Exhaust manifolds - big bolt and 'labirinth'

Hello everyone,

Does anyone know what is the purpose of the two features highlighted in the photos , big bolt and ‘labirinth’ ?

Rui w

Ok, found one, it is to connect anti polution device for the 3.4 liter.

Still no idea regarding the big bolt

The labyrinth is a provision for bolting on a heat sensing plate (C33312) which allows heat to be transferred over to the intake side for various purposes such as AEDs (early on) or pollution stuff (later). My 73 used it for pollution. Don’t know what the bolt is for, not present on a 73 or earlier. Perhaps used for a lambda sensor or similar on FI cars, but plugged here?

Thanks Robert, indeed, I then found the scheme on the parts catalogue .

I had the sane idea regarding the bolt. My car has a retrofit catalysor but the lambda sensor is located after the downpipe . One possibility could be to install test lambda sensors to check the injection on each set of 3 cylinders. .Just a possibility.


Indeed, Rui - originally used for checking CO levels, setting mixture. But can of course be used for other relevant purposes, as you say, or for checking exhaust back pressure etc etc.

xj6 85 Sov Europe (UK/NZ)

It’s too close to the exhaust port to be for a lambda sensor. You see a similarly located bolt hole on the 1974 v12 manifolds for exhaust gas recirculation.

kind regards

Mistery decyphered, thanks to Marek, Frank and Robert :blush:
All the best

Picture of the EGR valve :thinking:


Now I also understand why is there a bolt on the top of front exhaust manifold, much larger that the back one used to fix the exhaust shield: to screw in the EGR valve on cars with emission control possibly on early carburettor cars.

Small at the front, large at the rear, otherwise yes, exactly!
Curious if EGR had any measurable benefit for fuel economy. The system seems simple enough to retrofit.

I don’t think EGR would have impact on fuel economy,it also adds complexity to the system and those valves would eventually get stuck anyways.

It was listed as an emission control devise, David - to reduce combustion temps, reducing production of Nox. Like most emission control devises; I reject if it could improve engine performance or fuel economy. None was fitted to ‘European’ versions…

xj6 85 Sov Europe (UK/NZ)

EGR is pretty much the same as a hot air intake, reduces the power of the engine, requiring more throttle, reducing losses, making the engine more efficient, that’s why I’m asking.

It does add complexity and even today these valves fail frequently.

A little research says that EGR saves 3-15% fuel, the XK has no engine management so it might not benefit from all the potential (timing adjustments etc.) but it is a highly throttled engine most of the time (lots of torque but low gearing) so it could benefit…

Thats’ what I question, David - how reducing engine power can improve fuel economy…

xj6 85 Sov Europe (UK/NZ)

The engine is also a pump. It creates a vacuum in the intake manifold. Introducing an inert gas means the throttle can be opened further, reducing vacuum, reducing pumping losses.

According to the first article I found on google, friction loss in an engine is 3% and pumping losses 4%. We’ll agree that this means nothing but it at least tells us that it is a considerable loss of power. Reducing these 4% with warmer or even inert air helps. When full power is required, the valves close and cold air is used for full power.

How many hp does the engine make at idle, maybe 10? Creating vacuum must use at least 2-4 hp. It sucks the pistons up, that energy is removed from the crankshaft. A vacuum cleaner has about 2hp.

Precisely correct, albeit hard to believe that recirculating exhaust can be useful. Like vacuum advance, it only improves economy at cruise. The principle (avoid pumping losses) is also behind variable valve lift engines, where there is no throttle and consequently no vacuum in the manifold. IMHO.

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Another trick for reducing pumping losses is use of an “overdrive” gear ratio. This not only reduces revs whilst cruising, but can increase the throttle opening needed to achieve a certain cruising speed. Ideally (in terms of reduced pumping load) the overdrive ratio should be high enough so that full throttle is required to reach the desired cruising speed. That’s practical only with a CV tranny, of course. IMHO.


With a six, David; only 1,5 cylinder is producing power - the rest is using it to run the engine. The faster the engine is turning the more power is required to run it - in idle, and your 10 hp is likely in the ball park, all power is used just to run the engine. If more power is produced, the engine revs up to a new ‘idle’ level

To produce more power you need to burn more petrol, and opening the throttle adds more air - and petrol is added in proportion; engine revs up. Adding exhaust gas to the manifold reduces manifold vacuum (ref 'pumping loss), but the exhaust gas cannot burn petrol - it just lowers the combustion temps. Which reduces Nox production, which is/was the intent of the EGR - which was carefully metered by its valve. The lowered manifold vacuum reduces air ingress for any given throttle opening - but exhaust gas cannot burn petrol

The main power consumption for running the engine is the acceleration of moving parts, pistons/conrods, compressing the mixture and pushing out exhaust gas - which all increases with rpms; and surplus power is used to propel the car.

Rpm is pure power; more air goes through the engine, allowing more petrol to be burnt for more power. Higher gear, lower rpm, means less ‘waste’ in running the engine itself - saving petrol. However, lower rpm also means lower power production, so there is a limit to ‘overdrive’ saving; uphill or headwind slows the car, drops rpm - requiring more pedal or downshift. All of which is one secret of modern cars’ fuel economy; heaps of gears keeps the engine running at the lowest rpm which produces just enough power to run the car at a given speed…

It’s all intricately interlocking, but if EGR could lower petrol consumption without repercussions - it would be universal…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

I have no problem reaching cruise using full throttle (max torque), you probably mean the highest possible gear ratio so the engine is almost not throttled at cruise. Absolutely.

Since unfortunately I’m not a race driver I want more cruise economy, a two percent improvement in mpg means saving 4€ at every fillup, for the cost of a little EGR valve. I’ll keep my current XJ original but maybe the next one?

Btw, mercedes makes or made a two step camshaft that switches between two lobe profiles, one with high lift and one with very little, same principle but a simpler implementation.

It lowers manifold vacuum without doing anything else. This reduces suction on the pistons that are going down, so less work for the 1.5 pistons powering the vacuum pump that the intake cycle pistons are.

You seem to forget the vacuum again! Strong enough to boost the brakes, it acts on the engine as well, slowing it down.

More gears of course means the engine can be in the efficiency band at lower rpm, so there’s an economic ratio for 30, 40, 50, 60, 70, 80, 100 mph and it doesn’t rev like mad (using petrol) at the slightest inclines.