Don Terrill’s Speed-Talk

Like hear opinions on running a PCV valve on street performance V8,s up to say 700 HP, ive always used them rather than a catch can,
idea being to vent the crankcase better than catch cans that just vent to atmosphere.
Is using both a catch can and a PCV the best solution ? STD tension oil rings, in anything i build.
Keen to hear ideas.

I use a fixed orifice PCV valve pulling through an oil separator. At every oil change I drain 4-6 ounces of oil from the separator. Seems to work well. 500-ish HP big block chevy

PCV valve, if the carb has a port for it....If not, running one becomes a major vacuum leak and the carb will not function correctly. Learned that the hard way. The carb has to be calibrated for it, and not all carbs are. If you're running FI, the same rules apply. Best option is a vacuum pump. It will also help prevent oil leaks and can actually make power. You don't need to pull a lot of vacuum, but they do help ring seal (best when used with low tension, thinner rings) and keep the crankcase clear.

I like to run a PCV on street engines, mainly because of the moisture contamination of the oil from condensation. On a humid cool day and you drive say 50 miles, how much moisture is pulled into the engine? All engines have some blowby gasses that exit into the crankcase, and moisture is part of what is deposited. Hot oil and normal engine temp will boil off a lot of water vapor, but when you shut off the key and it sits, it will condense. Many times in the shop I have had guys with street engines asking why when they pull off the breather it is all milky. Adding a PCV will fix that issue. As mentioned a vacuum pump is the best, but an expensive option. The old backfire valves in the headers works well too. On street supercharged engines, I always try and talk them into at least using backfire valves to get rid of the blowby.

I just recently eliminated my PCV in favor of a road draft tube. I will probably run the oil for the summer and send it off for analysis (maybe 1500 to 2000 miles).

I tried a cyclone separator and caught a little oil, but not as much as I was consuming. I am now convinced all the oil my engine was using was through the PCV system. I tried a .094" fixed orifice PCV valve and it seemed to do a little better than the only other factory pvc valve I tried. I didn't run the fixed orifice very long.

My AFR shows more stable readings with fixed orifice PCV or no PCV.

FBird mentioned in another thread recently that PCV flow could increase HP at high RPM... Kind of an aha moment for me... I never considered it...

As far as condensation, I don't know, It seems to me once we shut the engine off atmospheric air (and moisture) is going into the engine and THEN the condensation will happen. I keep my fun car in a basement garage that stays about the same temperature as my house and has a dehumidifier set at 60% rel. I also have a F150 that sits on my carport under a cover and has only been driven 30,000 miles in 12 years...I worry more about condensation in my factory truck engine than I do my hotrod, but I could be all wrong. The professional engine builder that did my machine work told me to lose the PCV system and have an easier life...

If you are running a pcv valve and sucking oil into the engine, it's very likely you have it installed in a valve cover without a baffle. You have to have the baffle there to prevent the valve from sucking in oil, which it will do without it...I've never had a pcv valve suck oil into the engine when used with a baffle, but I have without. I'd never run a road draft tube on an engine anymore...for any reason unless it was a factory restoration.....I'll either use a pcv (99% of street apps) or a vac pump....Keeping the crankcase in a vacuum has zero disadvantages and many advantages.....

RT Machine wrote: ↑Tue Jun 16, 2020 7:24 am Like hear opinions on running a PCV valve on street performance V8,s up to say 700 HP, ive always used them rather than a catch can,
idea being to vent the crankcase better than catch cans that just vent to atmosphere.
Is using both a catch can and a PCV the best solution ? STD tension oil rings, in anything i build.
Keen to hear ideas.

It's more the cam duration and overlap that causes problems than the horse power output. What I'm saying is a low vacuum at idle may not pick up the PCV valve "pintle" off it's seat. The PCV valve has a spring to seat the pintle at zero vacuum (engine off) or reverse pressure (backfire). More cam overlap = lower idle vacuum and the PCV valve may not open - it takes a vacuum differential to open by design. Now none of the PCV valve manufacturers publish the operating specifications for open let alone for maximum flow. But you could take a Mighty Vac brake bleed tool and see where the PCV valve starts to open. Measuring flow is way to difficult. But you would want a PCV valve that starts to open at a vacuum amount less than your idle vacuum for your engine.

It reads like you expect to use a catch can w/o a PCV valve. That would make quite a vacuum leak and not sure you could tune it out by tuning the carb for that much flow at high vacuum idle. I think a lot of owners get ahead of themselves without understanding the entire system flow path let alone its affects. It may help to draw out the system flow path you have and a system flow path you plan to use and consider how it will affect the carb mixture.

Hope this can help.

Another option is the ME Wagner dual flow adjustable PCV valve;
http://mewagner.com/?p=444
I use one of these,tuned as per the directions.
There is a baffle underneath it in the valve cover.
Last year I plumbed a Trick Flow oil can in between it and the port on the carb.
Catches a bit of oil and overall it seems to work just fine.
Guy

Coloradoracer wrote: ↑Tue Jun 16, 2020 11:27 am If you are running a pcv valve and sucking oil into the engine, it's very likely you have it installed in a valve cover without a baffle. You have to have the baffle there to prevent the valve from sucking in oil, which it will do without it...I've never had a pcv valve suck oil into the engine when used with a baffle, but I have without. I'd never run a road draft tube on an engine anymore...for any reason unless it was a factory restoration.....I'll either use a pcv (99% of street apps) or a vac pump....Keeping the crankcase in a vacuum has zero disadvantages and many advantages.....

The valve covers are edelbrock with intact baffles. I ran them for decades on my previous build with no know oiling through the PCV. The previous build had less overlap, which might makes all the difference.

How do you choose the correct PCV valve? Anything wrong with the fixed orifice type? The only one I found was from a 65ish corvette.

Back in "the day". 66 Fairlane w/427FE I ran an electric fuel pump and hooked the mechanical pump backwards to a valve cover. Don't know if it helped but got a lot of questions and copiers

cardo0 wrote: ↑Tue Jun 16, 2020 12:54 pm

RT Machine wrote: ↑Tue Jun 16, 2020 7:24 am Like hear opinions on running a PCV valve on street performance V8,s up to say 700 HP, ive always used them rather than a catch can,
idea being to vent the crankcase better than catch cans that just vent to atmosphere.
Is using both a catch can and a PCV the best solution ? STD tension oil rings, in anything i build.
Keen to hear ideas.

It's more the cam duration and overlap that causes problems than the horse power output. What I'm saying is a low vacuum at idle may not pick up the PCV valve "pintle" off it's seat. The PCV valve has a spring to seat the pintle at zero vacuum (engine off) or reverse pressure (backfire). More cam overlap = lower idle vacuum and the PCV valve may not open - it takes a vacuum differential to open by design. Now none of the PCV valve manufacturers publish the operating specifications for open let alone for maximum flow. But you could take a Mighty Vac brake bleed tool and see where the PCV valve starts to open. Measuring flow is way to difficult. But you would want a PCV valve that starts to open at a vacuum amount less than your idle vacuum for your engine.

It reads like you expect to use a catch can w/o a PCV valve. That would make quite a vacuum leak and not sure you could tune it out by tuning the carb for that much flow at high vacuum idle. I think a lot of owners get ahead of themselves without understanding the entire system flow path let alone its affects. It may help to draw out the system flow path you have and a system flow path you plan to use and consider how it will affect the carb mixture.

Hope this can help.

Thanks for replies, i like the idea of measuring how much vacuum it takes to open the PCV valve.

cardo0 wrote: ↑Tue Jun 16, 2020 12:54 pm

RT Machine wrote: ↑Tue Jun 16, 2020 7:24 am Like hear opinions on running a PCV valve on street performance V8,s up to say 700 HP, ive always used them rather than a catch can,
idea being to vent the crankcase better than catch cans that just vent to atmosphere.
Is using both a catch can and a PCV the best solution ? STD tension oil rings, in anything i build.
Keen to hear ideas.

It's more the cam duration and overlap that causes problems than the horse power output. What I'm saying is a low vacuum at idle may not pick up the PCV valve "pintle" off it's seat. The PCV valve has a spring to seat the pintle at zero vacuum (engine off) or reverse pressure (backfire). More cam overlap = lower idle vacuum and the PCV valve may not open - it takes a vacuum differential to open by design. Now none of the PCV valve manufacturers publish the operating specifications for open let alone for maximum flow. But you could take a Mighty Vac brake bleed tool and see where the PCV valve starts to open. Measuring flow is way to difficult. But you would want a PCV valve that starts to open at a vacuum amount less than your idle vacuum for your engine.

It reads like you expect to use a catch can w/o a PCV valve. That would make quite a vacuum leak and not sure you could tune it out by tuning the carb for that much flow at high vacuum idle. I think a lot of owners get ahead of themselves without understanding the entire system flow path let alone its affects. It may help to draw out the system flow path you have and a system flow path you plan to use and consider how it will affect the carb mixture.

Hope this can help.

Thanks for the replies, some things to think about for sure, like the idea of measuring PCV valve opening point, Cheers.

I've experimented with this although my results are somewhat skewed as the blowby of the last build was unmanageable. I referenced several different PCV valves with vehicles , primarily from the latest 60's/ early 70's, that had idle vacuum I guessed matched mine (about 9-10"@850 RPM). I also incorporated an oil separator. I learned a few things, namely how a PCV valve works. And also that the adjustable version mentioned earlier, might be the ticket. Then of course there's always the pan evacuation method although for the street and highway, I'm not sure that would be the way to go.
For reference, the attached picture may enlighten some.

As well as measuring the opening point of the pcv valve pindle , I like the idea of being able to take the pcv valve apart and tweeking the internal spring (rate and or preload)
AND tweeking the shape/height taper of the internal pindle valve for effect.

Having a selection of low cost off the shelf pcv valves to test is nice too.

Keep that anti back fire back flow check function in tact.

The big trouble with 99 percent of ALL the aftermarket valve covers is the pcv valve hole and the breather holes are just in the wrong spot baffle (that works) or no baffle...
They are made to look pretty VS control oil within...