There is a margin of error there. I have no idea what the true limit is, because I've never gone past it and had problems as a result, but I've had customers come in with their reliefs completely shut down so that they can't function at all and have no problems. I do not recommend this, but I can tell you from experience that 1 or 2 inches isn't going to create problems from what I've seen. And I've seen a lot in 13 years of manufacturing systems and 6 more of selling them prior to manufacturing.
So, I get a little tickled when someone says, "This here blower generates 2" whole inches more lift than others do.".
On a similar note is blower RPM. I built systems for years that have over- driven blowers. They were spinning at 15% faster than the engine and the engine was typically turning at 3,400 to 3,450 RPM. If you do the calculations, that's in the neighborhood of 4,000 RPM. I also built one at 20% over- driven, just to see how it would do and it held up well, too.
The grief I caught about that from other manufacturers about it was pretty intense at times. But the truth is that:
1.) They had noticeably better vacuum than comparable systems that weren't over- driven.
2.) There was no noticeable increase in blower failure, based upon my observations over a 6- year period of building them from 1996 to 2001. (And in truth, I still "observe" them, as they come in for repairs. I've seen the old Hammers and PowerPaks come in recently, with the second and third engines on them, but with the original blower.)
So, while I caught grief from some competitors about it, my customers loved the idea, because they had killer vacuum, compared to what comparable systems did. And it cost them nothing extra... being done by simply using a larger pulley on the engine than the blower.
It was "standard issue" on the early Hammers and the PowerPak systems.
In speaking with a couple of the blower representatives I've dealt with, they state that blowers are built for 30,000 hours of continuous, 24 hour a day use, when taken care of properly. And with that in mind, the idea that we'll see them go bad in intermittent use within 2,500 to 5,000 hours is unlikely at best, if they are properly maintained, with regard to lubrication, not being run too hot, not being subject to excessive vacuum levels and being filtered properly.
(Over- driven blowers, BTW, were provided with 40 wt. ND oil, instead of the usual 30 wt. Blower manuals back then said to use 40 when heat levels were anticipated beyond a certain point and 30 wt. otherwise, so we used 40 on the over- driven blowers just to add better protection.)
So, "Yes", there is a "safety margin" there. Or, as they refer to it, a "work factor".
Duane
