Suck my vacuum
Discussion
I've had the TVR for 5 years now.
It has always had the 'shunting' to a greater or lesser extent so I decided to try to sort it out once and for all (or at least until I got hacked off with it and put in an MBE system)
I started by doing lots of reading (mostly on here) and then getting the bits I might need for diagnosis (Rover Gauge, oscilloscope [not quite sure what I'd use this for but I wanted one, so there] , was about to buy a vacuum gauge etc etc).
2 days ago I decided I'd try to run without the vacuum advance connected so I pulled the vac pipe off and then looked for a suitable bung to go over the stub pipe on the plenum. It was then that I spotted a small rubber pipe with a screw in the end of it on the vacuum advance pod.
All this time I've had the pipe connected but with the atmosphere side stopped off!!
I put the vacuum pipes back on and pulled the bung off the atmo side of the diaphragm and took it for a spin.
Different car!!
I can now drive at 30 MPH in 4th gear and it pulls smoothly.
If the vacuum advance is disabled, should the timing be altered to compensate?
My guess is that some of the guys who have serviced it and set the timing have noticed the bung and adjusted the timing slightly and others have not, which is why some times the shunting has been worse then at other times (it has been good for a year, bad for a year etc).
It has always had the 'shunting' to a greater or lesser extent so I decided to try to sort it out once and for all (or at least until I got hacked off with it and put in an MBE system)
I started by doing lots of reading (mostly on here) and then getting the bits I might need for diagnosis (Rover Gauge, oscilloscope [not quite sure what I'd use this for but I wanted one, so there] , was about to buy a vacuum gauge etc etc).
2 days ago I decided I'd try to run without the vacuum advance connected so I pulled the vac pipe off and then looked for a suitable bung to go over the stub pipe on the plenum. It was then that I spotted a small rubber pipe with a screw in the end of it on the vacuum advance pod.
All this time I've had the pipe connected but with the atmosphere side stopped off!!
I put the vacuum pipes back on and pulled the bung off the atmo side of the diaphragm and took it for a spin.
Different car!!
I can now drive at 30 MPH in 4th gear and it pulls smoothly.
If the vacuum advance is disabled, should the timing be altered to compensate?
My guess is that some of the guys who have serviced it and set the timing have noticed the bung and adjusted the timing slightly and others have not, which is why some times the shunting has been worse then at other times (it has been good for a year, bad for a year etc).
AFIK, it makes no difference if the secondary port on the vac-advance is blanked off, it's the retard connection point.
I've recently replaced my vac assembly as the diaphragm had split and testing the new one, once fitted, the actuator still works whether the spare port is capped or not. So I've left mine sealed.
I've recently replaced my vac assembly as the diaphragm had split and testing the new one, once fitted, the actuator still works whether the spare port is capped or not. So I've left mine sealed.
The vacuum advance is only there for light throttle when it adds advance because the mixture burns slower when the effective compression is low as little air is reaching the combustion chamber- so if you disconnect it the timing effectively becomes retarded and the engine efficiency drops- typically you will loose about 10% MPG. You cant compensate by altering the timing as it would become over advanced in the process.
Even at low RPMs at light throttle openings (where the so called shunting exists) the engine is still pumping pretty well, but because the throttle is almost shut the restriction still causes a significant vacuum.
Now consider the vacuum advance module is plumbed into the side of the throttle where all this depression exists and that everything else on the inlet side is well sealed, even at idle your big old V8 engine is sucking hard on that little vac module like a Thai hooker.
All that pull is transferred to a rod which in turn is connected to the distributor base plate, the pull moves the base plate against spring pressure to advance the moment the spark is initiated withing the combustion chambers.
As Blitz quite rightly points out, the vacuum advance module is really just there to give you you a bit more fuel economy when you are cruising at speed on the motorway with your foot barely opening that throttle butterfly at all.
In this condition the RPMs are higher than at idle so the engine is pumping super hard against that almost closed throttle butterfly, the depression engine side of the throttle is therefore enormous. What people don't always realise is even at lowish rpms a big V8 still generates enough pumping forces to overcome the base plate spring pressure and so will initiate a small amount of ignition retardation at 1,500 RPM where the shunting seems to start.
It pretty clear to me this engine likes this retarded ignition state between 1,500 & 1,800rpm as it seems to help with the shunting. To prove this point you can easily experiment by getting a willing assistant to manually apply vacuum to the the vac advance module as you drive the car through that difficult rpm band with next to no throttle opening.
Try without vacuum first to get it shunting, then try again with your assistant applying some vacuum advancing the spark and the shunting should vanish. That's why davelittlewood massively improved his shunting by pulling that plug out of his vac pipe.
There is an old wives tale still circulating amongst mechanics that says you can gain a bit of performance by stopping the vac advance module doing it's thing, if you think about how the unit works this can only be complete bollix because the unit is only ever in operation when the throttle butterfly is almost closed.
I have no idea how performance can be improved on an engine with it's throttle virtually closed but hey that's the urban myth and the reason some misinformed mechanics still block the vac advance pipe with a bung.
Along with the vac pipe blocking nonsense people with a shunting engine should also look out for partially seized distributor base plate, a vac module actuating rod that's slipped off the base plate peg, or a split vac module diaphragm.
All of which will make the vac advance module completely ineffective and could easily be the cause of your shunting, as can a host of other things like induction leaks, defective HT leads ect ect ect.
Now consider the vacuum advance module is plumbed into the side of the throttle where all this depression exists and that everything else on the inlet side is well sealed, even at idle your big old V8 engine is sucking hard on that little vac module like a Thai hooker.
All that pull is transferred to a rod which in turn is connected to the distributor base plate, the pull moves the base plate against spring pressure to advance the moment the spark is initiated withing the combustion chambers.
As Blitz quite rightly points out, the vacuum advance module is really just there to give you you a bit more fuel economy when you are cruising at speed on the motorway with your foot barely opening that throttle butterfly at all.
In this condition the RPMs are higher than at idle so the engine is pumping super hard against that almost closed throttle butterfly, the depression engine side of the throttle is therefore enormous. What people don't always realise is even at lowish rpms a big V8 still generates enough pumping forces to overcome the base plate spring pressure and so will initiate a small amount of ignition retardation at 1,500 RPM where the shunting seems to start.
It pretty clear to me this engine likes this retarded ignition state between 1,500 & 1,800rpm as it seems to help with the shunting. To prove this point you can easily experiment by getting a willing assistant to manually apply vacuum to the the vac advance module as you drive the car through that difficult rpm band with next to no throttle opening.
Try without vacuum first to get it shunting, then try again with your assistant applying some vacuum advancing the spark and the shunting should vanish. That's why davelittlewood massively improved his shunting by pulling that plug out of his vac pipe.
There is an old wives tale still circulating amongst mechanics that says you can gain a bit of performance by stopping the vac advance module doing it's thing, if you think about how the unit works this can only be complete bollix because the unit is only ever in operation when the throttle butterfly is almost closed.
I have no idea how performance can be improved on an engine with it's throttle virtually closed but hey that's the urban myth and the reason some misinformed mechanics still block the vac advance pipe with a bung.
Along with the vac pipe blocking nonsense people with a shunting engine should also look out for partially seized distributor base plate, a vac module actuating rod that's slipped off the base plate peg, or a split vac module diaphragm.
All of which will make the vac advance module completely ineffective and could easily be the cause of your shunting, as can a host of other things like induction leaks, defective HT leads ect ect ect.
Edited by ChimpOnGas on Monday 3rd August 19:50
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