KingTal0n

Hi there,

I am hoping to find some experienced input on this issue.
The discussion is, how much does Exhaust gas pressure ratio (compared to intake manifold pressure) affect the wastegate spring desired boost setting. I.e. in what applications is a wastegate likely to have trouble staying closed, due to a 2:1 EGP ratio or higher, and what are the possible solutions.


I found some good literature,
http://www.max-boost.co.uk/max-boost...st/exhaust.htm

This site is very detailed. They cover almost every nook and cranny about turbocharger setups, and even touch on the subject,

"Very roughly, for a stock(ish) turbo and a cat-less exhaust, the exh backpressure is about 2.5 times the boost pressure. So if we're running 1 bar boost, we've got 2.5bar pushing at the wastegate (against the actuator spring)
Then we calculate the area of the flap that the gases can see. Say it's 1 inch sq.
We've got (2.5)*14.7= 35.8psi (that's pounds per square inch!)
...so the gas force pushing the flap is 35.8 pounds. Simple.

That's why actuators that can hold high boost pressures need to have much stronger springs. "

In those cases they upgrade both the spring and diaphragm together, like here,

http://www.bankspower.com/techarticl...astegate-Works

" This happens because turbine inlet pressure also increases as boost pressure rises. The fix is to use a bigger spring in the wastegate actuator to hold it closed until the desired peak boost is achieved, however, that also requires a bigger actuator diaphragm to override the heavier spring when the desired boost level is reached. "


A high performance wastegate supplied by the manufacturer has this as part of it's design. The engineer knows the surface area of the valve, calculates resulting pressure vs the intended spring selection, that way the customer does not have to worry about the EGP except in extreme situations that crop up rarely. Do you agree or not, and why?

Is it so rare? Do we often find ourselves buying a 15psi wastegate, and then seeing only 7 or 10psi at the intake manifold due to EGP ratio? I dont think so. But I'd like to hear from you guys, and more technical with respect to how springs and designs are changing interaction with EGP.
Also one more question: Does a wastegate technically reference the exhaust pressure? They certainly reference boost pressure, and atmospheric pressure (or boost controller modified manifold pressure) but exhaust gasses pressure? They "sense" it through the diaphragm-valve system, but is that a true reference signal? Since the signal will vary from valve size to valve size, diaphragm size to diaphragm, there is no standard I can see. Discuss.


Edit: sorry I didnt mean to make a double post! I am very sorry! I didn't realize I had put them both on the same forum (I am asking many folks opinions to get cross platform).

Evo8cy

Since I am not someone who designs wastegates for a living, I only have this to say, someone who designs wastegates either of internal or external type of universal or specific use surely takes into consideration the basic backpressure to intended boost ratio and uses the appropriate diaphragm and spring type. Note that either too small of a wastegate port and/or backpressure after the wastegate , can lead to boost kreep.


Hopefully, people with more knowledge on spring/diaprhagm design will chime in to give their insight on how this is done.








Marios

Ben Corbett

i know I don't have a car for this forum but this is the best thread I've seen yet to answer my question. I'm having a boost problem in my s40. It's the wastegate actuator. Car has stock boost of 7.5 psi. I've found a company that makes the wastegate a for my car but they only have 3,5,9,11 and up psi wastegates. Which one do I get to replace. The 5? Or the 9?