What wastegate spring rate for electronic boost controller?
 

I need some advice on what spring to buy for my Tial 35mm external wastegate, and I couldn't find a thread covering it.

I'm setting up to run electronic boost control from the Megasquirt using a solenoid.

I want to work my way up to 10psi gradually, since megasquirt will take some figuring out, and I want the ability to run low boost in the winter.

The way I look at it, the weaker the spring the quicker the wastegate will open but the slower it will close, although it should close much faster than it opens since it has both the spring and boost pressure acting on it.

My question is this: What spring should I use if my ultimate goal is 10psi, and I want the ability to gradually turn up boost AND I want best wastegate performance. Would a 4psi spring be too low? If so I could buy a "winter" spring at say 4psi.

Thanks for helping!

It's not how high you want to go, it's how low you want to go. What will be your lowest psi? Then pick the spring under that.

Frank

4 might be fine. Typically (if i remember correctly) 3x the WG spring is doable with electronic boost control. With manual you can get like 1.5 the spring.

IMO: I'd just run a 6 and go from there. Gives you some more headroom for later when you get bored with 10psi.

I have a 7 spring in mine and i run a EBC, my engine management will handle 7 psi without problems, if something happens to my solenoid or boostcontroller then i know i can drive the car home and not worry about staying completely out of boost because the WG will keep me at 7.

Not if your solenoid fails closed :eek5:

True, man that would suck. Hopefully I would catch it when i started going Mach 10 and my engine jumps out of my hood.

Not to get off on a tangent, but I thought 10psi was the "safe" limit for stock internals.

So, what happens when you use a very light spring w/ high boost?

This has been hashed out more times than I can remember, but the stock internals are not limited by boost, per se. They are limited by cylinder pressure and knock. Boost contributes to cylinder pressure...as does the onset of boost and at what RPM it comes on.

Furthermore, 10psi from a TD04-15G is different than, say, 10psi of boost from an SC61 turbocharger. Further-furthemore, the '99+ cars have higher static compression and generally cannot run as much boost as the '90-'97 cars (although their heads flow much better).

Yeah, I didn't really mean to sidetrack the thread.

So, what happens if you use too light a spring w/ an EBC?

at very high pressure ratios (boost) there is some chance of it blowing open from the exhaust side.

you can run a low pressure spring with an ebc..... because you can usually adjust the solenoid duty cycle in order to adjust wastegate crack pressure....

the problem I am having with my MBC and a .4bar spring, is that the wastergate opens too early, and slows boost response..... you can remedy that with a properly set up EBC

now if I can just get my AVC-R installed and tuned correctly.....

I see what you mean; if you exhaust manifold pressure is much greater than boost reference pressure, and the wastegate spring isn't strong enough then the wastegate could open....didn't think of it that way.

I guess that could happen when opening the throttle quickly.

well it's more complicated than that. boost pressure is directed to one side of the diaphragm in the actuator and (pounds per square inch)/(square inches) = (pounds of force), but those are different square inches than the wastegate port on your turbo generally speaking. the WG port is smaller and the pressure is lower post-turbo than boost.

but at some point (why I say "high boost") it'll still open up. you wanna find out, get your 4 psi spring and dont run a vac line to the actuator :)