Since this thread's been given the Lazarus treatment (not by me) I'm going to throw this question out there:
I've got the Jeep 4.0L engine which, by all accounts and through the use of handily provided formulae, I've figured that the MAX intake cfm is around 370. This is if I've got it wrung out to redline which, unless I mess up drastically, should not often be the case. So, realistically, I'd be looking at a max RPM of 3000 which works out to approx 160 cfm.
That said, if I understand the prior discussion re: Donaldson pre-cleaners, I'd be better off to go with the smaller 7" pre-cleaner which has a max effective CFM of 170 vs. the larger 10" which has a max CFM of 320 for a couple of reasons:
1) since I will not be pushing the intake cfm to the max, the smaller pre-cleaner should suffice for probably 90-95% of the driving that I will be doing,
2) the larger pre-cleaner would actually be beyond the point of diminishing returns both in terms of dollar value and efficacy because: a) I would not be using all of the flow capacity of the larger pre-cleaner. so why spend the extra $$$ for the unused capacity; and b) the higher flow rating actually HAMPERS the pre-cleaner from doing its thing because the demanded flow rate is significantly lower than the MAX.
3) the 7" pre-cleaner will, obviously, have somewhat less aerodynamic drag and therefore would not stress the snorkel body and the mounting points as much as the 10" would.
Sound about right?