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Message: <blockquote data-quote="jinkl" data-source="post: 315425" data-attributes="member: 3008">1. The wastegate size is relevant to how much horsepower you are targeting2. If you want to run high boost, then a smaller wastegate is better than a larger wastegate. The idea being that in order to generate high boost you need most of the exhaust gas going through the turbine, not through the wastegate.3. If you want to run low boost then a larger wastegate is better than a smaller wastegate. The idea being that in order to keep the boost low you need a lot of the exhaust gas going through the wastegate not through the turbine.But what is high boost? Well for the sake of this discussion I have settled on 1.2 to 1.4 bar (18 to 21 psi) as being the divider, thus 1. 2 bar and under is low and 1.4 bar and above is high. How did I arrive at this number? Well based on the results of the surveys, this seems to be the most common point where the wastegate sizes change from theory 2 to theory 3 (above).The next bit of theory is that it takes 1 lb per minute of airflow to make 11 bhp in a current generation 4 valve engine. This is a pretty well established piece of turbo sizing philosophy. But how do we relate this to wastegate sizing? Well referring to the results of the surveys, it seems a straight 1 to 1 relationship is not too far from the average, so 1 lb of airflow = 1 mm of wastegate diameter.What I am trying to do with the formula is give a baseline, something to think about. An 85% fit type of thing. At the moment there seems to be a majority of guys wanting external wastegates who don't even know where to start. They have been told for big horsepower you need a big wastegate, which is not necessarily true. They have been told you can't have too big a wastegate, which is definitely not true. They have been told for high boost you need a big wastegate, which again is not necessarily true.In order to do this there is one main assumption, that the turbo is pretty much the optimium size for the target power and boost. If it is totally wrong then whether the wastegate is sized correctly or not is really irrelevant. The turbo / engine mismatch is gunna kill horsepower / response anyway.A a 50mm wastegate valve is going to weigh twice as much as a 35 mm one. So there is an inertia issue to be considered. I think that is really the crux of the problem. The diaphram has to move twice as much weight, in and out very rapidly. This has 2 undersirable effects incomparison to a smaller (more correctly) sized wastegate.Firstly the too large a wastegate has to open and close more often, as it has too little exhaust flow when closed and too much exhaust flow when open. This wears the diaphram, due to the requirement for more movements and more weight.Secondly all this opening and closing of the wastegate affects the amount of exhaust flow though the turbine, this leads to fluctuations in the boost control. eg; I have seen a relatively low powered engine, with a very large wastegate, move up and down 0.3 bar in its boost as the boost control circuit struggles to keep up with this open, closed, open, closed requirement.</blockquote>

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