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MG MGB GT V8 Factory Originals Technical - Do I need a valve cover 'flame trap' fitting?
|What, exactly, is a flame trap and why is one needed? I was perusing the "How to Convert ..." book the other day and they advised fitting one. My starboard-side valve cover just has, in the big threaded hole, a plastic insert that holds a $2.98 Pep Boys PCV valve, which is connected to the intake manifold for suction. Seems to work fine, but should I be using one of these flame trap things? ... many V8s (and the RV8) have them in mounted in the same hole I'm talking about.|
I think the flame trap is filled with Steel wool or similar stuff to cause the oil vapours that are vented through the flame trap to condense and drip back into the rocker cover and hence be returned to the sump for contiuned use. Without this flame trap this excess oil vapour would be sucked into the intake manafold and be burned (that is what is meant to happen the oil that gets past the flame trap) and this would increase your oil consumption and increase the emissions of the engine.
The PVC valve is the American version of the flame trap. It's job is to prevent a backfire from igniting the oil vapors in the crankcase while providing ventilation of any blow by. Earlier versions and other imports (ie Volvo) use a screen for this purpose. Sort of like the Welsh miners lamp theory.
If these get plugged, you can develop high crankcase pressure and start blowing oil out of various places.
Sounds like you're covered.
THANKS for the information! You just confirmed my thoughts on an oil leak problem in my truck as it is constantly blowing oil out of the side of the valve cover (of course right onto my exhaust manifold STINKY!!)
|Kelly is right about the high crankcase pressure and oil blowing out everywhere. I recently swapped my factory carb set for the Edelbrock/Offy rig and had to remove the existing breather ports on the rocker covers and fill the holes to give clearance for the throttle linkages. I drilled a new hole in the top of the LH rocker cover, fitted a new spigot then connected a hose to the underside of the Low Rider filter with one of the two old flame traps in line. Result - oil everywhere. The flame trap was blocked solid with all sorts of crud. Fitted the other flame trap and the problem disappeared!|
|I hadn’t thought of the secondary benefit of a PCV blocking the spark of a backfire from setting off the fumes in the crankcase. |
Its primary function is emission control. Something is needed to prevent the buildup of pressure and the old way of dealing with it was to simply vent the fumes to the outside air (hot rodders will still do this). Naturally, someone noticed that simple venting polluted the air and if it were bad enough, it would gag the car’s occupants.
Someone came up with the bright idea to recycle the fumes through the intake to be burned. That’s a good idea, but engines run better on cool non-polluted air same as people. The problem really shows up at slow engine speed so they invented the PCV. PVCs have a heavy ball inside that prevents nasty hot dirty air killing the engine and idle but at faster speeds the ball is drawn up allowing Positive Crankcase Ventilation.
My ’63 Buick 215 has a filter built into the oil cap for air to enter on the opposite side as the PCV. The factory V8 draws fumes from both covers through flame traps to the carburetors. Air has to get into the crankcase however and if I remember correctly when I asked Paul Kyle about it, he said there was a vent of some sort on the back of the engine that the Buicks don’t have.
|George, your comment got me to thinking. I'll bet the motor would be a lot happier without having half-burned oil dropped into the intake. Bet it accelerates plug fouling, etc. ... which is a problem I've had with the Rover V8. Wouldn't a better system be to just use two of those mini K&N filters, and run a hose from a valve cover to one of them inside a fender, etc., so that the negative pressure in that area when the car's moving would suck clean air through the system? Obviously you'd want to shield it from road goop, but do you think this would work?|
|Hi Harry, |
That's the way it was done in "the old days". If you look at most of the early British cars, most of them vented out of the tappet cover to the outside. Not a clean way to go by far.
Your engine should not suck oil unless you have a lot of blow by. Then you have other problems to worry about anyway. Is your oil fouling build up on one side of the spark plug or is it even?
Yes, an engine runs better on non-oily air and it runs better on air that isn’t as hot as the air in the sump. Remember the main purpose of the PCV is to cut the flow at idle to prevent stalling while opening at higher speeds when the motor can tolerate running on polluted air. However factory PCV setups usually include a filter to limit oil particles going in and fouling the spark plugs.
As one piston goes down, another goes up keeping the volume of air in the crank case is fairly stable, but it still varies by temperature and there is always some blow by so it’s better if the pressure is released through design rather than squirting out the dipstick hole. On street performance cars I have seen two K&N style filters, one on each valve cover to allow the pressure to equalize without subjecting the intake to the hot oily fumes. That doesn’t cycle fresh air through the crankcase, but is there an advantage other than to burn the oily fumes to limit air pollution? Perhaps there is…
Before replying I checked into this and was told that drag racers have been running a hose from one valve cover to a connector attached to their exhaust header at a 45° angle to draw air through their crankcase and out through the exhaust and now this system has now been adopted by NASCAR. I don’t know if they bother to do this to avoid any resistance to the pistons on their down stroke or to replace the hot air in the sump to help keep oil temperatures down. I doubt if it’s because the EPA is pressuring them and putting the oily air into the hot exhaust burns it or the manufactures would do it that way. I’m told those connecters are available in speed shops and in catalogs such as Summit’s. Using the low pressure from the fender is an inventive idea and should work if the pressure is really low enough to make a difference.
Here are a couple of points about emissions and emission controls. Piping suction vents to the fender or having open filters may catch the attention of inspectors and cause you to fail the visual inspection. Piping the oily air through the exhaust probably doesn’t clean fumes as well as through the intake and may cause you to fail the exhaust probe test. Even joking about altering your car’s pollution equipment on a public forum may result in angry mobs gathering around and threaten you. They would even spray paint “Gross Polluter” on your lovely MG if aerosol didn’t cause ozone depletion.
Anyway, I’m glad you asked this because it got me to thinking that I have my PCV setup incorrectly. Most of the PCV setups I have seen come from the factory using the vacuum from the air cleaner whereas I used a handy port off the bottom of the carburetor. Air rushing down the carburetor creates a vacuum that draws the correct amount of fuel and I’m allowing a huge steady flow of air to gush into the intake below this causing a lean mixture! Other uses of vacuum, such as distributor advance and power brakes are not constant. The more I think about this the more it bothers me, at idle the PCV closes and the mixture is rich then at a faster speed the PCV opens causing the mixture to be lean then vacuum drops at faster speed resulting in the mixture to go rich again.
|George, the NASCAR type rig sounds like a plan. I looked at the Summit and JEGS catalogs but I didn't see any such fitting as you describe. Any idea how much suction this would provide? ... seems to me the key is to not have TOO much suction, so you don't risk pulling out oil. Mobil1 is damned expensive, and I'd far prefer not to be sucking it out of the crankcase! My PCV setup does seem to allow the passage of a bit more oil than I would like.|
The first I heard of this system was when I checked with someone before answering this BBS thread. You mentioned Summit so I checked their website and found this decription of the Moroso Crankcase Evac System…
“A great way to clean out your crankcase.
These Moroso crankcase evac systems reduce crankcase pressure throughout the entire rpm range for increased piston ring seal, reduced intake charge contamination, and fewer oil leaks. They've been track and dyno tested to produce significant increases in engine performance. Includes two of each: oil separators/breathers, breather/filler cap grommets, one-way check valve and weld-in nipple for header collector.”
That doesn’t answer your question about how much suction is applied, but it is probably a good balance. Too much air flow through the crankcase would also concern me that the oil may be pulled off moving parts!
They also list one for the Mr. Gasket Crankcase Evacuation System…
“Increase engine performance by reducing pressure buildup.
These are primarily drag race systems designed to reduce pressure buildup in the engine crankcase. The benefits of these systems are better piston ring seal, less intake charge contamination, and reduced chance of oil leaks while increasing your engine performance. The kits include two chrome plated push-on valve cover breathers, breather grommets, check valves and 3 1/2 in. long threaded tubes. The tubes must be welded into collectors.”
The JEGS site also showed the Mr. Gasket Crankcase Evacuation System.
They don’t mention the effect on emissions.
|From what I know about this, having seen it on a lot of drag cars, is that it isn't a streetable system. You need a check valve in line and the vacuum is not constant--you can have pressure, hence the check valve. The actual intent here is to be a scavenging system and create a low pressure crankcase environment which will give you a couple extra horses. At idle and low revs it doesn't do anything.|
A PCV valve is a controlled vacuum leak and various carbs are designed for this. The valve has a spring that keeps it open. When there is high vacuum at idle, the spring is overcome and the valve restricts its max, but never closes completely, letting "x" amount of air through. When the vacuum signal is reduced the spring opens the valve a proportional amount, and since the vacuum isn't as great, it lets "x" (not "x +/- y) air through. So the amount of air let through is the same. At least, that's the way it's supposed to work. The carb is metered and jetted to account for this air leak throughout its range. For many years, some American engines had a spacer under the carb that accomodated a pcv valve hookup. Others had carbs with pcv fittings.
What you need is a good-quality PCV valve--one that will work properly depending on its installation. Some operate really poorly if lying on their sides, some are just plain junk. The engine needs to be in good shape with good gaskets and seals and the oil separator has to be effective and not be sitting in a puddle of oil (oil drainbacks need to be clean and unrestricted). And you can always choose a pcv valve that allows less flow than others but that will affect carb jetting somewhat.
|Wayne, do you have a recommendation for a PCV valve for a Rover V8? I don't think I have an oil separator -- there's a baffle on the starboard side valve cover underneath the threaded fitting where the PCV valve attaches and that's about it. I'm absolutely positive I'm sucking some oil out of the crankcase, because according to my A/F meter the motor runs richer when the PCV is connected. Perhaps I need a PCV valve that is a bit more resistant to letting stuff through. Or else presumably open breathers would work alright, though I don't fancy getting oil spray all over the engine compartment.|
|Sorry, I can't recommend a pcv valve but I think one from a V8 engine prior to the "closed" system would work (i.e., with a vented oil filler cap). Are you using stock valve covers or aftermarket ones with an add-on-type baffle (that also may be a problem)? Years ago I built up a 351 Winsor Ford for my 79 Mustang and had tall Moroso valve covers with their baffle--wasn't worth a darn, sucked oil at the rate of a quart per 50/100 miles. I dissected a Chevy small block valve cover and removed the oil baffle/separator which I was able to put in the Moroso vavle cover. That worked well with a stock Ford pcv valve--hardly used any oil. Boy, I wish I had that car back (but I sold it so I could get my 73 B)!|
It sounds like you have a much better understanding of the PCV system than I do. Is the best to route the PCV to the air cleaner or the carburetor base?
|There really is no vacuum "signal" inside the air cleaner that would allow use of a pcv valve. You could eliminate the pcv valve (like on the 4 cylinder engines) and hook to the air cleaner, but probably would get a oily film on the inside of the air filter element and carb top--there are 4 more cylinders to create blowby. To truly get an exchange of crankcase air for moisture elimination and evacuation of blowby gases, you need to hook to the carb base. The "inlet" hose to the crankcase could be hooked to the air cleaner unless you use the carbon cannister or have vented oil filler cap. Unless you really have a lot of blowby, you're not likely to contaminate the air filter element.|
|A couple of points here, a typical american pcv setup would be a more or less closed system. Fresh air is drawn from the air cleaner, sometimes from inside the element, sometimes from a smaller filter inside the housing. This air goes into the crankcase, usually through a simple fitting in a valve cover on a v-8, and vapors are drawn out of the other valve cover through a pcv valve, through a much smaller hose and into a fitting at the base of the carb or in the intake. The check valve in the PCV valve *opens* when vacuum is applied, which seems a little backwards until you realize that there is much greater airflow into the carb under no vacuum conditions and therefore any blowby would still be drawn into the cylinders. Closing the throttle produces a high vacuum condition, again drawing fresh air into the engine. |
As stated, it is an emissions control system, and not designed to add power. The carb is calibrated for the pcv, and actually, since a lot of the vapors are blowby they are gasses that have already been burned. Fuel injection does a better job of taking this all into account.
About the time the exhaust vacuum system appeared, there was a push on to completely seal the combustion gasses in the cylinders and to pull a vacuum in the crankcase. The idea was that without air in the crankcase there would be no windage, no oil foaming, and no air mass to oppose the motion of the pistons.
Some engine builders came pretty close to achieving the objective, and those 45°°°°°°°°°°°° fittings are the downscaled consumer version.These things along wity overlspped rings were the hot setup for awhilee.
|Does it make a difference wich side has the PCV, and wich side is connected to the air cleaner?|
Here is what I did: Rover SD1 valve covers reversed so the tall oil filler neck is on passenger's side at back. I then opened a bigger hole where the little EFI filter used to be,Fitted a grommet and the PCV. valve and connected to carb all this on passanger's side. On driver"s side the tall flame trap with all rhe wire mesh left inside now is at back and connected with a hose to air cleaner. does this sound Ok? I guess i have a closed system now. By the way the grommet and PCV are all Ford V8 items.
|Sounds fine to me.|
This thread was discussed between 09/05/2001 and 19/05/2001
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