Ok, I've searched and read so many different threads on adding PCV to flathead engines I'm thoroughly confused. I have a 49 Mercury flathead (8BA), put on an Edelbrock 3x2 intake with Holley 94 carbs and cannot figure out what to do when I remove the road draft tube. So I think I will take out the road draft tube, install a PCV valve in that hole and put a breather tube in where the mechanical fuel pump was (installed an electric pump). Also leaving the original oil fill tube. So my questions are 1) Will this setup work? And if it does where does the tube go for the PCV valve? There's no port on the intake for it and no connection at the bottom of the carbs. 2) Should I just plug the road draft tube and put the breather tube on the Fuel pump port? 3) Should I just leave the road draft tube and forget it all? Hopefully someone on here can help me with this setup.
just leave the road draft tube. If you really want a PCV, this is what i would do. plug up the RD tube, tap a hole or drill a hole in the side of the RD tube. Then attach a length of steel line maybe 3/8 or 1/2 inch to the hole via weld or other way you can figure. Then attach the pcv to that with a rubber hose and then another rubber hose to the vacuum. Or you could just leave the road draft tube. I think it would be cool though if you put a breather where the pump used to sit. Never seen it before.
Just remove the road draft tube and and install a grommet for the PCV valve. The breather on the oil fill tube is air intake for the system just like it always has been. It's about the easiest PCV valve installation going. They make a kit for this.
you can go either way...I had pcv on my flathead for years and had no problems.....I put a pcv grommet into the road draft tube and a breather vent where the fuel pump went....I used an electric fuel pump......you could also use thr road draft tube instead of pcv....It's your choice.....my intake had a vacuum port below the center carb....if you don't have a vacuum port, find a thick part of the manifold and just drill a hole and tap threads into it for 1/4" pipe threads...then use the correct fitting to plumb a vacuum line.
None of the above will work - you're all just recirculating the gasses (and pressure). edit: except for mschiffel who posted while I was typing : ) You need to drill and tap a hole in the base of your intake manifold to accept the hose from the pcv. It needs manifold vacuum to pull it into the cylinders for burning.
I cut the road draft tube and put in a grommet for the pcv valve,stock oil fill tube with breather and getting my vacuum from the stock 8ba intake.
My pee brain tells me that you should try to keep the engine vacuum up in a flathead, it ain't that high to begin with. So, I have the PCV in the manifold's road draft tube hole as has been described and I am routing the tubing from it to the base of my air cleaner. "Inny" air comes from using an early style fuel pump/oil filler tube stand at the rear. Previously I had the PCV tube going to a Southwind heater carb base plate for vacuum. Strictly a seat of the pants evaluation, but I feel that my flatmotor is pulling stronger since I changed this up.
I wondered the same thing, but talked with several guys on here that had routed the PCV line right into the intake and it worked fine. I have a 3 stroms, and my air filter bases are so small I really didn't have room to get the PCV mounted to them.
I made a 1" aluminum spacer for under the carb & tapped a 1/4" hole in it for a hose fitting. Extra carb hieght shoud help vacuum. Also removed the road draft tube & put a grommit in for the PCV valve.
Honestly, this is one of the least confusing flathead issues. Actually it's a no-brainer. The oil-filler already has a breather, no need for another at the fuel pump mount. The engine design (see picture above) is for the oil filler to be the inlet and the draft tube the outlet. The draft tube hole is perfect for a grommet and PCV valve and most aftermarket manifolds already have a manifold vacuum port. As for vacuum loss, mine pulls nearly 20 at idle with the PCV. The main function of the PCV valve is to control any serious vacuum loss, otherwise why have a valve at all?. The valve closes during low and high vacuum conditions and only opens during "medium" vacuum.
While researching exhaust scavenging I ran across a set-up for useing your exhaust for a pcv system. It was on a ot car with no way to tap the intake. I understood the purpose of a pcv valve was to stop a flashback (aka backfire ) from igniting the fumes in the block ?
Something to think about, on the flatheads all 8 cylinders doesn't connect into a common plenum, one side of the carbs feed 4 cylinders and the other side feeds the other 4. When you run the pcv valve into the manifold be sure all 8 cylinders see it or four will be running leaner than the other four cylinders. Below is a picture of how I set mine up. I y'd the tube to each side of the intake and also between the two carb bases I also connected a common vacuum port for vacuum acc. The pcv valve is from a '90 Jeep inline 6 which is close to the flathead cubic inches. Vergil
Hey Vergil, dead center of your photo shows the transfer ports that interconnect. Stick your fingers in there and when they touch each other you'll see how they do have a common plenum.
both front and back drivers side carb barrels connect but they do not connect with either of the passenger side carb ports. Vergil
8flat, I know what you mean, the single air cleaners do not have enough room on their bases to run the PCV to. That is why I made an air cleaner that fits over all 3 and gave me the room to plumb to the base of it. Mikes 51, 20 at idle?! All I can say is WOW. You're right, if I had that number on my engine I would not have been thinking the way I did to change things up.
OK, now you're blowing my mind. For years guys have been saying that dual carbs feed the front and rear cylinders separately, which may be true on some manifolds, but not any I've ever seen. Now you're saying that left and right banks are separated. Am I crazy, but don't both Strombergs and Holleys operate on single venturis? How are the left and right butterflies on a single two barrel carburetor with one venturi going to send out a different mixture? Edit: Looking again at the "pretzel knot" of your Offy (I have the same one), I see that there are left butterflies crossing to the right bank, and vice versa. Perhaps we should map this manifold and see if it is truly separated, I think there is equalization going on in there, if not, then you could never really get an accurate vacuum reading from a single port.
The drivers side throttle feed the two center cylinders on the drivers side and the front and back cylinders on the passenger side, the right side throttle plates feed the front and back cylinders on the drivers side and the two center cylinders on the passenger side. The pcv valve will add air that has not passed through the carb venturi to pick up the atomized gas, it is either air sucked in through the breather or blowby from internals of the engine. So if it is air without fuel in it mixed with air with fuel it will have less fuel in the air which will make the mixture a little leaner than the other cylinders that pulls all its air through the venturi adding gas to all the air. Vergil
Well, I guess this clears it up.... Thank you for your versions guys. I guess I'm going to have to decide on either a drilled port into the intake or a large air cleaner for all 3 carbs to provide a vacuum source. One other thing I was thinking about though, what if I put a cap on the fuel pump hole and tap the center of that for a vacuum source/fume intake? Think that would work?
If you mean could you install a pcv valve in the fuel pump location the answer is yes but if you trace the air flow in the picture Tommy posted you would be pulling air from the valve chest where there is a lot of oil and oil mist splashing around which would end up going through the pcv valve and to get the correct air circulation through out the engine the draft tube would need a filter installed on it to supply clean air into the engine and the oil fill tube would need a cap with no holes in it for air. The best place for the pcv valve is where the road draft tube goes into the intake. Mounted there in pulls air from the bottom of the engine as clean air enters the oil fill tube breather or to a hose run from the fuel pump block off plate to the air breather on the carburetor --- as long as it is filtered air. A lot of different ways are used to hook up the pcv valve that work, I have only posted my opinion and what I have used plus what I have read. Most pcv valves on the flatheads are hooked up so only four cylinders see the fumes from it and they work just fine, I just like mine to see all eight cylinders, again just my thoughts. We do like our flatheads don't we... Vergil
Vergil, I actually meant to put the PCV into the draft tube hole and run the hose from the top of the PCV to the cap on the fuel pump hole. Just not sure if there would be enough vacuum there. I didn't know anything about flatheads before starting this project but I'm learning every day. I'd like to avoid drilling a hole into the intake manifold if at all possible. I don't like permanent changes like that. With the 3x2 carbs adding a spacer to the center carb would just make everything misaligned, so that's my quandry.
No this will not work! You need a vacuum source. The road draft tube created a negative pressure and pulled air through the engine. It worked OK but the breather on top of the fuel pump stand is not in the air flow under the car so it will not pull the air through the engine and not remove the condensation and gasses that form sludge in your engine. If you don't want to modify the intake or use thin spacers under all 3 carbs just stay with the road draft tube. You need some kind of air flow through the engine for any kind of system that you choose. Your idea will not cause that air flow.
We have read many times about the guys who do not have any crankcase ventilation loosing oil and not being able to figure out where it is going or why they are loosing it. Isn't that caused by pressure being built up due to the expansion of crankcase gases and no where for it to vent? So, they make their own way out past a seal or blow past the rings. And, the gases and any moisture create the sludge in the oil that the old Fords with no PCV and no filter built up. That tells me that you don't need so much of a vacuum source, just a place for the gasses to vent to. That is why I prefer using the low pressure area that should be in an air cleaner and not tapping off of the engine's internal vacuum. Also, if you use a plate under a carb to get a manifold vacuum source like I did with the Southwind heater plate, just put a phenolic spacer under the other carb(s) that matches the vacuum plate. Even though I changed my PCV connection from the base plate to the air cleaner, I'm keeping the base plate to be a vacuum source for Bubba's Chevy distributor. That's the next engine improvement on my list and it is another reason why I wanted to preserve the engine vacuum.
Crankcase pressure comes from combustion gases blowing past the rings into the crankcase--- not the opposite. If crankcase pressure was enough to blow itself out, then a chimney would suffice and they never would have designed draft tubes or vacuum PCV in the first place. Combustion gases contain water vapor, especially now with ethanol fuels. You want a fresh air source to purge the moisture from the crankcase and you need a decent vacuum source to pull it in.
Mike, combustion gases blowing past the rings to pressurize the crankcase makes more sense. However, the crankcase is still being pressurized and without a PCV system, oil gets blown out. It still makes my case that the crankcase is being pressurized and therefore not much of a vacuum if any is required to purge.
All crankcases need to be vented or something's gonna blow out, if not from combustion blowby then from the expansion as the air gets hot in there. If you merely vent, then only excess pressure will be vented while the rest of the bad stuff still remains. Essentially you're blowing out old crankcase vapors with new crankcase vapors. My point is that "self-ventilating" was tried back in the early days and somebody figured out that a draft tube worked better because it pulled out the nasty stuff and pulled in fresh air.
Your idea is more than likely just going to give those hot oily vapors a place to go and you are going to end up with a dirty air cleaner, you either need a vacuum on a pcv valve or leave the draft tube in place. The air cleaner is a pretty good place to source your makeup air with a pcv system.
Yep, you're probably right. Guess I'll just have to clean out that K&N filter every so often. All I know is that my engine pulls better since I quit using the manifold vacuum for my PCV and I expect that I will have more vacuum available for the distributor advance when I put it on.
As for running a line to the air cleaner, guys who plan on doing that need to look at almost any late 60's through 80's carburetored American car with the factory air cleaner on it and you will see a tube running from the air cleaner to the valve cover for air to go into the engine's crankcase so that it can be drawn through the engine and through the pcv valve into the intake manifold by manifold vacuum. I can see not wanting to drill and tap a vintage intake but carb riser plates aren't that hard to come up with or make and they don't have to be that thick. Maybe one of our machinist members who has a mill needs to start a run of riser plates that are drilled and tapped for the pcv lines for the 3 bolt carb mounts. Or maybe they already exist and I haven't seen them. As far as running the road draft tube, yes that does work to a degree if the tube is properly mounted, isn't bent or cut off short and the breather is clean and working on the other end. It doesn't work well in slow traffic as it is a "road" draft tube that relies on a flow of air past the end of the tube to draw the crankcase fumes out into the air stream as you run down the road at road speed. If you have an engine with some wear on it you usually end up in a cloud of crankcase fumes when the car is sitting at a light or caught in slow traffic. That takes away your "cool old car" factor real quick as far as onlookers and potential hottie passengers go.
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