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Test it!
If you are not sure you could always go to the local autoparts store and get an el' cheapo valve, short length of hose to the base of the carb and just let it flap in the breeze while you see what it does to you idle/power/drivability. Lots cheaper and very easy compared to going to the speed shop for all new breathers, braided shiny and pretty stuff. Seems to me the one at the parts stores are only a couple of bucks.
Just a thought! Rick |
I don't have it anymore But when I was doing my brakes I had a booklet that came with the little mightyvac style handpump (Equus) for Vacuum. In it there's a section for testing egr and pcv valves. One could always do an "as found" test to determine what their own PCV opens at? The ones I have seen don't have any spring to lift against, I think its their actual valve weight that determines the vaccum required to open the port. The body is lifted up off the seat at peak vacuum thru the suction in the line to the intake/carb
So having said that, could a guy simply look at that great wall full of PCV valves and pick an application with lighter/less vac setting?? Lastly how about adjusting the air/fuel idle adjustment screws with a vac gauge at idle, looking for that optimum amount of vacuum? As you guys say you may only get what you get due to a wilder cam. Then your stuck. Tim |
Tim, you're right, the manifold vacuum (actually air flow) lifts the sliding valve up, but actually it moves up to the seat rather than up off the seat...at lower vacuum (under engine load conditions), the weight of the slider allows it to drop away from the seat and allows more air flow at lower vacuum (sounds backwards), up to the max flow permitted by the orifice in the PCV valve. I had the parts guy look up the application for a high performance 427 Ford (Mustang, I think) and bought that one, figured it would have flow characteristics close to what I needed...but is there any real difference? I dunno...
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BiB makes the best example. Too much crank case pressure blowing oil past the dipstick. Those of us in CA should be careful as I was told by several engine builders that the oil vapor lowers your octane rating.
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When running an independent runner induction system, such as 48IDA Weber carbs, on a 351w, dart block, bored/stroked to 427cu.in., already plumbed with an 8 port vacuum set up, how many "good" options do I have to run vacuum to my power brake booster and hopefully still be able to run a PCV ? Will I have enough vacuum for both? Should I only use one or the other? If I can't run vacuum for a PCV would a crankcase evac system be wise ? Would running two crankcase breathers on the valvecovers be sufficient ? Would the breathers be messy? Would a long breather tube routed to under the car be keep the engine compartment cleaner? Does anyone make an electric vacuum pump for this type of application ? Where can I find an assortment of different types of breathers, longer ones preferred ? I know, lots of questions, but I had to get these off my mind- whew !!
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It depends on your cam. A stock or mild cam provides so much vacuum, the PVC will not make much difference. With my lumpy (long duration, lots of overlap) cam, vacuum is at a premium for my brake booster. I have much better brakes without the PVC connection. I have also installed an electric vacuum pump to help out when the vacuum drops too much. Sliding off a mountain road one time was enough!
I used the Moroso system to draw the fumes out of the crankcase. The tubes from the rocker covers are connected to the exhaust headers instead. :D It is much easier to tune the carb without the PVC (steadier vacuum signal) and you make more power (richer mixture). However, it better be there for the smog test! Paul |
Make a Road Draft Tube
Tommyboy -
What I have done with my Webers is to add tall breathers (reworked steel valve covers) and fabricate a Road Draft Tube for one side (connected to an elbow on the other side with 5/8" heater hose). I cut up an old auto tranny fill tube and used it (every body was looking for vintage Road Draft Tubes for Concours cars - the prices are crazy - like $40 for one). http://www.clubcobra.com/photopost/d...bers_2-med.jpg This has worked well for me - a minimum of oil to wipe off the valve covers, occassionally. The first time I set-up my Webers on my Pantera, I linked two small fuel logs together to gather vacuum from eight ports for the power brakes. I tried to run two breather caps for crankcase ventilation - that was not sufficient - the crankcase pressure built up to the point it would push oil past the rear main seal (it was a high mileage engine). Adding a road draft tube cured it. Good luck, Jeff Burgy |
CobraJeff, I was also thinking of a road draft tube system. Would you happen to have any pictures of the passenger side and rear of your engine?
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Well, no.....but I guess I could get some....
.......didn't have any on hand, but went out to the garage and made a couple shots for you (thanks goodness for AutoLifters!).
Will post one here, a couple more in my gallery - hope this helps. http://www.clubcobra.com/photopost/d...umbing-med.jpg regards, jeff |
Hey Guys:
This belief that you need a PCV to relieve crankcase pressure makes no sense to me. I find it hard to believe that a breather cap on each valve cover cannot relieve what pressure might build. Cars ran for decades without PCVs and did not explode from crankcase pressure. How much blowby are some Cobra owners getting? The PCV was invented to burn off spent gasses not to relieve pressure. Just my 2 cents worth. Bob |
Bob, here's my take on it: you are correct, the PCV valve was originally intended as an emissions control device, but it had two unintended beneficial consequences...by pulling much of the blowby out of the crankcase it prevented the buildup of acids and other contaminants in the motor oil, promoting longer engine life; it also solved the oily mess around the breather or the road draft tube that older, non-PCV motors typically have...even engines in very good condition have some blowby, so most of the guys here would do it just for the "tidy" factor, and the better engine health is a major reason to put one on for a street vehicle.
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Bob,
I started out with the old 292 & 312 engines from the 50's ,they had a crankcase breather,1-1/2" diameter mounted right to the side of the bock just obove the oil pan, when those old engines got a few miles on them ,they really puffed pretty hard out of those pipes. |
Cobra Jeff,
Help me out. Is a Road Draft Tube a breather that dumps out under your engine?? Thanks, Steve |
I must be missing something here. Those who are running PCV are running to the base of the carbs. But doing this way might (or might not) screw up the carb at idle (?). So why not run the PCV to the base of the air cleaner instead?
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Steve -
A road draft tube is a breather that drops below the engine, with a "bologna slice" cut on it so that air flow under the car sucks the crankcase fumes out. This was typical on Ford V-8's in the 50's and 60's before pollution controls became mandated. Jeff |
Jeff,
Thanks for the info. Steve |
Pete,
through the carb makes quite a mess over time with a resultying loss of reliability and performance, to the manifold or under the carb dosnt gum up the carb, but can have an effect on performance esp. with a big cam. Karl |
Karl,
Ah-ha! I knew there had to be a good reason! |
I have a breather on the drivers side cover and a PCV on the passengers side that is connected under the carb. The PCV does seem to be spring loaded and I end up with more oil build-up around the PCV. Isn't that opposite of what would be expected?
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Hey is the mess simply because of the way the valve fits the grommet or the grommet fits the valve.
I would think oil and fumes should flow nicely under vacuum to the base of that intake. Your vacuum is max at idle and slow cruise.......should pull then. Shake the valve it should rattle. Doesn't matter which side it sits on as long as it is from valve to intake or carb The breathers are sometimes routed to the air breather inlet Tim |
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