Rover V8 + breather config (PCV?)

[MissMoppet]

I have now run my Stag (RV8 SD1 10A + Holley 4160 4 barrel carb) for two years. Never given any problems but feel it should be bit smoother and quicker (odd cough and hrumph when hot). Till last week there were two hoses from the rockers up to the bottom of the pancake filter but someone pointed out that on the SD1 there were flame traps. So I fitted these (see pic) and a welcome and unforeseen benefit was that the pool of oil in the valley en route to the garage floor disappeared.
Being of a curious nature I delved into this breather pipe problem and found all manner of suggestions on the web. The original SD1 had one pipe via a flame trap to a twin branched pipe to vac pipe of each SU. The other rocker cover outlet just had an open pipe with a flame trap. Others on various forums (using the SD1 in a variety of cars) had blocked off the outlets completely.
However on the TR7 forum someone who seemed to know what they're doing said that the RV8 was designed to have Positive Crankcase Ventilation and the breather pipes should be routed into the vac port with a PCV above the flame trap on one rocker cover. I hunted through Mr Haynes and Mr Rimmer and there is no mention of a PCV on the standard SD1 engine, only on the EFi.
That seemed to make sense to me - so I removed one pipe but left a flame trap and ran the other up to the large vac port of the Holley - I don't have a PCV (yet). And, woe and trice woe - the bugger would barely start . . .
So for the moment it's back to how it was - breathers up to air filter, Holley vac port blocked.
Gentlemen: your expert advice please.

Holley flame traps.jpg


Dave

[V Mad]

A PCV system can be useful in helping to reduce oil leaks, and also keep your oil cleaner by positively removing the fumes with the aid of a controlled vacuum. But it is by no means essential.

The so called flame trap also serves to help condense the oil mist and prevent oil passing along the breather pipe, casing a mess etc.

Now onto your use of the vacuum port. If you connect that directly without a PCV valve, the full vacuum will be applied to the system, which draws a lot of air/fumes into the engine inlet; this can dramatically weaken the mixture at idle (high vacuum condition). Even worse, when you also have a second breather (on the other cam cover) this allows even more air into the engine.

So, if you are going to use a positive breather system with the Holley, use a PCV valve as this controls and limits how much additional air flows into the inlet. A less important point, but if you use a second breather, this will lower the vacuum in the crankcase, and therefore not be so effective in helping reduce any oil leaks. Since some additional air enters the inlet (bypassing the carburettor) this weakens the idle mixture, so make sure you readjust the idle mixture back to the correct lamda.

Finally, there is no need for a PCV valve with CD types of carb (SU, Stromberg) as a moderate, constant vacuum port is already present.

[73stagman]

I run a Rover V8 with a fresh air breather on one rocker cover an da flame trap with the breather hose to the pcv port on the carb (edelbrock) with no PCV valve.
I have no oil leaks, it hardly uses any oil but there is an oily film on the silicon hoses in the engine bay. On a positive note when I wipe off the oil film it leaves the silicon hoses nice and shiny!

[V Mad]

Thats interesting Neil. When I connected the vacuum port directly without a PCV, initially the engine would not idle, and I had to enrich the mixture considerably to get some sort of idle quality. Also the vacuum pull on the oil cap was a bit scary. Even with a PCV valve connected, it took some experimentation to find a PCV valve type which I felt had the desired characteristics.

Maybe the Rover engine is not so sensitive? Have you checked the idle mixture is not too weak?

[73stagman]

I can guarantee the idle is not too weak.
The open to atmosphere (With a K&N crank case breather filter) other rocker cover breather allows fresh air venting. Remember that the Rover V8 engine set up with conventional push rod and rocker arms is a completely different animal from the over head cam arrangement in the Stag V8. Both rocker covers have breathing points and the crankcase is open to both rocker chambers.
My set up is nothing new. It s tried and tested by other British V8 engine users.
Remember as well that on the Rover V8 there is also a tappet chest breather at the back of the engine towards the left side as you look at the engine. I have this blocked off and a new hole drilled at the front of the tappet chest to drain oil back into the sump via the timing cover end (Thanks Mr David Harcastle!)

The Rover "flame trap" is also just that. There may be some misting inside it but it does nothing to restrict the flow of oil mist towards the carb. It does however prevent the possibility of a flame front reaching the hot oil gases in the crankcase.

Some of our younger Rover V8 operators don't even know there was anything but an EFI system let alone a 10.5:1 variant and often get their advice out of kilter.

If you want to see how the engine runs with no crank case breathing at the carb, start the engine and get it warm. Then remove all the breather pipes from the rocker cover and watch the oil mist.

[MissMoppet]

Many thanks for all that. Neil: can I ask what carb are you running?

[73stagman]

Edelbrock 500. At present its on an old Buick manifold but the plan is to change it for an offenhauser to lower the overall height under the bonnet

[MissMoppet]

Edelbrock 500. At present its on an old Buick manifold but the plan is to change it for an offenhauser to lower the overall height under the bonnet

The mystery deepens then. On the assumption that your Edelbrock and my Holley are pretty similar and I've tried your routing of just a breather on one rocker and hose + trap to PCV input on Holley - it won't run. It will run if I revert to both hoses going into the air filter. Well I might as well get a cheap PCV from Real Steel - I'm looking at BYFV184 - and try it.

[V Mad]

I dont know about the Edelbrock but the breather on the Holley is large and can flow a lot of air if connected directly.

[73stagman]

Unfortunately the assumption that the two carbs are "similar" is a good bit away from the truth!
There is port on the Edelbrock which is specifically a crankcase ventilation connection port. There is also a vacuum servo port on the rear of the carb that is not suitable for crank case venting. (I've never tried to see what happens with that port open or connected to the crank case because it says not to in the manual as the engine wont run!!) I guess the two carbs are as similar as chalk and cheese.
My engine will run with both connected, both disconnected or as they are now. I just have to adjust the carb up to suit.... (The oil fumes add to the fuel/air mixture so more oil in the mist equals richer mixture so the carbs have to be set slightly leaner. The idle also has to be adjusted to suit. The current setup gives best idle and power delivery. Its not brilliant at present as I need to change the needles (you cant do this as your carb uses jet plates) to be slightly richer at WOT as the engine stumbles if I smash the throttle to the floor. If I let it "speed up" and modulate the throttle pedal the power delivery is pretty good and smooth. The fuel consumption reflects the amount of throttle use though!

[73stagman]

Many thanks for all that. Neil: can I ask what carb are you running?

Do you have a breather on the crank case anywhere? OR do you just have the oil filler and the flame trap on one side and just the flame trap on the other?

[MissMoppet]

Do you have a breather on the crank case anywhere? OR do you just have the oil filler and the flame trap on one side and just the flame trap on the other?

Can I please refer you to my original post where you should be able to see off the jpg that I have two hoses going from each r/c outlet to the air filter - via two flame traps that I have plumbed in. I have ordered a cheap PCV from Real Steel to try to copy the original SD1 layout but they said that the SD1 pipes are 1¼ inch - mine certainly aren't - they're 13mm o/d ie take a 13mm id hose. I know I 'll have to plumb in with a short length of hose maybe with a reducer, so I don't understand why my r/cs do not seem to be standard.

[V Mad]

.......on the SD1 there were flame traps. So I fitted these (see pic) and a welcome and unforeseen benefit was that the pool of oil in the valley en route to the garage floor disappeared.

Dave

This mirrors my own eperience experimenting with breathers. Putting a gauze in the line traps the oil mist and condenses it. A flame trap is simply a cavity filled with gauze. But you should make sure the oil can drain away somewhere safe, eg back to where it came.

[V Mad]

Breather pipes of 13mm bore are more than adequate for the job.

When you add a PCV valve, note that the device should be operated correct way up; inlet at the bottom, and preceded by a flame/oil trap. Whether you choose to plumb the sytem as a crossflow, or closed system, thats up to you, but I would use a closed system.

[73stagman]

Breather pipes of 13mm bore are more than adequate for the job.

When you add a PCV valve, note that the device should be operated correct way up; inlet at the bottom, and preceded by a flame/oil trap. Whether you choose to plumb the sytem as a crossflow, or closed system, thats up to you, but I would use a closed system.

One of the benefits of a properly fitted and functioning PCV valve is that it is a flame trap in its self. If a back fire occurs the PCV valve will react to the pressure and shut to close off the crank case vent. You dont need to add another flame trap.