High Mileage 996 Engines
Discussion
Not really - it's a difficult subject to cover here as there is a gradual loss of bottom end power/torque. It is not really noticeable during ownership because it is so gradual a loss that you don't notice it creeping up over the years - or in your period of ownership.
To explain why - the power/torque an engine develops is due to the compression pressure before ignition - i.e. mixed fuel does not produce the same burn pressure to push the piston down when it is at different pressures before ignition. This is why the old easy tune up solution was "high compression pistons" - the same amount of fuel and air was trapped but it was compressed to a higher pressure before ignition.
Everything people do to tune an engine tries to squeeze/suck more air into it so that when it is compressed it reaches a higher pressure before ignition. This is indeed what turbo's and super chargers do. So long as the compression is not so high that it causes the fuel to explode rather than burn - the higher the pressure the more power (from the same volume of air/fuel) you get (this is also why turbo charged engines have lower geometric compression to compensate for the extra air being blown in that would otherwise lead to an exploding mixture - detonation).
All the variable cam lift and timing sytems do is to make the engine breath better at different revs - without harming the breathing at peak revs - which in turn feeds mor air into it and results in a higher compression pressure before ignition at lower revs.
This is particularly important at low revs because it is easy to manufacture an engine have the perfect compression ratio when it is running at its most efficient breathing (somehwere near peak revs) but after that all the breathing at lower revs traps less air and reaches a lower compression pressure before ignition. (In fact this is why two strokes suddenly produce a huge turbo like power increase - not as often stated because the exhaust system suddenly resonates but because it finally traps enough air to produce a "powerful" burn).
It is quite true that if an engine had a variable compression ratio - this would enable better torque and power at lower revs but as this has not been done yet - better torque comes from better compression pressure.
Now as the bores go oval (as the pistons are already made oval anyway) the piston clearance increases a lot and the rings do not seal as well in the oval bore so the precious amount of trapped air at low revs leaks down the bore and lowers the compression pressure before ignition.
In leaking down the bore it pushes the oil film down and overheats the oil, the face of the bore and the pistons and this is why most older 996 engines that seize - unusually - do so on just one face.
You only notice this gradual loss of torque after having the engine rebuilt after correcting the roundness - as a noticeable increase in sharpness and bottom end torque and acceleration.
As long as high mileage engines are not driven excessively fast with less than full throttle in high gears - they may run OK until the bore eventually cracks (which might be a long way off). But if they are driven with the same gusto as when they were new - they may well fail prematurely.
The actual bore (and ring) wear is not high because the extra clearance comes about by the round bore getting bigger in one direction and smaller in the other so when they are re-rounded - the wear is relatively little.
Since the cars are capable of in excess of 160mph, any normal sensible driver wanting to preserve their high mileage engine can still obtain excellent performance without over straining it - so usually as cars age and are worth less the owners also tend to on average drive them less agressivley than some who can afford a new Porsche and for all the money they paid quite rightly expect to be able to thrash the bo***cks of it without it comming to any harm.
When is the best time to fix this problem is difficult to asses due to the different driving styles influencing the rate of ovality (and who knows how a previous owner treated the car) - and as several engines have failed with cracked bores at 40K to 60K + but some others at 130K are still Ok - it is difficult to set a figure.
However taking on board all the other failures (that a rebuild can refresh/avoid like chains, IMS bearing, crankshaft shells etc) and relevant ages (for seal wear etc) and mileages - it just seems to me that anyone wanting to keep a car for a lot longer - with around 100K on the clock - would benefit from a rebuild around then before failures increase the repair costs. Or they could get on a waranty scheme and not worry about the extra repair costs and simply await the outcome. If a clutch was needed soon and exhaust system bolts and joints were corroding etc it might make more sense to address all the problems in one go - as buyers of older Porsches expect to do and frequently authorise.
In all this - reducing the strain on the engine as it ages will usually produce benefits for longevity.
I hope this explains it better.
Baz
To explain why - the power/torque an engine develops is due to the compression pressure before ignition - i.e. mixed fuel does not produce the same burn pressure to push the piston down when it is at different pressures before ignition. This is why the old easy tune up solution was "high compression pistons" - the same amount of fuel and air was trapped but it was compressed to a higher pressure before ignition.
Everything people do to tune an engine tries to squeeze/suck more air into it so that when it is compressed it reaches a higher pressure before ignition. This is indeed what turbo's and super chargers do. So long as the compression is not so high that it causes the fuel to explode rather than burn - the higher the pressure the more power (from the same volume of air/fuel) you get (this is also why turbo charged engines have lower geometric compression to compensate for the extra air being blown in that would otherwise lead to an exploding mixture - detonation).
All the variable cam lift and timing sytems do is to make the engine breath better at different revs - without harming the breathing at peak revs - which in turn feeds mor air into it and results in a higher compression pressure before ignition at lower revs.
This is particularly important at low revs because it is easy to manufacture an engine have the perfect compression ratio when it is running at its most efficient breathing (somehwere near peak revs) but after that all the breathing at lower revs traps less air and reaches a lower compression pressure before ignition. (In fact this is why two strokes suddenly produce a huge turbo like power increase - not as often stated because the exhaust system suddenly resonates but because it finally traps enough air to produce a "powerful" burn).
It is quite true that if an engine had a variable compression ratio - this would enable better torque and power at lower revs but as this has not been done yet - better torque comes from better compression pressure.
Now as the bores go oval (as the pistons are already made oval anyway) the piston clearance increases a lot and the rings do not seal as well in the oval bore so the precious amount of trapped air at low revs leaks down the bore and lowers the compression pressure before ignition.
In leaking down the bore it pushes the oil film down and overheats the oil, the face of the bore and the pistons and this is why most older 996 engines that seize - unusually - do so on just one face.
You only notice this gradual loss of torque after having the engine rebuilt after correcting the roundness - as a noticeable increase in sharpness and bottom end torque and acceleration.
As long as high mileage engines are not driven excessively fast with less than full throttle in high gears - they may run OK until the bore eventually cracks (which might be a long way off). But if they are driven with the same gusto as when they were new - they may well fail prematurely.
The actual bore (and ring) wear is not high because the extra clearance comes about by the round bore getting bigger in one direction and smaller in the other so when they are re-rounded - the wear is relatively little.
Since the cars are capable of in excess of 160mph, any normal sensible driver wanting to preserve their high mileage engine can still obtain excellent performance without over straining it - so usually as cars age and are worth less the owners also tend to on average drive them less agressivley than some who can afford a new Porsche and for all the money they paid quite rightly expect to be able to thrash the bo***cks of it without it comming to any harm.
When is the best time to fix this problem is difficult to asses due to the different driving styles influencing the rate of ovality (and who knows how a previous owner treated the car) - and as several engines have failed with cracked bores at 40K to 60K + but some others at 130K are still Ok - it is difficult to set a figure.
However taking on board all the other failures (that a rebuild can refresh/avoid like chains, IMS bearing, crankshaft shells etc) and relevant ages (for seal wear etc) and mileages - it just seems to me that anyone wanting to keep a car for a lot longer - with around 100K on the clock - would benefit from a rebuild around then before failures increase the repair costs. Or they could get on a waranty scheme and not worry about the extra repair costs and simply await the outcome. If a clutch was needed soon and exhaust system bolts and joints were corroding etc it might make more sense to address all the problems in one go - as buyers of older Porsches expect to do and frequently authorise.
In all this - reducing the strain on the engine as it ages will usually produce benefits for longevity.
I hope this explains it better.
Baz
Ballcock said:
Fantastic info Baz , it's really great that you take the time to post in such detail , we would not have access to this type of knowledge if you chose not to share it.
+1I always take the time to read the hartech posts, I've not seen these things explained in such a detailed and easy to understand way anywhere else.
I'm sure you could put a book together from all the posts you've made on PH - I'd buy it
hartech said:
Some stuff....
All of a sudden you realise what a good thing Barrel base gaskets / through bolt seals leaking is. It acts as a catalyst for you to go in there and freshen the engine up !!It`s the same problem with a lot of the 3.2 Carreras. They don`t leak, keep running but breath like a steam train and really need the engine opening up.
He
996GT2 said:
Ballcock said:
Fantastic info Baz , it's really great that you take the time to post in such detail , we would not have access to this type of knowledge if you chose not to share it.
+1I always take the time to read the hartech posts, I've not seen these things explained in such a detailed and easy to understand way anywhere else.
I'm sure you could put a book together from all the posts you've made on PH - I'd buy it
amir_j said:
996GT2 said:
Ballcock said:
Fantastic info Baz , it's really great that you take the time to post in such detail , we would not have access to this type of knowledge if you chose not to share it.
+1I always take the time to read the hartech posts, I've not seen these things explained in such a detailed and easy to understand way anywhere else.
I'm sure you could put a book together from all the posts you've made on PH - I'd buy it
I wouldn't buy it as it's all right here for free on PH!! (
)What it has done, however, has instilled in me a lot of confidence in the Hartech outfit, and when I'm ready to buy some pork (shortly, hopefully), Hartech is probably the first place I'll look.
Yes Henry - as you know well - the 3.2 wears rings at an alarming rate and with similar loss of bottom end breathing and lower compression pressures resulting. Eventually because this loss of bottom end breathing changes the air flow meter mixture ratios and people start to fiddle with the air flow meter settings or put up with very rich mixtures.
When badly worn they drive more like a 2 stroke - only achieveing good compression pressures and performance when the cams make the engine breath well anyway - 3500 to 4K revs. The difference in bottom end that results from a rebuild is amazing.
People seem to accept the benefits from having a 3.2 top end rebuild, a 964 top and bottom end, a 944 head gasket or pinion bearing, 944 S, S2 and 968 cams and pinion bearings - yet for some reason - to refresh a 996 (a much more powerful car) at over 100K for similar costs is heavily criticised. Bearing in mind that during the rebuild several potential critical failures can be modified to prevent them occuring again - I find it odd that the cost is so criticised. Indeed if at 100K a car (that has then reached low or minimal depreciation) got a rebuild at say £3-4K - that would then last it until say 180K (the end perhaps of its life anyway) that works out at about 2p/mile.
For anyone on our Lifetime Maintenance Plan the costs of fixing a fault are so low that it seems like a no brainer (as they say) to me!
When I go into our local crankshaft regrinding/reboring centre there are always lots of powerful engines being rebuilt, XR2 turbos, various Japanese engines at lower mileages - I don't know if their web sites contain similar uproar - I somehow doubt it.
I guess it is just the expectation of longevity from the period when Porsche were going bust - that has been knocked the reality of life with a modern bottom of the market Porsche in the present World?
Baz
When badly worn they drive more like a 2 stroke - only achieveing good compression pressures and performance when the cams make the engine breath well anyway - 3500 to 4K revs. The difference in bottom end that results from a rebuild is amazing.
People seem to accept the benefits from having a 3.2 top end rebuild, a 964 top and bottom end, a 944 head gasket or pinion bearing, 944 S, S2 and 968 cams and pinion bearings - yet for some reason - to refresh a 996 (a much more powerful car) at over 100K for similar costs is heavily criticised. Bearing in mind that during the rebuild several potential critical failures can be modified to prevent them occuring again - I find it odd that the cost is so criticised. Indeed if at 100K a car (that has then reached low or minimal depreciation) got a rebuild at say £3-4K - that would then last it until say 180K (the end perhaps of its life anyway) that works out at about 2p/mile.
For anyone on our Lifetime Maintenance Plan the costs of fixing a fault are so low that it seems like a no brainer (as they say) to me!
When I go into our local crankshaft regrinding/reboring centre there are always lots of powerful engines being rebuilt, XR2 turbos, various Japanese engines at lower mileages - I don't know if their web sites contain similar uproar - I somehow doubt it.
I guess it is just the expectation of longevity from the period when Porsche were going bust - that has been knocked the reality of life with a modern bottom of the market Porsche in the present World?
Baz
hartech said:
It is quite true that if an engine had a variable compression ratio - this would enable better torque and power at lower revs but as this has not been done yet - better torque comes from better compression pressure.
Believe it was SAAB had a go with an engine that had a cylinder block that could rotate about the crankcase.It would be sat on top normally giving a higher compression ration but when full beans were summoned, the cylinders were rotated (crank pin and cylinder block arc didnt share a common point) effectively moving the head further away from the crank and reducing the geometric compression so more boost could be thrown in without detontation.
Very hazy memory of this, it cropped up in a random lecture at uni ~5yrs ago. Dont think the engine ever got close to production though.
I didn't know about the Saab engine emicen but I must say that in my view a variable compression ratio engine is the way to go in the future!
When I left the motorcycle industry I had worked out that this was the way to go for a two stroke and indeed had 2 ways worked out of doing it - but - both needed extensive digital electronic controls that were simply not available then and masses of dyno work to perfect - which I knew no British Business had the stomach for.
Baz
When I left the motorcycle industry I had worked out that this was the way to go for a two stroke and indeed had 2 ways worked out of doing it - but - both needed extensive digital electronic controls that were simply not available then and masses of dyno work to perfect - which I knew no British Business had the stomach for.
Baz
hartech said:
1
It is quite true that if an engine had a variable compression ratio - this would enable better torque and power at lower revs but as this has not been done yet - better torque comes from better compression pressure.
2
As long as high mileage engines are not driven excessively fast with less than full throttle in high gears - they may run OK until the bore eventually cracks (which might be a long way off). But if they are driven with the same gusto as when they were new - they may well fail prematurely.
Baz
Hi Baz, I sincerely appreciate you taking the time to tell us this much about the Porsche engines, Thank you very much.It is quite true that if an engine had a variable compression ratio - this would enable better torque and power at lower revs but as this has not been done yet - better torque comes from better compression pressure.
2
As long as high mileage engines are not driven excessively fast with less than full throttle in high gears - they may run OK until the bore eventually cracks (which might be a long way off). But if they are driven with the same gusto as when they were new - they may well fail prematurely.
Baz
I have two questions about your post that intrigues me.
1 re: compression ratio. How does this work? When I read about the compression ratio of a normally aspirated Porsche engine, it the bar number the times the air is compressed or the actual force the piston is subjected to when the fuel burns? I'm confused on this.
2. Why does it matter how much throttle you apply on a worn engine in relation to the speed you are driving? Does this mean that it's better to go full throttle/no throttle as on a race-track? Can't you cruise with a worn engine at 120mph? Why is this?
Thank you very much.
Decanter said:
Hi all
Reading this thread with interest regarding higher mileage 996's, and some superb posting from Hartech; very informative and very easy to understand for a numpty such as myself.
But regarding the 996 longevity I have a question................. most is without doubt answered regarding the engine but I'm also wondering about the gearbox on these cars, I remember some information in the past that these weren't able to be reconditioned, and that it was necessary to purchase an exchange gearbox from Porsche at substantial cost, else a used item, is this still the case? Or are spare parts now available for the rebuild of these gearboxes? Sorry if this is a daft question, but for someone like myself it could make all the difference to a potential purchase knowing details such as this.
To save Baz a load of typing, have a look at what he wrote in this thread a few moonths ago.Reading this thread with interest regarding higher mileage 996's, and some superb posting from Hartech; very informative and very easy to understand for a numpty such as myself.
But regarding the 996 longevity I have a question................. most is without doubt answered regarding the engine but I'm also wondering about the gearbox on these cars, I remember some information in the past that these weren't able to be reconditioned, and that it was necessary to purchase an exchange gearbox from Porsche at substantial cost, else a used item, is this still the case? Or are spare parts now available for the rebuild of these gearboxes? Sorry if this is a daft question, but for someone like myself it could make all the difference to a potential purchase knowing details such as this.
http://pistonheads.co.uk/gassing/topic.asp?h=0&...
The compression ratio is worked out as follows.
You put the piston at top dead centre and fill the engine with fluid from a burette to measure the volume inside the engine on top of the piston at top dead centre (you do this through the plug hole with the cylinder vertical making allowance for how far up the spark plug thread you go to allow for the volume inside the spark plug). This is called the clearance volume.
The area of the bore multiplied by the stroke then gives what is called the swept volume.
The compression ratio is the swept volume plus the clearance volume divided by the clearance volume. So if a cylinder is 100ccs and the clearance volume is 10ccs the compression ratio is 11 to 1.
As you can see this is relatively meaningless as it only tells you a story about the physical construction of the engine. The compression pressure that results depends upon how much air is trapped in the cylinder when all the valves are shut before it is compressed. At low revs relatively little air is trapped so the result of compressing it 11 times is still quite a low pressure. When an engine is breathing at its best it will be close to sucking in its own volume in air and the compression pressure may well be 11 times more than it was at atmospheric pressure (and there are a few complications for temperature etc as Boyles law relates pressure and temperature etc - but these do not change the basic theory). One of the engine dynamic - heat engine thermodynamic laws reflects this relationship between power and compression ration but expressing it as a figure to the power of the compression ratio (but I cannot remember what it is called - only how it all works)
How much air it can trap compared to the swept volume is called the "volumetric efficiency" of the engine. The first engine I am aware of to achieve 100% volumetric efficiency (i.e. to suck in the swept volume entirely) was the NSU rennmax 250 cc 4 stroke twin of the late 50's era.
Everything that can be done to a normally aspirated engine (one that sucks its own air in) to increase the amount of air it can trap at high revs - unfortunately reduces the amount it can trap at low revs and visa versa. So tuning has always been a compromise to achieve the best all round entrapment for the application - lots of power at high revs for racing or lots of power/torque at low revs for heavy pulling etc.
Varying the timing of various features effectively makes the engine like different engines would be if they were not variable - so you can improve the breathing at a wider range of revs. So altering the valve timing and/or the valve lift, changing the induction length etc can broaden the power band.
Pumping air in under pressure can exceed 100% volumetric efficiency and hence be more powerful than a normally aspirated engine and therefore can increase pressures so much that the fuel explodes and that is no good for producing power.
For any situation the more you open the throttle the more air is trapped in the engine so the higher the pressure before ignition and the higher the burn pressure of the fuel.
The piston is pushed down by that pressure (like your leg pushing down on a bicycle pedal). The higher the pressure the greater the push. The crankshaft is rotated by the piston pushing hard against the cylinder wall and transferring that downward linear pressure into rotation through the connecting rod that connects the piston to the crankshaft (again rather like a leg and a bicycle pedal).
The more the throttle is opened the more the piston pushes against the cylinder wall and the greater the pressure trying to force burning fuel past the piston and rings and down into the cylinder bore.
So you can cruise at 120mph - what is relevant is how quickly you want to reach 120mph. Acceleration is proportional to torque (which at any given revs is proportional to power) - so the faster you want to accelerate the more power or torque you must produce and the greater the resulting forces on the piston wall creating piston blow by.
So if say you used three quarters throttle - you would trap less air, have lower compression pressure, lower forces on the piston and cylinder wall and less piston blow by - also less power - but you could still reach 120mph - but more slowly than if you used full throttle (which would strain the engine more by getting there more quickly).
The effective weight (or inertia forces) of engine parts when they are accelerating and rotating is magnified by the revs in more or less a cubic ratio. So higher revs results in very much higher mechanical loads from those parts moving about in the engine and hence strains the engine more than a straight ratio increasing linearly with speed.
This is why the way you drive the car as it ages influences its lifespan - both in terms of how soon you drive fast when warming it up, how much you open the throttle and how many revs you often use before changing gear. Similarly someone who happens to drive modestly from new will have an engine in better condition and likely to last longer than one that was accelerated at full throttle to peak revs from cold, from new and for all its life.
People used to driving lots of the same models can tell the difference in the feel of the car and good dealers will select those that have had a relatively sedate life and may refuse to buy or part exchange a car that has been worn out prematurely as it will only backfire on their reputation and pocket.
The main gearbox parts usually needed are available now. The biggest problem seems to be that the main bearings are shrouded with a seal (just like the ill fated IMS bearing) and that this prevents the washing away of minute wear particles, allows the oil to get hotter and does not replenish it quickly - but they can be replaced.
The other main problem is that older gearboxes allowed you to move the gear lever and gear as far as it could go until it was fully engaged and this was effectively the gear change stop mechanism. This worked well but you could usually feel the gear jamming into place slightly back though the gear lever when it was fully engaged.
These gearboxes have a different stop that prevents the gear lever moving the engagement dogs fully home into gear (so you do not feel it coming to a stop) and this makes the gear change feel sweeter. The last part of the engagement is taken up by spring loaded pawls inside the synchromesh.
This would be good if the set up was set perfectly, but two things alter that perfection. (1) The shaft is moved by shims to get the best position and (2) the engaging shafts, bearings and selector forks etc wear.
If the original setting was perfectly in the centre (so for example the movement for 1st and 2nd was exactly equal in both directions) then even with wear it would probably work OK. Unfortunately - for some reason the setting seems to favour more movement in one direction and less in the other until with wear the engagement with the least movement doesn't quite move far enough to allow the spring pawls to drag it further into place and the synchromesh sits half in and half out of gear. Then when you let the clutch out - sometimes it slips into gear and sometimes it doesn't and slips out again. Because new parts are less worn, changing to them can improve the gear change but a better solution is to re-set the positioning so that it is central and fully engaged in both 1st and 2nd anyway.
I hope this answers your recent questions,
Baz
You put the piston at top dead centre and fill the engine with fluid from a burette to measure the volume inside the engine on top of the piston at top dead centre (you do this through the plug hole with the cylinder vertical making allowance for how far up the spark plug thread you go to allow for the volume inside the spark plug). This is called the clearance volume.
The area of the bore multiplied by the stroke then gives what is called the swept volume.
The compression ratio is the swept volume plus the clearance volume divided by the clearance volume. So if a cylinder is 100ccs and the clearance volume is 10ccs the compression ratio is 11 to 1.
As you can see this is relatively meaningless as it only tells you a story about the physical construction of the engine. The compression pressure that results depends upon how much air is trapped in the cylinder when all the valves are shut before it is compressed. At low revs relatively little air is trapped so the result of compressing it 11 times is still quite a low pressure. When an engine is breathing at its best it will be close to sucking in its own volume in air and the compression pressure may well be 11 times more than it was at atmospheric pressure (and there are a few complications for temperature etc as Boyles law relates pressure and temperature etc - but these do not change the basic theory). One of the engine dynamic - heat engine thermodynamic laws reflects this relationship between power and compression ration but expressing it as a figure to the power of the compression ratio (but I cannot remember what it is called - only how it all works)
How much air it can trap compared to the swept volume is called the "volumetric efficiency" of the engine. The first engine I am aware of to achieve 100% volumetric efficiency (i.e. to suck in the swept volume entirely) was the NSU rennmax 250 cc 4 stroke twin of the late 50's era.
Everything that can be done to a normally aspirated engine (one that sucks its own air in) to increase the amount of air it can trap at high revs - unfortunately reduces the amount it can trap at low revs and visa versa. So tuning has always been a compromise to achieve the best all round entrapment for the application - lots of power at high revs for racing or lots of power/torque at low revs for heavy pulling etc.
Varying the timing of various features effectively makes the engine like different engines would be if they were not variable - so you can improve the breathing at a wider range of revs. So altering the valve timing and/or the valve lift, changing the induction length etc can broaden the power band.
Pumping air in under pressure can exceed 100% volumetric efficiency and hence be more powerful than a normally aspirated engine and therefore can increase pressures so much that the fuel explodes and that is no good for producing power.
For any situation the more you open the throttle the more air is trapped in the engine so the higher the pressure before ignition and the higher the burn pressure of the fuel.
The piston is pushed down by that pressure (like your leg pushing down on a bicycle pedal). The higher the pressure the greater the push. The crankshaft is rotated by the piston pushing hard against the cylinder wall and transferring that downward linear pressure into rotation through the connecting rod that connects the piston to the crankshaft (again rather like a leg and a bicycle pedal).
The more the throttle is opened the more the piston pushes against the cylinder wall and the greater the pressure trying to force burning fuel past the piston and rings and down into the cylinder bore.
So you can cruise at 120mph - what is relevant is how quickly you want to reach 120mph. Acceleration is proportional to torque (which at any given revs is proportional to power) - so the faster you want to accelerate the more power or torque you must produce and the greater the resulting forces on the piston wall creating piston blow by.
So if say you used three quarters throttle - you would trap less air, have lower compression pressure, lower forces on the piston and cylinder wall and less piston blow by - also less power - but you could still reach 120mph - but more slowly than if you used full throttle (which would strain the engine more by getting there more quickly).
The effective weight (or inertia forces) of engine parts when they are accelerating and rotating is magnified by the revs in more or less a cubic ratio. So higher revs results in very much higher mechanical loads from those parts moving about in the engine and hence strains the engine more than a straight ratio increasing linearly with speed.
This is why the way you drive the car as it ages influences its lifespan - both in terms of how soon you drive fast when warming it up, how much you open the throttle and how many revs you often use before changing gear. Similarly someone who happens to drive modestly from new will have an engine in better condition and likely to last longer than one that was accelerated at full throttle to peak revs from cold, from new and for all its life.
People used to driving lots of the same models can tell the difference in the feel of the car and good dealers will select those that have had a relatively sedate life and may refuse to buy or part exchange a car that has been worn out prematurely as it will only backfire on their reputation and pocket.
The main gearbox parts usually needed are available now. The biggest problem seems to be that the main bearings are shrouded with a seal (just like the ill fated IMS bearing) and that this prevents the washing away of minute wear particles, allows the oil to get hotter and does not replenish it quickly - but they can be replaced.
The other main problem is that older gearboxes allowed you to move the gear lever and gear as far as it could go until it was fully engaged and this was effectively the gear change stop mechanism. This worked well but you could usually feel the gear jamming into place slightly back though the gear lever when it was fully engaged.
These gearboxes have a different stop that prevents the gear lever moving the engagement dogs fully home into gear (so you do not feel it coming to a stop) and this makes the gear change feel sweeter. The last part of the engagement is taken up by spring loaded pawls inside the synchromesh.
This would be good if the set up was set perfectly, but two things alter that perfection. (1) The shaft is moved by shims to get the best position and (2) the engaging shafts, bearings and selector forks etc wear.
If the original setting was perfectly in the centre (so for example the movement for 1st and 2nd was exactly equal in both directions) then even with wear it would probably work OK. Unfortunately - for some reason the setting seems to favour more movement in one direction and less in the other until with wear the engagement with the least movement doesn't quite move far enough to allow the spring pawls to drag it further into place and the synchromesh sits half in and half out of gear. Then when you let the clutch out - sometimes it slips into gear and sometimes it doesn't and slips out again. Because new parts are less worn, changing to them can improve the gear change but a better solution is to re-set the positioning so that it is central and fully engaged in both 1st and 2nd anyway.
I hope this answers your recent questions,
Baz
emicen said:
hartech said:
It is quite true that if an engine had a variable compression ratio - this would enable better torque and power at lower revs but as this has not been done yet - better torque comes from better compression pressure.
Believe it was SAAB had a go with an engine that had a cylinder block that could rotate about the crankcase.It would be sat on top normally giving a higher compression ration but when full beans were summoned, the cylinders were rotated (crank pin and cylinder block arc didnt share a common point) effectively moving the head further away from the crank and reducing the geometric compression so more boost could be thrown in without detontation.
Very hazy memory of this, it cropped up in a random lecture at uni ~5yrs ago. Dont think the engine ever got close to production though.
http://en.wikipedia.org/wiki/Saab_Variable_Compres...
http://www.carenthusiast.co.uk/2000/shows/geneva20...
Highlights:
The five cylinder in-line, 1.6-litre Saab Variable Compression engine on display generates no less than 225bhp (almost 150bhp per litre!) and yet can return the fuel economy of a conventional 1.6-litre engine with less than half that power.
Linked to a powerful supercharger, the Saab Variable Compression (SVC) engine can deliver the power of a 3.0-litre engine while operating at a relatively low 8:1 compression ratio. However, it can also run with a much higher compression ratio, up to 14:1, for good economy at smaller throttle openings when the supercharger is ‘off-boost’.
Thats really interesting as I didn't realise that anyone had made a variable compression engine. I did it the easy way by running an engine on a dyno with different static compression ratio cylinder heads (swapping them over and doing another test run) - so I could plot on a graph the change in compression ratio (with revs and throttle opening) that gave the best results - and then I worked out 2 ways to achieve this variable compression engine with a 2 stroke engine - one varying the cylinder height hydraulically (but it would need quite a big pump to move quickly enough) and the other rotating an eccentric sleeve around the crankshaft main bearings via a pivoted arm and smaller hydraulic cylinder (both of which also altered the exhaust and transfer port heights - similar in a 4 stroke to changing valve lift and timing - so providing a double whammy of variable compression and valve timing). It was interesting (even remarkable) to see the results in which a dyno run at low revs with relativley little air trapped and therefore low true compression presure that normally gave small power output - would transform into a very powerful delivery (despite the small amount of fuel) once the compression pressure was raised sufficiently to achieve suitable temperatures and pressures before ignition.
I have been trying to remember the formula for efficiency of an OTTO cycle engine (4 stroke gas burning engine - in this case the gas being petrol and air). I think it uses rules for the adiabatic cycle (on the grounds that there is insufficient time for the gas to draw much heat out of the surounding cylinder walls and head) and that the formula for efficiency is 1-(1/r) (where r is the compression ratio) the result in brackets being raised to the power of constant for the gas used (say X) -1.
In other words the higher the compression ratio the smaller the figure in the brackets and therefore when you take that from 1 the larger the answer (or efficiency). (Sorry if thats wrong it was over 40 years ago)!
This is largely due to Boyles law which I think is that as the volume reduces under compression the pressure rises proportionally. So the higher the compression ratio the higher the pressure of the gas before ignition and the greater burn efficiency and burn pressure results. The work done raising the pressure is seen as an increase in the temperature of the gas - so higher compression results in greater efficiency and higher temperatures (and a better - more powerful burn).
Pre-ignition occurs when the pressure rises so much that the gas reaches its ignition point/temperature before ignition by spark and therefore beats the ignition timing.
So much for the theory (possibly flawed by senility) but the fact remains that compression pressure is the key to performance and worn or oval bores, rings, head seals or valves - all throw away the potential performance otherwise attainable - particularly outside of the natural hot cam timing points.
Baz
I have been trying to remember the formula for efficiency of an OTTO cycle engine (4 stroke gas burning engine - in this case the gas being petrol and air). I think it uses rules for the adiabatic cycle (on the grounds that there is insufficient time for the gas to draw much heat out of the surounding cylinder walls and head) and that the formula for efficiency is 1-(1/r) (where r is the compression ratio) the result in brackets being raised to the power of constant for the gas used (say X) -1.
In other words the higher the compression ratio the smaller the figure in the brackets and therefore when you take that from 1 the larger the answer (or efficiency). (Sorry if thats wrong it was over 40 years ago)!
This is largely due to Boyles law which I think is that as the volume reduces under compression the pressure rises proportionally. So the higher the compression ratio the higher the pressure of the gas before ignition and the greater burn efficiency and burn pressure results. The work done raising the pressure is seen as an increase in the temperature of the gas - so higher compression results in greater efficiency and higher temperatures (and a better - more powerful burn).
Pre-ignition occurs when the pressure rises so much that the gas reaches its ignition point/temperature before ignition by spark and therefore beats the ignition timing.
So much for the theory (possibly flawed by senility) but the fact remains that compression pressure is the key to performance and worn or oval bores, rings, head seals or valves - all throw away the potential performance otherwise attainable - particularly outside of the natural hot cam timing points.
Baz
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