PCV valve AND breather cap too?

[JCoop]

My friend's 460 has a PCV valve hooked up to vacuum at the carb base, AND a breather cap on the other valve cover, but the cap is the type with a hole on top; and you can really see the blow by at idle.
My 460 has both as well but the breather is the type where the vent is under the cap, venting at the base of the cap.
Are both necessary? Doesn't one negate the effect or effectiveness of the other?

Insight from 460 gurus would be appreciated.

[Burgs]

The carb hookup creates the vauum that pulls air in thru the other breather. Yours looks right.

[ByronRACE]

PCV stands for Positive Crankcase Ventilation.

Engine vacuum sucks air/vapor out of the crankcase.

It doesn't matter how air gets in, as long as it does...otherwise, you'll pull enough crankcase vacuum to pull oil away from the seals and cause them to wear quickly.

If you've got the crankcase hooked to engine vacuum through a check valve and still see vapor out of the breather, you simply have more crankcase vapor being generated (blowby) than the PCV system is able to evacuate. Excessive blowby, or low engine vacuum...or maybe normal blowby and low engine vacuum due to radical cam. Depends.

I'm not a PCV fan. I don't like oil in my intake. When it gets into the combustion chamber, it creates depostis and induces detonation. I vent the crankcase through large hose to an external can/breather and don't run PCV. My cam doesn't generate much vacuum; so a PCV system wouldn't work well anyway. And, the supercharger creates the potential for a supercharged crankcase in the event that the PCV checkvalve fails to seal. The same thing happens in a race engine if you have a little ring damage or blowby at WOT. Picture this...you're at WOT, blowby enters the crankcase, engine vacuum is zero (because you're at WOT) and the PCV check valve is being held shut by a weak spring and/or weight. If your crankcase is adequitely ventilated, the vapor blows out the breathers. If the breathers are not sufficient, some of the vapor blows through the PCV into the intake manifold carrying oil along with it. That oil gets into the combustion, lowers the effective octane rating of the fuel, and now you detonate and damage the engine. Take this one step further into a situation where you have a badly hurt engine already...and it's a great way to finish it off. The excessive blowby will send significant oil into the intake, causing the thing to self-destruct ahead of schedule. I'd much rather vent to a can, and periodically see what's in the can...it tells me if I have trouble; hopefully early enough that I can do something about it before all my parts go to heaven.

[JCoop]

Thanks Burgs, but it's not that motor; another car, although hooked up like that. BTW, old pic of that engine bay. I've since moved the firebottle to the trunk and replaced it with an Accusump

And thanks ByronRACE. Good explanation. Unfortunately, in NY, when registered under the legal and proper "homebuilt" vehicle procedure as a '98 Custom, the DMV inspection computers kick in a default '81 standards emission requirement (PCV valve and idle sniff test).
However, I think I will vent the other side to a tank as that sounds cleaner than venting all over the engine bay.

[speed220mph]

As was said, the breather is to allow air to flow into the low-pressure crankcase under idle and part-throttle conditions. However, under wide-open throttle conditions, it vents high crankcase pressure resulting from blowby. The ideal situation is to run a "closed" system, or one that routes filtered air from the suction side of the air cleaner(s) to the breather. Problem here is most air cleaners used on Cobras don't provide a fitting to do this. Another approach would be to vent the crankcase to a firewall-mounted puke tank. Regardless of how it's done, the crankcase must be vented.

[JCoop]

So let me ask this: What would be the cause of excessive blow by? Broken or cracked rings? This particular 460 seems to also have abnormally high oil pressure (65 cold, 55 warm) compared to my other one which normally runs 45/35. I plan to do a leak down test as soon as I can borrow the apparatus; worst case scenario being a rebore, rings, pistons this winter.

[ByronRACE]

Causes of blow-by:

-improper bore finish, rings don't seat during break in
-mixture too rich, rings don't seat during break in
-wrong ring package
-wrong piston to bore clearance
-rings installed wrong
-excessive ring end-gap
-detonation damage (rings, ring lands, pistons)

But what is "excessive" blow-by...are you sure it isn't normal? All engines have blow-by.

[Randy Rosenberg]

Interesting... I was told that oil vapors contain a specific acid that if not positively evacuated from the engine could condense and re-enter the oil; I was told that these acids will lead to early bearing wear, and that it's better to burn them (PCV) than attempt to evacuate them to atmosphere. I was told that if one does not run a PCV, then one ought to change oil frequently (like 500 to 1000 miles).

I didn't "google" this to see if there's validity to it...

Any thoughts or comments?

[JCoop]

Originally Posted by ByronRACE

Causes of blow-by:

-improper bore finish, rings don't seat during break in
-mixture too rich, rings don't seat during break in
-wrong ring package
-wrong piston to bore clearance
-rings installed wrong
-excessive ring end-gap
-detonation damage (rings, ring lands, pistons)

But what is "excessive" blow-by...are you sure it isn't normal? All engines have blow-by.

The car had 4 qts of oil when we picked it up. Upon changing the oil and filter, filling it to the full mark with 6.5 qts, the oil pressure was up at 65. It had one of these breather caps that had a hole on top, and foam inside. You could see the oil mist pulsating upwards from the hole. So we installed a regular Ford Racing breather cap with the little holes at the base of the cap. You could still see the oil mist pulsating outward from the base. the 460 on SPF#1052 has the same cap and no oil ever pulsates out of it.

[ByronRACE]

Do you have a vacuum gauge on this engine? What does it pull at idle?

Assuming it has an oil filler neck, what happens if you cap it off with your hand. Does the PCV ever create a vacuum on your palm? It should; assuming the engine has no other breathers. If it doesn't, you either have an ineffective PCV, or too much blowby to make it work. If you have noticable puffing out an open breather opening (like a locomotive), you have problems.

Byron

[ByronRACE]

Originally Posted by Randy Rosenberg

Interesting... I was told that oil vapors contain a specific acid that if not positively evacuated from the engine could condense and re-enter the oil; I was told that these acids will lead to early bearing wear, and that it's better to burn them (PCV) than attempt to evacuate them to atmosphere. I was told that if one does not run a PCV, then one ought to change oil frequently (like 500 to 1000 miles).

I didn't "google" this to see if there's validity to it...

Any thoughts or comments?

I have no idea Randy. Though, that's news to me. The byproducts of combustion is mostly water and CO2, with CO, some oxides of nitrogen, and assorted trace elements...as well as unburned fuel vapor. But Acids? Beats me. Keeping water and fuel out of the crankcase is a true benefit. If you have an engine that pulls reasonable vacuum, is built tight and doesn't produce blowby, and doesn't run a power adder...then PCV is a good thing provided it doesn't pull oil into the intake. But, it takes a lot of fuel vapor to contaminate 11qts of oil; and keeping the crankcase ventilated to atmosphere is good enough mitigation for water vapor most of the time. Additionally, most cobra guys drive the cars less than 2K miles/year...so how much benefit can it be.

[JCoop]

Originally Posted by ByronRACE

Do you have a vacuum gauge on this engine? What does it pull at idle?

Assuming it has an oil filler neck, what happens if you cap it off with your hand. Does the PCV ever create a vacuum on your palm? It should; assuming the engine has no other breathers. If it doesn't, you either have an ineffective PCV, or too much blowby to make it work. If you have noticable puffing out an open breather opening (like a locomotive), you have problems.

Byron

No vacuum gauge, Byron, but the mist looks like a locomotive. Thought hard about it last night and it looks like a $4K bottom job so I think I'm returning the car; which could be a hassle and an interesting saga for another thread as I had signed all paperwork including a warranty waiver with the dealer who took it on consignment. Thanks for your insight, although it confirmed what looks like a nightmare.

[ByronRACE]

Originally Posted by JCoop

No vacuum gauge, Byron, but the mist looks like a locomotive. Thought hard about it last night and it looks like a $4K bottom job so I think I'm returning the car; which could be a hassle and an interesting saga for another thread as I had signed all paperwork including a warranty waiver with the dealer who took it on consignment. Thanks for your insight, although it confirmed what looks like a nightmare.

Ouch, sorry to hear that! I didn't realize that was your circumstance. If you've got that much blowby chuffing out the filler; I'm sorry to say it sounds irreversible without a teardown. Hone&rings minimum (if the engine is fresh and just didn't seat), but more than likely...something worse assuming the engine isn't new.

[wboss429]

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