Road Draft Tube

[kilkm]

I have a '62 Electra Convertible with the 401 engine. A few times I've noticed while cruising with the top down in the summer, exhaust fumes, fairly strong at times. The car has a new exhaust system, and the only source of the fumes I can think of is the draft tube. I've seen custom valley covers for these engines that do not have a hole in them for the draft tube. The engine has chrome valve covers on it, each side has a breather cap, but no place for a PCV valve. I'm wondering if I replaced the valley cover with a custom unit that is not set up for a draft tube, would the duel breather caps proved adequate crankcase ventilation, or would I also have to replace the valve covers with a set that are set up for a PCV valve? Engine is already equipped with a modern Edelbrock performer carb. Anybody have thoughts on this, I've never owned an engine before that has one of these draft tubes.

[55PackardGuy]

I'd be tempted to convert to positive crankcase ventilation. It's definitely been done, on cars a lot older than your '62. In fact, the 401 was eventually offered with a PCV setup from the factory. On some models, there is a rubber grommet where one of your valve cover breathers is located, which was used for the hose attachment on models that had PCV. You might want to check around and see if you can find an example of a mid-'60s engine with PCV that will show you how the system was plumbed in. The problem I see with relying on valve cover breathers only is there will be no low-pressure area around them to help evacuate the crankcase gases. In a PCV system, the engine vacuum does this for you. With a road tube, it's the drop in air pressure at the outlet caused by the air stream across the end of the tube when the car is moving at speed.

Just some thoughts. I bet there are people on this forum who have made the conversion.

[JohnD1956]

I would think the best thing to do is find out why you have so much seepage from this tube. I have to ask all, what causes this smokey condition in the engine. I thought it was overfilling the crankcase or incorrect timing causing overheating. Am I even on track?

JD

[Smartin]

There should be no exhaust fumes coming from the tube. YOu could have an exhaust leak at the manifold, or possibly at the exhaust crossover on the intake. They have a habit of rusting out on bottom.

[JohnD1956]

I was thinking he was smelling burnt oil from the tube, not exhaust gas.

I remember my 56 smoking a lot from that tube one time. This was after bringing it to life from a long rest. After changing the oil and driving the car a while it just stopped, and today it does't blow any smoke at all, but I never understood exactly what caused the condition in the first plcae.

JD

[55PackardGuy]

Not an "expert opinion" or anything, but I would think you'd get more fumes from the draft tube for the same reason you usually get excess crankcase pressure--blow-by. There are usually residual gases going through the tube, but they're dilluted enough to not give much odor. However, whatever fumes it breathes are pretty rank. I remember when PCV came along, there were people touting the theory that there were actually worse pollutants coming from the breather tube than from the exhaust pipe. Don't know if that was true, but crankcases breed a lot of noxious gases. PCV has been a pretty successful anti-pollution device, and may even has some advantages for reducing engine wear by sending some lubricants back through the intake. If it's not too hard to retrofit, I think it would be worth the trouble.

Another possibility for the odor is simply a rust hole in the floor near the outlet of the tube, or a misrouted or partially blocked tube.

You have to remember too, that the owner says this is a problem only with the top down. If you're idling with the top down, it's pretty easy for exhaust fumes to creep around from the back, especially with a little breeze from behind. If it's fumes from the draft tube coming in from outside, you'd think that just having the windows open would be enough to make it noticeable.

[kilkm]

Thanks everyone for their input, I do believe the odor is from blow by from the draft tube (odometer shows 91,000 and I'm sure they are original.) I never drive this car with the top up, only once or twice when a rainstorm popped up when I was at a show, so I never noticed the odor with the top up on this rare occasion. The car is in storage from November to April here in the frozen North. An "add on" A/C system had been added to this car and someone mentioned could be getting fumes from a rust hole in the floor. Now I'm betting it's from the drain from the A/C unit located near the draft tube. I think the easiest solution would to be to replace the valley and valve covers from a later model engine, say a '65 or '66, that had a PCV system. Does anybody know if there is any reason this would not work?

[Guest imported_bjr]

What I have done in the past is remove the road draft tube and put a grommet in the hole. Then put a PCV valve in the grommet and run to the intake manifold or the base of the carb. Look at a newer model that came with a factory PCV to see where they connected the line to from the PCV valve. With this setup sucking fumes from the valley cover, it's important to run a breather in both valve covers or one side will sludge up and one will be clean. Brian

[Guest Straight eight]

A PCV will lean out the mixture of fuel to the engine. When a PCV goes on an engine, the carb is rejetted to allow for the air the pcv is leaning the mixture. The 41 Buicks had an interesting set up for this problem. They used a non venting oil filler cap, no road tube, but they put an opening in the valve cover, and one in the air cleaner and connected the two with a pipe about the size of electrical conduit. As the story goes these early straight eights liked to give off fumes from the crankcase too. With these changes the carb would not have to be rejetted as no vacuum from below the carb is being used. Take a look at a 41 with the dual carbs. I think you'd wanta try that first. I had a 41 for 20 years and never had any blow by come into the car.

[55PackardGuy]

PCV systems are often vented into the air cleaner rather than the carb base. I believe the advantage of having a valve in the line is to regulate flow. I'm curious if the early Buick system of "positive ventilation" used an oil bath air cleaner, which would catch the oily residue and simply add it to the oil already in the air cleaner. On worn engines with PCV ventilation to the air cleaner, there is sometimes oily residue that accumulates in the housing and also sometimes fouls the paper element near the breather inlet.

On a vintage ('61 Ford Y-block) marine engine I have with dual side draft carbs, there's a trick setup that vents directly through the valley cover into the bottom of each of the carb inlets through a metal screen (similar to the screen material in vented valve cover caps). The inlets bolt directly over the screens. Naturally in a boat, there's no place to vent a "draft tube" except inside the boat, so positive ventilation is required.

I guess the bottom line is, there have been lots of different crankcase ventilation alternatives implemented through the years and there are quite a few options open to fabricate one to replace a draft tube.

[NTX5467]

Two distinctions in designs -- "open" pcv system and "closed" pcv system. The "open" system uses a regular breather on one of the valve covers for air intake into the engine internals as the pcv pulls it out of the other valve cover and into the carb baseplate. The "closed" system pulls clean air from inside of the engine air filter assembly upstream from the carb so that only filtered air goes into the crankcase (the engine air filter does a better job of filtering the air than the "hog's hair" or metal shavings in the breather caps ever could). Later versions of the closed system use a separate filter (used to be the AC FB59) that snaps into a hole on the air filter housing for the air intake, filtered, of the pcv system.

Many of the earlier "closed" systems used a "spark arrester" in the upstream supply area of the system. Something about snuffing out any possible "fires" that might result from a "backfire" situation. This was a somewhat short-lived concept, except on truck engines.

The key thing is "circulation" of clean air (reasonably) through the motor, picking up any vapors and byproducts from the combustion process and then re-burning them in the engine.

Enjoy!

NTX5467

[55PackardGuy]

OK, so it looks like I have my plumbing fouled up here... sorry. But, a question: in the "closed" system is the direction of flow always from the air cleaner into the engine? I have seen systems that do not just have a hose clipped to the outside of the air cleaner housing (with the little extra filter) where the connection is actually routed to a hole in the "hump" inside the filter housing close to the carb thoat. OK, so this would provide filtered inlet air IF there was enough suction at the hole to be able to overcome vacuum into the carb. With a pcv hose in this location, though, I have actually seen crankcase fumes being sucked through the hole and INTO the carb throat with the engine running at a couple thousand RPM, even though the air cleaner cover was removed (which would actually decrease the engine vacuum effect). That's why I think that in a system like this, the crankcase gases are going the opposite direction, toward the air cleaner and directly into the top of the carb. I don't believe there is any venting into the carb base in this case.

Another question: does the pcv valve control the direction of flow or equalize it, or is it simply a poppet that opens when there is enoug pressure on the crankcase side?

[NTX5467]

The pcv valve is a vacuum-biased flow controller. It has a spring inside it that moves the plunger open, closed, or in-between depending upon engine manifold vacuum. Under higher loads and heavy throttle (close to WOT), little manifold vacuum would exist, but there should be some (approx 5" Hg) anyway, but if the spring lets the valve "close", as it would be when the engine is stopped, the crankcase pressure would revert to the entry point for exit (i.e., the filter in the air cleaner). Such near-WOT operation would not be typical, though, so the existing manifold vacuum would keep flow going through the engine (from the air cleaner/entry to the pcv to the carb and inside the engine to be burned).

To observe how this all works, you can pull the valve from the valve cover and throttle into the engine quickly to drop the manifold vacuum level and watch the plunger in the pcv valve move. Of course, leaving the vac hose from the valve hooked up to the carb/manifold vacuum source while you do this. At least, that's the theory.

If the pcv valve is not clogged with deposits, it should always have suction on the side of the valve that plugs into the engine (with the engine running). To get an idea, when you have the valve unplugged from the engine, gently put your finger over the end of the valve with the engine running OR (with the valve still plugged into the engine) put your finger (or a stiff piece of paper) over the intake hose that the filter plugs into (with the engine running) and you should feel vacuum there too (but not quite so much as it's a bigger hole/hose at that location).

For example, I have a '66 Chrysler service manual that mentions pulling the breather cap (an open system on non-CA cars) from the left hand valve cover. Then, with the engine running, using a parts tag (or other stiff piece of paper) to check how firmly that paper is pulled to the valve cover hole that the breather cap slides onto. If vacuum is present, things are generall thought to be working ok. This is with the engine at idle.

On that engine, the air intake is on the driver's side valve cover, where the oil is put into the motor (which is also baffled underneath it). The pcv valve is on the passenger side valve cover, similarly baffled in the valve cover under it. In the early '70s, they reversed that arrangement and used a filtered can on the passenger side valve cover and a hose that went to the air cleaner housing's outer edge and had the pcv valve on the driver's side valve cover. Not sure why the reversal, unless it was more for packaging or if it just worked better that way.

In the case of the air inlet for the pcv system, the air in the air cleaner housing is "flowing" but the only vacuum per se will exist under the throttle plates and inside the intake manifold.

Now . . . let me also mention "pan evacuation systems" that many drag racers use on their engines. This system does not use a pcv valve, but has two of the Chrysler-style breather/filters, one on each valve cover, that have a hose hooked to them that connects to an A.I.R. check valve that screws into the header collector (with the appropriate fitting). The check valve keeps exhaust gases from going up the hose to the engine, but allows the vacuum created by the exhaust gas flow (and pulses thereof) to pull any combustion byproducts that happen to get by the piston rings into the crankcase to be "evacuated" from the crankcase for additional power gains. I hope this doesn't confuse the issue too much.

Enjoy!

NTX5467