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

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

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

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.

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.

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.

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