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 PCV 
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Joined: Mon Feb 18, 2013 1:13 am
Posts: 158
Post PCV
Is a PCV valve mandatory for a mostly street driven car? For idling at stop lights?

Could I put a valve inline and restrict some of the flow, to give my carb a better signal.

Old's is pretty much wide open with out any restriction. My engine is fairly new and I don't see any blow by as of yet.

I don't have a gauge to read the air/fuel mixture, so maybe I would be running blind if I restrict it.
Or maybe my plugs could tell the story?

I am aware of the speedtalk thread, would like to know if this is a race only option.

Steve


Mon Sep 09, 2013 3:14 pm
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Joined: Sun Jun 23, 2013 1:41 pm
Posts: 366
Location: Phila. Penn.
steve,
I think this is an important topic, but to your specific questions, I'm not really sure what you're asking. Let me try and maybe you can clarify and then others will chime in.
Quote:
Is a PCV valve mandatory for a mostly street driven car? For idling at stop lights?

By mandatory do you mean the law? For practical purposes that varies from state to state.
As far as needed to idle nicely, no.
Quote:
Could I put a valve inline and restrict some of the flow, to give my carb a better signal.

Lost me here. On the first part, inline with what? On the second part, I think your asking if PCV helps signal and its a to part answer but basically I don't see why it would. The PCV generally lets in a relatively small amount of crankcase air into the carb's base. The signal for the idle circuits comes from the manifold vacuum and the signal for the mains by the air velocity past the boosters.

Quote:
Old's is pretty much wide open with out any restriction. My engine is fairly new and I don't see any blow by as of yet.

My understanding is that reducing the pressure in the crankcase can help insure the rings do their job, particularly the oil ring. Before PCVs, draft tubes were used. At higher rpms, reducing crank pressure can help hp. But it would seem that a PCV is less effective approaching wide open throttle. This is because there is little manifold vacuum. For that purpose racers use vacuum pumps or draft tubes in the exhaust.

Quote:
I don't have a gauge to read the air/fuel mixture, so maybe I would be running blind if I restrict it.
Or maybe my plugs could tell the story?

Not sure of the specifics of your situation, but it will run a little leaner and just tune as usual. If the engine has poor manifold vacuum at idle, that's when it becomes challenging. PCVs are based on the assumption that the manifold vacuum is high and will decrease with throttle opening. Hot rods don't always do that.
Quote:
I am aware of the speedtalk thread, would like to know if this is a race only option.

Uh. Last PCV thread I saw on ST got continually derailed into a lecture about PCVs and reducing emissions. My observation is the opposite. It's rare on race cars. Probably because its an extra thing to figure out and hard to make work well with a race cam. But perhaps others will have more insight.


Mon Sep 09, 2013 9:11 pm
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Joined: Mon Feb 18, 2013 1:13 am
Posts: 158
Thanks for the response and I think you answered a lot of my questions.

We don't have emissions here, (Once me and a friend ran open exhaust down main street!) I was wondering if it was needed to evacuate the Valve covers at idle or if there might be some kind of build up that I am not aware of. My car is mostly street driven.

I was thinking of placing a regulator of sort in between the carb and the PCV valve to adjust the amount of vacuum. Just kind of like a dead man valve on an air compressor tool, only not all the way off.

I was also curious if using two breathers instead of a PCV valve would increase the ware on my engine rings. I think you answered that, however it would effect my idle circuits. I might need to put a Vacuum gauge on and see where I am at.

Right now I have the throttle all the way closed at idle, the idle screw is not even applied. It idles at 800 in park and lower in drive with out any problems. Seems a little strange that I am not using the idle screw, but that how this one reacts.

My Car is a 72 Cutlass with a 455, I have many threads here. It has a pretty loopy cam, and Aluminum top half, 3.90 gears. I built in mind for the Strip, but I mostly run 91 and drive it on the street, once a week. It's currently a Holley test mule, I love to try different combo's and Carb's.

I will re read your post, I am still a little confused on weather or not this would benefit my Carb tuning or not. I am not as advanced as most on this site, I am learning out of a passion for carb tuning, not out of necessity. I built my car from scratch out of the junk yard, and I am on my third 455, the last one lasted me 9 years. This one is Machined and balanced. For the most part I like to do everything myself, though I did have the last engine delivered as a short block. I respect professional results in some cases. I also had someone else do the body work, I hate sanding bondo.

There was a little about this topic this morning on ST, but it left off with Mark saying you might want to decide weather to use a PCV valve and the other guy saying I may try running two breathers instead. Some were discussing an adjustable Valve and that's were I got the idea to regulate the flow.

Steve


Mon Sep 09, 2013 10:21 pm
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Joined: Sun Jun 23, 2013 1:41 pm
Posts: 366
Location: Phila. Penn.
There is always some pressure generated in the crankcase, and your engine obviously doesn't have draft tube, so you definately want breathers somewhere. Otherwise when there is some pressure it will look for a way out and find its own. If it finds a way out, it will carry the oily vapors with it. On the strip, you'll probably run to what 5000 ? in second gear and then go through the lights at a bit less. IMO, that and on the highway is where the breather system will benefit the engine.

The comparison should be two breathers vs. PCV with Breather (at least for breather in the valve cover arrangements).
Would one be better for reducing engine wear, I don't know. On a streetable engine, not raced, it would seem the PCV is more effective. Mixing in an aftermarket cam and some racing, I don't know.

I'm actually rather curious whether connecting the breathers, as done on older road race cars and circle trackers, effects the circulation of the crankcase gases. The one positive thing it does is keep the oil in the engine (unlike a catch can). This is all because oil starts to accumulate in the valve covers during braking and hard cornering. (This won't happen on street car unlesss you're autocrossing or running road courses on 'r' compound tires). But, I've wondered if the conected breathers actually help the crankcase vapors circulate in concert with crank rotation.

I'm puzzled about your carb's throttle blades. Since the idle mix screws have no effect, my guess is some transfer slot is showing. Probably too much. Otherwise, where would the fuel be coming from when the idle mix screw is closed?
In this case, adding the PCV might let you readjust throttle and idle mix.

Sometimes carbs are set up to run without a PCV. This how QF returned my Holley. No PCV nip. Why? I dunno, but my guess is they thought it was for circle track. But your Holley's is probably set up assuming PCV and has the 3/8 nip in the base. Easy experiment. Run a 3/8 id tube from a breather to the PCV nipple. Splice in a restriction made from a dowel. The 1970's chilton's had a chart based on engine size. More displacement, bigger hole. Since you're experimenting, maybe try 1/16" dia hole and work up incrementally. It won't be terrible effective as a PCV at higher rpm, but will let you see the effect at idle.

I'll have to look at ST when I have a chance. That sounds like a better thread. There is now an aftermarket two staged adjustable PCV that is interesting. But it was hard for me to understand how it was addressing the issues caused by a sporty cam.

Yes. Get yourself a vacuum gage, a timing light if you don't already have, and a low rpm tach (or tach/dwell) if the timing light does not have one built in. You can sometimes pick these up used for very little. These are very useful tools. Also see if Old's ever offered a vacuum gage for the cutless intstrument panel. Sometimes they were called 'performance gages'. Having a vac gage in the dash is always informative.


Tue Sep 10, 2013 8:11 am
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Joined: Mon Feb 18, 2013 1:13 am
Posts: 158
Good info.

The idle mixture screws are responding, it's the top idle raise and lower screw that I have all the way out, because the with the blades all the way closed I am at the perfect rpm, 800 in park and 600 in gear.

This is a little curious to me, because on most Carbs I have to creep open the blades with the idle screw just a hair. But maybe this is a good thing.

Olds did offer a factory Vac gauge, but I am not hung up on period correctness, only sometimes the aftermarket stuff just junk.

I have a shift light that I hid in the top heater vent, and it comes on at about 4900, just as a way to know when I am getting close, but I have ran it up to 5500 and 6000, and it ran healthy without any slow down, valve chatter or float. With the internal balance it runs very smooth as well, as if it hardly uses up any energy.

The thing that I was thinking about is after my Carb gets into the main's my PCV valve weight drops down and I only have one breather working any way. So maybe two breathers might be more affective.

Not sure if I am correct on this or not.


Tue Sep 10, 2013 9:41 am
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Joined: Sun Jun 23, 2013 1:41 pm
Posts: 366
Location: Phila. Penn.
Steve,
I think some of this will become clearer as you get more familiar with the principles behind the carb circuits. See if you can find Holley Carburetors and Manifolds by Mike Urich and Bill Fisher (HP Books, Los Angeles) 1987. (also sold as Holley p/n 36-73) Any edition by the same authors will do for your purposes. Although there is a slight sales pitch, the drawings and explanations are very clear and accurate. Once you understand the principles and approaches Holley used, you'll also be taking a big step toward understanding approaches taken by other companies.

Also, once you know the functions, you won't be driving us nuts with terms like 'upper' and 'lower' screw, which is leaving people like me (who don't know what carb you're using) totally lost. :) In this case I can only guess that perhaps one is the base idle speed (actually the throttle stop for idle) and the other is a fast idle speed which is frequently used with a choke.

The important thing is to have the transition slots exposed at idle. Sounds like you may be there. If you are, then I don't know what will happen with a PCV if the throttle is already as closed as it can get.

I wasn't suggesting the OEM vacuum gage for originality. Just that its sometimes convenient (and cool) to have it in the instrument panel. For what you're asking, its not needed, just thought you might like having it while you drive.

Just to be clear, getting into the mains should not cause the weight to drop. Its when the throttle opens so much that the manifold vacuum drops. PS. There are PCV's that don't rely on weight at all. AMC v-8s for example used a hortizontal PCV valve and the spring did all the work. Fascinating how many ways there are to skin a cat. (figuratively speaking of course. No cats were harmed in the process of writing this post)


Tue Sep 10, 2013 11:59 am
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