Should I replace these cam gears?
07-13-2015, 08:19 AM
#31
Rennlist Member
Some interesting inputs here. Referring back to Adrian's original point about trying to understand whether the cam wheels are past their sell by date- I had this dilemma a year or so ago with obvious signs of coating wear on my cam wheels. After posting pics in this regard some folks recommended that the wheels were shot but the local maintenance manager [a friend of mine] felt there was still plenty of use left in them despite the wear pattern.
This started me thinking about why the coating wear invariably seems to be limited to the top flats of the wheel but not in the valleys that seem to show no signs of polishing whatsoever. Remember it is the valleys that take the drive load not the top flats so what is going on here?
As I thought about this it made me wonder if in fact Porsche may possibly have slipped up here. Could it possibly be that in their calcs they did not take into consideration the thickness of the applied finish coating when they specified the cog geometry?
Does not sound like the kind of mistake Porsche would make but I dare say stranger things have happened in industrial engineering.
Just a thought
Regards
Fred
This started me thinking about why the coating wear invariably seems to be limited to the top flats of the wheel but not in the valleys that seem to show no signs of polishing whatsoever. Remember it is the valleys that take the drive load not the top flats so what is going on here?
As I thought about this it made me wonder if in fact Porsche may possibly have slipped up here. Could it possibly be that in their calcs they did not take into consideration the thickness of the applied finish coating when they specified the cog geometry?
Does not sound like the kind of mistake Porsche would make but I dare say stranger things have happened in industrial engineering.
Just a thought
Regards
Fred
07-13-2015, 12:43 PM
#32
Former Vendor
Some interesting inputs here. Referring back to Adrian's original point about trying to understand whether the cam wheels are past their sell by date- I had this dilemma a year or so ago with obvious signs of coating wear on my cam wheels. After posting pics in this regard some folks recommended that the wheels were shot but IJ the local maintenance manager [a friend of mine] felt there was still plenty of use left in them despite the wear pattern.
This started me thinking about why the coating wear invariably seems to be limited to the top flats of the wheel but not in the valleys that seem to show no signs of polishing whatsoever. Remember it is the valleys that take the drive load not the top flats so what is going on here?
As I thought about this it made me wonder if in fact Porsche may possibly have slipped up here. Could it possibly be that in their calcs they did not take into consideration the thickness of the applied finish coating when they specified the cog geometry?
Does not sound like the kind of mistake Porsche would make but I dare say stranger things have happened in industrial engineering.
Just a thought
Regards
Fred
This started me thinking about why the coating wear invariably seems to be limited to the top flats of the wheel but not in the valleys that seem to show no signs of polishing whatsoever. Remember it is the valleys that take the drive load not the top flats so what is going on here?
As I thought about this it made me wonder if in fact Porsche may possibly have slipped up here. Could it possibly be that in their calcs they did not take into consideration the thickness of the applied finish coating when they specified the cog geometry?
Does not sound like the kind of mistake Porsche would make but I dare say stranger things have happened in industrial engineering.
Just a thought
Regards
Fred
Many factors need to be taken into consideration, but especially important is the intended use.
If the owner intends on driving the car 2,000 miles a year, the belt will need to be changed again, before the gears are a real problem. Replacing the gears, given this information, would be a waste of money.
However, If the owner expects to change the belt again in 45,000 miles, these gears need to be changed.
Irregardless if what happens with the cam gears, the oil pump gear needs to be changed to the steel gear.....that's a "must do" operation.
With regards to your question about the wear on the gears, the timing belt is designed to hit the top if the gear. The bottom teeth to not hit the bottom of the valley.
If you stop and look/think about it, the relatively square edges of the top of the gear in contact with the relatively square edges of the valleys in the belt, gives a much better "drive action" than the round edges of the belt hitting the round edges of the valleys on the gears.
I believe this is how all drive belts, like this, are designed.
07-13-2015, 06:49 PM
#33
Rennlist Member
It's always a dilemma,in the shop, when we see gears in this condition, while installing a cam belt. These gears have another 20,000 miles before they are worn to the point of being detrimental to the running if the engine.
Many factors need to be taken into consideration, but especially important is the intended use.
If the owner intends on driving the car 2,000 miles a year, the belt will need to be changed again, before the gears are a real problem. Replacing the gears, given this information, would be a waste of money.
However, If the owner expects to change the belt again in 45,000 miles, these gears need to be changed.
Irregardless if what happens with the cam gears, the oil pump gear needs to be changed to the steel gear.....that's a "must do" operation.
With regards to your question about the wear on the gears, the timing belt is designed to hit the top if the gear. The bottom teeth to not hit the bottom of the valley.
If you stop and look/think about it, the relatively square edges of the top of the gear in contact with the relatively square edges of the valleys in the belt, gives a much better "drive action" than the round edges of the belt hitting the round edges of the valleys on the gears.
I believe this is how all drive belts, like this, are designed.
Many factors need to be taken into consideration, but especially important is the intended use.
If the owner intends on driving the car 2,000 miles a year, the belt will need to be changed again, before the gears are a real problem. Replacing the gears, given this information, would be a waste of money.
However, If the owner expects to change the belt again in 45,000 miles, these gears need to be changed.
Irregardless if what happens with the cam gears, the oil pump gear needs to be changed to the steel gear.....that's a "must do" operation.
With regards to your question about the wear on the gears, the timing belt is designed to hit the top if the gear. The bottom teeth to not hit the bottom of the valley.
If you stop and look/think about it, the relatively square edges of the top of the gear in contact with the relatively square edges of the valleys in the belt, gives a much better "drive action" than the round edges of the belt hitting the round edges of the valleys on the gears.
I believe this is how all drive belts, like this, are designed.
I have no detailed knowledge of what the design engineers consider in these drive systems thus no real position on whether they are designed correctly or not- as an engineer I try to visualise what happens in practice at the micro level and concluded that I would expect to see most signs of contact wear at the top [right hand side?] of the valley where logically the maximum shear load is taken. Presumably the resistance to motion changes throughout the cycle and that probably explains why some teeth appear to wear more than others but I still struggle to understand why the top surface polishes and not the side wall. Logically there has to be a reason why Porsche changed the tooth design to the later variant and my presumption was that it was somehow connected to the wear patterns experienced on the earlier design.
Regarding the oil pump cog I looked at mine most carefully and could see no signs of wear or damage and visually it looked to me as though the cog was a steel unit although I never thought to check. I was under the impression that Porsche fitted steel cogs to the 90 S4 motors in the factory but please do correct me if my understanding is incorrect. if there were issues with earlier oil pump cogs one would think such updates would have been incorporated by the time the 90 model came along [surely?].
I noted Mark Kibbort's comments about wear and belt tension being overdone that seemed to make some sense. I have always felt the stock tensioner was adequately designed/robust. In 16 years of ownership [but not a huge mileage] I tend to change belts every 5 years or so initiated in some [2?] cases by water pump failure. Given I live in a somewhat dusty atmosphere I might expect some abrasion factor is in play so try not to take any chances with this item but if the dust were the main causal factor then I might have expected a more even wear pattern.
Regards
Fred
07-13-2015, 07:06 PM
#34
Former Vendor
Hi Greg,
I have no detailed knowledge of what the design engineers consider in these drive systems thus no real position on whether they are designed correctly or not- as an engineer I try to visualise what happens in practice at the micro level and concluded that I would expect to see most signs of contact wear at the top [right hand side?] of the valley where logically the maximum shear load is taken. Presumably the resistance to motion changes throughout the cycle and that probably explains why some teeth appear to wear more than others but I still struggle to understand why the top surface polishes and not the side wall. Logically there has to be a reason why Porsche changed the tooth design to the later variant and my presumption was that it was somehow connected to the wear patterns experienced on the earlier design.
Regarding the oil pump cog I looked at mine most carefully and could see no signs of wear or damage and visually it looked to me as though the cog was a steel unit although I never thought to check. I was under the impression that Porsche fitted steel cogs to the 90 S4 motors in the factory but please do correct me if my understanding is incorrect. if there were issues with earlier oil pump cogs one would think such updates would have been incorporated by the time the 90 model came along [surely?].
I noted Mark Kibbort's comments about wear and belt tension being overdone that seemed to make some sense. I have always felt the stock tensioner was adequately designed/robust. In 16 years of ownership [but not a huge mileage] I tend to change belts every 5 years or so initiated in some [2?] cases by water pump failure. Given I live in a somewhat dusty atmosphere I might expect some abrasion factor is in play so try not to take any chances with this item but if the dust were the main causal factor then I might have expected a more even wear pattern.
Regards
Fred
I have no detailed knowledge of what the design engineers consider in these drive systems thus no real position on whether they are designed correctly or not- as an engineer I try to visualise what happens in practice at the micro level and concluded that I would expect to see most signs of contact wear at the top [right hand side?] of the valley where logically the maximum shear load is taken. Presumably the resistance to motion changes throughout the cycle and that probably explains why some teeth appear to wear more than others but I still struggle to understand why the top surface polishes and not the side wall. Logically there has to be a reason why Porsche changed the tooth design to the later variant and my presumption was that it was somehow connected to the wear patterns experienced on the earlier design.
Regarding the oil pump cog I looked at mine most carefully and could see no signs of wear or damage and visually it looked to me as though the cog was a steel unit although I never thought to check. I was under the impression that Porsche fitted steel cogs to the 90 S4 motors in the factory but please do correct me if my understanding is incorrect. if there were issues with earlier oil pump cogs one would think such updates would have been incorporated by the time the 90 model came along [surely?].
I noted Mark Kibbort's comments about wear and belt tension being overdone that seemed to make some sense. I have always felt the stock tensioner was adequately designed/robust. In 16 years of ownership [but not a huge mileage] I tend to change belts every 5 years or so initiated in some [2?] cases by water pump failure. Given I live in a somewhat dusty atmosphere I might expect some abrasion factor is in play so try not to take any chances with this item but if the dust were the main causal factor then I might have expected a more even wear pattern.
Regards
Fred
Belts and drives are an interesting thing to study. Gates has quite a bit of information available and there is good reading there.
The 928 cam belt is certainly an older design belt and Gates has had several versions of cam belt design since the 928 belt was designed. If you study these newer designs, they seem to be trying to spread the loads on the belt out to a higher percentage of the belt surface (which is probably a good thing.)
I looked seriously at a newer design belt and drives for the 928 engines, when it looked like the 928 cam gears might be "gone forever" and it appeared that someone was going to have to build new gears. However, the cost of having a new design belt and gears made was absurd.
That "study" does not make me an expert, just a mechanic being a student.
If you decide to study this topic, I'd certainly like to hear what you figure out, from an engineer's perspective.
BTW.....I agree that the oil pump gear change occurred around the 1990 model year. I had assumed that the change to a steel gear was because of wear, until I started seeing cracks in the aluminum gears, whereupon I decided that Porsche had seen something and made a change long before I saw a problem. While it's fun to try and guess why they do some things....in the end, it's just a guess.
(I still see cars with aluminum ball joints....on a frequent basis. Porsche certainly did not "recall" those ball joints or make some statement like "all of the 1R connecting rods must be replaced if more than 2 nuts are removed from the entire set of rods"....they just said that they had been superceeded. I tell people that the ball joints are weak and should be replaced....but I certainly can't produce any paperwork that proves that.....nor have I ever seen one, personally, that had failed. I can "suggest" that they get replaced, but that is about as far as I can go. However, I have seen broken oil aluminum oil pump gears....that I can tell people they need to change.)
At any rate, I now install a steel oil pump gear every time I do a belt change on a car originally equipped with an aluminum gear. These vehicles have enough stuff going wrong and wearing out, without me leaving in something that I know can fail.
I'm also realistic enough to know that at the end of the day, I'm just a tiny, squeaky little voice in the 928 world....and people are going to do what they desire to do, regardless of what I suggest.
Last edited by GregBBRD; 07-13-2015 at 07:27 PM.
07-14-2015, 07:06 AM
#35
Rennlist Member
I would be delighted to try to design something if I had the necessary background. I consider the 928 engine bay a work of art in terms of design packaging- how many vee 8's hide under a bonnet so low? Not one to this day I suspect. I think the only real world possibility would be if it is possible to make a cam wheel out of high tech lightweight materials like carbon fibre or something that could be 3D printed? Sadly I doubt such material exists and if it did it would probably cost more than the car is worth [sad to say].
Rgds
Fred