Distributer vacuum tube missing...?
Discussion
For the first time yesterday I showed my car to other club members, while looking at the engine it was pointed out to me that the small pipe that should go from the vacuum diaphragm on the distributer to the air intake manifold is actually missing. There are blanking plugs inserted at either end too. What's more mystifying is that a new diaphragm ring has been fitted to the distributer, so why bother to do this if you're not going to fit the pipe?
Is there any reason at all why the pipe may not have been fitted?
Is there any reason at all why the pipe may not have been fitted?
It actually benefits cruising at low throttle openings. Have a read of this thread if you have a spare hour, some cars run better without it but most cars keep it as it does improve fuel economy.
https://www.pistonheads.com/gassing/topic.asp?h=0&...
https://www.pistonheads.com/gassing/topic.asp?h=0&...
bobfather said:
It's not good to have unmetered air going into the plenum. If you decide to leave the vacuum pipe off you should blank off the plenum tapping to prevent air being drawn in
I had a word with my dealer, he said that they normally leave the pipe disconnected, then adjust the timing, which on my car is spot on. The plenum tapping has been blanked off, so has the diaphragm end at the distributer.griff59 said:
I had a word with my dealer, he said that they normally leave the pipe disconnected, then adjust the timing, which on my car is spot on. The plenum tapping has been blanked off, so has the diaphragm end at the distributer.
You need to reconnect your vacuum advance, it's there for a reason!If the dealer says they disconnect it ask them why, they clearly don't understand how the system is designed to work which would give me no confidence in their abilities to maintain your car in any way.
Study the vacuum advance system yourself so you understand how it works and why it's there, reconnect it, then stop using the people who disconnected it because they clearly have no idea what they are doing.
RichB said:
Just out of interest, my Griff 500 has never had a vacuum advance, so the chaps at the Blackpool factory did not fit them, or at least not in March '99 they didn't.
Interesting, have you tried connecting it?I'll have a go a see if it makes any difference.
PS, I've trawled the net for photos of Griffith engines, it's very interesting that far more have the hose disconnnected than connected.
Edited by griff59 on Wednesday 11th July 17:30
It's laughable people are still debating if the vacuum advance should be connected or not 
This is 1940's ignition technology, so I'm amazed that almost 80 years later people still don't understand the basic principles and benefits of advancing ignition timing on an internal combustion engine at higher RPM when the engine is under light load and running leaner.
A distributor is a very crude device that can only advance ignition timing in direct relation to increasing engine speed, like all distributors since they were perfected in the early 1920s it works on centrifugal force just like a steam engine governor. In the case of a Wedge, V8S, Chimaera or Griffith TVR did nothing with the Range Rover/Discovery/V8 Defender distributor whatsoever so the advance curve is exactly what Lucas designed for those Land Rover built vehicles. This distributor has 18 degrees of mechanical advance, so if you idle at 10 degrees all you'll ever get is 10 + 18 = 28 degrees of total timing.
The problem with this is under certain conditions like light load cruising an internal combustion engine will significantly benefit from a lot more ignition advance, by the early 1920's this was well understood and all aircraft were fitted with a manual ignition timing lever where the pilot could retard timing to aid easier starts and advance it to improve fuel economy when cruising. This feature soon became standard on automobiles too, look at most vintage cars and you'll see this lever on the steering wheel marked early/late which we refer to now as advance/retard.
During the Second World War when aircraft range was extremely important (aircraft engines spend most of their time at a fixed engine speed at light load) engine makers came up with a system using engine vacuum acting on a diaphragm to advance the ignition timing automatically, by the 1940s the vacuum advance unit was starting to be adopted by the automotive industry, and because it worked so well by the 1950's it was standard on almost every car.
To be clear there are absolutely no disadvantages to running an automatic vacuum advance system whatsoever, only advantages. On our cars you still get the 10 degrees at idle plus the 18 degrees of mechanical advance to give you your 28 degrees all out at 3,000rpm, but as soon as you settle into cruise by lifting your foot off the throttle pedal, the vacuum advance unit will add a further 10 degrees of timing to give you 38 degrees.
The difference between 28 degrees and 38 degrees at light load cruise is a considerable increase in fuel economy, 38 degrees would be too much timing under acceleration when the engine is under load but because you lose vacuum when you open the throttle butterfly the vacuum advance unit adds nothing so timing instantly drops back to a safe 28 degrees all out at 3,000rpm.
As we can now see there is absolutely no benefit in disconnecting the vacuum advance unit, people who recommend this practice need to go back to school because quite clearly they don't have a clue how the system works. To be honest this is all very basic stuff, if your mechanic is advocating disconnecting the vacuum advance I would be very concerned about the level of his general skills and knowledge in his chosen profession.
However, the reason sometimes our cars do drive smoother at small throttle openings with the vacuum advance disconnected is because our cars use a ported vacuum signal to the vacuum advance unit, this system gives zero vacuum at idle and only places the port on the vacuum side if the engine when the throttle butterfly passes over it. The throttle position this transition from no vacuum to vacuum takes place as the throttle butterfly passes over the port is therefore absolutely critical to smooth low speed engine behaviour, get it wrong and the vacuum advance can suddenly and violently add and subtract timing as you drive at small throttle openings (normally at low road speeds in town) which can contribute what the TVR world like to call shunting.
This is why disconnecting the vacuum advance unit altogether can improve low speed engine behaviour but doing so is very much a bodge, while it may help smooth out some of the shunting on a poorly set up engine it'll also mean forfeiting a good slice of fuel economy especially when cruising on the motorway. What the mechanic needs to do first is properly understand how the system is designed to work, once he's educated himself on how the system is actually designed to work hopefully he'll finally realise why throttle butterfly rest position is such a critical adjustment when setting up these cars to drive smoothly through town.
The factory workshop manual from Land Rover clearly gives the setup procedure and butterfly air gap, with everything now set correctly that vacuum advance unit should be reconnected. The vacuum advance unit is after all the only 3D adaptive ignition control system you get with a distributor, so to leave it disconnected is a super dumb and ignorant thing to do. My final point would be the ported vacuum system on our cars only really exists as a way to reduce emissions at idle, it was invented in the early 1970's to help clean up certain tailpipe emissions so if that doesn't bother you there is another option.
If you connect the vacuum advance unit to a full vacuum signal you can go back to how things were done in the 1960's when all designers needed to concern themselves with was making the engine run at its best. Running full vacuum gives you a bit more idle timing which the Rover V8 definitely likes, it also means you remove the changeover from no vacuum to vacuum you get with ported vacuum that can promote issues, finally throttle butterfly resting air gap becomes way less critical so overall there are only benefits in switching to a full vacuum signal.
With mechanics you can't really expect to make something work correctly unless you take the time to properly understand how the system was designed to work in the first place, and the practice of disconnecting the vacuum advance is a classic case of someone failing to study and understand the system. Some so called TVR Specialists may well have discovered disconnecting the vacuum advance makes the engine a little smoother in some cases, but what they really should have done is studied how the system was designed to work with it connected, then followed the instructions on setting up the engine in the Land Rover factory workshop manual.
This is 1940's ignition technology, so I'm amazed that almost 80 years later people still don't understand the basic principles and benefits of advancing ignition timing on an internal combustion engine at higher RPM when the engine is under light load and running leaner.
A distributor is a very crude device that can only advance ignition timing in direct relation to increasing engine speed, like all distributors since they were perfected in the early 1920s it works on centrifugal force just like a steam engine governor. In the case of a Wedge, V8S, Chimaera or Griffith TVR did nothing with the Range Rover/Discovery/V8 Defender distributor whatsoever so the advance curve is exactly what Lucas designed for those Land Rover built vehicles. This distributor has 18 degrees of mechanical advance, so if you idle at 10 degrees all you'll ever get is 10 + 18 = 28 degrees of total timing.
The problem with this is under certain conditions like light load cruising an internal combustion engine will significantly benefit from a lot more ignition advance, by the early 1920's this was well understood and all aircraft were fitted with a manual ignition timing lever where the pilot could retard timing to aid easier starts and advance it to improve fuel economy when cruising. This feature soon became standard on automobiles too, look at most vintage cars and you'll see this lever on the steering wheel marked early/late which we refer to now as advance/retard.
During the Second World War when aircraft range was extremely important (aircraft engines spend most of their time at a fixed engine speed at light load) engine makers came up with a system using engine vacuum acting on a diaphragm to advance the ignition timing automatically, by the 1940s the vacuum advance unit was starting to be adopted by the automotive industry, and because it worked so well by the 1950's it was standard on almost every car.
To be clear there are absolutely no disadvantages to running an automatic vacuum advance system whatsoever, only advantages. On our cars you still get the 10 degrees at idle plus the 18 degrees of mechanical advance to give you your 28 degrees all out at 3,000rpm, but as soon as you settle into cruise by lifting your foot off the throttle pedal, the vacuum advance unit will add a further 10 degrees of timing to give you 38 degrees.
The difference between 28 degrees and 38 degrees at light load cruise is a considerable increase in fuel economy, 38 degrees would be too much timing under acceleration when the engine is under load but because you lose vacuum when you open the throttle butterfly the vacuum advance unit adds nothing so timing instantly drops back to a safe 28 degrees all out at 3,000rpm.
As we can now see there is absolutely no benefit in disconnecting the vacuum advance unit, people who recommend this practice need to go back to school because quite clearly they don't have a clue how the system works. To be honest this is all very basic stuff, if your mechanic is advocating disconnecting the vacuum advance I would be very concerned about the level of his general skills and knowledge in his chosen profession.
However, the reason sometimes our cars do drive smoother at small throttle openings with the vacuum advance disconnected is because our cars use a ported vacuum signal to the vacuum advance unit, this system gives zero vacuum at idle and only places the port on the vacuum side if the engine when the throttle butterfly passes over it. The throttle position this transition from no vacuum to vacuum takes place as the throttle butterfly passes over the port is therefore absolutely critical to smooth low speed engine behaviour, get it wrong and the vacuum advance can suddenly and violently add and subtract timing as you drive at small throttle openings (normally at low road speeds in town) which can contribute what the TVR world like to call shunting.
This is why disconnecting the vacuum advance unit altogether can improve low speed engine behaviour but doing so is very much a bodge, while it may help smooth out some of the shunting on a poorly set up engine it'll also mean forfeiting a good slice of fuel economy especially when cruising on the motorway. What the mechanic needs to do first is properly understand how the system is designed to work, once he's educated himself on how the system is actually designed to work hopefully he'll finally realise why throttle butterfly rest position is such a critical adjustment when setting up these cars to drive smoothly through town.
The factory workshop manual from Land Rover clearly gives the setup procedure and butterfly air gap, with everything now set correctly that vacuum advance unit should be reconnected. The vacuum advance unit is after all the only 3D adaptive ignition control system you get with a distributor, so to leave it disconnected is a super dumb and ignorant thing to do. My final point would be the ported vacuum system on our cars only really exists as a way to reduce emissions at idle, it was invented in the early 1970's to help clean up certain tailpipe emissions so if that doesn't bother you there is another option.
If you connect the vacuum advance unit to a full vacuum signal you can go back to how things were done in the 1960's when all designers needed to concern themselves with was making the engine run at its best. Running full vacuum gives you a bit more idle timing which the Rover V8 definitely likes, it also means you remove the changeover from no vacuum to vacuum you get with ported vacuum that can promote issues, finally throttle butterfly resting air gap becomes way less critical so overall there are only benefits in switching to a full vacuum signal.
With mechanics you can't really expect to make something work correctly unless you take the time to properly understand how the system was designed to work in the first place, and the practice of disconnecting the vacuum advance is a classic case of someone failing to study and understand the system. Some so called TVR Specialists may well have discovered disconnecting the vacuum advance makes the engine a little smoother in some cases, but what they really should have done is studied how the system was designed to work with it connected, then followed the instructions on setting up the engine in the Land Rover factory workshop manual.
After trying to eliminate the light throttle shunting problem I discovered the one way valve plumbed into the vacuum pipe had failed, and was not allowing any vacuum ( and was the wrong way around!). I've replaced it with just a pipe, and noticed an improvement.
The atmosphere side tube on the vac advance is capped off. I cannot find any information on what this does, or if capping it off is correct?
The atmosphere side tube on the vac advance is capped off. I cannot find any information on what this does, or if capping it off is correct?
chris212 said:
After trying to eliminate the light throttle shunting problem I discovered the one way valve plumbed into the vacuum pipe had failed, and was not allowing any vacuum ( and was the wrong way around!). I've replaced it with just a pipe, and noticed an improvement.
The atmosphere side tube on the vac advance is capped off. I cannot find any information on what this does, or if capping it off is correct?
Theres some logical speculation that says the reason there's a hose take-off on the other side of the vac advance was to accommodate future plans by Land Rover to go forced induction.The atmosphere side tube on the vac advance is capped off. I cannot find any information on what this does, or if capping it off is correct?
However, I suspect it's more emisions nonsense, in theory removing the cap gives less resistance on the atmosphere side so more vacuum advance. Personally I'd remove the cap and switch from ported vacuum to a full manifold vacuum signal.
I'd run 14 degrees at idle with the vacuum advance disconnected, this gives 14 + 18 of mechanical advance to deliver 32 degrees all out. Now reconnect the vac advance and idle timing should be in the region of 18-21 degrees which the engine will like, however you will need to reduce the air passing through the base idle circuit to bring the idle speed back down.
The above also takes the vacuum advance assisted light load cruising timing from 38 to 42 degrees which should buy you an additional 2-3mpg on the motorway. The car will idle nicer, drive smoother in the shunting zone, provide sharper throttle response, give slightly better top end performance a will generally be more economical.
The advantages are many, the disadvantage is one... ie your emissions at idle will go up. Personally I'd just go all out and delete all three catalytic converters too, then go find yourself a sympathetic MoT tester who is happy to overlook the emissions element of the test.
Edited by ChimpOnGas on Thursday 12th July 17:33
chris212 said:
After trying to eliminate the light throttle shunting problem I discovered the one way valve plumbed into the vacuum pipe had failed, and was not allowing any vacuum ( and was the wrong way around!). I've replaced it with just a pipe, and noticed an improvement.
Are you sure the one way valve had failed. You will struggle to suck air through it via ones mouth. This is normal. Its like that to stop any voilent ignition spikes as the throttle passes over it. This would give shunting. It takes a while of constant vacuum for it to work. This way you only get the advance rise when cruising with a constant ish throttle opening which is when you need it rather than when the throttle is constantly being quickly passed over it.
Hope that makes sense its getting late.
Gassing Station | Griffith | Top of Page | What's New | My Stuff