Kevin D.
Well-Known Member
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.
REALLY ?No breather. Seal it off tight.
Yes. You want it to pull a vacuum in the crankcase.
Yes. You want it to pull a vacuum in the crankcase.
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.
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.
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
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
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 systemIs 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.
So maybe if it's a daily or weekend around towner it would best be served by a standard PCV system, no?
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 ???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
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 ???
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 circuitinteresting. 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 ???