High Mileage 996 Engines
Discussion
Actually,
Saab made a variable compression engine about 6 years ago, though I do not know if it made it into production.
Essentially it relied on the speed of the crank to determine in internal device..... that lifted the head by about 2-3 degrees.think about it pivoting on one side.belows included...but no drawings do I have....
i remember reading about this in my local Audi dealer at the time,[mag escapes me] and thinking wow, how simple, yet revolutionary....anyways it has been done and Saab where there,
Kevin
Saab made a variable compression engine about 6 years ago, though I do not know if it made it into production.
Essentially it relied on the speed of the crank to determine in internal device..... that lifted the head by about 2-3 degrees.think about it pivoting on one side.belows included...but no drawings do I have....
i remember reading about this in my local Audi dealer at the time,[mag escapes me] and thinking wow, how simple, yet revolutionary....anyways it has been done and Saab where there,
Kevin
Would you guys allow me to indulge in a little mild bragging about my past and in the process go off subject a bit and tell a few tales of interest to those who enjoy engine tuning subjects and information?
My variable compression work was done in 1976 (just a few years before Saab) but I never managed to complete it due to lack of support and electronic ability to make the control system. Howver once I knew (and had proved in a fixed state) it worked - the incentive to prove it was less than the satisfaction of having found out something that at the time I had never known anyone else discover. That is not to say something similar hadn't been done before - in fact I saw a photo of a 2 stroke engine made in the 30's (I think) with a variable cylinder height (using a rotating cylinder rising and falling up and down on a thread) - but the pictures I saw never linked the reason for building it to compression ratio - but instead to port timing. I fully expect that the engineer knew all of that then and there is often nothing new in this World and usually someone else - you find - already did most things before.
I did however make a variable exhaust port for the armstrong two stroke engine using a flap valve - that worked. It basically restricted the exhaust flow at low revs by holding back the flow - this effectively delayed the timing at lower revs and improved entrapment so raising compression ratio (and I still have such a cylinder still at Hartech).
Apparently 2 years after leaving Armstrong Motorcycles - they were Involved with Honda to use a new 125cc two stroke engine with an apparently revolutionary exhaust flap valve they proudly showed off the the directors - one of whom slipped out to the then unused dyno room and from inside a dusty cupboard, pulled out my identical system tested several years before and to the utter amazement of the Japanese - blew the dust off it and plonked it on the board room table.
It was only when I was watching a dyno run in which lots of air and fuel were visibly going into the engine while it was producing very little power and that there was very little difference to the amount going in when it suddenly took off - that made me suspect that it wasn't the dynamic flow of inlet, exhaust and resonant tuning that determined the power band (as frankly everyone in the World had previously alluded to) but that it was actually possible that there is a true compression ratio (i.e. an actual compression pressure) below which little power results and above which the burning changes into a much more powerful burn. This puts a new theory in place to suggest that the formula for performance is not just that power increases with compression pressure but that there is a minimum pressure under which the burn is under impressive and a slightly higher pressure after which there is a huge jump in performance (something I never previously knew). It proved that actually the compression pressure was far more important than the inlet, exhaust and ignition timing - the inlet or exhaust tuning or the induction length - and they were all the things everyone had spent all their time developing and working on (and it may be that such things are known now and taught but not when I was in the education system).
Fitting increasingly higher compression cylinder heads resulted in this engine (that was supposed to be so highly tuned it would not pull the skin off a rice pudding - as they say) at low revs into a very powerful engine at those same low revs and with the same "high rev" tuning. Eventually the very high compression head seized the engine at high revs but not until a graph had been produced to prove the benfits of a variable compression engine (reducing as the revs increase). It showed a huge power band despite being basically highly tuned and previously with a very narrow power band.
This in turn meant that an even higher state of engine tune could be used but still produce a big enough power band to start and accelerate through the gearbox.
It would undoubtedly have been a "World beater" but then it would have either been copied or banned by the governing body and I probably would not have got the credit anyway. I say this because around that time I received from the motorcycle racing governing body (the FIM) a letter stating that in future the number of cylinders would be determined by the number of combustion chambers! This was extremely odd to say the least!
500cc GP events were limited to 4 cylinders at the time and two strokes were dominant. I don't know which manufacturer was behind the change but Honda wanted to compete with a 4 stroke but needed 8 cylinders to do so which would be illegal. Realising what they were up to I wrote to the FIM and asked if 2 pistons in 2 bores (or 8 pistons in 8 bores with 4 combustion chambers - in pairs) had a common combustion chamber (or 4 common combustion chambers) - would it be a regarded as a single cylinder (or a 4 cylinder) and they replied "yes" - plot exposed. I then asked - if two combustion chanbers were linked together would this be one cylinder - they said "yes". Finally I said - how big would the "link between the cylinders" have to be - would a 0.25mm hole drilled between 2 otherwise seperate combustion chambers turn it into one chamber and the two cylinders into one?
Having made it clear that I understood the scam and therefore could also make an 8 cylinder 2 stroke that they would have to rate as a 4 cylinder (and effectively stop Honda from benefitting) - I got no reply - but shortly afterwards the original change previously notified was reversed and we returned to the obvious logical relationship of pistons and bores determining the number of cylinders. A story just to demonstrate how devious big business interestes can be!
I also produced a carburettor throttle slide that as it moved - opened the disc valve inlet timing as well (so automatically varied the inlet timing in line with the throttle opening) and a thermocouple fitted to an electronic box that prevented seizures (common in two strokes of the era) by detecting the rise in cylinder temperature in time to switch off the ignition before it seized up.
But like many things - at the same time - usually different people around the world come to the same conclusions and all invent something similar and new at the same time.
I also patented the pull through suspension unit used on the Carbon Fibre Framed racing Armstrong motorcycle that we developed and that won the British Championship 3 years running - but I don't think this has been used again even though it was superior, lighter and resulted in a lower centre of gravity.
When I moved on to run a Carbon Fibre developement factory - I was called to a grubby workshop (the size of a kitchen and crammed with machinery) where one man on his own had - over several years of hard and unrewarded work - developed the first carbon fibre framed bicycle.
I advised him to sell the idea to a bigger business or it would be copied anyway and indeed it became the famous frame that won in an olympic event and the origin of which was claimed by a major manufacturer - but I never saw his name mentioned again even though I know it was all his work - poor sod.
I think nothing is really new and probably you can always find someone that thought up similar solutions years before but I do know that when I worked out how important the true compression pressure at low revs was - it was not copied or stolen from anyone but the results of my own testing and investigative thought processes and I don't care who believes that or not - I know the truth and that is all that matters to me. It also explained so many other things I had noticed in various engines over the years and for me became one of the most important issues in engine development or repair.
I am however suprised that this compression pressure issue has not to my knowledge been raised elsewhere (except it seems with Saab) as it is crucial to performance and extremely relevant to the performace of engines as they age and the wear reduces performance - particulcrly at low revs.
p.s. I am also currently in talks with a publisher - so you may actually find a book appear one day with many more similar stories, theories and explanations - as long as I can finish it before I forget it all!
Enough of my ego boosting reminiscing and trying to fool myself that I was more important to engine - engineering than I know I am - overalls back on and back to the real world in the workshop where there is a gearbox to rebuild and a special tool to make - damm it - it's much more more fun fooling yourself about what might have been!
Baz
My variable compression work was done in 1976 (just a few years before Saab) but I never managed to complete it due to lack of support and electronic ability to make the control system. Howver once I knew (and had proved in a fixed state) it worked - the incentive to prove it was less than the satisfaction of having found out something that at the time I had never known anyone else discover. That is not to say something similar hadn't been done before - in fact I saw a photo of a 2 stroke engine made in the 30's (I think) with a variable cylinder height (using a rotating cylinder rising and falling up and down on a thread) - but the pictures I saw never linked the reason for building it to compression ratio - but instead to port timing. I fully expect that the engineer knew all of that then and there is often nothing new in this World and usually someone else - you find - already did most things before.
I did however make a variable exhaust port for the armstrong two stroke engine using a flap valve - that worked. It basically restricted the exhaust flow at low revs by holding back the flow - this effectively delayed the timing at lower revs and improved entrapment so raising compression ratio (and I still have such a cylinder still at Hartech).
Apparently 2 years after leaving Armstrong Motorcycles - they were Involved with Honda to use a new 125cc two stroke engine with an apparently revolutionary exhaust flap valve they proudly showed off the the directors - one of whom slipped out to the then unused dyno room and from inside a dusty cupboard, pulled out my identical system tested several years before and to the utter amazement of the Japanese - blew the dust off it and plonked it on the board room table.
It was only when I was watching a dyno run in which lots of air and fuel were visibly going into the engine while it was producing very little power and that there was very little difference to the amount going in when it suddenly took off - that made me suspect that it wasn't the dynamic flow of inlet, exhaust and resonant tuning that determined the power band (as frankly everyone in the World had previously alluded to) but that it was actually possible that there is a true compression ratio (i.e. an actual compression pressure) below which little power results and above which the burning changes into a much more powerful burn. This puts a new theory in place to suggest that the formula for performance is not just that power increases with compression pressure but that there is a minimum pressure under which the burn is under impressive and a slightly higher pressure after which there is a huge jump in performance (something I never previously knew). It proved that actually the compression pressure was far more important than the inlet, exhaust and ignition timing - the inlet or exhaust tuning or the induction length - and they were all the things everyone had spent all their time developing and working on (and it may be that such things are known now and taught but not when I was in the education system).
Fitting increasingly higher compression cylinder heads resulted in this engine (that was supposed to be so highly tuned it would not pull the skin off a rice pudding - as they say) at low revs into a very powerful engine at those same low revs and with the same "high rev" tuning. Eventually the very high compression head seized the engine at high revs but not until a graph had been produced to prove the benfits of a variable compression engine (reducing as the revs increase). It showed a huge power band despite being basically highly tuned and previously with a very narrow power band.
This in turn meant that an even higher state of engine tune could be used but still produce a big enough power band to start and accelerate through the gearbox.
It would undoubtedly have been a "World beater" but then it would have either been copied or banned by the governing body and I probably would not have got the credit anyway. I say this because around that time I received from the motorcycle racing governing body (the FIM) a letter stating that in future the number of cylinders would be determined by the number of combustion chambers! This was extremely odd to say the least!
500cc GP events were limited to 4 cylinders at the time and two strokes were dominant. I don't know which manufacturer was behind the change but Honda wanted to compete with a 4 stroke but needed 8 cylinders to do so which would be illegal. Realising what they were up to I wrote to the FIM and asked if 2 pistons in 2 bores (or 8 pistons in 8 bores with 4 combustion chambers - in pairs) had a common combustion chamber (or 4 common combustion chambers) - would it be a regarded as a single cylinder (or a 4 cylinder) and they replied "yes" - plot exposed. I then asked - if two combustion chanbers were linked together would this be one cylinder - they said "yes". Finally I said - how big would the "link between the cylinders" have to be - would a 0.25mm hole drilled between 2 otherwise seperate combustion chambers turn it into one chamber and the two cylinders into one?
Having made it clear that I understood the scam and therefore could also make an 8 cylinder 2 stroke that they would have to rate as a 4 cylinder (and effectively stop Honda from benefitting) - I got no reply - but shortly afterwards the original change previously notified was reversed and we returned to the obvious logical relationship of pistons and bores determining the number of cylinders. A story just to demonstrate how devious big business interestes can be!
I also produced a carburettor throttle slide that as it moved - opened the disc valve inlet timing as well (so automatically varied the inlet timing in line with the throttle opening) and a thermocouple fitted to an electronic box that prevented seizures (common in two strokes of the era) by detecting the rise in cylinder temperature in time to switch off the ignition before it seized up.
But like many things - at the same time - usually different people around the world come to the same conclusions and all invent something similar and new at the same time.
I also patented the pull through suspension unit used on the Carbon Fibre Framed racing Armstrong motorcycle that we developed and that won the British Championship 3 years running - but I don't think this has been used again even though it was superior, lighter and resulted in a lower centre of gravity.
When I moved on to run a Carbon Fibre developement factory - I was called to a grubby workshop (the size of a kitchen and crammed with machinery) where one man on his own had - over several years of hard and unrewarded work - developed the first carbon fibre framed bicycle.
I advised him to sell the idea to a bigger business or it would be copied anyway and indeed it became the famous frame that won in an olympic event and the origin of which was claimed by a major manufacturer - but I never saw his name mentioned again even though I know it was all his work - poor sod.
I think nothing is really new and probably you can always find someone that thought up similar solutions years before but I do know that when I worked out how important the true compression pressure at low revs was - it was not copied or stolen from anyone but the results of my own testing and investigative thought processes and I don't care who believes that or not - I know the truth and that is all that matters to me. It also explained so many other things I had noticed in various engines over the years and for me became one of the most important issues in engine development or repair.
I am however suprised that this compression pressure issue has not to my knowledge been raised elsewhere (except it seems with Saab) as it is crucial to performance and extremely relevant to the performace of engines as they age and the wear reduces performance - particulcrly at low revs.
p.s. I am also currently in talks with a publisher - so you may actually find a book appear one day with many more similar stories, theories and explanations - as long as I can finish it before I forget it all!
Enough of my ego boosting reminiscing and trying to fool myself that I was more important to engine - engineering than I know I am - overalls back on and back to the real world in the workshop where there is a gearbox to rebuild and a special tool to make - damm it - it's much more more fun fooling yourself about what might have been!
Baz
On the subject of variable compression engines, Mercedes are currently working on what they call a 'dies-otto' engine. In testing, a 4-cylinder nominal 1800 cc engine of this type was fitted to an S-class and returned about 50 mpg (IIRC) with the same performance as an S-320 CDI (I think it even had slightly more torque and low end acceleration!)
Here's an odd one. I have a 99 996 cab with 87k and full service history.It has always had good oil pressure (3bar at idle and 4.5 at speed) It went in for its MOT and when it came back oil pressure was only 1 bar at idle or speed. Sounds and runs ok at present. Started it from cold this am and idle pressure was 3bar (will reudce when warm) but when the engine was revved it DROPPED to 1bar only to return to 3 bar when idling again. Anybody heard of a 996 suddenly with such low oil pressure or more peculiarly DROPPING when revved.Any Ideas??
brown996 said:
Here's an odd one. I have a 99 996 cab with 87k and full service history.It has always had good oil pressure (3bar at idle and 4.5 at speed) It went in for its MOT and when it came back oil pressure was only 1 bar at idle or speed. Sounds and runs ok at present. Started it from cold this am and idle pressure was 3bar (will reudce when warm) but when the engine was revved it DROPPED to 1bar only to return to 3 bar when idling again. Anybody heard of a 996 suddenly with such low oil pressure or more peculiarly DROPPING when revved.Any Ideas??
sounds like you've got a bad oil pressure sensor.It does seem likely that there is a fault with the sender or guage - but strange that it went wrong while at the MOT station?
If in doubt it is always worth checking the oil filter for bronze particles and the oil pressure relief valve (in the oil pump) to make sure it has not stuck possibly with a bit of debris (as it is possible for a small piece of the silicon they stick the crankcase joints together with to come lose and float round to the valve and jam it).
I feel sure it will be OK,
Baz
If in doubt it is always worth checking the oil filter for bronze particles and the oil pressure relief valve (in the oil pump) to make sure it has not stuck possibly with a bit of debris (as it is possible for a small piece of the silicon they stick the crankcase joints together with to come lose and float round to the valve and jam it).
I feel sure it will be OK,
Baz
Thanks Baz will do.I cant see that the MOT station could have done anything although they did replace both of the rear to front brake pipes and renewed an AC pipe joint just behind the front drivers door. I have let them know about the problem just in case.If this sparks anything else in your mind let me know. Thanks again.
Well, I thought you might like to know how we are getting on with this peculiar problem.
It not the sender,the gauge or the oil pressure release valve. All been tested and there is genuinely low oil pressure
No debris or metal particles in oil or filter. No water in oil or oil in water.
No noises or smoke.
Car still has v low oil pressure which DROPS when engine is revved and rises (slightly) at idle.
Now checking for faied or failing oil pump.
It not the sender,the gauge or the oil pressure release valve. All been tested and there is genuinely low oil pressure
No debris or metal particles in oil or filter. No water in oil or oil in water.
No noises or smoke.
Car still has v low oil pressure which DROPS when engine is revved and rises (slightly) at idle.
Now checking for faied or failing oil pump.
Pretty much has to be a failed or failing pump then... IMHO.
How is the pump secured to its shaft? Is there a woodruff key or alike? It sounds like it could be slipping as rpm is rapidly increased? Or, it could just be worn internally...
Will be interesting to hear what the problem is.
How is the pump secured to its shaft? Is there a woodruff key or alike? It sounds like it could be slipping as rpm is rapidly increased? Or, it could just be worn internally...
Will be interesting to hear what the problem is.
brown996 said:
Well, I thought you might like to know how we are getting on with this peculiar problem.
It not the sender,the gauge or the oil pressure release valve. All been tested and there is genuinely low oil pressure
No debris or metal particles in oil or filter. No water in oil or oil in water.
No noises or smoke.
Car still has v low oil pressure which DROPS when engine is revved and rises (slightly) at idle.
Now checking for faied or failing oil pump.
I hope you have someone very good on the job.It not the sender,the gauge or the oil pressure release valve. All been tested and there is genuinely low oil pressure
No debris or metal particles in oil or filter. No water in oil or oil in water.
No noises or smoke.
Car still has v low oil pressure which DROPS when engine is revved and rises (slightly) at idle.
Now checking for faied or failing oil pump.
brown996 said:
Well, I thought you might like to know how we are getting on with this peculiar problem.
It not the sender,the gauge or the oil pressure release valve. All been tested and there is genuinely low oil pressure
No debris or metal particles in oil or filter. No water in oil or oil in water.
No noises or smoke.
Car still has v low oil pressure which DROPS when engine is revved and rises (slightly) at idle.
Now checking for faied or failing oil pump.
Well for those interested, the oil pump was removed and checked and is fine. However on draining the oils, the oil filter was removed and was found to be crushed up inside the housing.The pump was refitted, with a new filter and oil pressure is back to normal.It not the sender,the gauge or the oil pressure release valve. All been tested and there is genuinely low oil pressure
No debris or metal particles in oil or filter. No water in oil or oil in water.
No noises or smoke.
Car still has v low oil pressure which DROPS when engine is revved and rises (slightly) at idle.
Now checking for faied or failing oil pump.
Feeling is that the oil separator (there is a lot of oil around it but no tell tale smoke)is damaged and somehow created a vacuum that has crushed the filter element. We may never know for sure, but it too is being replaced (manual car) so all seems to be on the mend now...... without a new engine!
brown996 said:
brown996 said:
Well, I thought you might like to know how we are getting on with this peculiar problem.
It not the sender,the gauge or the oil pressure release valve. All been tested and there is genuinely low oil pressure
No debris or metal particles in oil or filter. No water in oil or oil in water.
No noises or smoke.
Car still has v low oil pressure which DROPS when engine is revved and rises (slightly) at idle.
Now checking for faied or failing oil pump.
Well for those interested, the oil pump was removed and checked and is fine. However on draining the oils, the oil filter was removed and was found to be crushed up inside the housing.The pump was refitted, with a new filter and oil pressure is back to normal.It not the sender,the gauge or the oil pressure release valve. All been tested and there is genuinely low oil pressure
No debris or metal particles in oil or filter. No water in oil or oil in water.
No noises or smoke.
Car still has v low oil pressure which DROPS when engine is revved and rises (slightly) at idle.
Now checking for faied or failing oil pump.
Feeling is that the oil separator (there is a lot of oil around it but no tell tale smoke)is damaged and somehow created a vacuum that has crushed the filter element. We may never know for sure, but it too is being replaced (manual car) so all seems to be on the mend now...... without a new engine!
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