Yes, Greenie...
It held 22" of lift. Granted, we didn't run it at that... but it did hold it at that for a few seconds, without bogging down.
My guess is that it would do it as well now, even with a motor going on 8 years old, considering internal slippage of the blower has increased over time. (There's a "hole", to be considered.)
I do NOT recommend building systems without vacuum reliefs, BTW. I'm well aware of the possibility of seal damage by exceeding manufacturer's spec's. That system was a test, with the understanding that I'd "eat" the blower, if something went wrong.
My point here is, as it always has been... that blower spec's are not "cast in concrete"... In other words, they have a "work- factor" built in. And, my personal experience, from building literally hundreds of systems with over- driven blowers, is that that safety factor is at least 15%... because that's the level that most of the systems I built operated at on a daily basis, in a wide variety of conditions.
On the other hand... the faster a blower turns, the more noise it makes. So, in 2000 to 2001, I built systems with over- sized, under- driven blowers. The logic there was that they could turn slower, but have the same CFM, AND produce less noise. It worked, but I didn't find the noise level reduced as much as expected... and, even though I announced what I did, I still felt pangs of guilt about it. One of the things people look at as a big factor in what system to buy, is what size the blower is. And an under- driven blower is simply under- performing. (If yours is (which can only be done on a belt- driven system, BTW...), you can verify it quickly, by comparing the pulley sizes of the blower and motor. If they're not the same size, and the one on the blower is larger, then it's under- driven.)
(With the Predator's, and Accelerators I dial them down to 3,200 RPM, which does keep the noise down... but the system airways are very efficient, in terms of minimal restrictions... and Predator "CP" tanks are much smaller, resulting in even better vacuum performance.... but that's another story...)
Nowadays, there's a push in heat exchange technology, to capture heat from every conceivable source... including blower exhaust. Weeeelllllll... the faster a blower turns, the more heat it generates. So, we're back to turning them faster (at least, I am, on the new exchangers).
To me, this approach is quite a bit more "honest" and straightforward than some of the things I've seen other manufacturers do. For instance, choking down the internal airways on the blower intake. This introduces airflow restriction, which makes the blower work harder, which makes it run hotter. But it also robs the end user of valuable CFM to do it. So a guy with say, a 47, 4007, or 4L blower, is only getting performance similar to the next size under that. (I've heard of one manufacturer that does it in PVC, which can't handle high temperatures... and the PVC melts. Another does it in an internal intake fitting, fabricated to smaller openings than the external piping would indicate... so that to look at the system, you'll never know it. Another does it via having what I call, "spaghetti plumbing", with lots of turns in it to introduce restriction and therefore cause heat.)
This kind of "stuff" isn't un-detectable, if you just look at your vacuum gauge. When your system is running full- out, with nothing connected to it, that gauge should read as low as possible. To me, if it reads more than 3", you need to find out why, because it's being robbed of more than 3" of the total lift it can produce... and if your vac relief is set for 15", you're losing more than 20% of it's power, before you even connect a hose to it, much less, before the wand even gets to the carpet and makes a connection with it...
Why not just design a system that takes all of this into account...? I don't get the logic in manufacturers who don't. They have more money to do the research... and more resources to put that into play. And yet... they take shortcuts...
My guess is that they're over- focused on heat (like the one I heard about that places the temperature gauge sensor close to the exhaust manifold... presumably to show higher heat levels... even though they're misleading... and the ones who jet their wands down, in order to claim higher heat levels), and not so much on true airflow.
It's not impossible to do both... A bit more involved from a design standpoint... but worth it, if you're seriously focused on vacuum performance as well as temperature.
This isn't "new" to me... If you go back to my earliest posts anywhere... on ICS or CleanFax, for example, in 1998-9, you'll find that I've said the same thing all along... that airflow efficiency is a "key" element...
) No relief = no possible improvement by adding one.
