god damn shunt is killing me
Discussion
davep said:
Vac pipe take-off - There's always something to spoil the party! On some HIF carbs, the vac take-off sighted just after the butterfly - that's between the butterfly and the carb to manifold mounting flange - has been eliminated. Instead, the engines these carbs are fitted to use a vac take off in the manifold. The reason for this is again wholly idle emissions orientated. The higher vacuum that exists in the manifold area can be used to pull more advance. Theoretically this will reduce emissions at idle.
No, no no no, no 
This is entirely wrong, if you run more advance you will not improve emissions you will make them worse, indeed whoever wrote the above nonsense has got their whole understanding of the concept completely back to front
. Manifold vacuum is also far more stable than ported vacuum not less stable, even on our compromised plenum and velocity stack arrangement!By switching your vacuum advance from the ported vacuum to a full manifold vacuum signal you should find the following:
1. An increase in idle timing of approximately 4-6 degrees
2. A corresponding increase in idle speed, obviously you'll need to correct this by winding in the base idle screw
3. A smoother idle
4. Far lower exhaust manifold temperatures
5. Improved throttle response just off idle
6. Better drivability just off idle and at small throttle openings
7. An elimination of the need to precisely adjust the throttle butterfly air gap which is critical with the ported vacuum system
8. Improved engine behavior during the warm up phase
9. An increase in emissions
Basically there are six clear benefits in moving to a full manifold vacuum signal, against the one negative which is an increase in emissions at idle.
As we can see from the above graph CO is not the problem, but the further we advance our ignition timing the higher the NOx and HCs figures go.
This is whole the reason ported vacuum was created, by running retarded idle timing you can considerably lower nitrogen oxide (NOx) emissions and hydrocarbon (HC) emissions too, while at the same time still retaining the benefits of the ignition advancing vacuum advance unit. At idle and running a ported vacuum setup the port is very much still on atmosphere side of the throttle butterfly, so like this the vacuum advance sees zero vacuum and adds no timing at all, you are idling at an emissions friendly 10-12 degrees. However, as soon as the throttle butterfly is opened and it passes the port, the port is now on the vacuum side of the throttle butterfly adding vacuum and so advancing the timing.
The problem with ported vacuum is at small throttle openings say as you regulate your speed through town, the throttle butterfly is constantly passing back and forth over the port, at one moment it's on the atmosphere side and a millisecond later it's on the full vacuum side. This situation is changing rapidly and causes unwanted on/off pulses at the vac advance unit witch is in turn adding and subtracting spikes of timing which will cause hesitancy and jerky engine behavior, indeed what some TVR types may call shunting
In the above image I have identified the problem area with a red circle, now imagine what's going to happen as the throttle butterfly passes back and forth over the circled ported vacuum port or worse still is sat right on it as you hold a fixed throttle position when driving at small throttle openings. If you want to see the effects for yourself just 'T' in a vacuum gauge to the ported vacuum advance hose and go for a drive in the shunting zone, watch the needle twitch frantically, now imagine each twitch of that needle will equate to timing violently added and subtracted which is a terrible situation for smooth engine operation.
Ported vacuum is just a way of giving you the emissions friendly 10-12 degrees of idle timing while retaining the well known and understood economy benefits that come by advancing the ignition timing by as much as a further 10-12 degrees under light load cruise conditions. An additional environmental benefit of running ported vacuum and the retarded idle timing it gives, is an increase in heat transferred to the exhaust system, this is useful when you're running catalytic converters as they only start to work when hot.
But if you didn't need to worry about emissions you would never run ported vacuum because of the other issues it gives, the engine is also far happier with more advanced idle timing too, ported vacuum only exists because of emissions and the need to reduce them especially at idle.
When ported vacuum was developed in the States in the late 1960's / early 1970's that pulsing vacuum signal to the vacuum advance capsule was a real problem, as the throttle butterfly passes over the port it caused a big spike in timing as the vac advance unit tugs on the base plate in the distributor, engineers needed to find a solution!
The American designers that came up with the emissions reducing ported vacuum idea needed to come up with a solution to these big on/off swings in ignition timing as the throttle butterfly passes the port, or worse still is sat right on it as you hold a small throttle opening which as already covered is typically in a Chim/Griff when driving through town. The solution came in the form of a vacuum delay valve, this little device acted as a pneumatic damper smoothing out those big spikes in timing, Land Rover also adopted this idea in the form of LR Part No: ERC6997
This is quite a rare part now and some will charge you as much as £56 plus postage for this little plastic device, which is absolutely insane
https://www.johncraddockltd.co.uk/series/series-3/...
You will often see ERC6997 fitted to early Griffs & Chimaeras but it seems fairly early on TVR deemed it unnecessary (or too expensive) so they stopped adding it around early 1995. Vacuum delay valves were fitted to pretty much every Yank V8 during the 70's & 80's when ported vacuum became common place so they definitely work and they were definitely needed, but they are only needed if you run ported vacuum!
If you are going to persevere with ported vacuum you really need that vacuum delay valve, but the truth is it's yet another one of those sticking plaster fixes that only exists because designers were trying to reduce idle emissions using what I consider to be the nasty ported vacuum strategy. If you want to fit a cheaper vacuum delay valve than the stupid money being asked for a genuine ERC6997 then FSE make a universal one (Part No: APV001) that for just £20 should do the same job:
https://www.ebay.co.uk/itm/FSE-VACUUM-ADVANCE-ANTI...
There's no question that running a ported vacuum signal to the vacuum advance unit helps lower idle emissions, but it comes with so many other issues if you can find a sympathetic MoT tester who's happy to overlook the emissions element of the test I'd strongly recommend people go back to the good old and far more stable manifold vacuum signal.
davep said:
Belle427 have you found the levels of full manifold vacuum consistent and stable from the plenum port? I was thinking of doing this mod but on reading the article below I have some reservations, especially if 'induction pulsing' as mentioned is a factor and considering the plenum works as a Helmholtz resonator it could be:
Article from https://www.minimania.com/SU_Carbs_HIFa_more_finely_honed_instrument:
Vac pipe take-off - There's always something to spoil the party! On some HIF carbs, the vac take-off sighted just after the butterfly - that's between the butterfly and the carb to manifold mounting flange - has been eliminated. Instead, the engines these carbs are fitted to use a vac take off in the manifold. The reason for this is again wholly idle emissions orientated. The higher vacuum that exists in the manifold area can be used to pull more advance. Theoretically this will reduce emissions at idle. Unfortunately it's exposed to induction pulsing. In the A-series, this is UGLY. The pulsing and high manifold vacuum causes rough idling. Particularly where anything 'sportier' (ANYTHING) than a bog-stock-standard cam profile is used.
Classic example - the MG Metro. Its adventurously sporty cam makes for a slightly lumpy idle. This increases idle speed emissions, so Rover use manifold-sourced vacuum to increase idle speed ignition timing via the vac advance. We're talking 28 degrees of ignition timing here! Doesn't work because the pulsing created by the cam causes erratic ignition timing, culminating in running on. So an anti-run-on valve was fitted as a modification. Vicious circle. After messing about trying to solve the emission problems this caused, it was found that changing the originally quoted valve clearances to 0.014"inlet /0.017" exhaust (effectively reducing the cam over-lap) helped dramatically. And, like I said earlier, the sportier the profile, the worse it becomes. Even worse - when running, this system can cause ignition advances in the order of 50 degrees on lift-off and on again - an absolute sure-fire recipe for savage detonation.
The solution is easy - remove the vac take-off from the manifold and fit it back to just in front of the butterfly where it should be.
Also, if you have RoverGauge and the time could you check that with manifold vacuum from the plenum fitted and when at steady idle, fuel cell selection in the fuel map is not jumping around from cell to cell as vacuum and therefore ignition advance rises and falls due to pulsing.
Many thanks.
I should have some time to plug rovergauge in and take a look tomorrow evening.Article from https://www.minimania.com/SU_Carbs_HIFa_more_finely_honed_instrument:
Vac pipe take-off - There's always something to spoil the party! On some HIF carbs, the vac take-off sighted just after the butterfly - that's between the butterfly and the carb to manifold mounting flange - has been eliminated. Instead, the engines these carbs are fitted to use a vac take off in the manifold. The reason for this is again wholly idle emissions orientated. The higher vacuum that exists in the manifold area can be used to pull more advance. Theoretically this will reduce emissions at idle. Unfortunately it's exposed to induction pulsing. In the A-series, this is UGLY. The pulsing and high manifold vacuum causes rough idling. Particularly where anything 'sportier' (ANYTHING) than a bog-stock-standard cam profile is used.
Classic example - the MG Metro. Its adventurously sporty cam makes for a slightly lumpy idle. This increases idle speed emissions, so Rover use manifold-sourced vacuum to increase idle speed ignition timing via the vac advance. We're talking 28 degrees of ignition timing here! Doesn't work because the pulsing created by the cam causes erratic ignition timing, culminating in running on. So an anti-run-on valve was fitted as a modification. Vicious circle. After messing about trying to solve the emission problems this caused, it was found that changing the originally quoted valve clearances to 0.014"inlet /0.017" exhaust (effectively reducing the cam over-lap) helped dramatically. And, like I said earlier, the sportier the profile, the worse it becomes. Even worse - when running, this system can cause ignition advances in the order of 50 degrees on lift-off and on again - an absolute sure-fire recipe for savage detonation.
The solution is easy - remove the vac take-off from the manifold and fit it back to just in front of the butterfly where it should be.
Also, if you have RoverGauge and the time could you check that with manifold vacuum from the plenum fitted and when at steady idle, fuel cell selection in the fuel map is not jumping around from cell to cell as vacuum and therefore ignition advance rises and falls due to pulsing.
Many thanks.
Edited by davep on Monday 23 July 10:29
I have not really done a lot of in depth testing, just seat of the pants stuff.
Anything that improves low speed drive ability, cooler running and slightly better mpg is a bonus in my book, although to be fair my car ran well with no shunting before the vacuum change.
A year on and the car has been faultless, no overheating and Ive been using itbto commute in 30+ degrees. Ive just sold that car and will be picking up a bit of a project Chim at the weekend.
I'm on this thread to make a note of my ignition settings as i lost my original note book.
1st thing im doing to the new Chim is replicate exactly the set up I had on my old one.
Stu
I'm on this thread to make a note of my ignition settings as i lost my original note book.
1st thing im doing to the new Chim is replicate exactly the set up I had on my old one.
Stu
Vacuum delay valve's (damper) are to stop pinking on initial acceleration when the throttle is cranked rapidly 
Ford used them too back in the 80's for the mentioned reason not helped by them jumping on lean burn fueling at the time 
1.8 CVH & 1.6e and 1.8 Pinto motors whilst the Japs had been at it since the early 70's
Ford used them too back in the 80's for the mentioned reason not helped by them jumping on lean burn fueling at the time 1.8 CVH & 1.6e and 1.8 Pinto motors whilst the Japs had been at it since the early 70's Steve_D said:
So for you TVR ownership is all about the pain.
Steve
I don't understand your comment. I sold my first Chimaera because I had a good offer on it. I bought another that was mechanically good and cosmetically challenged. The drive home made me realise how awful these cars drive at low speeds. Steve
So new vac unit, idle set at 12dec so 30 max mechanical, full vacuum so 18 at idle and the car is a pleasure to drive in traffic or low speeds.
Why would that be pain?
StuVT said:
Steve_D said:
So for you TVR ownership is all about the pain.
Steve
I don't understand your comment. I sold my first Chimaera because I had a good offer on it. I bought another that was mechanically good and cosmetically challenged. The drive home made me realise how awful these cars drive at low speeds. Steve
So new vac unit, idle set at 12dec so 30 max mechanical, full vacuum so 18 at idle and the car is a pleasure to drive in traffic or low speeds.
Why would that be pain?
Selling a car you have sorted then buying a project to me equates to a love of pain.
Steve
It does need reversing but is simply a case of removing the full vaccum pipe from the distributor and blocking it and reconnecting the ported vacuum pipe from the throttle body that you would have blocked and tucked away for 364 days of the year. I do mine when I waited at the garage. Driving it again in ported vacuum wasnt nice.
For the record of other members. TVR number 2. Chimaera 400 with 96k on the clock. New vacuum unit needed so purchased one from powerspark on ebay. They confirmed there version is the same Lucas unit in a plain box and is about £10 cheaper than the one in a Lucas branded box.
Timing at idle 12 degrees NO VAC
Full advance 30 degrees NO VAC.
VAC Connected and the idle timing changes by 14deg so idles in the low to mid 20's. Remebering that as soon as the throttle is opened the vacuum is lost and the mechanical advance comes into play which under load is limited to a nice safe 30 degrees.
The car runs smoother, starts better, runs cooler, and on my commute this morning 25 miles on some 30 limits, 60 limits and a majority of Motorway at 75mph the fuel gauge barely moved.
Also to note this Chimaera has really bad diff mounts, and the knocking was considerably reduced during the drive because their was only a hint of shunting rather than the rodeo bull type of shunting it had when I first picked it up. So the timing and subsequent shunting is the death of the diff bushes in my opinion.
For the record of other members. TVR number 2. Chimaera 400 with 96k on the clock. New vacuum unit needed so purchased one from powerspark on ebay. They confirmed there version is the same Lucas unit in a plain box and is about £10 cheaper than the one in a Lucas branded box.
Timing at idle 12 degrees NO VAC
Full advance 30 degrees NO VAC.
VAC Connected and the idle timing changes by 14deg so idles in the low to mid 20's. Remebering that as soon as the throttle is opened the vacuum is lost and the mechanical advance comes into play which under load is limited to a nice safe 30 degrees.
The car runs smoother, starts better, runs cooler, and on my commute this morning 25 miles on some 30 limits, 60 limits and a majority of Motorway at 75mph the fuel gauge barely moved.
Also to note this Chimaera has really bad diff mounts, and the knocking was considerably reduced during the drive because their was only a hint of shunting rather than the rodeo bull type of shunting it had when I first picked it up. So the timing and subsequent shunting is the death of the diff bushes in my opinion.
So having read this marathon post, Champongas and Blitzracing has convinced me to try a manifold vacuum. I am thinking of removing one of the blanking plugs on the back of the plenum ( drivers side) where the brake servo vacuum ( I think comes from). The question i have is that i have found this part https://rimmerbros.com/Item--i-ADU1402 which is a brake servo vacuum take off with non-return valve.
It was the non-return element that confused me, i guess this is non-return back into the plenum and won't cause me any problems if i use this component to create a manifold vacuum for my distributor.
This may all sound quite dumb so feel free to point this out if so!
It was the non-return element that confused me, i guess this is non-return back into the plenum and won't cause me any problems if i use this component to create a manifold vacuum for my distributor.
This may all sound quite dumb so feel free to point this out if so!
Steve_D said:
You will need to disable that valve in some way otherwise you will pull a vacuum on the dizzy but not release it. Probably something as simple as poking a match stick down the side of the valve seat.
Steve
that's what i thought and I am glad I asked ( so maybe I am not so dumb) Steve
blitzracing said:
Just drill a small hole and pop a metal pipe in it with a bit of jb weld or locktite. Something like a bit of metal brake pipe is fine.
Mark got the infrared temp monitor yesterday so will measure cat temp on white and green maps today. Will text you the results.
BW
Andy
Thanks. Ill be looking for a second 5ltr Guinea pig to try the modified green map on if the cat temp is OK on yours. Any takers with a shunting 500 who does not mind swapping out the ECU chip and fitting a switchable tune resistor? Access to an infrared temp monitor as well would be good to check the cat temp.
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