover it.