My 1968 911T engine is now dismantled, the valve springs removed are single and not the double ones that are shown on the Porsche PET, perhaps a later upgrade? The spring tension has been tested, out of the 12, 11 are within specification. I am replacing the cams, the manufacturer of the cams suggested to stick with the single springs - any DDK advice either way would be appreciated.
Some of the rockers have picked up on the shafts, I suspect some oil starvation at some point. Replacing the oil pump part of the engine rebuild is a no brainer. What pump should I fit, I believe the SC pump fits and is better? Does anyone have one or know where to buy one from (most reasonable source).
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Jimmy
Valve springs, single or double & oil pump
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Jimmy
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abzadams
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Re: Valve springs, single or double & oil pump
Re oil pump. When I did mine I did the upgrade. You need to the oil bypass modification on the case to fit the SC pump. I was looking for a secondhand one for ages and then I was told try to get a recon from a known source as the last thing you want is to button the whole engine back up and then find it was poo.
I got mine from Supertech in the states and Henry does a little mod on them to make them a little more efficient (15%). It landed in the UK for around £450
HTH
I got mine from Supertech in the states and Henry does a little mod on them to make them a little more efficient (15%). It landed in the UK for around £450
HTH
Rich
Sepia 72 2.5T/E (gone)
2004 996 Turbo
Sepia 72 2.5T/E (gone)
2004 996 Turbo
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fetuhoe
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Re: Valve springs, single or double & oil pump
The 1968T did use a single spring as standard from the factory but weren't used for very long.
If only 11 are within spec you could have a problem as I have not seen the single springs being sold.
They were only used for a short time and then replaced with the regular dual springs used on all other 911s from 65-89.
I don't understand why your cam guy is concerned about using standard dual springs as they work well and don't cause excessive cam wear.
Your rockers will have galled onto the shafts because from 1967 to 1969 Porsche in their wisdom used an investment cast rocker are without any bush.
The only lubrication that is available for the rocker shaft is the small holes in the rocker arm and this results in a 'mixed-mode' lubrication that is not effective when you try to run steel on steel.
After 1969 all rocker arms were fitted with Glyco bushes of the same type as fitted to the forged rockers used from 1965 until the 1967 Model Year.
They were used right through until the 964 and are very effective and perform well.
The cost of boring out the old rockers to fit these bushes is just not worthwhile.
The only other rocker used on 911s that didn't use a bush was the 'solid' 906 style rocker used on the 911R, ST, RSR and finally the 935.
These rockers had significantly more oil grooves and were used in conjunction with a surface treated rocker shaft produced by Porsche Motorsport and which has been NLA for many years.
Oil pumps and 911s are a very interesting subject.
I am not convinced that a 1968 911T needs anything better than a standard pump.
The engine is 110BHP, it doesn't have any auxiliary coolers and runs quite well with good oil pressure when both the engine and pump are in good condition.
I would agree that improving the scavenge section of the pump could have positive benefits but these only become apparent at high engine speeds where reducing windage is an important issue.
A standard 911T just doesn't rev fast enough for this to be a consideration.
If the axial and radial clearances in the pump are within sensible limits and there is no scoring to the housing then it is very unlikely that a new pump is needed.
I would disassemble the pump, taking care to mark two of the mating teeth so it can be re-assembled in the same position and inspect for wear.
Unless the gear teeth are chipped or damaged by having ingested debris the mesh will almost certainly be in good order.
If there is no sign of radial wear or axial wear in the housings, again the pump is unlikely to 'spill'.
I have been debating oil pump mods with Henry for some years now and it is fair to say that we don't agree about the validity of the concept.
The claim is that that by modifying the pump the flow of oil in the engine will increase but sadly this is simply inaccurate.
Oil flow in an engine is determined by bearing clearances, pistons squirter volumes and the spray bars in the cam towers.
The pump itself will not produce any pressure until it is flow limited and it is quite clear that the pump produces more flow than is required as pressure increases with engine speed until it reaches the limit set by the pressure limiting valve.
There are several ways on increasing pump flow. The first is to increase pump speed, the second is to increase the length of the gear mesh and third is to increase the number of teeth which generally mean larger diameter rotors.
The only way to achieve these objectives is to change the pump.
If you increase the capacity of the standard pump on an engine that isn't fitted with piston squirters all you will do is increase the parasitic losses and shove more oil down the Pressure Limiting Valve which will, of course have a negative impact on oil temperature.
The Modification carried out to the SC pump involves a small change to the angle of the inlet port to the pressure section of the pump.
The claim is that the poor fluid dynamic behaviour of the standard pump prevents oil for being effectively gathered and by smoothing the flow the output increases.
I just can't see how this works. The inlet to the pump is fed by the head in the oil tank which provides a positive pressure and primes the pump.
If oil flow were restricted then in my experience the pump would cavitate and this would have a catastrophic impact on the engine.
Cavitation is a process where insufficient oil is fed to a pump and the pressure in the mesh falls until the it reaches its vapour pressure and then these is no effective output.
This would mean that during periods of cavitation the bearings would see any oil and would be instantly damaged.
I have never heard of 911 oil pumps suffering from cavitation and they are generally bomb-proof.
Having spent more than 20 years making Pump Test Rigs I do have some experience in this area and I just don't follow the logic.
If you have a very high rpm race engine then using Turbo and GT3 pumps with very large scavenge capability is very good idea.
Modern pumps such as the Auto Verdi unit are also good at high speeds as they use a very effective Roots style pump for scavenge and a carefully sized pressure pump to control parasitic loss.
There are no parts available for the refurbishment of 911 Oil pumps so I am not sure what can really be achieved.
It is fair to say that Henry doesn't agree with my comments.
If only 11 are within spec you could have a problem as I have not seen the single springs being sold.
They were only used for a short time and then replaced with the regular dual springs used on all other 911s from 65-89.
I don't understand why your cam guy is concerned about using standard dual springs as they work well and don't cause excessive cam wear.
Your rockers will have galled onto the shafts because from 1967 to 1969 Porsche in their wisdom used an investment cast rocker are without any bush.
The only lubrication that is available for the rocker shaft is the small holes in the rocker arm and this results in a 'mixed-mode' lubrication that is not effective when you try to run steel on steel.
After 1969 all rocker arms were fitted with Glyco bushes of the same type as fitted to the forged rockers used from 1965 until the 1967 Model Year.
They were used right through until the 964 and are very effective and perform well.
The cost of boring out the old rockers to fit these bushes is just not worthwhile.
The only other rocker used on 911s that didn't use a bush was the 'solid' 906 style rocker used on the 911R, ST, RSR and finally the 935.
These rockers had significantly more oil grooves and were used in conjunction with a surface treated rocker shaft produced by Porsche Motorsport and which has been NLA for many years.
Oil pumps and 911s are a very interesting subject.
I am not convinced that a 1968 911T needs anything better than a standard pump.
The engine is 110BHP, it doesn't have any auxiliary coolers and runs quite well with good oil pressure when both the engine and pump are in good condition.
I would agree that improving the scavenge section of the pump could have positive benefits but these only become apparent at high engine speeds where reducing windage is an important issue.
A standard 911T just doesn't rev fast enough for this to be a consideration.
If the axial and radial clearances in the pump are within sensible limits and there is no scoring to the housing then it is very unlikely that a new pump is needed.
I would disassemble the pump, taking care to mark two of the mating teeth so it can be re-assembled in the same position and inspect for wear.
Unless the gear teeth are chipped or damaged by having ingested debris the mesh will almost certainly be in good order.
If there is no sign of radial wear or axial wear in the housings, again the pump is unlikely to 'spill'.
I have been debating oil pump mods with Henry for some years now and it is fair to say that we don't agree about the validity of the concept.
The claim is that that by modifying the pump the flow of oil in the engine will increase but sadly this is simply inaccurate.
Oil flow in an engine is determined by bearing clearances, pistons squirter volumes and the spray bars in the cam towers.
The pump itself will not produce any pressure until it is flow limited and it is quite clear that the pump produces more flow than is required as pressure increases with engine speed until it reaches the limit set by the pressure limiting valve.
There are several ways on increasing pump flow. The first is to increase pump speed, the second is to increase the length of the gear mesh and third is to increase the number of teeth which generally mean larger diameter rotors.
The only way to achieve these objectives is to change the pump.
If you increase the capacity of the standard pump on an engine that isn't fitted with piston squirters all you will do is increase the parasitic losses and shove more oil down the Pressure Limiting Valve which will, of course have a negative impact on oil temperature.
The Modification carried out to the SC pump involves a small change to the angle of the inlet port to the pressure section of the pump.
The claim is that the poor fluid dynamic behaviour of the standard pump prevents oil for being effectively gathered and by smoothing the flow the output increases.
I just can't see how this works. The inlet to the pump is fed by the head in the oil tank which provides a positive pressure and primes the pump.
If oil flow were restricted then in my experience the pump would cavitate and this would have a catastrophic impact on the engine.
Cavitation is a process where insufficient oil is fed to a pump and the pressure in the mesh falls until the it reaches its vapour pressure and then these is no effective output.
This would mean that during periods of cavitation the bearings would see any oil and would be instantly damaged.
I have never heard of 911 oil pumps suffering from cavitation and they are generally bomb-proof.
Having spent more than 20 years making Pump Test Rigs I do have some experience in this area and I just don't follow the logic.
If you have a very high rpm race engine then using Turbo and GT3 pumps with very large scavenge capability is very good idea.
Modern pumps such as the Auto Verdi unit are also good at high speeds as they use a very effective Roots style pump for scavenge and a carefully sized pressure pump to control parasitic loss.
There are no parts available for the refurbishment of 911 Oil pumps so I am not sure what can really be achieved.
It is fair to say that Henry doesn't agree with my comments.
-
Jimmy
- Married to the DDK
- Posts: 245
- Joined: Fri Nov 23, 2012 6:52 pm
- Location: Aldeburgh, Suffolk
Re: Valve springs, single or double & oil pump
Many thanks fetuhoe, your response / advice is much appreciated.
I have managed to locate 11 rockers inc. shafts with the bush you mention, just one more to locate. Perhaps I could bore out an original rocker and fit a bush.
Having 11 good valve springs is useless unless I locate one more, looks like the dual spring is the only option.
Regrettably, my oil pump has seen better days, some scoring on the gears. When we dismantled the case, part of a piston skirt was found lying in the bottom so, some debris would have washed through. I am taking the plunge to fit new pistons and cylinders to the 'S' specification, for this reason fitting a new oil pump seems a worthy investment. Any tips on where to locate a pump would be valued.
Anyone have a set of 2.0 Mahle pistons and cylinders available to sell.....
Thanks again fetuhoe
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Jimmy
I have managed to locate 11 rockers inc. shafts with the bush you mention, just one more to locate. Perhaps I could bore out an original rocker and fit a bush.
Having 11 good valve springs is useless unless I locate one more, looks like the dual spring is the only option.
Regrettably, my oil pump has seen better days, some scoring on the gears. When we dismantled the case, part of a piston skirt was found lying in the bottom so, some debris would have washed through. I am taking the plunge to fit new pistons and cylinders to the 'S' specification, for this reason fitting a new oil pump seems a worthy investment. Any tips on where to locate a pump would be valued.
Anyone have a set of 2.0 Mahle pistons and cylinders available to sell.....
Thanks again fetuhoe
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Jimmy