Discussion
The 'God damn shunt' topic highlighted the problem of the full vacuum leak to atmosphere via the tee'd connection from the plenum to the crankcase ventilation pipe. However the suggested positioning of the valve, at the firetrap, didn't address that particular problem. And I'm struggling to see any point in the valve at the firetrap position.
As I understand it, some form of positive crankcase is a good thing. The thick pipe between the firetrap and the throttle body will always provide crankcase ventilation but it will only be positive ventilation under certain conditions. The tee'd connection into the plenum provides the positive aspect under the remaining conditions, that is at small throttle openings. The only problem, therefore, with the existing system is that the vacuum connection is uncontrolled and this is then should be where the PCV valve is required.
I bought one of the suggested valves as shown.
It appears to work by restricting flow as vacuum applied to the silver end increases. So the silver end faces the plenum.
So this is how I fitted it between the main pipe and the plenum and using some pieces of hoses of various diameters to provide a socket for the valve and a connection to the plenum.
Or have I got it all wrong???
As I understand it, some form of positive crankcase is a good thing. The thick pipe between the firetrap and the throttle body will always provide crankcase ventilation but it will only be positive ventilation under certain conditions. The tee'd connection into the plenum provides the positive aspect under the remaining conditions, that is at small throttle openings. The only problem, therefore, with the existing system is that the vacuum connection is uncontrolled and this is then should be where the PCV valve is required.
I bought one of the suggested valves as shown.
It appears to work by restricting flow as vacuum applied to the silver end increases. So the silver end faces the plenum.
So this is how I fitted it between the main pipe and the plenum and using some pieces of hoses of various diameters to provide a socket for the valve and a connection to the plenum.
Or have I got it all wrong???
Looks like I got it wrong anyway and I have the valve the wrong way around, clearly my suck isn't as strong as idle vacuum suck! I'll reverse it in the morning.
The google description:
The restricted hole is 3.3mm which I suppose gives a compromise of too much flow at idle and not enough under load?
The google description:
The restricted hole is 3.3mm which I suppose gives a compromise of too much flow at idle and not enough under load?
blitzracing said:
I think if you have a closer look at the ventilation plug - its got a tiny hole in it only so it does not cause a significant air leak into the plenum.
This ^ that PCV valve is currently doing nish 
and above , the PCV also expects open ventilation to the crankcase not the little mushroom this style of PCV system just dont work unless 
that style PCV valve like you have has must have an open vented crankcase entry like the pic 
Edited by Sardonicus on Friday 16th March 23:04
So looking at that diagram, the 'mushroom' on the n/s rocker cover does the same job as the pipe from air filter to filler cap ie admits filtered air. Then the hose from the fire trap on the o/s rocker cover should go direct to the plenum via the pcv valve and the connection to the throttle body removed.
The PCV valve is then doing the job of the plenum/small orifice vacuum ventilation at low load and the throttle body/large hose/venturi effect ventilation at high load?
The PCV valve is then doing the job of the plenum/small orifice vacuum ventilation at low load and the throttle body/large hose/venturi effect ventilation at high load?
Dougal9887 said:
Three important points before you progress:
1. Take some time to understand how the standard Passive Crankcase Ventilation system works
2. Now study how a PCV valve is designed to work
3. Be 100% comfortable with the idea of fitting a valve, the standard passive crankcase gas management system used on our cars isn't very sophisticated when compared with other cars even from the same period, but it does work! Any valve you fit in the crankcase gas side will be a restriction and if that means the system can no longer effectively manage crankcase pressure there's a very real risk it will find a different path and potentially blow gaskets and oil seals, I hold no responsibility if you fit the valve and you find it promotes oil leaks.
As we can see from my images below, manifold vacuum is actually pulling from two places, it's not only pulling crankcase gasses from the O/S rocker cover but it's pulling air from the throttle body too. The idea here is actually to separate the plenum from the throttle body under high plenum vacuum conditions where it can only be described as a vacuum leak, the idea is not separate the plenum or the throttle body from the flame trap (O/S rocker cover). The concept of fitting a PCV valve is simply to stop strong manifold vacuum at the plenum from drawing air through the throttle body, while still allowing the crankcase gasses to be managed effectively no matter what the driving condition is (idle, cruise, heavy WOT acceleration).
Keep in mind the amount vacuum drawing crankcase gasses from the O/S rocker cover via the flame trap is crudely but adequately controlled by two simple orifice restrictions:
1. The brass restrictor fitted to hose that goes to the plenum
2. The little breather fitted to the back of the N/S rocker cover
While this is a crude system it does work acceptably so leave it alone, but vacuum from the plenum hose isn't just pulling crankcase gasses, it's pulling air from the throttle body too and there's nothing to control this circuit.
In the above photo crankcase gasses are shown in orange, with air being shown in blue.
It's the blue air element you are looking to control.
The above diagram shows flow at idle or when cruising, because in these conditions the throttle butterfly is mostly closed, and because the engine is still pumping away the vacuum inside the plenum will be high.... so in crude terms there's a lot of suck.
However, when you're accelerating hard the throttle butterfly is open so there is little or no restriction, in this condition there's little or no vacuum in the plenum so again in crude terms there's little or no suck. But when you're accelerating the engine is still making crankcase gasses, so they still need to be managed. This is why Land Rover fitted that circuit that connects the the O/S rocker cover to the throttle body, under hard acceleration while there's little or no vacuum in the plenum there is still a small amount of venturi vacuum at the throttle body which is used to draw the crankcase gasses into the engine that way.
Here's the path of crankcase gasses under hard acceleration, with the gasses shown in orange again.
![](https://thumbsnap.com/sc/x2vmsyQL.jpg[/img]</a> <a href="https://thumbsnap.com/x2vmsyQL" Target="_blank" rel="nofollow">[img]https://thumbsnap.com/sc/x2vmsyQL.jpg)
This works fine to manage crankcase gasses under acceleration, the system also works well enough to manage crankcase gasses when you're idling and cruising, no one is disputing this. But the problem is under idle/cruise conditions there's actually noting to stop air being drawn by the strong manifold vacuum from the throttle body connection which apart from a little bit of venturi vacuum is mostly an area that's at atmospheric pressure.... so you can only really describe this path of air as a completely un-managed vacuum leak.
Here's that path of air at idle & cruise again shown in blue.... This is what we're looking to address.
Fit the right PCV valve & fit it in the right place and crankcase gasses are still managed at idle, cruise, and even wide open throttle when accelerating... what the valve stops is the vacuum leak.
I’ve often wondered if our pcv system contributes towards shunting. We know in the shunt rpm area the plenum has contaminated air in it from exhaust blow back. This contaminated air is then inhaled by a cylinder wanting nice cold oxiginated air but it doesn’t get clean cold air it gets the contaminated air with some fresh air which gives a poor burn and we get shunt.
When this shunt is occurring the PCV system will be sucking hard pulling crankcase gases into the plenum and then on into the cylinders. Now; crankcase gases will be red hot and I would assume be of a low oxygen content as they will be mostly blow by gases which have escaped past the rings. Red hot air with contaminated low oxygen content is not what an engine wants yet we are adding this via the PCv to the already contaminated plenum air.
I just wonder if we do away with the pcv into the plenum we would get a better quality of air in the plenum and in turn smoother low down running.
Will we get oil leaks ? Possibly, possibly not. I know some on here run from the flame trap to a catch tank and then just vent the catch tank to atmosphere with no issues. I suppose it depends how good your seals are and how much crankcase pressure your producing. I’ve certainly run RV8’s in the past just dumping the flame trap over board with no issues.
Maybe worth a go !!!!
When this shunt is occurring the PCV system will be sucking hard pulling crankcase gases into the plenum and then on into the cylinders. Now; crankcase gases will be red hot and I would assume be of a low oxygen content as they will be mostly blow by gases which have escaped past the rings. Red hot air with contaminated low oxygen content is not what an engine wants yet we are adding this via the PCv to the already contaminated plenum air.
I just wonder if we do away with the pcv into the plenum we would get a better quality of air in the plenum and in turn smoother low down running.
Will we get oil leaks ? Possibly, possibly not. I know some on here run from the flame trap to a catch tank and then just vent the catch tank to atmosphere with no issues. I suppose it depends how good your seals are and how much crankcase pressure your producing. I’ve certainly run RV8’s in the past just dumping the flame trap over board with no issues.
Maybe worth a go !!!!
Running a PCV valve definitely worked for me, and it's nothing to do with luck or coincidence..... I studied the standard setup, identified it's weakness.... and fixed the vacuum leak with a PCV valve.
As explained above, it's really about solving the direct un-valved connection between full manifold vacuum at the plenum and atmosphere at the throttle body. If you study the standard arrangement there's nothing to differentiate this circuit from a vacuum leak.... because that's exactly what it is.
Solve this vacuum leak and run a full manifold vacuum signal to your vac advance unit to give a far more appropriate 18 to 20 degrees of timing at idle, instead of the retarded 10-12 degrees ported vacuum gives you, and any standard 14CUX Chimaera can be made to run infinitely better in just a morning... and all for around a tenner.
Not only are these changes simple and incredibly cheap to implement, the benefits are easily accessible without even touching the14CUX fueling. While I respect all the work that's been done to crack the 14CUX code, the positive results of these two changes are so pronounced it's tempting to suggest people have largely been barking up the wrong tree for years.
As explained above, it's really about solving the direct un-valved connection between full manifold vacuum at the plenum and atmosphere at the throttle body. If you study the standard arrangement there's nothing to differentiate this circuit from a vacuum leak.... because that's exactly what it is.
Solve this vacuum leak and run a full manifold vacuum signal to your vac advance unit to give a far more appropriate 18 to 20 degrees of timing at idle, instead of the retarded 10-12 degrees ported vacuum gives you, and any standard 14CUX Chimaera can be made to run infinitely better in just a morning... and all for around a tenner.
Not only are these changes simple and incredibly cheap to implement, the benefits are easily accessible without even touching the14CUX fueling. While I respect all the work that's been done to crack the 14CUX code, the positive results of these two changes are so pronounced it's tempting to suggest people have largely been barking up the wrong tree for years.
ChimpOnGas said:
I addition to the way the crankcase breather system was plumed as standard, when you understand how the carbon canister system works its not hard to see why it has the potential to promote shunting in the 1,700 - 1,800rpm area. The 14CUX ECU pulses the valve open for short periods below 1720 RPM and holds it open at higher speeds once the engine has achieved operating temperature and is in closed loop.
Here's 13 points to consider on the journey to delivering a nicer driving 14CUX RV8 TVR:
1. Disconnect the electrical connector at the purge valve on the carbon canister
2. Remove the carbon canister hose at the plenum
3. Block the short pipe screwed into the plenum so you don't have a vacuum leak
4. Re-route the carbon canister hose to the induction pipe that runs up the N/S inner wing so petrol fumes aren't an annoyance
5. Replace the unvalved barbed "T" connector/reducer used by Land Rover with a Standard PC849 from a 5.0 litre V8 CJ5 Jeep
6. Switch the emissions driven ported vacuum arrangement and apply a full vacuum signal to the vac advance unit
7. Using the base idle screw now lower your raised idle speed to achieve a 950 - 1050rpm
8. Let your stepper motor draw air though its own independent small breather filter, rather than taking it from the throttle body
9. Get rid of all three catalytic converters
10. Completely go through and service the ignition system on the car using quality new components like MSD Super Conductor HT leads
11. Ditch the dreadful failure prone plug extenders, replacing them with insulation tubing and heat socks to protect the plug ends
12. Fit a set of BPR6EIX plugs or better still the new Denso TTs
13. Go for a test drive and marvel at how smooth your Chimaera drives
In nearly every case the drivability issues can be traced to all the emission based additions Land Rover cobbled onto an old engine to try an make it reach ever stricter pollution targets, when what they really should have done is invest in designing a new cleaner engine from scratch. Prune away all the emissions add-ons and the engine returns to how it was designed to run and it'll purr like a kitten.
It will also fail the MoT.... unless that is you can find a sympathetic tester who doesn't care about the emissions part of the test
Here's 13 points to consider on the journey to delivering a nicer driving 14CUX RV8 TVR:
1. Disconnect the electrical connector at the purge valve on the carbon canister
2. Remove the carbon canister hose at the plenum
3. Block the short pipe screwed into the plenum so you don't have a vacuum leak
4. Re-route the carbon canister hose to the induction pipe that runs up the N/S inner wing so petrol fumes aren't an annoyance
5. Replace the unvalved barbed "T" connector/reducer used by Land Rover with a Standard PC849 from a 5.0 litre V8 CJ5 Jeep
6. Switch the emissions driven ported vacuum arrangement and apply a full vacuum signal to the vac advance unit
7. Using the base idle screw now lower your raised idle speed to achieve a 950 - 1050rpm
8. Let your stepper motor draw air though its own independent small breather filter, rather than taking it from the throttle body
9. Get rid of all three catalytic converters
10. Completely go through and service the ignition system on the car using quality new components like MSD Super Conductor HT leads
11. Ditch the dreadful failure prone plug extenders, replacing them with insulation tubing and heat socks to protect the plug ends
12. Fit a set of BPR6EIX plugs or better still the new Denso TTs
13. Go for a test drive and marvel at how smooth your Chimaera drives
In nearly every case the drivability issues can be traced to all the emission based additions Land Rover cobbled onto an old engine to try an make it reach ever stricter pollution targets, when what they really should have done is invest in designing a new cleaner engine from scratch. Prune away all the emissions add-ons and the engine returns to how it was designed to run and it'll purr like a kitten.
It will also fail the MoT.... unless that is you can find a sympathetic tester who doesn't care about the emissions part of the test
Dougal9887 said:
So why not just omit the red circled pipe going to the throttle body and fit the PCV valve in place of the tee? The valve can then regulate the ventilation flow; even at WOT there will be some suck with the valve fully open.
At WOT there is no vacuum, this is why venturi vacuum at the throttle body is used!
ChimpOnGas said:
At WOT there is no vacuum, this is why venturi vacuum at the throttle body is used!
There must be some surely! I'll have a look at the Megasquirt logs from my Lotus to check, but I can't recall MAP getting that low and it's only getting vacuum from ITB's.I thought that there was insufficient vacuum at WOT due to the restrictor used to control closed throttle vacuum, and that was why the venturi vacuum was used. The PCV is an automatically adjusting restrictor.
Dougal9887 said:
There must be some surely! I'll have a look at the Megasquirt logs from my Lotus to check, but I can't recall MAP getting that low and it's only getting vacuum from ITB's.
I thought that there was insufficient vacuum at WOT due to the restrictor used to control closed throttle vacuum, and that was why the venturi vacuum was used. The PCV is an automatically adjusting restrictor.
Ok Dougal, I'm not here to argue with you, clearly you know best so please progress as you see fit. Alternatively you could always just listen to the advice being offered that includes diagrams and a clear explanation of the theory.I thought that there was insufficient vacuum at WOT due to the restrictor used to control closed throttle vacuum, and that was why the venturi vacuum was used. The PCV is an automatically adjusting restrictor.
While choosing to follow what's being presented to you by the person who'd identified the issue in the first place may not allow you to feel you've found a better solution yourself, it will at least be a guaranteed shortcut to success.
Given you weren't aware of the problem in the first place until the person who's already solved it presented it to you, you may want to reconsider your relentless 'I know better' challenges, especially as your first effort clearly demonstrated you hadn't even grasped the basic theory.
Good luck with it.
I remain unconvinced by some of what's said in regards to shunting, mainly because in my own experience the best results have been with retarding the ignition on a plenum/wild cam rather than advancing (retarding forces larger throttle angles which promotes higher gas speeds, reduces the vacuum in the plenum so presumably reducing exhaust sucked back up on overlap, gives better fuel/air mix, prevents the fuel dropping out of the airstream as much etc etc) but this point :
8. Let your stepper motor draw air though its own independent small breather filter, rather than taking it from the throttle body
is most surely a recipe for disaster on the lucas. adding air to the plenum that ISN'T going through the air flow meter. (notwithstanding the fact that the std breather hoses do this anyway, but putting the breather upstream of the air flow meter was presumably the worse of two evils ...)
8. Let your stepper motor draw air though its own independent small breather filter, rather than taking it from the throttle body
is most surely a recipe for disaster on the lucas. adding air to the plenum that ISN'T going through the air flow meter. (notwithstanding the fact that the std breather hoses do this anyway, but putting the breather upstream of the air flow meter was presumably the worse of two evils ...)
Edited by spitfire4v8 on Monday 19th March 11:27
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