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Old 08-20-2010, 12:22 PM   #1
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Default please excuse the noob ? about hysteresis (kpa)

ok so i am new to using ms, i have searched the forum for a watered down explanation of what hysteresis(kpa) stands for/ what it does and what i do with it in overboost protection. i am using tunerstudio ms on msII, thanx.
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Old 08-20-2010, 12:59 PM   #2
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KPA=kilopascals , a measurement of atmospheric pressure. At sea level, the atmospheric pressure is roughly 100kpa. Anything <100kpa is considered vaccuum, anything over 100kpa is considered 'boost'.

Could you more clearly define what you want to know about overboost protection?
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Old 08-20-2010, 01:58 PM   #3
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100 kpa = 14.7 psi. At sea level 200 kpa is 14.7 psi worth of boost (taking out atm preasure). Basically every 1 psi = 6.8 kpa.
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Old 08-20-2010, 02:23 PM   #4
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I think the OP is asking more for a definition of "hysteresis."

It's the amount a value has to drop below your limit for whatever function is dependent on that variable to be reset, turned off, etc.

Example: If I have my fans set to come on at 205* CLT with a hysteresis of 5, they will switch on when the coolant hits 205* and switch back off when CLT drops to 200*.

As my turbo is still sitting in my basement, I will let others comment on what a reasonable hysteresis value would be for overboost protection.
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Old 08-20-2010, 02:35 PM   #5
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That's pretty much it. Hysteresis, generally, is a resistance to change in state. For our purposes it describes the phenomenon whereby once an output or function has turned "on", the variable or variables which affect it must change by more than a certain amount before the output or function will turn "off" again, or vise-versa.

So, let's say that you want your engine to never see anything more than 200 kPa of manifold pressure. You'd enter "200" into the "Maximum Boost" field.

Now, hysteresis comes into play after you have hypothetically exceeded 200 Kpa and the overboost protection has kicked in. If hysteresis is set to 0, then the engine will come back on the instant MAP drops below 200 kPa. If hysteresis is set to 10, then it will wait for MAP to fall below 190 kPa before turning back on. Etc. This doesn't mean that the overboost protection will cut in at 190 kPa, you could run 199 KPa with no problem. It just means that if you do hit 200, you then have to wait for it to drop back down to 190 before the engine will fire again.

Frankly, hysteresis is very useful when it comes to things like triggering your cooling fans, where you don't want them to rapidly cycle on-off-on-off as the temp hovers right around the switchpoint. In fact, the MS1 didn't even offer a hysteresis setting here. I've not yet tuned an MS3 so I can't speak from experience here, but intuitively, I'd suggest that unless you're having problems with it, just set it to 1.
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Old 08-23-2010, 07:45 AM   #6
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wow thanks so much that completley cleared it up for me, and mine is set at 10 so i'm just going to leave it then. day by day it is becoming easier to understand msII and tune.
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