Removing pistons without pulling engine
11-08-2021, 08:19 AM
#46
Rennlist Member
I suspect there is some "overthinking" going on here.
The cams are designed to get the optimal amount of charge [air] into the cylinders so that an optimal amount of fuel can be added to get the best bang for the buck. Similarly the exhaust process has to get the gases out and again this process has to be optimised and invariably the exhaust cam has a different profile to the inlet cam- for a reason.
When cold cranking the motor what is missing? The answer of course is fuel and ignition. What happens when the fuel is ignited? The pressure goes through the roof and drives the motor. At some point close to BDC the exhaust valve opens and the gases get ejected but normally do so under pressure. During cold cranking the inlet process is not disturbed but the exhaust process most certainly is because the over pressure is not present- basically you do not get as much gas exhausting as you do during induction and thus on subsequent strokes the cranking pressure goes up and typically maximises after three cranks- thereon there is enough pressure to drive out what is coming in and the peak cold cranking pressure is attained that becomes the a measure of the relative health of the compression process.
Cam timing and LSA will impact the peak pressure attained. The GT cams with LSA of 106 degrees attain about the same pressure as the S4 cams even though they lift higher and probably induce more air but with a tigher LSA there is more overlap. The S3 cams with identical profile to the GT cams typically produce around 220 psig [or so it seems] but then with LSA of 114 degrees there is less overlap thus higher cranking pressure.
Compressing the gas most certainly takes work and for sure that will develop a little heat but it will be of no practical significance to the cranking pressure generated and measured. Thus the first compression stroke should generate pressure consistent with the static compression ratio and the following two strokes or whatever will be additive until the peak cranking pressure is reached at 185 psig or whatever.
Part "rocket science" but not overly complicated or doctoral thesis material.
The cams are designed to get the optimal amount of charge [air] into the cylinders so that an optimal amount of fuel can be added to get the best bang for the buck. Similarly the exhaust process has to get the gases out and again this process has to be optimised and invariably the exhaust cam has a different profile to the inlet cam- for a reason.
When cold cranking the motor what is missing? The answer of course is fuel and ignition. What happens when the fuel is ignited? The pressure goes through the roof and drives the motor. At some point close to BDC the exhaust valve opens and the gases get ejected but normally do so under pressure. During cold cranking the inlet process is not disturbed but the exhaust process most certainly is because the over pressure is not present- basically you do not get as much gas exhausting as you do during induction and thus on subsequent strokes the cranking pressure goes up and typically maximises after three cranks- thereon there is enough pressure to drive out what is coming in and the peak cold cranking pressure is attained that becomes the a measure of the relative health of the compression process.
Cam timing and LSA will impact the peak pressure attained. The GT cams with LSA of 106 degrees attain about the same pressure as the S4 cams even though they lift higher and probably induce more air but with a tigher LSA there is more overlap. The S3 cams with identical profile to the GT cams typically produce around 220 psig [or so it seems] but then with LSA of 114 degrees there is less overlap thus higher cranking pressure.
Compressing the gas most certainly takes work and for sure that will develop a little heat but it will be of no practical significance to the cranking pressure generated and measured. Thus the first compression stroke should generate pressure consistent with the static compression ratio and the following two strokes or whatever will be additive until the peak cranking pressure is reached at 185 psig or whatever.
Part "rocket science" but not overly complicated or doctoral thesis material.
11-08-2021, 02:15 PM
#47
Three Wheelin'
i doubt "boost" charging at 400 rpm ... quick compression almost double the temperature in a 10:1 cilinder. Even minus the heat losses to cilinder walls i assume at cranking even a bad air fill , far from 100% .
11-08-2021, 02:36 PM
#48
Rennlist Member
I suspect there is some "overthinking" going on here.
The cams are designed to get the optimal amount of charge [air] into the cylinders so that an optimal amount of fuel can be added to get the best bang for the buck. Similarly the exhaust process has to get the gases out and again this process has to be optimised and invariably the exhaust cam has a different profile to the inlet cam- for a reason.
When cold cranking the motor what is missing? The answer of course is fuel and ignition. What happens when the fuel is ignited? The pressure goes through the roof and drives the motor. At some point close to BDC the exhaust valve opens and the gases get ejected but normally do so under pressure. During cold cranking the inlet process is not disturbed but the exhaust process most certainly is because the over pressure is not present- basically you do not get as much gas exhausting as you do during induction and thus on subsequent strokes the cranking pressure goes up and typically maximises after three cranks- thereon there is enough pressure to drive out what is coming in and the peak cold cranking pressure is attained that becomes the a measure of the relative health of the compression process.
Cam timing and LSA will impact the peak pressure attained. The GT cams with LSA of 106 degrees attain about the same pressure as the S4 cams even though they lift higher and probably induce more air but with a tigher LSA there is more overlap. The S3 cams with identical profile to the GT cams typically produce around 220 psig [or so it seems] but then with LSA of 114 degrees there is less overlap thus higher cranking pressure.
Compressing the gas most certainly takes work and for sure that will develop a little heat but it will be of no practical significance to the cranking pressure generated and measured. Thus the first compression stroke should generate pressure consistent with the static compression ratio and the following two strokes or whatever will be additive until the peak cranking pressure is reached at 185 psig or whatever.
Part "rocket science" but not overly complicated or doctoral thesis material.
The cams are designed to get the optimal amount of charge [air] into the cylinders so that an optimal amount of fuel can be added to get the best bang for the buck. Similarly the exhaust process has to get the gases out and again this process has to be optimised and invariably the exhaust cam has a different profile to the inlet cam- for a reason.
When cold cranking the motor what is missing? The answer of course is fuel and ignition. What happens when the fuel is ignited? The pressure goes through the roof and drives the motor. At some point close to BDC the exhaust valve opens and the gases get ejected but normally do so under pressure. During cold cranking the inlet process is not disturbed but the exhaust process most certainly is because the over pressure is not present- basically you do not get as much gas exhausting as you do during induction and thus on subsequent strokes the cranking pressure goes up and typically maximises after three cranks- thereon there is enough pressure to drive out what is coming in and the peak cold cranking pressure is attained that becomes the a measure of the relative health of the compression process.
Cam timing and LSA will impact the peak pressure attained. The GT cams with LSA of 106 degrees attain about the same pressure as the S4 cams even though they lift higher and probably induce more air but with a tigher LSA there is more overlap. The S3 cams with identical profile to the GT cams typically produce around 220 psig [or so it seems] but then with LSA of 114 degrees there is less overlap thus higher cranking pressure.
Compressing the gas most certainly takes work and for sure that will develop a little heat but it will be of no practical significance to the cranking pressure generated and measured. Thus the first compression stroke should generate pressure consistent with the static compression ratio and the following two strokes or whatever will be additive until the peak cranking pressure is reached at 185 psig or whatever.
Part "rocket science" but not overly complicated or doctoral thesis material.
"Compressing the gas most certainly takes work and for sure that will develop a little heat but it will be of no practical significance to the cranking pressure generated and measured"
You should look up the data on gas compression and heat.
10 Bar makes a -lot- of heat. Speed matters here too, but heat is..even at 4mph piston speed, generated. Like I noted, this is how diesel engines work. Not all have starting plugs, and if they do, are not always in use. It's cranking air compression at work.
You're overthinking the valve action at a slow cranking speed..its very simply a _dead simple_ atmospheric fill of the cylinder, a very high Ve value. What the valves are doing works at "speed"..not su much at crank. There are no shock waves, pules, scavenging, or pressure waves to take advantage of at cranking.
11-08-2021, 02:45 PM
#49
Instructor
I don't "suspect" over thinking ....................
I am totally sure of it !
I would not get too hung up on the "actual " figure ..... what I would be looking for is a pattern (consistancy) ... if they are all "poor" , but within (say !!) 5% of each other ............. then look elsewhere for problems , if one cyl or 2, ? is down say 15% ... then start investigating. It's important to be "consistant" in the process : pref : up to temp engine , fully open throttle - all plugs out , and then x secs churning per cyl , etc etc. Note results and keep them for y years down the road , then you can repeat and look for changes. You or I , can't simply calculate what the figure should be based on compression ratio !!
I am totally sure of it !
I would not get too hung up on the "actual " figure ..... what I would be looking for is a pattern (consistancy) ... if they are all "poor" , but within (say !!) 5% of each other ............. then look elsewhere for problems , if one cyl or 2, ? is down say 15% ... then start investigating. It's important to be "consistant" in the process : pref : up to temp engine , fully open throttle - all plugs out , and then x secs churning per cyl , etc etc. Note results and keep them for y years down the road , then you can repeat and look for changes. You or I , can't simply calculate what the figure should be based on compression ratio !!
11-08-2021, 04:21 PM
#50
Rennlist Member
Ahh, Yes - I was mistakenly thinking of operating speeds. And while 400rpm cranking speed is still pretty fast to move air around, it probably isn't fast enough to get the Ve increase that valve overlaps and such get at actual running rpm. My bad.
11-08-2021, 05:58 PM
#51
Rennlist Member
I don't "suspect" over thinking ....................
I am totally sure of it !
I would not get too hung up on the "actual " figure ..... what I would be looking for is a pattern (consistancy) ... if they are all "poor" , but within (say !!) 5% of each other ............. then look elsewhere for problems , if one cyl or 2, ? is down say 15% ... then start investigating. It's important to be "consistant" in the process : pref : up to temp engine , fully open throttle - all plugs out , and then x secs churning per cyl , etc etc. Note results and keep them for y years down the road , then you can repeat and look for changes. You or I , can't simply calculate what the figure should be based on compression ratio !!
I am totally sure of it !
I would not get too hung up on the "actual " figure ..... what I would be looking for is a pattern (consistancy) ... if they are all "poor" , but within (say !!) 5% of each other ............. then look elsewhere for problems , if one cyl or 2, ? is down say 15% ... then start investigating. It's important to be "consistant" in the process : pref : up to temp engine , fully open throttle - all plugs out , and then x secs churning per cyl , etc etc. Note results and keep them for y years down the road , then you can repeat and look for changes. You or I , can't simply calculate what the figure should be based on compression ratio !!
Yes, a pattern, even an consistent one, but if everything is 30% down from expectations..thats a bad even pattern.
11-08-2021, 06:01 PM
#52
Rennlist Member
I don't "suspect" over thinking ....................
I am totally sure of it !
I would not get too hung up on the "actual " figure ..... what I would be looking for is a pattern (consistancy) ... if they are all "poor" , but within (say !!) 5% of each other ............. then look elsewhere for problems , if one cyl or 2, ? is down say 15% ... then start investigating. It's important to be "consistant" in the process : pref : up to temp engine , fully open throttle - all plugs out , and then x secs churning per cyl , etc etc. Note results and keep them for y years down the road , then you can repeat and look for changes. You or I , can't simply calculate what the figure should be based on compression ratio !!
I am totally sure of it !
I would not get too hung up on the "actual " figure ..... what I would be looking for is a pattern (consistancy) ... if they are all "poor" , but within (say !!) 5% of each other ............. then look elsewhere for problems , if one cyl or 2, ? is down say 15% ... then start investigating. It's important to be "consistant" in the process : pref : up to temp engine , fully open throttle - all plugs out , and then x secs churning per cyl , etc etc. Note results and keep them for y years down the road , then you can repeat and look for changes. You or I , can't simply calculate what the figure should be based on compression ratio !!
If you do a little more path on volume, piston speed, and added heat and the pressure derived from that....you could get there.
But Porsche already did that, on a fresh strong motor, 180psi..just measuring a fresh tight motor.
Thats why it wont be: CR x 14.7 = result. It is (CR x 14.7) + other_physics = result.
11-08-2021, 06:41 PM
#53
That is correct.
But the "correct difference" would be a night and day difference.
You need to pull the upper two plastic timing covers, turn the engine to the line between O and T and see if the cam timing marks now line up. (This only occurs every two revolutions, so you may have to turn the engine two times to find the cam timing marks.)
But the "correct difference" would be a night and day difference.
You need to pull the upper two plastic timing covers, turn the engine to the line between O and T and see if the cam timing marks now line up. (This only occurs every two revolutions, so you may have to turn the engine two times to find the cam timing marks.)
Yes I pulled the covers and checked the cam timing marks. All 3 line up as they should.
Mechanical advance timing is in spec also at 26degBTDC at 3000RPM.
So it seems that it was moved by 1 or 2 teeth with no significant change in behavior.
11-08-2021, 06:51 PM
#54
Instructor
Actually, we just did.
If you do a little more path on volume, piston speed, and added heat and the pressure derived from that....you could get there.
But Porsche already did that, on a fresh strong motor, 180psi..just measuring a fresh tight motor.
Thats why it wont be: CR x 14.7 = result. It is (CR x 14.7) + other_physics = result.
If you do a little more path on volume, piston speed, and added heat and the pressure derived from that....you could get there.
But Porsche already did that, on a fresh strong motor, 180psi..just measuring a fresh tight motor.
Thats why it wont be: CR x 14.7 = result. It is (CR x 14.7) + other_physics = result.
It's a long time since I completed my Mech Eng degree ..... (many years since we had an engine on the test bed (dyno) with variable compression ratio for example ... and we busy students were plotting horsepower against CR ............ anyways , that is irrelevant here !) But my competition engine building experience tells me the psi reading will change ..................... (I have no idea what " a little more path....." means)
11-08-2021, 06:54 PM
#55
Instructor
This thread seems to have turned into a thermodynamics debate!
Yes I pulled the covers and checked the cam timing marks. All 3 line up as they should.
Mechanical advance timing is in spec also at 26degBTDC at 3000RPM.
So it seems that it was moved by 1 or 2 teeth with no significant change in behavior.
Yes I pulled the covers and checked the cam timing marks. All 3 line up as they should.
Mechanical advance timing is in spec also at 26degBTDC at 3000RPM.
So it seems that it was moved by 1 or 2 teeth with no significant change in behavior.
11-08-2021, 06:58 PM
#56
That sure is a heavy loss time wise. Did you get chance to check the cams are indexed correctly?
I very much doubt that kind of deficit is recoverable but...?
You also need to eliminate any other potential problems with the engine management system. Hopefully you will get some more "to check" suggestions.
I very much doubt that kind of deficit is recoverable but...?
You also need to eliminate any other potential problems with the engine management system. Hopefully you will get some more "to check" suggestions.
I don't think there is any engine management system as such in this car. Hopefully somebody will correct me if I am wrong.
Cam indexing is not something I have done before - so here are some more questions!
My understanding is that if the indexing is wrong, then piston 1 is not at TDC when the harmonic balancer mark suggests that it is. Is that correct?
If this was an issue, would it be on both banks of cylinders? Or it could be only on 1 bank?
Can it be verified with a clock gauge measuring the movement of piston 1 through the plug hole or injector hole as it approaches and moves away from TDC? Or is it a more invasive operation?
If the indexing is wrong, can it be overcome, even as a test, by advancing the ignition timing more than specification?
Thanks,
Robert.
Last edited by Wavey; 11-08-2021 at 07:02 PM.
11-09-2021, 05:39 AM
#57
Rennlist Member
Fred, I don't have an other engine with known compression to test my rig unfortunately, but the fact that it can register up to 170psi on a wet test gives me some confidence that the test rig is reasonably accurate.
I don't think there is any engine management system as such in this car. Hopefully somebody will correct me if I am wrong.
Cam indexing is not something I have done before - so here are some more questions!
My understanding is that if the indexing is wrong, then piston 1 is not at TDC when the harmonic balancer mark suggests that it is. Is that correct?
If this was an issue, would it be on both banks of cylinders? Or it could be only on 1 bank?
Can it be verified with a clock gauge measuring the movement of piston 1 through the plug hole or injector hole as it approaches and moves away from TDC? Or is it a more invasive operation?
If the indexing is wrong, can it be overcome, even as a test, by advancing the ignition timing more than specification?
Thanks,
Robert.
I don't think there is any engine management system as such in this car. Hopefully somebody will correct me if I am wrong.
Cam indexing is not something I have done before - so here are some more questions!
My understanding is that if the indexing is wrong, then piston 1 is not at TDC when the harmonic balancer mark suggests that it is. Is that correct?
If this was an issue, would it be on both banks of cylinders? Or it could be only on 1 bank?
Can it be verified with a clock gauge measuring the movement of piston 1 through the plug hole or injector hole as it approaches and moves away from TDC? Or is it a more invasive operation?
If the indexing is wrong, can it be overcome, even as a test, by advancing the ignition timing more than specification?
Thanks,
Robert.
I use the term "engine management system" to mean whatever devices are used to measure air flow, set the fuel injection pulse width and determine the ignition advance- If the system used in your MY is other than the LH/EZ combo I guarantee I will know squat about it but you need to ensure that whatever system is in play that it is working correctly- unfortunately I cannot help you with that in full detail but it sounds as though you may need some help in that regard.
The point i have been labouring about compression testing is that you need to know what pressure you are looking for and you need to ensure you have used the correct procedure. The throttle plate has to be wide open, the test rig adapter has to seal correctly in the spark plug threads, the cam timing has to be set correctly and the electrical system has to be healthy with the motor cranking at the correct speed [many of these cars have degraded electrical systems if they are still "running" on kit that was supplied originally]. You would likely be astonished if/when you hear the difference between a motor that is cranking correctly and one that is not- it can often be a "night and day" type of thing!
Your compression pressures as reported are suspiciously low and if correct will degrade performance some but they are not catastrophic and to my way of thinking do not explain a 0 to 60 of 12 seconds or anything like it! Before you rip anything apart you need to systematically eliminate any "false" contributors to the degradation that may be present at the moment whatever they may be. Your test rig may well be OK but unless it has been calibrated against a known pressure you cannot take the numbers it produces as "gospel". The numbers you report are "reasonably even" for starters. In my experience rings that are knackered or fouled do not generate numbers that high and as GB advised you earlier, the rings in these motors are very robust.
Wet testing will increase the compression pressure readings and as I understand anything up to about 10% is considered "normal". Your numbers [circa 20%?] suggest a ring seal issue but please do not take my word for it- do a bit more research. How much oil did you add? A teaspoon per cylinder should be sufficient.
Dirty Bosch fuel injectors alone can easily cause a 20% power deficit per cylinder- a 5% loss can almost be guaranteed minimum if they have not been cleaned .
The cam sprockets have 48 teeth, the crank sprocket has 24 teeth- if the cam sprocket is one tooth out the timing on that bank changes by 15 crank degrees, if the belt skips a tooth on the crank sprocket the timing [on both banks] is out by 30 crank degrees, two teeth by 60 crank degrees etc. If this had happened then your cranking pressures would be down however the wet test results tend to suggest this is not the case- however you need to eliminate such from your line of enquiry.
The indexing marks on the crank damper will always be correct unless some dork put the damper on back to front [it happens!]- as you look at the timing marks on the damper from the front of the engine the numbers should be in the correct orientation when you look at them- if they are "upside down" the damper is on back to front and then you read what in effect is jibberish.
Get hold of an inductive timing light and check to ensure the spark at idle happens at around the 6 degrees before TDC mark- if not determine why.
When your engine is set such that No1 cylinder is at TDC on its compression stroke the indexing marks on the cam sprocket must align with the timing notch on the back cover. If you need to advance the engine by 30 crank degrees to get them to align then the timing belt has jumped a tooth on the crank- unlikely but possible. This can happen if some dork removed the odd looking idler underneath and to the left of the crank sprocket as looked at from the front of the motor. Normally this thing is not in contact with the belt and some folks therefore think it is not needed- but it is if the motor backfires! Unfortunately to see this pulley the timing cover has to be removed- not a huge job but takes time as accessory belts have to be removed first.
A leak down test is relatively easy and cheap to perform- you need a test rig and an air compressor- this will help you better understand where you are losing compression pressure if such is the case and it may be in more than one area- this test will tell you whether you are losing pressure from the rings, the inlet valves, the exhaust valves and [possibly] from a leaking head gasket- or a combination thereof!
Trust the above points are helpful- there are plenty of folks on the list who better understand your model year and its nuances who hopefully will chime in as needs be- these motors are relatively bullet proof all things considered but time and usage eventually takes its toll with everything mechanical!
11-09-2021, 05:01 PM
#59
Robert,
I use the term "engine management system" to mean whatever devices are used to measure air flow, set the fuel injection pulse width and determine the ignition advance- If the system used in your MY is other than the LH/EZ combo I guarantee I will know squat about it but you need to ensure that whatever system is in play that it is working correctly- unfortunately I cannot help you with that in full detail but it sounds as though you may need some help in that regard.
The point i have been labouring about compression testing is that you need to know what pressure you are looking for and you need to ensure you have used the correct procedure. The throttle plate has to be wide open, the test rig adapter has to seal correctly in the spark plug threads, the cam timing has to be set correctly and the electrical system has to be healthy with the motor cranking at the correct speed [many of these cars have degraded electrical systems if they are still "running" on kit that was supplied originally]. You would likely be astonished if/when you hear the difference between a motor that is cranking correctly and one that is not- it can often be a "night and day" type of thing!
Your compression pressures as reported are suspiciously low and if correct will degrade performance some but they are not catastrophic and to my way of thinking do not explain a 0 to 60 of 12 seconds or anything like it! Before you rip anything apart you need to systematically eliminate any "false" contributors to the degradation that may be present at the moment whatever they may be. Your test rig may well be OK but unless it has been calibrated against a known pressure you cannot take the numbers it produces as "gospel". The numbers you report are "reasonably even" for starters. In my experience rings that are knackered or fouled do not generate numbers that high and as GB advised you earlier, the rings in these motors are very robust.
Wet testing will increase the compression pressure readings and as I understand anything up to about 10% is considered "normal". Your numbers [circa 20%?] suggest a ring seal issue but please do not take my word for it- do a bit more research. How much oil did you add? A teaspoon per cylinder should be sufficient.
Dirty Bosch fuel injectors alone can easily cause a 20% power deficit per cylinder- a 5% loss can almost be guaranteed minimum if they have not been cleaned .
The cam sprockets have 48 teeth, the crank sprocket has 24 teeth- if the cam sprocket is one tooth out the timing on that bank changes by 15 crank degrees, if the belt skips a tooth on the crank sprocket the timing [on both banks] is out by 30 crank degrees, two teeth by 60 crank degrees etc. If this had happened then your cranking pressures would be down however the wet test results tend to suggest this is not the case- however you need to eliminate such from your line of enquiry.
The indexing marks on the crank damper will always be correct unless some dork put the damper on back to front [it happens!]- as you look at the timing marks on the damper from the front of the engine the numbers should be in the correct orientation when you look at them- if they are "upside down" the damper is on back to front and then you read what in effect is jibberish.
Get hold of an inductive timing light and check to ensure the spark at idle happens at around the 6 degrees before TDC mark- if not determine why.
When your engine is set such that No1 cylinder is at TDC on its compression stroke the indexing marks on the cam sprocket must align with the timing notch on the back cover. If you need to advance the engine by 30 crank degrees to get them to align then the timing belt has jumped a tooth on the crank- unlikely but possible. This can happen if some dork removed the odd looking idler underneath and to the left of the crank sprocket as looked at from the front of the motor. Normally this thing is not in contact with the belt and some folks therefore think it is not needed- but it is if the motor backfires! Unfortunately to see this pulley the timing cover has to be removed- not a huge job but takes time as accessory belts have to be removed first.
A leak down test is relatively easy and cheap to perform- you need a test rig and an air compressor- this will help you better understand where you are losing compression pressure if such is the case and it may be in more than one area- this test will tell you whether you are losing pressure from the rings, the inlet valves, the exhaust valves and [possibly] from a leaking head gasket- or a combination thereof!
Trust the above points are helpful- there are plenty of folks on the list who better understand your model year and its nuances who hopefully will chime in as needs be- these motors are relatively bullet proof all things considered but time and usage eventually takes its toll with everything mechanical!
I use the term "engine management system" to mean whatever devices are used to measure air flow, set the fuel injection pulse width and determine the ignition advance- If the system used in your MY is other than the LH/EZ combo I guarantee I will know squat about it but you need to ensure that whatever system is in play that it is working correctly- unfortunately I cannot help you with that in full detail but it sounds as though you may need some help in that regard.
The point i have been labouring about compression testing is that you need to know what pressure you are looking for and you need to ensure you have used the correct procedure. The throttle plate has to be wide open, the test rig adapter has to seal correctly in the spark plug threads, the cam timing has to be set correctly and the electrical system has to be healthy with the motor cranking at the correct speed [many of these cars have degraded electrical systems if they are still "running" on kit that was supplied originally]. You would likely be astonished if/when you hear the difference between a motor that is cranking correctly and one that is not- it can often be a "night and day" type of thing!
Your compression pressures as reported are suspiciously low and if correct will degrade performance some but they are not catastrophic and to my way of thinking do not explain a 0 to 60 of 12 seconds or anything like it! Before you rip anything apart you need to systematically eliminate any "false" contributors to the degradation that may be present at the moment whatever they may be. Your test rig may well be OK but unless it has been calibrated against a known pressure you cannot take the numbers it produces as "gospel". The numbers you report are "reasonably even" for starters. In my experience rings that are knackered or fouled do not generate numbers that high and as GB advised you earlier, the rings in these motors are very robust.
Wet testing will increase the compression pressure readings and as I understand anything up to about 10% is considered "normal". Your numbers [circa 20%?] suggest a ring seal issue but please do not take my word for it- do a bit more research. How much oil did you add? A teaspoon per cylinder should be sufficient.
Dirty Bosch fuel injectors alone can easily cause a 20% power deficit per cylinder- a 5% loss can almost be guaranteed minimum if they have not been cleaned .
The cam sprockets have 48 teeth, the crank sprocket has 24 teeth- if the cam sprocket is one tooth out the timing on that bank changes by 15 crank degrees, if the belt skips a tooth on the crank sprocket the timing [on both banks] is out by 30 crank degrees, two teeth by 60 crank degrees etc. If this had happened then your cranking pressures would be down however the wet test results tend to suggest this is not the case- however you need to eliminate such from your line of enquiry.
The indexing marks on the crank damper will always be correct unless some dork put the damper on back to front [it happens!]- as you look at the timing marks on the damper from the front of the engine the numbers should be in the correct orientation when you look at them- if they are "upside down" the damper is on back to front and then you read what in effect is jibberish.
Get hold of an inductive timing light and check to ensure the spark at idle happens at around the 6 degrees before TDC mark- if not determine why.
When your engine is set such that No1 cylinder is at TDC on its compression stroke the indexing marks on the cam sprocket must align with the timing notch on the back cover. If you need to advance the engine by 30 crank degrees to get them to align then the timing belt has jumped a tooth on the crank- unlikely but possible. This can happen if some dork removed the odd looking idler underneath and to the left of the crank sprocket as looked at from the front of the motor. Normally this thing is not in contact with the belt and some folks therefore think it is not needed- but it is if the motor backfires! Unfortunately to see this pulley the timing cover has to be removed- not a huge job but takes time as accessory belts have to be removed first.
A leak down test is relatively easy and cheap to perform- you need a test rig and an air compressor- this will help you better understand where you are losing compression pressure if such is the case and it may be in more than one area- this test will tell you whether you are losing pressure from the rings, the inlet valves, the exhaust valves and [possibly] from a leaking head gasket- or a combination thereof!
Trust the above points are helpful- there are plenty of folks on the list who better understand your model year and its nuances who hopefully will chime in as needs be- these motors are relatively bullet proof all things considered but time and usage eventually takes its toll with everything mechanical!
Engine management for this car would be the CIS and timing I guess.
CIS
I have rebuilt the WUR and have all fuel pressures set to the lower end of the acceptable range to provide a richer mixture. (I have installed an A/F meter and can see the car leaning out as it heats up so I am keeping it a little rich to counter that. Warm control pressure does not rise but A/F ratio does.)
Injectors are all new (The shorter MB units which I read work just fine.)
New coil and plugs, HT leads checked for resistance.
New FD regulator seals and check valve for pump.
Have not touched the FD itself and that is on the maybe to-do list.
Timing
All 3 timing marks line up perfectly now with piston 1 at TDC (TDC just found with a screwdriver). As I mentioned before this was previously out by 1 or 2 teeth but then corrected by a mechanic with no change in performance which everybody says is not possible. (Perhaps that is a due to an underlying issue that I have not found yet). If these marks were mistakenly lined up with piston 1 at TDC of its exhaust stroke-would the engine run at all?
With a timing light the advance at 3000RPM was found to be 26BTDC which is in spec. I guess the idle timing will just be what it needs to be to achieve the 3000RPM advance. Am I correct here?
The cam sprocket piece you mention is new to me and I need to learn more about that before attempting anything. I don't even know where to start.
I think I need to verify somehow that the vacuum part of the advance is doing what it should. WSM is very unhelpful on this bit.
Compression
An update here - Another forum member has the same car as me and has kindly confirmed the same compression readings as me (140ish) and his car is still delivering the as-sold acceleration!
Good news in a way, and frustrating news in another way!
On your advice I have just done an engine oil flush, and then another for good measure.
On the plus side my hard warm start as gone away with all the work put into the CIS!
Very eager to here more from you guys - I know the missing are horses are in there somewhere!
Robert.
11-09-2021, 06:08 PM
#60
Former Sponsor
You've obviously checked for full throttle?
There's a chronic CIS issue, which radically reduces performance. I'm not near a computer to type or to post a picture.
There's a chronic CIS issue, which radically reduces performance. I'm not near a computer to type or to post a picture.