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I am thinking of changing to a bigger turbo, is there a way to determine/calculate wastegate duty to build a table before dialing in accurately?
I will be using closed loop boost control but my wastegate tables need to be close for this to work properly.
In my experience, no. There's so many aspects that will affect the final boost pressure that trying to calculate a required duty cycle and expecting it to be accurate is probably a bit of a stretch. For example when you go from a small turbo to a larger turbo the exhaust back pressure will be reduced which tends to result in a higher boost pressure when using the same wastegate/spring combination but you'd need to know what the exact effect on back pressure with your new turbo will be in order to account for this. The wastegate size, spring pressure, and location of the wastegate can also impact on the boost for a given duty cycle. My recommendation is to simply start at the wastegate spring pressure, find out where you are, and then start adding duty cycle. You'll quickly see for example that maybe an increase of 15% duty cycle gives you an extra 10 kPa or whatever it may be. This helps guide your tuning.
Note that there is also a limit in the range of boost you can achieve with a given wastegate and turbo. What this means is that as you start increasing the duty cycle initially from the wastegate spring pressure you'll initially see a reasonably consistent increase in boost per %DC. As you start driving the turbo harder though the back pressure will increase and this forces the wastegate to open which means that you'll start needing more %DC to achieve the same increase in boost. in some situations with a restrictive turbo you'll get to the point where you can't achieve any more boost.
I remember a tuner told me he used a formula to get a rough calculation but wasn't sure what way he was going about it. I currently have my wastegate control setup for bleed off so from memory(I haven't touched the WGDC table in quite some time) my table values represent 0%WGDC = fully opened and 100%WGDC = fully closed. I have the option in my ECU software to switch this around.
From watching the open loop boost control video I you work the other way round, 100%dc = fully opened?
Thanks for your reply