|There is no performance benefit to upgrading to a larger oil cooler. None. However, it's necessary to maintain reasonable oil temperatures with a modified setup.
Is upgrading the oil cooler in a moderately modified Z necessary? IMHO, the answer is yes. Not because of the moderate modifications but rather I think the base oil cooler is woefully insufficient. This is subjective depending on what oil temperature you believe to be "acceptable". I'm hitting over 300° routinely on hot days not driving hard. If you track your Z I think it's a must upgrade.
Oil temp is not so much a function of boost level as it is time accumulated at high boost. So a modified turbo may be generating only a little more heat than a stock turbo, both run at full bore. I don't know how much more heat is generated comparing modified to stock, but measuring oil pan temperature is all you need to do to know if it's necessary. I'll explain why in a detailed future post.
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