03-02-2012, 02:33 PM
As a result of developing the supercharger kit, I have learned an awful lot about the 968 engine and what it can and cannot do or tolerate. Developing it allowed me the "opportunity" to try a number of things and see what the results and effects were. Doing over 140 dyno pulls, and over 20 hours of on the road tuning showed a LOT.
What I wasn't suspecting though was to find any serious weak links in the engine. I expected certain limitations, but not truly weak components.
We knew about some oiling issues, but as this was a street kit, that seemed to be a non-issue.
We knew about overheating issues, but again, given the recommendations for improvements given with the kit, that seemed to be fine as well.
We knew about the head gasket issues, and knew what to do about that.
We knew about the valve guides and such, and knew what to do about that.
Over the years we had been told that the bottom end of this engine was pretty bullet-proof. I took that as gospel, and didn't worry about it. Sure, the rod bearing thing was a potential concern, but that was also manageable. While the crank itself is truly stellar at resisting wear, the bottom end is by no means bullet-proof.
Here's where it gets messy. As a part of development, I wanted to see what kind of boost the engine could take. I wanted to push things up just a bit. I was able to handle the mixture and heat issues. That was no problem. I expected more trouble with that, but it was actually pretty easy. What I did not expect was an OEM major component failure.
We knew that the early rods were found to be weak and fail at the connection of the upper and lower half of the rod. Porsche beefed that up and issued an updated rod. That was not the problem. The problem is the material used in making the rods, and the design of them.
Every engine has a designed in "weak link". This is to help reduce the chance of catastrophic failure and grenading. Porsche decided to make the rod the weak link. Many engine designers do this. They essentially have to choose between the rod and the bearing/crank. The rod is usually less expensive and less damaging. So, to how I figured this out.
In trying to resolve an inexplicable oil consumption issue, I changed out quite a few things with no success. I changed out the head, the Air/Oil Separator, and tried multiple cleanings of the engine internals with products well known at succeeding at curing problems due to dirty rings and such. No luck. It became obvious that something else was going on, and it was likely worn oil rings. so, out came the engine.
Upon inspection at disassembly the problem became obvious. Rods had bent, causing the piston to go cock-eyed through the cylinder as it went through each stroke. They basically collapse under the load. That meant that the oil rings were hitting the side wall wrong and wore out, which resulted in the oil passing by the rings.
The head gasket was perfect, as were all parts of the combustion chamber and pistons. We never over-revved the engine, as the redline was always kept stock. There were absolutely no signs of detonation. We knew that already though, as there were monitors on the knock sensors to tell us if we had that problem, and they never did.
The damage was purely as a direct result of boost creating a final compression ratio beyond the limits of the rods to withstand it.
This will be the case with both early and late rods. It has nothing to do with the connection point. The failure occurs well up the rod nearer the wrist pin. This will happen to any engine that pushes boost up too far. It may not happen right away, but it will happen. You probably won't know it either for quite a while. My engine continued to run just fine, even though it had bent rods. There was no sign whatsoever that there was a problem. No noise, no pressure loss, not even a discernible power loss. There was no clue whatsoever that this kind of problem existed, but for the oil consumption.
In looking back at what was happening, we know exactly when this happened, and at what boost level. We know exactly the point at which it will quickly happen again, and can take a good guess at which point it would gradually begin to happen.
These engines are capable of handling boost, and are still excellent candidates for low boost kits. They do extremely well under the right conditions. They have their very real limits though, and exceeding those is problematic.
So, the bottom line is that if you want to play around and try to run higher boost than about 6 psi, and you want to keep the 11:1 ratio, you have to change the rods to Carrillo or something like that which can withstand the pressure. Porsche rods will not do. It has nothing to do with heat, fueling, detonation, or anything like that. It is the design of the rod itself. Changing to the stiffer aftermarket rod though also means that the rod bearings now become the weak link, as they will be taking the entire load of the constant pounding, and will need more regular changing.
As with pretty much everything else, there is no free lunch.
So, basically another $10k is going into this project. I could have had a V8 - LOL.
I'll post pics next week when I get everything all cleaned up and have good ones to post.
What I wasn't suspecting though was to find any serious weak links in the engine. I expected certain limitations, but not truly weak components.
We knew about some oiling issues, but as this was a street kit, that seemed to be a non-issue.
We knew about overheating issues, but again, given the recommendations for improvements given with the kit, that seemed to be fine as well.
We knew about the head gasket issues, and knew what to do about that.
We knew about the valve guides and such, and knew what to do about that.
Over the years we had been told that the bottom end of this engine was pretty bullet-proof. I took that as gospel, and didn't worry about it. Sure, the rod bearing thing was a potential concern, but that was also manageable. While the crank itself is truly stellar at resisting wear, the bottom end is by no means bullet-proof.
Here's where it gets messy. As a part of development, I wanted to see what kind of boost the engine could take. I wanted to push things up just a bit. I was able to handle the mixture and heat issues. That was no problem. I expected more trouble with that, but it was actually pretty easy. What I did not expect was an OEM major component failure.
We knew that the early rods were found to be weak and fail at the connection of the upper and lower half of the rod. Porsche beefed that up and issued an updated rod. That was not the problem. The problem is the material used in making the rods, and the design of them.
Every engine has a designed in "weak link". This is to help reduce the chance of catastrophic failure and grenading. Porsche decided to make the rod the weak link. Many engine designers do this. They essentially have to choose between the rod and the bearing/crank. The rod is usually less expensive and less damaging. So, to how I figured this out.
In trying to resolve an inexplicable oil consumption issue, I changed out quite a few things with no success. I changed out the head, the Air/Oil Separator, and tried multiple cleanings of the engine internals with products well known at succeeding at curing problems due to dirty rings and such. No luck. It became obvious that something else was going on, and it was likely worn oil rings. so, out came the engine.
Upon inspection at disassembly the problem became obvious. Rods had bent, causing the piston to go cock-eyed through the cylinder as it went through each stroke. They basically collapse under the load. That meant that the oil rings were hitting the side wall wrong and wore out, which resulted in the oil passing by the rings.
The head gasket was perfect, as were all parts of the combustion chamber and pistons. We never over-revved the engine, as the redline was always kept stock. There were absolutely no signs of detonation. We knew that already though, as there were monitors on the knock sensors to tell us if we had that problem, and they never did.
The damage was purely as a direct result of boost creating a final compression ratio beyond the limits of the rods to withstand it.
This will be the case with both early and late rods. It has nothing to do with the connection point. The failure occurs well up the rod nearer the wrist pin. This will happen to any engine that pushes boost up too far. It may not happen right away, but it will happen. You probably won't know it either for quite a while. My engine continued to run just fine, even though it had bent rods. There was no sign whatsoever that there was a problem. No noise, no pressure loss, not even a discernible power loss. There was no clue whatsoever that this kind of problem existed, but for the oil consumption.
In looking back at what was happening, we know exactly when this happened, and at what boost level. We know exactly the point at which it will quickly happen again, and can take a good guess at which point it would gradually begin to happen.
These engines are capable of handling boost, and are still excellent candidates for low boost kits. They do extremely well under the right conditions. They have their very real limits though, and exceeding those is problematic.
So, the bottom line is that if you want to play around and try to run higher boost than about 6 psi, and you want to keep the 11:1 ratio, you have to change the rods to Carrillo or something like that which can withstand the pressure. Porsche rods will not do. It has nothing to do with heat, fueling, detonation, or anything like that. It is the design of the rod itself. Changing to the stiffer aftermarket rod though also means that the rod bearings now become the weak link, as they will be taking the entire load of the constant pounding, and will need more regular changing.
As with pretty much everything else, there is no free lunch.
So, basically another $10k is going into this project. I could have had a V8 - LOL.
I'll post pics next week when I get everything all cleaned up and have good ones to post.
94 Midnight Metallic Blue Cab Porsche 968 w/deviating cashmere/black interior and WAY too many mods to list - thanks to eric for creating www.968forums.com
"It isn't nearly as expensive to do it right as it is to do it wrong."
