Bore torn up by Piston
05-21-2010, 02:06 PM
#1
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Bore torn up by Piston
This is a bit academic, as I am about to install a properly rebuilt engine anyway, but I am interested in understanding what might have gone wrong with my previous engine! I figure most of the lurking 944 engine gurus are in this forum, so below are a couple of pics of my no. 4 piston and bore after a teardown. This is a supercharged 968 engine.
The car has previously run fine for nearly two seasons of racing until I had a local tuner install some arrow rods recently to toughen up the bottom end. At the same time I changed from stock to a cometic gasket. He also did a tune following - at that time I was using the factory ECU with a Haltech Interceptor.
The car ran ok on his dyno, but when run later on the race control dyno for power checking we noticed it was running extremely lean. For a lot of reasons I was already fed up with the original tuner so rather than screw around I had a Motec installed by another tuner and the car was retuned. All looked good.
However, car went on to blow three head gaskets in next three sessions. First time was the cometic, that just exhibited general poor sealing. I figured maybe the deck and head surfaces weren't good enough for a cometic so installed a stock gasket, which blew next time out on no.4. Rechecked the tuning and installed a widefire, but on next session out had the failure here accompanied by lots of oil smoke from the catchcan filter. Pulled it apart and the widefire had started to deform in the same place on no 4. Pulled the piston out and this is the damage. The rings have seized into the ring lands and the piston has tore up the bore.
What I am wondering is whether this is likely to have been a single event last time out, or accumulated piston and bore damage over the last few sessions. Never had a chance to inspect the bore as each time I changed the gasket no 4 was at TDC. Overheating obviously, but just bad tuning or could the rings have been incorrectly installed?
The car has previously run fine for nearly two seasons of racing until I had a local tuner install some arrow rods recently to toughen up the bottom end. At the same time I changed from stock to a cometic gasket. He also did a tune following - at that time I was using the factory ECU with a Haltech Interceptor.
The car ran ok on his dyno, but when run later on the race control dyno for power checking we noticed it was running extremely lean. For a lot of reasons I was already fed up with the original tuner so rather than screw around I had a Motec installed by another tuner and the car was retuned. All looked good.
However, car went on to blow three head gaskets in next three sessions. First time was the cometic, that just exhibited general poor sealing. I figured maybe the deck and head surfaces weren't good enough for a cometic so installed a stock gasket, which blew next time out on no.4. Rechecked the tuning and installed a widefire, but on next session out had the failure here accompanied by lots of oil smoke from the catchcan filter. Pulled it apart and the widefire had started to deform in the same place on no 4. Pulled the piston out and this is the damage. The rings have seized into the ring lands and the piston has tore up the bore.
What I am wondering is whether this is likely to have been a single event last time out, or accumulated piston and bore damage over the last few sessions. Never had a chance to inspect the bore as each time I changed the gasket no 4 was at TDC. Overheating obviously, but just bad tuning or could the rings have been incorrectly installed?
05-21-2010, 02:18 PM
#2
Drifting
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Just throwing some theory as this is what the thread is likely to be #4 would be the coollest bore as it gets the coollant first so perhaps if the pistons got to hot and grew to much it went bad ..
standard 968 pistons ? trashed bores like that and extreme scuffing is what i hear about the incorrect piston coating .
standard 968 pistons ? trashed bores like that and extreme scuffing is what i hear about the incorrect piston coating .
05-21-2010, 03:09 PM
#4
Drifting
I am not a lurking expert, but I had the same issue. It was the tuning of my TEC GT stand alone. The only cylinder that had any damage was number four. The cometic gasket blew at number four.
I'm now running full sequential so we can tune each cylinder seperately. These 968 motors built for turbo applications are not cheap. I did not tune the car.
This tune will be with the help of Chris White.
Bad tunes are F****** expensive.
I'm now running full sequential so we can tune each cylinder seperately. These 968 motors built for turbo applications are not cheap. I did not tune the car.
This tune will be with the help of Chris White.
Bad tunes are F****** expensive.
05-21-2010, 03:50 PM
#5
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Just throwing some theory as this is what the thread is likely to be #4 would be the coollest bore as it gets the coollant first so perhaps if the pistons got to hot and grew to much it went bad ..
standard 968 pistons ? trashed bores like that and extreme scuffing is what i hear about the incorrect piston coating .
standard 968 pistons ? trashed bores like that and extreme scuffing is what i hear about the incorrect piston coating .
#4 cylinder gets the coolant after 1,2,&3. Then the coolant goes to the head where #4 combustion chamber gets the coolant before 3,2 &1.
Looks like piston overheating (due to lean mixtures not coolant temps) – you mentioned that the builder replaced the rods – did he reuse the old pistons and rings?
05-21-2010, 05:02 PM
#7
#4 piston the hottest, lean tune, piston expanded a bit too much causing scored cylinder. And I will also add that maybe to top it off the coolant system wasn't burped correctly.
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05-21-2010, 05:25 PM
#8
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Other way around...unless coolant pumps spin backwards in Australia!
#4 cylinder gets the coolant after 1,2,&3. Then the coolant goes to the head where #4 combustion chamber gets the coolant before 3,2 &1.
Looks like piston overheating (due to lean mixtures not coolant temps) – you mentioned that the builder replaced the rods – did he reuse the old pistons and rings?
#4 cylinder gets the coolant after 1,2,&3. Then the coolant goes to the head where #4 combustion chamber gets the coolant before 3,2 &1.
Looks like piston overheating (due to lean mixtures not coolant temps) – you mentioned that the builder replaced the rods – did he reuse the old pistons and rings?
05-21-2010, 05:29 PM
#9
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Thread Starter
At the time of failure, the coolant temp was rock steady around 90C. I was runnning a new safe tune and the AFR's were rich if anything, the timing advance had also been reduced. This was a logging session on a conservative tune. The Motec was using lambda control and there are no indications on the log of leaning out or overheating. Unless no 4 was leaning out individually which is possible I suppose.
That's why I wonder if there was already damage from the previous sessions when the car did overheat, that only showed up on this session. Can this type of damage only happen suddenly, or could it have been developing? If it happened suddenly it means there are still major tuning issues, but if it was residual damage from previous sessions, the new tune may have been fine, just too late to undo existing damage.
I am pretty careful about bleeding the coolant system. With the first two head gasket failures there was fairly sudden overheating and coolant loss and the engine was shut down straight away. In this case the water temp was still normal at the point I shut it down which was as soon as I got smoke in the cabin, which turned out to be huge amounts of blowby coming through my less than perfectly sealed firewall. It probably would have overheated pretty quickly if I had kept running, but in this case thre was no coolant loss or temp spike at all.
That's why I wonder if there was already damage from the previous sessions when the car did overheat, that only showed up on this session. Can this type of damage only happen suddenly, or could it have been developing? If it happened suddenly it means there are still major tuning issues, but if it was residual damage from previous sessions, the new tune may have been fine, just too late to undo existing damage.
I am pretty careful about bleeding the coolant system. With the first two head gasket failures there was fairly sudden overheating and coolant loss and the engine was shut down straight away. In this case the water temp was still normal at the point I shut it down which was as soon as I got smoke in the cabin, which turned out to be huge amounts of blowby coming through my less than perfectly sealed firewall. It probably would have overheated pretty quickly if I had kept running, but in this case thre was no coolant loss or temp spike at all.
05-21-2010, 05:32 PM
#10
Other way around...unless coolant pumps spin backwards in Australia!
#4 cylinder gets the coolant after 1,2,&3. Then the coolant goes to the head where #4 combustion chamber gets the coolant before 3,2 &1.
Looks like piston overheating (due to lean mixtures not coolant temps) – you mentioned that the builder replaced the rods – did he reuse the old pistons and rings?
#4 cylinder gets the coolant after 1,2,&3. Then the coolant goes to the head where #4 combustion chamber gets the coolant before 3,2 &1.
Looks like piston overheating (due to lean mixtures not coolant temps) – you mentioned that the builder replaced the rods – did he reuse the old pistons and rings?
maybe the piston rings were moved around a little after already seating in. Due to changing the rods..... Could that cause catastrophic failure?
05-21-2010, 07:26 PM
#11
Drifting
At the time of failure, the coolant temp was rock steady around 90C. I was runnning a new safe tune and the AFR's were rich if anything, the timing advance had also been reduced. This was a logging session on a conservative tune. The Motec was using lambda control and there are no indications on the log of leaning out or overheating. Unless no 4 was leaning out individually which is possible I suppose.
That's why I wonder if there was already damage from the previous sessions when the car did overheat, that only showed up on this session. Can this type of damage only happen suddenly, or could it have been developing? If it happened suddenly it means there are still major tuning issues, but if it was residual damage from previous sessions, the new tune may have been fine, just too late to undo existing damage.
I am pretty careful about bleeding the coolant system. With the first two head gasket failures there was fairly sudden overheating and coolant loss and the engine was shut down straight away. In this case the water temp was still normal at the point I shut it down which was as soon as I got smoke in the cabin, which turned out to be huge amounts of blowby coming through my less than perfectly sealed firewall. It probably would have overheated pretty quickly if I had kept running, but in this case thre was no coolant loss or temp spike at all.
That's why I wonder if there was already damage from the previous sessions when the car did overheat, that only showed up on this session. Can this type of damage only happen suddenly, or could it have been developing? If it happened suddenly it means there are still major tuning issues, but if it was residual damage from previous sessions, the new tune may have been fine, just too late to undo existing damage.
I am pretty careful about bleeding the coolant system. With the first two head gasket failures there was fairly sudden overheating and coolant loss and the engine was shut down straight away. In this case the water temp was still normal at the point I shut it down which was as soon as I got smoke in the cabin, which turned out to be huge amounts of blowby coming through my less than perfectly sealed firewall. It probably would have overheated pretty quickly if I had kept running, but in this case thre was no coolant loss or temp spike at all.
Completely tore down the motor, replaced all bearings, had the crank rebalanced, and installed new pistons, rings, and sent the block to www.uschrome.com to be re NiCom coated. All because of a tuning issue.
We've rewired new leads to the fuel pump. My situation was happening over time, not a single event.
Chris White will hopefully share some of his experience on this topic.
05-21-2010, 07:40 PM
#12
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Search around for the guy in the UK who built an (um...) S2? low boost turbo and scuffed a bore also. My feeling is that lean-induced detonation is cocking the piston in the bore.
I have a supercharged 968 with 928 Motorsport's blower on. I am considering getting a thick head gasket and perhaps resizing the rods a bit shorter to be more boost tolerant, although I might not really need to at my level of boost. I will also probably run water injection which I have used to great effect on my old Saab. I have a set of forged rods to install so I might do the other changes while I am in there.
One thing I think could be an issue and might have killed my brother's supercharged 928S4 is tip-in detonation, where you go to WOT suddenly and get a shot of boost while still on a part-throttle timing map. An accelerator pump type setting in an engine management system can help with this, water injection might too if the system responds quickly enough.
-Joel.
I have a supercharged 968 with 928 Motorsport's blower on. I am considering getting a thick head gasket and perhaps resizing the rods a bit shorter to be more boost tolerant, although I might not really need to at my level of boost. I will also probably run water injection which I have used to great effect on my old Saab. I have a set of forged rods to install so I might do the other changes while I am in there.
One thing I think could be an issue and might have killed my brother's supercharged 928S4 is tip-in detonation, where you go to WOT suddenly and get a shot of boost while still on a part-throttle timing map. An accelerator pump type setting in an engine management system can help with this, water injection might too if the system responds quickly enough.
-Joel.
05-21-2010, 08:56 PM
#13
Burning Brakes
When my gasket went, my temps were fine. My particular issue was happening since the tune was done. I put about 3K miles on the motor. I sent the car to Garrity, and he told me that the tune was not done properly, and the voltage to my fuel pump was falling when on boost. My A/F readings were fine from inside the car. We pulled the head to find signs of detonation at number four only, some scorching in number four, and a few burn marks on the top of the piston. I've heard detonation before, and I didn't hear anything. I also found that my knock sensor wasn't hooked up, which it now is.
Completely tore down the motor, replaced all bearings, had the crank rebalanced, and installed new pistons, rings, and sent the block to www.uschrome.com to be re NiCom coated. All because of a tuning issue.
We've rewired new leads to the fuel pump. My situation was happening over time, not a single event.
Chris White will hopefully share some of his experience on this topic.
Completely tore down the motor, replaced all bearings, had the crank rebalanced, and installed new pistons, rings, and sent the block to www.uschrome.com to be re NiCom coated. All because of a tuning issue.
We've rewired new leads to the fuel pump. My situation was happening over time, not a single event.
Chris White will hopefully share some of his experience on this topic.
Is this the same motor that Tim (Speedforce) built and tuned?
05-21-2010, 10:33 PM
#14
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Clearly #4 has always had this "condition" hence all the head gaskets failures at #4. Im not so quick to blame the tuners. More so our situation. Air fuels probably AVERAGE out ok. but rich in #1 and lean in #4 is not acceptable.. but the A/F in the up pipe will read the average and look good. So..... I'm going to tune with an EGT at every port. I'm sure I will see whats going on then
Anyone else do done something like this?
Fortunately, with my set up I can tune each cylinder individually.. Another huge PLUS. (Someday I'll get to tune it .. ) SOooooo not sorry I went this route. It did hurt my wallet at start though!..
Anyone else do done something like this?
Fortunately, with my set up I can tune each cylinder individually.. Another huge PLUS. (Someday I'll get to tune it .. ) SOooooo not sorry I went this route. It did hurt my wallet at start though!..
Last edited by 95ONE; 05-22-2010 at 02:11 AM.
05-21-2010, 10:38 PM
#15
Drifting
SFR never built a motor for me. Garrity Repta was always the engine builder. We are dealing with racing motors, and no one is to blame. I'm just now taking charge of the tuning and making sure who does it, knows WTF they are doing. Part of the issue was the low voltage going to my 044 pump. We've taken care of this, and we should be fine going forward.
This issue also allowed me to deal with some heat issues, and build a custom intercooler designed for the new turbo and this motor.
No one is to blame but me. I take full responsibility for the issues pertaining to this issue.
This issue also allowed me to deal with some heat issues, and build a custom intercooler designed for the new turbo and this motor.
No one is to blame but me. I take full responsibility for the issues pertaining to this issue.