Adjusting Ignition Timing
06-06-2005, 03:23 PM
#16
Rennlist Member
not entirely true, but mosty true. actually, there is a thing called a venturi, that the throttle becomes at WOT. (the neck down in the throttle ) so, the more air flow through the throttle, the more the vacuum at the venturi. the max vaccum is at WOT. when the throttle starts to open, there is still a huge pressure differential on the other side ofthe pressure plate. if there wasnt, the car would be at full throttle (lowest pressure drop across the throttle ) the word throtttle means"choke". by def its the variable presure drop device. on the intake side of the throttle, you have vaccum all the time at idle.. Just off idle, it goes away. this is used to suck the gas fumes from the canister for emissions. it also pulls back the diaphram in the fuel regulator to reduce pressure at idle and at part or WOT, their is little vaccum and full pressure is restored. then, at full throttle, little vaccum is pressent at that bottom port and the port on the side of the air filter , creates vacuum. this adds to the centrifugal advance.
correct me if im wrong, but thats the way i understand it to work.
Mk
correct me if im wrong, but thats the way i understand it to work.
Mk
Originally Posted by BrianG
Yes, Jim, there is a HUGE valve that controls this vacuum signal. It's actually the throttle body that performs this function. It's called "ported vacuum or timed vacuum" and it refers to the distribution or location of the vacuum source.
The "main" vacuum source is between the engine and the throttle plate (on the throttle body or the carb) and generally is taken right off of the intake manifold, somewhere. The "timed" vacuum source is taken from a port just barely ahead (on the air filter side) of the throttle plate so that it only "sees" vacuum when the throttle plate begins to open.
Hence, no vacuum at closed throttle........... max vacuum at partial throttle......decreasing toward...... nominal atmospheric pressure at WOT
The "main" vacuum source is between the engine and the throttle plate (on the throttle body or the carb) and generally is taken right off of the intake manifold, somewhere. The "timed" vacuum source is taken from a port just barely ahead (on the air filter side) of the throttle plate so that it only "sees" vacuum when the throttle plate begins to open.
Hence, no vacuum at closed throttle........... max vacuum at partial throttle......decreasing toward...... nominal atmospheric pressure at WOT
06-06-2005, 03:47 PM
#17
Rennlist Member
Glen,
I dont (think) i really (KNOW) anything here . I do have a (feeling) though.
if pulling the vaccum hose at idle lowers timing, and reduceds idle speed, maybe your vacuum line is connected to the wrong port! (is this possible)
also, if you noticed, if you do the timing test per the manual, you go 2000rpm with vacuum disconnected, and get 20 degrees or so (US 84 LJET spec, 23degrees for earlier cars) so, this means there is vacuum at part throttle. the line is connected to the wrong port, you could have vaccum at idle, and when pulling it, the rpms would drop. meaning your idle settings are based on this and could be wrong too. (just a thought)
next, I dont know about the retard hose. heard its for emmisions only and with scots car we just left it capped .
I seemed to remember a part in the manual that talks about the vacuum advance adding to the centrifugal advance for a total advance of "X" . on my US, it was 43" degrees.. vaccum disconnected was 20 degrees, and this implied that the vaccum part was 23degrees added. this happend at full throttle too .
MK
I dont (think) i really (KNOW) anything here . I do have a (feeling) though.
if pulling the vaccum hose at idle lowers timing, and reduceds idle speed, maybe your vacuum line is connected to the wrong port! (is this possible)
also, if you noticed, if you do the timing test per the manual, you go 2000rpm with vacuum disconnected, and get 20 degrees or so (US 84 LJET spec, 23degrees for earlier cars) so, this means there is vacuum at part throttle. the line is connected to the wrong port, you could have vaccum at idle, and when pulling it, the rpms would drop. meaning your idle settings are based on this and could be wrong too. (just a thought)
next, I dont know about the retard hose. heard its for emmisions only and with scots car we just left it capped .
I seemed to remember a part in the manual that talks about the vacuum advance adding to the centrifugal advance for a total advance of "X" . on my US, it was 43" degrees.. vaccum disconnected was 20 degrees, and this implied that the vaccum part was 23degrees added. this happend at full throttle too .
MK
Originally Posted by GlenL
Mark,
Can't find a pic or a reference, so I'll tell you what I know and what I think.
(Know) Pulling the advance hose at idle reduces the advance and the engine runs slow and rough.
(Know) Pulling the retard hose at idle doesn't do anything.
(Think) The advance hose, which attaches at the back, gets manifold vacuum from below the throttle plate.
(Think) The retard line gets vacuum from above the throttle plate.
(Think) As the throttle opens, the retard works against the advance to reduce vacuum advance while mechanical advance occurs.
(Think) This would allow the spark advance to follow the opening of the throttle better then just having advance and a stiffer spring. It take out some of the effect on manifold vacuum of engine RPM by using the pressure difference above and below the throttle and not just the manifold pressure alone.
Can't find a pic or a reference, so I'll tell you what I know and what I think.
(Know) Pulling the advance hose at idle reduces the advance and the engine runs slow and rough.
(Know) Pulling the retard hose at idle doesn't do anything.
(Think) The advance hose, which attaches at the back, gets manifold vacuum from below the throttle plate.
(Think) The retard line gets vacuum from above the throttle plate.
(Think) As the throttle opens, the retard works against the advance to reduce vacuum advance while mechanical advance occurs.
(Think) This would allow the spark advance to follow the opening of the throttle better then just having advance and a stiffer spring. It take out some of the effect on manifold vacuum of engine RPM by using the pressure difference above and below the throttle and not just the manifold pressure alone.
06-06-2005, 09:58 PM
#18
Borys,
Check the connections and operation of the auxillary air valve. This allows extra air (controlled vacuum leak) to the engine when cold to increase the idle. Actually small to medium vacuum leaks will increase your idle.
On CIS engines the vacuum line to the advance is ported so you would see little or no vacuum at idle. IIRC the timing should advance less than 4 degrees when the vacuum line is reconnected at idle (800 rpm).
Dennis
Check the connections and operation of the auxillary air valve. This allows extra air (controlled vacuum leak) to the engine when cold to increase the idle. Actually small to medium vacuum leaks will increase your idle.
On CIS engines the vacuum line to the advance is ported so you would see little or no vacuum at idle. IIRC the timing should advance less than 4 degrees when the vacuum line is reconnected at idle (800 rpm).
Dennis
06-06-2005, 11:51 PM
#19
Nordschleife Master
One thing I really want is a repair manual for my Euro car. Everyone note, Borys has one too. Anyone have one?
Opening the manual fiche I have to page 26-2 it clearly shows the vacuum hose for US CIS cars running from the front of the throttle body. Looking at page 28-1 it clearly states that the CIS cars have only a vacuum retard system.
On pages 28-25 and 28-26 it shows the operation of the advance and retard behavior for US units from 80 to 84.
The 80 to 83 Euro units, like Borys and I have, use the combined advance and retard distributors, not the retard-only unit found on US CIS cars.
Borys himself shows the difference in this thread: https://rennlist.com/forums/showthre...vacuum+diagram
THe hand-drawn diagram agrees with how my car is plumbed. The diagram from the manual shows the US CIS I refered to before.
It's clear that Porsche did some different things with their cars that are pre-electronic ignition.
Couple of points:
The vacuum connection on the front of the throttle body is ported. Both Dennis and I report the same observation that it doesn't pull any vauum at idle. It is used for retard on both US and Euro versions. Wouldn't make much sense to have extra retard at idle (US car) now would it?
The line from the back (Euros) is an un-ported signal. Whether this is in the venturi or opens higher-up and gets more of a manifold vacuum signal I don't know.
Looking at my (handy) Tech Spec booklet, the numbers for an '80 Euro are 26 degrees at 3000 rpm with the hoses off. Then 14 to 18 at idle, still with hoses off. Connecting the retard hose does nothing, connecting the advance hose pulls it back to something like 22 (IIRC!)
Whew!
Opening the manual fiche I have to page 26-2 it clearly shows the vacuum hose for US CIS cars running from the front of the throttle body. Looking at page 28-1 it clearly states that the CIS cars have only a vacuum retard system.
On pages 28-25 and 28-26 it shows the operation of the advance and retard behavior for US units from 80 to 84.
The 80 to 83 Euro units, like Borys and I have, use the combined advance and retard distributors, not the retard-only unit found on US CIS cars.
Borys himself shows the difference in this thread: https://rennlist.com/forums/showthre...vacuum+diagram
THe hand-drawn diagram agrees with how my car is plumbed. The diagram from the manual shows the US CIS I refered to before.
It's clear that Porsche did some different things with their cars that are pre-electronic ignition.
Couple of points:
The vacuum connection on the front of the throttle body is ported. Both Dennis and I report the same observation that it doesn't pull any vauum at idle. It is used for retard on both US and Euro versions. Wouldn't make much sense to have extra retard at idle (US car) now would it?
The line from the back (Euros) is an un-ported signal. Whether this is in the venturi or opens higher-up and gets more of a manifold vacuum signal I don't know.
Looking at my (handy) Tech Spec booklet, the numbers for an '80 Euro are 26 degrees at 3000 rpm with the hoses off. Then 14 to 18 at idle, still with hoses off. Connecting the retard hose does nothing, connecting the advance hose pulls it back to something like 22 (IIRC!)
Whew!
06-07-2005, 12:29 AM
#20
Nordschleife Master
Knew I'd seen it. Finally found it. Page 20-6 in my fiche set shows the full CIS hose routing scheme. Apparently didn't take the time to tailor it for the US. Shows two lines from the distributor: retard to the front of the throttle body and advance to the back.
06-07-2005, 03:13 PM
#21
Rennlist Member
Glen, if you look at the diagrams, and then also look at an actual throttle body, you can see a few things. section 24-16 has the diagram.
The front port on the throttle body, is on top of the throttle plate (meaning on the vacuum side of the throttle plate) if you didnt measure any vacuum on this port at idle, something is definitely funny. its used to pull back timing about 4 degrees at idle. (normally 8-10 degrees, and with the retard vacuum connected, more like 4 degrees)
the rear ports are set up as follows.
1. the one on top of the throttle plate works the fuel regs. (ie full vacuum when at idle , which goes away under throttle)
2. the one that feeds from the bottom of the throttle , has no vacuum (or very little ) at idle, and is responsible for advancing the timing on the distributor, 4degrees advance, and this goes to 15degrees or so at high rpms and near full throttle conditions and higher vacuum. It also connects to the char. canister so it pulls fumes under high throttle settings. the centrifugal advance works on rpms and is close to 15 degrees advance as you get near 3000rpm and higher. the centrifugal and vaccuum adavance are added together for total advance.
you said that your car stumbled by pulling the vacuum line to the distributor (advance line) the timing change should only be in the 4 degree range, hardly enough to change much, unless your idle is way low to begin with. however, you are right, there is some change in timing with the vacuum advance at idle according to the manual. Now, you should however see vacuum on the front port that connects to the retard timing line to the distr. if you dont, that is strange.
thats a great hand drawing on the other thread, but it would be nice to see the picture on 24-16 of the manual, plus a picture of the throttle body and those ports.
Mk
The front port on the throttle body, is on top of the throttle plate (meaning on the vacuum side of the throttle plate) if you didnt measure any vacuum on this port at idle, something is definitely funny. its used to pull back timing about 4 degrees at idle. (normally 8-10 degrees, and with the retard vacuum connected, more like 4 degrees)
the rear ports are set up as follows.
1. the one on top of the throttle plate works the fuel regs. (ie full vacuum when at idle , which goes away under throttle)
2. the one that feeds from the bottom of the throttle , has no vacuum (or very little ) at idle, and is responsible for advancing the timing on the distributor, 4degrees advance, and this goes to 15degrees or so at high rpms and near full throttle conditions and higher vacuum. It also connects to the char. canister so it pulls fumes under high throttle settings. the centrifugal advance works on rpms and is close to 15 degrees advance as you get near 3000rpm and higher. the centrifugal and vaccuum adavance are added together for total advance.
you said that your car stumbled by pulling the vacuum line to the distributor (advance line) the timing change should only be in the 4 degree range, hardly enough to change much, unless your idle is way low to begin with. however, you are right, there is some change in timing with the vacuum advance at idle according to the manual. Now, you should however see vacuum on the front port that connects to the retard timing line to the distr. if you dont, that is strange.
thats a great hand drawing on the other thread, but it would be nice to see the picture on 24-16 of the manual, plus a picture of the throttle body and those ports.
Mk
06-07-2005, 03:35 PM
#22
Rennlist Member
comments inserted after looking at manual.>>>>>>>>>>>
Originally Posted by GlenL
Mark,
Can't find a pic or a reference, so I'll tell you what I know and what I think.
(Know) Pulling the advance hose at idle reduces the advance and the engine runs slow and rough.
>>>>>>should pull back timing 4 degrees, shouldnt be noticable.
(Know) Pulling the retard hose at idle doesn't do anything.
>>>>>>>>>>>this should make the car idle faster , as the vaccum retard works on idle vacuum and pulls the timing back 4 degrees or so.
(Think) The advance hose, which attaches at the back, gets manifold vacuum from below the throttle plate.
>>>>>>>>>>>>>>>below the throttle plate, yes, on the AFM side of things, so no (or little) vacuum at idle and more at fuller throttle settings
(Think) The retard line gets vacuum from above the throttle plate.
>>>>>>>>>>>>yes, on top of the throttle plate is the high vacuum side at idle. this is the vacuum fitting on the front of the throttle.
(Think) As the throttle opens, the retard works against the advance to reduce vacuum advance while mechanical advance occurs.
>>>>>>>>>>>>>>>Maybe, the retard only pulls back timing at idle to help with emmisions i guess. however, it may work with mechanical advance so that by the time you are at higher rpms, there is no retard, only vacuum advance
(Think) This would allow the spark advance to follow the opening of the throttle better then just having advance and a stiffer spring. It take out some of the effect on manifold vacuum of engine RPM by using the pressure difference above and below the throttle and not just the manifold pressure alone.
>>>>>>>>>>>>>>Maybe ????? dont know for sure. what i do know is that we didnt use the vacuum line for retard on scots euro intake system,and it ran fine. however with the larger throttle body, and less venturi, the vacuum advance was less vs the very small neckdown on the US throttle body.
I wonder if if the euro advance is with different springs on the distrib. (for the 80-83 euro), and/or done by the LH jetronic ignition system on the euro 84-85.
Mk
Can't find a pic or a reference, so I'll tell you what I know and what I think.
(Know) Pulling the advance hose at idle reduces the advance and the engine runs slow and rough.
>>>>>>should pull back timing 4 degrees, shouldnt be noticable.
(Know) Pulling the retard hose at idle doesn't do anything.
>>>>>>>>>>>this should make the car idle faster , as the vaccum retard works on idle vacuum and pulls the timing back 4 degrees or so.
(Think) The advance hose, which attaches at the back, gets manifold vacuum from below the throttle plate.
>>>>>>>>>>>>>>>below the throttle plate, yes, on the AFM side of things, so no (or little) vacuum at idle and more at fuller throttle settings
(Think) The retard line gets vacuum from above the throttle plate.
>>>>>>>>>>>>yes, on top of the throttle plate is the high vacuum side at idle. this is the vacuum fitting on the front of the throttle.
(Think) As the throttle opens, the retard works against the advance to reduce vacuum advance while mechanical advance occurs.
>>>>>>>>>>>>>>>Maybe, the retard only pulls back timing at idle to help with emmisions i guess. however, it may work with mechanical advance so that by the time you are at higher rpms, there is no retard, only vacuum advance
(Think) This would allow the spark advance to follow the opening of the throttle better then just having advance and a stiffer spring. It take out some of the effect on manifold vacuum of engine RPM by using the pressure difference above and below the throttle and not just the manifold pressure alone.
>>>>>>>>>>>>>>Maybe ????? dont know for sure. what i do know is that we didnt use the vacuum line for retard on scots euro intake system,and it ran fine. however with the larger throttle body, and less venturi, the vacuum advance was less vs the very small neckdown on the US throttle body.
I wonder if if the euro advance is with different springs on the distrib. (for the 80-83 euro), and/or done by the LH jetronic ignition system on the euro 84-85.
Mk
06-07-2005, 04:20 PM
#23
Nordschleife Master
Mark,
Does your manual show how the tubes are routed inside the throttle body? Mine doesn't. Thinking of the carbs I've explored, the passages will run up and down to where they need them. Can't judge the signal just by where the hose connects.
Also, those advance graphs are for the US models. Next time I'm into it, I'll record the numbers for each mode of hose attachment.
Does your manual show how the tubes are routed inside the throttle body? Mine doesn't. Thinking of the carbs I've explored, the passages will run up and down to where they need them. Can't judge the signal just by where the hose connects.
Also, those advance graphs are for the US models. Next time I'm into it, I'll record the numbers for each mode of hose attachment.
06-07-2005, 05:02 PM
#24
Rennlist Member
actually, i traced them down when i was building my first part euro with euro intake. you can tell by the way and position the vacuum attachement ports are cast in the throttle body. high on the TB means its ported to the top of the throttle body (ie like the front one leading to vacuum retard, and the rear one for fuel regs pressure pull back) the only bottom of the throttle plate is the vacuum advance and its a small port on the bottom of the plate and this is the air filter side of the plate. (little vacuum at idle, max vacuum under throttle conditions)
the only real diff, is where the ports are located near or far from the plate itself, and the size of the port. maybe from the old throttle pics here on the list, we can see the holes. the manuals just dont show much, other than the casting shapes and position of where the lines attach
MK
the only real diff, is where the ports are located near or far from the plate itself, and the size of the port. maybe from the old throttle pics here on the list, we can see the holes. the manuals just dont show much, other than the casting shapes and position of where the lines attach
MK
Originally Posted by GlenL
Mark,
Does your manual show how the tubes are routed inside the throttle body? Mine doesn't. Thinking of the carbs I've explored, the passages will run up and down to where they need them. Can't judge the signal just by where the hose connects.
Also, those advance graphs are for the US models. Next time I'm into it, I'll record the numbers for each mode of hose attachment.
Does your manual show how the tubes are routed inside the throttle body? Mine doesn't. Thinking of the carbs I've explored, the passages will run up and down to where they need them. Can't judge the signal just by where the hose connects.
Also, those advance graphs are for the US models. Next time I'm into it, I'll record the numbers for each mode of hose attachment.
06-07-2005, 06:12 PM
#25
Rennlist Member
the fact that you saw no vacuum on the retard vaccum line, is puzzling. anything attached to front TB vacuum port will get vacuum. again, its used to pull back timing at idle, nothing more. also, if you look at the other ports that feed from on top of the throttle, you will see that once you come off idle, any measureable vacuum just goes away, even though there is still vacuum in the plennum. If you think about it, its nesessary as the fuel regs, that operate on reduced pressure due to vaccum on this line, have full pressure available just off idle.
the manual does have the mitivac vacuum numbers for different speeds, etc.
Be interesting to see if you have the lines connected correctly based on this information. bottom line, you should see about 40 degrees advance when you have it all hooked up and at 3000rpm.
mk
the manual does have the mitivac vacuum numbers for different speeds, etc.
Be interesting to see if you have the lines connected correctly based on this information. bottom line, you should see about 40 degrees advance when you have it all hooked up and at 3000rpm.
mk
Originally Posted by GlenL
Mark,
Does your manual show how the tubes are routed inside the throttle body? Mine doesn't. Thinking of the carbs I've explored, the passages will run up and down to where they need them. Can't judge the signal just by where the hose connects.
Also, those advance graphs are for the US models. Next time I'm into it, I'll record the numbers for each mode of hose attachment.
Does your manual show how the tubes are routed inside the throttle body? Mine doesn't. Thinking of the carbs I've explored, the passages will run up and down to where they need them. Can't judge the signal just by where the hose connects.
Also, those advance graphs are for the US models. Next time I'm into it, I'll record the numbers for each mode of hose attachment.
