Recently a few odd things have been happening to some of the HyPo engines being brought to me or I have been building. Is there a formula to figure out how much "breathing" an engine has to have to "neutralize" crank case pressure at idle AND @ high RPM? It was brought to my attention that if you plug all breathing (PCV, breathers, etc.), than put a boost or pressure gauge on the dip stick tube it would tell you at least how much pressure your dealing with. Truth or fiction?
No simple formula..! Quite complex and ever changing. And yes, while you will need an accurate and sensitive gauge, yes you can measure the pressure. BUT, the pressure WILL find likely a way out before you are able to get a good, repeatable reading. It all depends on the ring seal...blowby. A naturally aspirated engine will have somewhat less than a supercharged/pressurized engine, this should be somewhat obvious. If the engine has a good seal, 1% to maybe 3% or 4% leakdown, the crankcase pressure will be lower than an engine with 8% or 10%+ leakdown. So there's that, pretty simple in thought, difficult to predict exactly. Now add the pumping pressure. Every time a piston travels "down" the cylinder, there is pressure put directly into the crankcase. This was proven many years back that a piston moving down the cylinder, say #1 in a GM V-8 engine, then piston #6 should provide an equal but opposite "vacuum" as that piston travels up its cylinder...but it doesn't, been proven. An engine with near perfect ring seal, will still have internal pressure. All this is why racers started using A.I.R. pumps (from 60's smog era cars) to pull a "vacuum" in the crankcase. Much better action thAn the PCV and the header evacuation systems. You actually need BOTH on an engine without an actual pump. The PCV system will work at lower rpm's, and the header evacuation for higher rpm vacuum. Then when the AIR pumps didn't last, dedicated pumps were designed, which are still high maintenance. Not really suited for street use, though I know people that have done it. Then as to further mix up your question, as the rings wear, the crankcase pressure will...go up ! Does that help any ? Mike
I’m not sure about the engine breathing part of your question, but there is a correlation between crankcase pressure and piston ring seal. On a Diesel engine, it’s much more complicated to take a cylinder compression reading like on a gasoline engine. The engine companies, (International, Cummins, etc), will list a crankcase pressure specification that a healthy engine should not exceed. The way it works, is to connect a certain sized orifice to the road draft tube and connect it to a manometer to the orifice. I’m going to use International as my example because that’s what my specialty is, but they say no more than 6 inches of water at high idle. High idle is the throttle on the floor with no load on the engine. Anything more than that and they say you have ring seal or power cylinder problems. And yes, the oil fill cap in place and dipstick in place during the test. I’m not sure what a gasoline engine should have, I don’t know that the manufacturers have ever published a spec for that. Sent from my iPhone using The H.A.M.B. mobile app
Running an accurate leakdown test will give you the information you need to alert the customer as to the condition of their engine . We've all seen engines run really well for a bunch of miles with too much blow-by , if it doesn't foul plugs , and the cause is known , its the owners choice as to whether you " fix it" or live with it ..... I guess my ? Would be , what's the point of measuring the pressure ?
The leak down will show you what is leaking into the pan. A manometer reading in the pan will show the delta of what's getting in and what's venting out.
it's not the pressure you need to worry about, really...it's the amount of blowby--the flow rate. The leakdown test will give you a static % number for blowby, but won't tell you much about what happens when it's running. The bigger the engine, the higher the rpm, the worse condition the rings are in, etc the more ventilation it needs. And baffling is pretty important, also.
yup. But the pressure that builds up, depends on the flow past the rings, and then out through the breather system. The breather system has to be built to handle sufficient flow rate, so that pressure does not build up. If you're looking at it as having to measure the pressure, you're looking at it wrong. You need to measure the airflow.
You have it backwards. Pressure is a result of flow finding a restriction down stream of the pump. I.E. All pumps create flow (even these big air pumps we call engines). It is downstream restrictions that create pressure. Imagine a hydraulic pump with no hoses attached. It will dump a ton of oil out the port when the pump is running, but won't develop any pressure at the outlet since there is nothing to restrict the oil. In the case of blow-by, the crankcase pressure is a balance between the amount of blow-by going into the crankcase and how much restriction there is in the breather / PCV system. When I worked for a small engine manufacturer a few years ago, blow-by was something that the engine designers always wanted more data about. As the lead Test Engineer I had to find a way to accurately measure that flow. This was not as easy as you may think (and extremely difficult on the 2-strokes). It often involved elaborate setups that included ping tanks and multiple pressure and flow meters. Especially on the larger single cylinder engines which created all sorts of interesting pressure waves and dynamic responses within the system.
" The breather system has to be built to handle sufficient flow rate, so that pressure does not build up." I should have worded the question more in this fashion! Example; SBC 413 Lunati, no blow by, no oil consumption. My mind says it should have a lot more breathing than the little holes provided in the push in breather. Yes, I can mount breathers all day long on the motor just was wondering where the sweet spot is!
I was really enjoying that engine ( in the green T) until I saw the fuel lines That's how my PCV is set up , works fine ( depending on how high & often I crank 6500 rpm ) then I get a little blow-by .
I am not sure about modern engines with better pistons/ring combos. We used to have a crankcase pressure problem with boosted motors. I have got to assume that the added combustion pressure blew past the rings. We did a lot of things to overcome it, gapless rings, tighter tolerances (which seemed to be anti-productive to me), vac pumps. A naturally aspirated engine in goods shape, should never really have a crankcase pressure problem with a breather or two and a PCV valve.
Cylinder leakage is one source of pan pressure. Windage generated by the spinning crankshaft and piston movement are other sources.
Cylinder leakage is the only one that causes flow out of the engine...the other two will move air around inside the crankcase a whole bunch, though
Some of the later model unmentionable engines had enough air movement inside the pan that openings were cast into the main bearing webs to allow for movement back and forth as the pistons rose and fell. As long as the leakdown test provides reasonable results, I'd just try crankcase breathers.
I performed that blow by test many, many times myself. I carried the tool with various sized orifices on my service truck.
I am looking at putting a PCV valve on my flathead. In my search for one that would work with my car, I found this: http://mewagner.com/?p=444
I mentioned this on another thread, I installed a Wagner valve in the Boss engine in my Cyclone, it ran this week for the first time so I am not able to express my opinion on it yet. From the research I did before purchasing it every thing I read about it was positive. I also installed an accumulator in the system. Jim
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