I have...
Using intake vacuum to pull crankcase vacuum describes how PVC works. If you notice on most PCV systems, the hose going between the intake and crankcase is big, and the fresh air feed is small. They do this to induce vacuum at part throttle. Choking off the fresh air feed line further increases vacuum further at part throttle. In your case, you choked it off entirely. In this case, the crankcase is not being evacuated of vapors, but the vacuum is higher.
The unfortunate side effect is oil enters the intake manifold; which makes a mess as well as helps coke the valves and induce detonation. Granted, I'm not talking about a tight healthy engine; I'm talking about a tired and/or loose engine with plenty of blowby. Also, on an engine that makes more than about 12" of vacuum, you really should run a relief valve if this is a street driven vehicle. The reason is that the crank seals and valve seals will be under vacuum, which will suck the oil away from the seals and cause them to wear prematurely. Sometimes you can even hear the seals singing like a reed flute as they draw in air. But, you're right...it creates vacuum at part throttle.
Other options...
Vacuum pumps are nice. Expensive. More of a hassle to mount and deal with belt routing.
Exhaust venturi evac systems are *great* if done right, and my personal favorite if I'm building a system from scratch and the exhaust system is not yet complete. Most people that have tried and failed with these systems failed due to one of a few reasons.
- The Mr. Gasket and/or Moroso kits use cheap offshore check valves in the system that don't seal, and have too much cracking pressure. Every inch of vacuum it takes to crack the valve open is an inch of vacuum that your crankcase never sees. The typical Mr. Gasket valve takes 4 or 5" of vacuum just to open. That will never work. Or, they leak so much that the exhaust gas they pass far outweighs any vacuum benefit.
- Almost all the kits on the market use a 1/2" steel pipe and have you weld it into an exhaust tube somewhere near the collector and give you a drawing and/or spec to go by that simply says to poke the pipe into the tube at an angle protruding into the pipe somewhere around 1/4" to 3/8" deep. Well, that's great...but not correct for most of us using 3.5" or 4" collectors or stepped exhaust and FAR from optimal.
To do an exhaust evac system successfully, you have to understand the venturi effect and where the areas of low pressure and high pressure are generated in the exhaust as the exhaust gas travels. Also, the higher the gas velocity, the more vacuum you'll generate. This works against your premise of using large exhaust in the first place; and the last thing most people are going to do is choke down the exhaust to get the velocity up just so they can create more crankcase vacuum. In fact, that's exactly what you should do. It should be tuned to the point where it doesn't choke the exhaust flow but maximizes exhaust velocity. Next, the shape of the exhaust at the collector matters a bunch as well.
If you're building your exhaust system with an evac system in mind, consider this shape. Let's say you run a 3.5" collector and couple that to a 3.5" exhaust pipe. If you want an evac system that works great, weld a 3.5" to 3.25" REDUCER at the tip of the collector, then slip your 3.5" exhaust pipe over the tip of the reducer and weld it where the flare reaches 3.5" OD. In other words, you have a tube inside of a tube over a very short distance of maybe 3-4". Next, drill your hole for your vacuum feed tube as close to the header side of this overlap area as possible. No protrusion into the tube is required or suggested. Use an OEM exhaust check valve from any late 70's early GM or Ford car that uses these valves. The old Corvettes use them, for example. 5.0 Mustangs have them as well. Run the other end to your crankcase just as usual.
What's happening here?
The exhaust gas is accelerated when it goes through the short choke area of the collector. The higher velocity gas shoots through the center opening, and the area behind this opening created by the "tube in a tube" configuration becomes an area of low pressure. The high velocity stream is trying to pull that gas toward the stream but can not. As soon as you drill a hole and connect a tube, you've created a vacuum pump. The faster the gas travels (the more you choke the center tube), the more vacuum it will generate. I've made 25" of vacuum at idle doing this. It was choked too much for open throttle, however...you have to tune it, or make an educated guess that won't choke your exhaust and live with it.
Exhaust flow capability is determined by diameter AND length. I think you'll find that the small restriction area (for most of us) won't make an appreciable difference in power production as long as you don't go overboard. If you're making huge power numbers, then scale things accordingly and do your own experimentation.
Monitor crankcase vacuum. If you start pulling more than 12" or so with factory seals, you might start sucking air past the seals which dries them out...damage can occur. Use a relief valve if necessary or choke/restrict the vacuum source.
This has already become a book, so what the hell... take a look at this diagram.
D1 is your 3.5" collector
D2 is the 3.25" choke point I suggested be made from a 3.5 to 3.25" reducer. Instead of a flat plate like this, just use a conventional smooth transition reducer...it's better for flow.
The areas where they are showing all the swirling gas areas is the area of low pressure as indicated by the sketch of a simple fluid filled barometer. Imagine that the U shaped connection at the bottom is a fluid filled tube. What it's showing you is that the fluid is being drawn up due to vacuum.
If you want to read more, read about "The Venturi Effect" or Bernoulli, the guy that discovered this. He'd probably vomit if he saw what we were doing with is discovery.
Byron
