NefMoto

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.

You are correct you have no control over boost until the crack pressure.  I would consider that to be the caveat.

The rationale behind it is that it will keep the WG closed longer (sooner) and the turbos will spool faster/sooner.  Obviously this is beneficial with low displacement and big turbos.  You should be able to achieve the same result with a lower crack pressure paired with a gated boost controller (manual), which will keep manifold pressure off the WG actuator until the target boost is reached.  This will work as long as the exhaust pressure itself doesn't overcome the WG and push the WG open.

The down side to the lower crack pressure scenario is that without the gated boost controller, the wastegate will start to open at the low crack pressure, which reduces the turbo response, and could actually prevent you from reaching your target boost.  The result is the spool is smoothed out and ramps more progressively - IE, the turbo is laggier.

This is based on my big turbo experience with EVOs but should apply the same.  I run an MBC on my K04 hybrid car for that reason - it is noticeably faster at spooling and without it the car feels like a wet noodle in comparison.

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 :)
 

If running 100% on the n75 still doesnt get you where you need to be, should be able to put a restriction in the charge control lines that usually got to the compressor housings to help it out.

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).

My initial springs are around 14 lbs iirc. So I'll first be tuning on those (above that, when my motor is broken in), and I'll likely switch to slightly stiffer after, so I might have some commentary in a little bit.

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.

I run the Tial recommended cracking pressure of 18.5 psi (so I see around 21-22 psi with 0% WGDC)... really there is no need to run less boost then that... it's easy to run on 100% pump gas and I don't see any benefits to running less boost to be honest.

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? ;-)

Agreed, I always tune with as soft a wastegate as possible.

^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?

You want your wastegate to be set at an optimal position where it won't sacrifice spool, yet it won't create backpressure/give you boost all the time. I'm a firm believer in using the N75.

Run like N/A while cruising, request boost when load goes up. Car feels smooth, good gas economy, and its how Bosch intended it to be.

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...

If you add another controller in parallel, you will only ever achieve the lowest boost value of the two controllers.  That is why people with shitty overboosting tunes run an MBC in parallel - to clip the boost.

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.

True, but herein lies the problem...  If you have a lot of turbine inlet pressure as is the case with a large internally wastegated GT or TiAL turbo, you may need a much higher pressure actuator spring to actually build boost initially.  Bleeding the control pressure off isn't going to yield you a wide range of control with the cracking pressure at 18.5...  The difference between 0 and 100% N75 DC is probably not going to be what you (or the ECU) are looking for.  Having said that, has anyone unplugged the N75 on a GT or TiAL car and seen what you can actually build for boost without any charge control and the wastegates set as recommended?

When you say "...the n75 alone just didn't have enough oomph to cover it." do you mean it couldn't bleed off enough boost to compensate for the cracking pressure?  I'm not sure how that could really be the case, there shouldn't be a difference in the amount of air it needs to bleed off whether you have K03's or GT28's unless the wastegate actuator travels way further.

I think thats what I mean lol. Buddies car is a terrible example because his issue is prob mech/tuning, what he did was just a bandaid.

What I was getting at has to do with the ratio of what the lines will flow vs what the n75 can bleed. A restriction should shift that ratio in favor of the n75 to help relieve pressure from the actuator. Still not going to fix much if the wg spring is not enough to handle the gate size vs tip.

Quick and dirty (and prob a bad idea lol), set fuel cut at ~20psi, wedge gates shut, see what curve looks like?

I'll post logs in like 2 weeks of my GT car running 0% WGDC everywhere on ~13lb springs in external gates.

Cool man, looking forward to your results!  Especially after beating up the requested boost for a while.

For my 605's recommended is 18.5 psi cracking, I see about 22 psi with 0% WGDC...

I actually just read through your 19 page thread about your 605 tuning last night and was wondering how things went.  I noticed about half way through you were getting TC because of the boost deviation and was surprised that you were still going with N75 vs MBC.  Really all the N75 can do is chop a couple lbs for you before you hit the sensor limit anyway. Your tune speaks for itself, that traffic light launch video where you blow the back end sideways is pretty rugged. :)  

yeah, the TC is/was more the TQ mgmt then boost deviation for the most part... also I am probably going to run either an MBC or use my VEMS WB02 gauge as it includes a PID boost controller.  Bothering with the N75 past 22psi is really not worth it at all, I've been doing it for simplicity and learning of course.

Thanks for the comments, more will be happening next year.... Switching to a different MAF setup (HPX sensor) and doing rods or possibly a 3L build :)

Nasty. You going to HPX so you can blow through it?

I think the n75 is useful past the map limit.  especially when tuning lower power cars without enough fueling to go the distance, you can spike and taper to keep afr good.

I've had mbc springs not be stiff enough for high boost applications, so we just throw in n75 and go to town.

Maybe with K04's on stock fueling and 8 lb wg's I'd be inclined to agree. With 18.5 lb wg's and a minimum 22 psi charge the n75 is not going to offer much help.

No, it will be draw through just like it is now.. but I won't have to worry about maxing the sensor again and the increased resolution will be welcome.


I agree in that point... I'm not really a fan of MBC's as you have better control and usually better spool with an EBC system because it should be able to keep it closed longer.

So, on higher powered cars going past the MAP limit, who also have overkill fueling, you would be for MBC?

Nice, it will be cool to have more people fine tuning these HPX's.

Bringing this from deads as I finally have 770s and 2.9L in my car.

I am running 0% WGDC and seeing something I don't like which is extreme twitchiness of the car when applying 40-50% throttle, I hit max boost (23psi) without much control. It appears that smoothness of N75 controlled mid throttle is effectively gone, not liking it at all.

So back to the original discussion. I might go ahead and lower WG preload to something that makes more sense for non-drage strip car. I would like to chop it in half but I realize that it might not be realistic.

Any new info since the original discussion that might be helpful in achieving 6lb wastegate like driveability of k03/k04s?

Edit: I read a bit about wastegate cracking pressures and spring rates etc and it looks like the 2x limit of spring rate vs max boost attainable before flat opens pertains mostly to external wastegates. Internal ones can usually handle about 4x the pressure diffrerentials before they get cracked regardless of what controller is doing:

There is a boost pressure amount that correlates to high enough pressure in the turbo exhaust manifold that the pressure in the manifold will push the wastegate open on it's own. This is the crux of the problem. Usually this happens on external wastegates somewhere around twice the boost pressure that the wastegate spring can hold on its own (because the diaphragm has usually about twice the surface area of the wastegate valve). So if the wastegate spring on its own would produce 6 psi, the max boost obtainable by some methods will be around 12-14 psi. This is because the pressure in the manifold overcomes the spring pressure and forces the wastegate open. This isn't true for all setups, but it's a general rule of thumb to go by.

Internal wastegates are a little different because their diaphragms are usually 3 or 4 times the surface area of the valve seat. And they have a lever that makes it easier for the wastegate actuator to open the wastegate flap, but it makes it hard for the exhaust manifold pressure to force open the wastegate so usually you can make over four times as much boost on an internal wastegate instead of only two times.

Just as a data point, I've currently got  ~13lb springs in my external wastegates, and my part throttle isn't too bad at all (when I've got the rest of the tune dialed in).  I'll be throwing in prob 5 more lbs of spring when I start turning the boost up to 25+psi.

May not help you much, but thought I'd throw my data out there.

You're in the "2x the spring rate max boost" bucket though. I will wait until I can start tuning the car properly as I am still at initial stage of engine break in. Few weeks at least until I can start turning up the steam :(

I know, just relaying my partial throttle driving experience on 13lb preload, 16psi boost on external wastegates.

Just finished a tune with 32psi and a 13 psi spring.

Drives beautifully.

Rick

Anyone want to share some info on how they setup the KFLDIMX and KFLDRL?

I have tuned my KFLDIMX pretty much trial and error and lots of reflashes, and my KFLDRL is not in use at all - I have just done the table linear to the axis.

External gate?

andrews S3 per chance?

 ;)

My own racecar runs tial 44mm external gate, hybrid gt3582
stock seat ibiza n75 controls.. Actuator pressure of 12psi...  26psi from 60% duty cycles.. plenty left in it.
you dont need stuff springs in these gates...