z 28 jari

iTrader: (1)

Just measured crank case vacuum at oil dipstick at idle,it was -15kPa.Passenger side valve cover air inlet blocked,and original Ac cv774c pcv at place.Idle speed was ~900 rpm ,and idle vacuum was -32...-40kPa(measured at plenum)With brand new cv774c pcv valve vacuum at crank case was -25kPa.I read that crank vacum should be from -1" to -3 inhg (-3.3 to -10.1kPa)Have you measure crank case vacuums and if yes,how much vacuum and what pcv valve?

Edit:some engine specs:sbc 385ci,stealthram,xfi280,10.6 static comp,cranking compression pressure 182psi,piston ring gap:1:st 0.2" 2nd:0.22"

skinny z

I was experimenting with different PCV valves and paying attention to which old school engines might have a vacuum profile similar to mine.

https://www.thirdgen.org/forums/tech...ions-what.html

What I didn't do is what you've done and that's examine what the crankcase pressure actually is during idle while testing the various valves.
I never did come to any real conclusion regarding which valve worked the best. It would be interesting to know what the pressures are during cruise and WOT as well.

sofakingdom

iTrader: (1)

The only purpose of a STOCK PCV valve is to maintain "SOME" vacuum across the crankcase, to evacuate the nasty stuff. Whatever "numbers" you read wherever, are only an indication of what the engine did in its FACTORY STOCK configuration. What … 145 HP?? Yeah let's all go back to the factory specs.

With a STOCK PCV valve and more or less STOCK induction system (cam, intake, carb, ignition timing, etc.) there should be SIGNIFICANT vacuum in the crankcase at idle. I don't know what number to put on it but it should be enough to make a sucking sound when released. I'd guess, from various motors I've worked with over time, that 2 - 5 in of mercury is fairly typical of a well tuned motor in good running condition, with no blowby. Generally, more is better; if your engine gives more than "spec", it's probably a good thing. Different combinations can give widely different numbers. The STOCK numbers only apply to as-built, showroom condition, pump gas, 198x motors.

If you add ignition timing, which any sensible person would do most of the time given that the factory ignition timing was crippled by its emissions requirements, then idle vac will increase as a consequence; and along with it, crankcase vac.

Your numbers don't seem particularly unusual. Nothing to worry about, for sure, as if, there might be something wrong. Without knowing the details of your motor I can't say whether it's what I'd "expect", but they mostly pass the "smell test".

skinny z

Stock numbers also apply to the heavily cammed higher compression engines of old that generally had **** poor vacuum. Going through the catalogs turns up some interesting variants of the valve itself (as shown in the link posted above). I'd forgotten about these experiments as the engine went south in a different way and the whole thing has been mothballed for the time being.

IROCZman15

iTrader: (25)

I am following along with this now, as I am working to solve a crankcase pressure issue on my new 406 (gapped for nitrous), and would like to learn what pcv and size hose and pressure/vac are recommended, instead of just throwing random combinations of those on the engine.


keep posting fellas!

NoEmissions84TA

iTrader: (1)

This a worthwhile experiment. I'm following also.

z 28 jari

iTrader: (1)

I add some engine specs to first post.

skinny z

This is what I'm interested in. An adjustable PCV valve.

http://mewagner.com/?p=444

skinny z

You may find this interesting.

https://www.thirdgen.org/forums/tech...ions-what.html

IROCZman15

iTrader: (25)

wow, a ton of info and experimenting in there. thanks for linking that. the PVC system (valves) are way more application specific than I had imagined. even altitude is a factor. I've been subscribed to a thread about the group purchase of that adjustable PVC valve here on TGO for a bit, but haven't seen an update in a while. I agree that it is an expensive item, but after i do my further experimenting.. it might be the route I have to go if things arent solved. lots of good info here and on the other posts!

IROCZman15

iTrader: (25)

i do not have any intention to hijack the OP's thread, but I am actively trying to solve my PCV issue and figured I would add on in hopes of helping him or getting mroe advice from the group..



I need to get an accurate vacuum gauge one day, but here is what I did today and after a 30 minute test drive, no signs of oil leaking or noticeable issues. I do need to check flow with a vac gauge.

currently having luck with the PCV valve that was on my old 305 engine (but was a replacement from autoparts store many years ago). found a grommet that fit much more snugly in the valve cover, ran a -6 ptfe hardline hose from the pcv valve over to the back of the sniper but since the 3/8 vac port at the back of the sniper is already in use, I used a T fitting to split the -6 ptfe line into two 3/16" lines that go into the two 3/16" vac ports of the holley sniper. seems to be working (which I know is not the best scientific way of proving anything), if not, I might look deeper into that adjustable PCV valve.









[url=https://flic.kr/p/2iG7P3d][



thoughts anyone? the reason I do not want to use the 3/8 vac port in the front of the holley sniper is because I think it would ruin the clean look of the engine and be unsightly. if I REALLY needed to, maybe I would use it.

skinny z

I would say there's some functionality there. And certainly better than nothing. As for the front port and the unsightly part, I would think that matching the PCV hose to your fuel lines with similar fittings,would result in it blending in with the rest.
Now the question is, as the OP has posted earlier, what amount of vacuum is in the crankcase? And what's reasonable? Crankcase vacuum pump manufacturers have stated that anything more than 12 in/Hg doesn't result in additional improvements. With a conventional PCV system, the idle side of it is easy enough to balance as the blow-by is at a minimum, and somewhat similarly, the conditions at cruise RPMs and engine load. It's the WOT blast that makes a mess of things. And no PCV valve is going to help there despite the valve being wide open. I have about an inch of vacuum on a full throttle pass and that's not much to help with crankcase pressure. That would be in pan evacuation or vacuum pump territory.
The 3 in/Hg that Jari posted and observations by Sofa in multiple engines (2-5") seem to indicate what one can expect. Once I pull the heap out of storage and prior to taking the engine apart, I'll run a few tests. I've got what I think are wiped out rings (among other things) and there's significant blow-by. I've incorporated a catch can to help reduce the amount of crap that gets sucked into the intake.

z 28 jari

iTrader: (1)

Just started engine(cold)with new cv774c pcv valve,and chek engine light come on(code 33,maf signal high).Engine idled 1200rpm.I shut engine down,and replased old cv774c pcv valve back,started engine,it idled 1000rpm,and no check engine light.So last 35 years have trimmed that old pcv just right to my engine

redneckjoe

I had always heard you couldnt run a crankcase evac kit with mufflers. Just another old wives tale. On a motor with big ring end gaps for nitrous or boost, i was blowing the dipstick out, and the motor was starting to get bad blow by. Put a crank evac kit on and problem solved. If i had the money i'd probably run a vacuum pump. Just put another evac kit on a big block. Well worth the little bit of money they cost IMO.
https://www.summitracing.com/parts/mor-25900

RJ IROC

I'm running an XFI280 cam in a stealth rammed 383. I modified my stock valve to allow more flow at the idle area (60-65kpa). I ground the lip (widest part of the valve) to separate the two halves of the valve. I had to grind a flat on each side of the pintle to allow more flow at medium to high vacuum conditions. I welded the two sections back together, and the valve works great. My main concern was retaining the one-way flow in the event of an engine backfire. This valve does that, and increases PCV flow at idle. Any change in airflow characteristics, I was able to tune out since I'm running an EBL. I made sure to run an external baffled catch can to prevent plug fouling and charge contamination due to the increase in flow.

skinny z

Is that the original equipment spec PCV valve?
Something I've been thinking about and that's a vacuum reading with the PCV blocked off. Seeing as it is a controlled vacuum leak, it would be interesting to know how much of a leak it is.

z 28 jari

iTrader: (1)

Yes it is car original pcv valve now at place,new one was ordered from rockauto,and it is usa made.Both has same #cv774c part number.Old have probably weaken spring->less flow at idle.One way to measure flow is to put rotameter inline between intake manifold and pcv valve,but before buying rotameter you shoud know the approx. flow, that you can buy meter with right scaling.Something like this:
https://www.aliexpress.com/item/3304...archweb201603_

NoEmissions84TA

iTrader: (1)

I heard the same. How can you create a vacuum in a pipe where there is backpressure present? That's why the evac system is for race vehicles (open exhaust) only.

redneckjoe

As long as you have good free flowing mufflers it will work fine.

skinny z

There's some lively debate about this on the web.
At one time I could recite the principle that makes for a functional pan evacuation system. This has to do with the volume of air (exhaust) rushing past the open end of the evac tube. Some have compared this to the venturi effect in a carburetor put that wouldn't be entirely accurate. The other part that makes this work are the negative pressure exhaust pulses that are constantly moving up and down the length of the entire system. These harmonics are present whether the system ends at the collector or with a tailpipe at the back of the car.
Results may vary.