crankcase ventilation-inflow air mod?

[mtnjag]

I have a 62 etype which was fitted with an XJ6 SIII oil filter housing with oil cooler some time ago. Since the oil bypass went horizontally to the rear rather than straight down, the orig bypass oil inlet was blocked off and a new hole drilled just above the oil fill level on the side toward the rear of the pan with the return fitting moved to there to interface with the XJ6 oil bypass return.

I have now sourced from Denis Welch Motorsports a bypass hard line that goes to the orig bypass oil input on the bottom of the pan to the horizontal fitting with a short hose connecting the two. Should be nice.

That leaves the rear existing hole above oil level unused.

Looking for information on whether the following is desirable or not desirable, beneficial or harmful, useful or useless??

Thinking of using this hole as an INLET air point to aid in crankcase fume scavenging. Vertical fitting-hose-filter. All the other cars I have use an inlet for air and an outlet to vacuum or road tube. The XK engine only has an outlet to the airbox. I would assume this is not very efficient since the head breather is pulling against a dead hole.

The other systems do have the inlet up on top, usually in the valve cover or oil fill spout, to pull down and through the crank case to the outlet on the other bank or to the timing cover on the Sprite. But there are no other areas I can see to open up an inlet breather on the Jag. The cam covers don't seem to be optimum since the air would just be pulled forward to the breather in the chain cavity rather than through the crankcase.

Jag designed it and used this design for many years but would there be a benefit to aiding the breathing in this way? The hole is already there.

[Tom W]

There’s a couple of objectives with a crank case breather system. One is to reduce the pressure in the crank case, so the oil seals get an easier life. Lower pressure helps the engine make a bit more power too, as the windage on the moving components is less.

The 2nd is to prevent or reduce the escape of nasty crank case emissions into the atmosphere. I guess we’re discussing the former here.

You’re right, the Jag system is quite different to that on most other cars of a similar age, being only vented to the airbox. This doesn’t create a big vacuum at idle, and isn’t brilliant at wide open throttle either if the air filter is clean. Other cars use the vacuum across a component in the intake to create a flow through the engine. On my Rover P6, the vacuum is generated across the carburettor bridge. Air into the engine is taken from the airbox, passes through the engine, and is drawn from the timing chest, through a flame trap, to the carburettor. On my 4.6 Range Rover, the vacuum is generated across the throttle body. Air goes into one rocker cover from before the throttle plate, through the engine, and out through the other bank behind the throttle body. On the Range Rover, at wide open throttle, flow is out from both banks as there’s no depression across the throttle body.

I think the ventilation on the Jag engine is pretty marginal, particularly on tired or tuned engines. Factory race versions of the engine all have extra breathers on the rocker covers. My car used to blow out oil until I enlarged the breather sufficiently. I have weber carbs, so there’s no port on the airbox to run the breather to, it just breaths into a catch tank.

I don’t think it would harm to try an extra breather, but I don’t know whether you’ll generate much flow through the engine as the vacuum source is pretty weak. I’d fit a little filter on top so if it does draw any air in, it’s drawing in clean air.

[mtnjag]

Thanks for the feedback.

Yes, my objective here is to reduce pressure and vacate the crankcase better than the stock system would seem to do. Currently a 9/16" hole in the pan, larger than the stock head breather, which I think would be adequate.

My intent would be to add the line into the side of the pan at existing hole with a weld on fitting, bring a 90 degree hose end out and up with a short hose and a small filter of which there are many available. Maybe have that pan intake vent under the carbs or up the back at bulkhead.

Concerns include will it draw IN through that vent and out the stock head breather or is there too little vacuum pull and would I end up with another breather going OUT, which of course is not desirable at that location.

Input welcome!

[Tom W]

I think you’ll see different flow through the new breather depending on engine conditions. Windage from the crank will probably cause it to blow out. Under maximum vacuum at wide open throttle the engine might draw air in through the new breather.

[PeterCrespin]

I think you’ll see different flow through the new breather depending on engine conditions. Windage from the crank will probably cause it to blow out. Under maximum vacuum at wide open throttle the engine might draw air in through the new breather.

I think blow-by is the main cause of crankcase pressure in a Jag. I would expect it to be by far its highest at WOT. Racing Jag engines used dry sump, which in theory creates crankcase vacuum by scavenging a larger volume of oil/air than the volume pumped in (oil only). If the oil tank was vented to atmosphere, that would create a significantly reduced crankcase pressure which would help conserve oil during long races.

In practice, when the returning oil/air mixture is de-aerated on the tank baffles or swirl-pot, the oil drops back into the tank and the separated air is usually recycled to the crankcase through a fitting on the side of the sump. Meanwhile, the cam box breathers blow out, not breath in. For a road car I'd use the stock early system with the large bore curved fitting into the plenum. If you thrash it, try the breather in a catch tank with equally large or greater fittings to see what comes out. On bikes I just used to strap an empty oil bottle to a frame rail and loosely drop the hose into that. You could do the same with an E. Duckhams made the fastest catch tanks

It was an inverse sort of snobbery/psychology in the early days of the Classic Racing Motorcycle Club, to turn up with a spotless gleaming and expensive race bike, with the scruffiest possible "I've been at this game for years" catch bottle hung off the end of the breather hose to satisfy the scrutineers. Me being me, I normally reversed things by turning up with cheap DIY machinery but a spotless fresh used catch bottle.

Never came last though...

[mtnjag]

So you do not believe an air inlet in the pan would aid the scavenging of air by the head breather?

Better just to open up the front of head breather,I think later cars have a bigger bore fitting, and run it to a catch can?

In other words, forget about an inlet vent direct to the pan?

[Tom W]

It’s a simple theory, but difficult to predict the precise flows.

To get flow, you need a differential of pressure. With a breather open to atmosphere, any pressure generated in the crank case will cause a flow out of the breather. Since your objective is to prevent excess pressure in the crank case, the breather needs to be large enough that the pressure in the crank case never exceeds the point at which the seals start leaking.

If you fit the breather to a vacuum source, as with the factory breather, you have a greater pressure differential and less chance of exceeding the pressure where things leak.

If you then add another breather to atmosphere, things get difficult to predict. The engine will still draw air in through the standard breather, as the intake is at a lower pressure than the crank case. Whether the new breather flows in or out will depend on whether the remaining crank pressure is now above or below atmospheric pressure. This really depends on blow by at the given RPM, throttle position and how restrictive the air filter is.

You won’t know without trying.

[mtnjag]

"The engine will still draw air in through the standard breather"

My understanding of this engine is that there is no standard breather for the engine to draw air through. Only to breather in the front of the head which draws air. That's why I was thinking of providing a vent through which the eng could draw air.

Am I missing a breather?

[Tom W]

Air gets into the crank case from combustion blow by.

The crank case doesn’t actually need to be open to atmosphere, you’re after a pressure differential, not necessarily flow. It the engine doesn’t have much blow by, the standard breather will just lower the pressure inside the crank case. Not by much, but it might be enough to stop leaks. It’s the same principle by which the manifold vacuum evacuates the reservac tank for the brakes.

[mtnjag]

OK, I see what you're saying. Plenty of air getting into the crankcase with blow-by. Don't need to let more in.

More questions, trying to understand the principles here. Thanks

So is it because the Sprite and Chevys are pulling crankcase air with strong manifold vacuum, rather than the mild vacuum created by the air cleaner vacuum the Jag has, that they also have an inlet air vent for the crankcase?

[Tom W]

I think that’s to do with removing more fumes, rather than just lowering pressure. By allowing a flow through the engine, you can purge out more of the fumes.

Any system that uses the depression across the throttle butterfly to generate the vacuum needs a carefully sized system, otherwise the idle speed will be high. The flow through the engine is bypassing the throttle butterfly. Similarly, any system that uses vacuum across the carburettor bridge requires the system to be carefully sized, or the relative pressures across the carburettor will be wrong. If I disconnect the breather from the carbs on my Rover, the engine nearly dies.

There isn’t anywhere on the Jag carbs where you can tap in for a greater vacuum.

[PeterCrespin]

There is a double vac tapping on twin Strangleberg 'Federal' XJ & E-types, and on twin HS8 XJ engines used in the UK and RoW. The narrow bore (3/8"?) pipe runs from the front breather to both carbs near the mounting flange and this draw in fumes for recirculation. The V12 E has something similar and in both cases the narrow bore pipes work because of the strong vacuum. Not ideal for performance but adequate foe a road car.


















on timns IIRCcars a

[mtnjag]

So better just to leave as is and maybe increase bore size of front of head breather?

[Tom W]

There isn’t anywhere on the Jag carbs where you can tap in for a greater vacuum.

I was referring to the SUs. Not really considered the stromberg setup. Strange as my car used to have it. I have the bits in the shed somewhere. That’s a lot more complex, and includes a charcoal canister and breather for the fuel tank all plumbed into the same system. Though from the point of engine breather plumbing it still only taps into the engine at one point. When I got my car, it was already converted to Weber’s, but with the remains of the Stromberg vacuum system now open to air. I had problems with the engine blowing oil out wherever it could, usually the dipstick hole, as the original small bore breather was much too small without the strong vacuum, as Peter says.

Are you trying to solve a breathing problem, or just find a function for the now redundant hole in the sump?

[MarekH]

Take a look at how the v12 breather system changed, in particular the final two years of production when it was substantially added to. The is a pdf document on the web about v12/AJ16 engine which neatly explains it all. Properly implemented you should have fewer oil leaks out of the rear main seal and various joints around the engine.

Under maximum vacuum at wide open throttle the engine might draw air in through the new breather.

There is no vacuum at WOT at full rpm nor at 50% throttle at medium rpm, nor even at 30% throttle at low revs. (See what I posted about ported vacuum and look at the graph if that sounds odd to you.)

kind regards
Marek

[Tom W]

There is no vacuum at WOT at full rpm nor at 50% throttle at medium rpm, nor even at 30% throttle at low revs. (See what I posted about ported vacuum and look at the graph if that sounds odd to you.)

kind regards
Marek

There is in the breather system, which is what we’re referring to here. The breather on an SU equipped car feeds into the airbox downstream of the air filter, but upstream of the carburettor. So under those conditions, where the pressure drop across the air filter is greatest, then the vacuum in the breather is greatest. How much, as I said before, is dependent on how restrictive the air filter is.

[mtnjag]

I have considered an inlet breather for a while based on what I see in my other cars, both a crankcase inlet for fresh air and a crankcase outlet. The jag having only an outlet. Thought of cam cover breathers as I said in my original post but they would seem to not serve the function.

My perceived purpose is to provide free flowing scavenging of air from the crankcase rather than the front breather trying to pull from a “dead hole” ,so to speak, in a restricted manner. Purpose of this is to lower pressure in the crankcase, reduce leaks, improve flow through the front breather as the pipe seems to foul up easily. Perhaps improve performance and oil mileage slightly.

As I worked up this latest project to reinstate the oil return pipe I flashed on this existing vent I had in the pan and thought, hmmmmmm I wonder if this is the answer.

[Tom W]

I don’t think it will harm to try your idea, providing you follow a few common sense precautions. The two things I’d be trying to avoid would be any dirt being able to get in during scenarios when the the new vent serves as an inlet, and any condensation or oil mist making its way out during scenarios when it serves as an outlet.

Running the new pipe to a commercially available breather tank with a little filter on top should serve both purposes.

[mtnjag]

As long as the car is running would you anticipate outflow taking place?

[Tom W]

I don’t know.

Any XK engine will have some blow by, no matter how good condition it’s in. With the breather connected to the airbox, there’s a flow from the crank case to the airbox. This lowers the pressure in the crank case, but what is hard to predict is whether it will reduce the pressure to below atmospheric pressure. If it is below atmospheric pressure, the new breather will flow in. If it’s still above atmospheric there will be a flow out.