Crankcase evac. Question?

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Kevin D.

Kevin D.

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I just put a pan evac. system on my 416 Dart. It runs from valve covers to headers. Was wondering should I still run a valve cover breather or not. Your input appreciated.
 
yellow rose said:
Yes. You want it to pull a vacuum in the crankcase.
Click to expand...

Is it not true that some of those evac systems actually cause pressure at low RPM's instead of vacuum?
I read that somewhere on a hotrod website so it must be true.:D
So maybe if it's a daily or weekend around towner it would best be served by a standard PCV system, no?
 
What happens when you let off the gas at say 6200rpm and the sudden shift in pressure dosen't have anywhere to go?
And what does this massive inward suction suck against constantly? the seals?
And before you answer this take a couple meds and chill out I'm not questioning your mechanical Authority here I'm looking for answers Myself.

yellow rose said:
Yes. You want it to pull a vacuum in the crankcase.
Click to expand...
 
There SHOULD be a one way check valve at the collector.

Also, the angle of the tube that goes into the collector is important, as is how far that tube goes into the collector.

I've seen as high as 8 inches of vacuum on a 632 Chevy at 7200.

Most I've seen on a small block was mine at 6 at 8800.

I haven't had one back up yet. But...when using alcohol I changed the check valve ever 75 runs or so. They would get a build up on them and not shut. I learned that the hard way.
 
TrailBeast said:
Is it not true that some of those evac systems actually cause pressure at low RPM's instead of vacuum?
I read that somewhere on a hotrod website so it must be true.:D
So maybe if it's a daily or weekend around towner it would best be served by a standard PCV system, no?
Click to expand...

I'm using both right now. The PCV and the pan evacs. So far, nothing has gone wrong. Yet. I'm sure if something is going to go sideways it will happen to me!
 
j par said:
What happens when you let off the gas at say 6200rpm and the sudden shift in pressure dosen't have anywhere to go?
And what does this massive inward suction suck against constantly? the seals?
And before you answer this take a couple meds and chill out I'm not questioning your mechanical Authority here I'm looking for answers Myself.
Click to expand...


I'm good on meds today!

You are correct. As pan vacuum builds, the seals and gaskets have to seal or you can't build vacuum. Some belt drive vacuum pumps can pull 15-18 inches of vacuum and a good dry sump with a well sealed system can make over 20 inches.

Much over that, it gets hard to oil the wrist pins and keep stuff sealed up.
The longer the stoke, the bigger the bore the more displacement you have! Just like the are above the piston, you have that much displacement below the piston. It's like a hurricane down there. So, a negative pressure in the crank case makes oil return easier, along with crank scrapers. And...the rings will seal better. So....you can use thinner rings. Which causes less drag. And with less drag you get more power. Especially on the exhaust and intake strokes. Plus...once you get thinner than 1/16 inch rings (1.5 mm for you metric types) you can use gas ports. Which allows even thinner rings, and they need less back clearance, so they have less radial tension which saves more power. And so it goes
 
That makes a lot of sense in my case. I have the correct weld-in angled check valve in the right spot at the correct depth (like most things I did it myself so I'm not telling you what the "muffler guy" said) just one on the passenger side and a regular breather on the driver-side.
The stroker motor wants to "try" and blow oil and sometimes dose. The teen just screems and haven't seen a drop.

yellow rose said:
I'm good on meds today!

You are correct. As pan vacuum builds, the seals and gaskets have to seal or you can't build vacuum. Some belt drive vacuum pumps can pull 15-18 inches of vacuum and a good dry sump with a well sealed system can make over 20 inches.

Much over that, it gets hard to oil the wrist pins and keep stuff sealed up.
The longer the stoke, the bigger the bore the more displacement you have! Just like the are above the piston, you have that much displacement below the piston. It's like a hurricane down there. So, a negative pressure in the crank case makes oil return easier, along with crank scrapers. And...the rings will seal better. So....you can use thinner rings. Which causes less drag. And with less drag you get more power. Especially on the exhaust and intake strokes. Plus...once you get thinner than 1/16 inch rings (1.5 mm for you metric types) you can use gas ports. Which allows even thinner rings, and they need less back clearance, so they have less radial tension which saves more power. And so it goes
Click to expand...
 
That makes a lot of sense in my case. I have the correct weld-in angled check valve in the right spot at the correct depth (like most things I did it myself so I'm not telling you what the "muffler guy" said) just one on the passenger side and a regular breather on the driver-side.
The stroker motor wants to "try" and blow oil and sometimes dose. The teen just screems and haven't seen a drop.

yellow rose said:
I'm good on meds today!

You are correct. As pan vacuum builds, the seals and gaskets have to seal or you can't build vacuum. Some belt drive vacuum pumps can pull 15-18 inches of vacuum and a good dry sump with a well sealed system can make over 20 inches.

Much over that, it gets hard to oil the wrist pins and keep stuff sealed up.
The longer the stoke, the bigger the bore the more displacement you have! Just like the are above the piston, you have that much displacement below the piston. It's like a hurricane down there. So, a negative pressure in the crank case makes oil return easier, along with crank scrapers. And...the rings will seal better. So....you can use thinner rings. Which causes less drag. And with less drag you get more power. Especially on the exhaust and intake strokes. Plus...once you get thinner than 1/16 inch rings (1.5 mm for you metric types) you can use gas ports. Which allows even thinner rings, and they need less back clearance, so they have less radial tension which saves more power. And so it goes
Click to expand...
 
TrailBeast said:
Is it not true that some of those evac systems actually cause pressure at low RPM's instead of vacuum?
I read that somewhere on a hotrod website so it must be true.:D
So maybe if it's a daily or weekend around towner it would best be served by a standard PCV system, no?
Click to expand...
The one way valves at the headers will stop any pressure. I measured mine at idle and it pulls about 2 inches of vacuum. That's with a full exhaust system
 
fishy68 said:
The one way valves at the headers will stop any pressure. I measured mine at idle and it pulls about 2 inches of vacuum. That's with a full exhaust system
Click to expand...
interesting. before I tore into my 505 to check bearings (for the hell of it), I was checking vacuum the last time I drove it. It had more than I was expecting , 8-10- at idle, as hi as 16 at other times. this is w/ a .628 solid roller, 254-260 @ .050.----- the inside of the engine looks great, but after pulling the plugs, I noticed # 7 was colored like a paper sack, the rest were soot black, -------- comments ???
 
famous bob said:
interesting. before I tore into my 505 to check bearings (for the hell of it), I was checking vacuum the last time I drove it. It had more than I was expecting , 8-10- at idle, as hi as 16 at other times. this is w/ a .628 solid roller, 254-260 @ .050.----- the inside of the engine looks great, but after pulling the plugs, I noticed # 7 was colored like a paper sack, the rest were soot black, -------- comments ???
Click to expand...


Holy crap...you are pulling 8-10 inches of vacuum at idle through the pan evac? If you can do that, that's impressive.

As for plug color, it sounds a bit rich. It's not unusual to have one or two holes look lean when the system is rich overall.
 
famous bob said:
interesting. before I tore into my 505 to check bearings (for the hell of it), I was checking vacuum the last time I drove it. It had more than I was expecting , 8-10- at idle, as hi as 16 at other times. this is w/ a .628 solid roller, 254-260 @ .050.----- the inside of the engine looks great, but after pulling the plugs, I noticed # 7 was colored like a paper sack, the rest were soot black, -------- comments ???
Click to expand...
Wow! As YR said that's impressive. To be honest I have never measured any other than mine so I don't know what an EVAC would pull in other combos. I'm sure there are several factors that influence how much vacuum it pulls. Sooty plugs is rich. Could be as simple as the idle is set too rich. Could also be the main jets. You really need a wideband A/F meter to tell in what circuit(s) lies the problem. First and easiest thing to do is try leaning out the idle when it's warmed up and in gear. If you can't lean it out without it running rough it's probably in the high speed circuit
 
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