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What caused #4 piston to go bye bye
I have a stock block 1.8 with a cx racing turbo kit with rx8 injectors. 2 days after getting it tuned I was only running 5 psi, and I came out of boost and the car almost died. It started shaking bad. Sounded like a Subaru and a knocking sound from the engine and exhaust. Compression test was 140psi for 1,2, and 3 but 4 had 30psi cold. Wet comp test they all stayed the same psi. Found misfire on #4 cylinder. I figured it was a valve problem, Then took the head off and found there was a whole chunk out of the piston. The spark plug was also bridged with ash deposits all over it and valves had piston pieces all over them. The injectors testing good and I have the block torn apart, and I just DONT KNOW WHAT HAPPENED!
https://cimg9.ibsrv.net/gimg/www.mia...03861a3c7e.jpg https://cimg1.ibsrv.net/gimg/www.mia...49715fa632.jpg |
When you say injectors tested good, like you had them put on a flow bench? There are a lot of fake RX8 injectors and if you didn't have them cleaned and flow tested before they were put in it could have been an issue if they were dirty and causing issues.
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Originally Posted by Arca_ex
(Post 1616876)
When you say injectors tested good, like you had them put on a flow bench? There are a lot of fake RX8 injectors and if you didn't have them cleaned and flow tested before they were put in it could have been an issue if they were dirty and causing issues.
https://cimg5.ibsrv.net/gimg/www.mia...38b89fa98e.jpg |
Post a tune and log.
Total waste of everyone's time without them. |
At this point seems like it's either in the tune or manufacturing defect in the piston. Post the tune then.
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on the bright side, time for a built motor
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Looks like detonation on both the intake and exhaust side of that piston.
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Originally Posted by sonofthehill
(Post 1616893)
Looks like detonation on both the intake and exhaust side of that piston.
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Originally Posted by andyfloyd
(Post 1616896)
Yep thats what i thought when i looked at it. That cylinder was detonating and since #4 is the hottest it was the first to let go. OP do any of the other pistons show signs of detonation?
https://cimg2.ibsrv.net/gimg/www.mia...3749f43c22.jpg https://cimg9.ibsrv.net/gimg/www.mia...9ab5a38617.jpg |
The missing piece of the piston looks to be caused by the ring butting; too much heat with not enough ring gap. If you look at the fracture surface you see beach marks (red) that radiate away from the initiation point of the fracture (green.)
https://cimg2.ibsrv.net/gimg/www.mia...ae7f43388b.jpg A bad tune would exacerbate the issue, but I'd say the root cause was ring gap that was too tight. |
Originally Posted by Fidgitk
(Post 1616920)
The missing piece of the piston looks to be caused by the ring butting; too much heat with not enough ring gap. If you look at the fracture surface you see beach marks (red) that radiate away from the initiation point of the fracture (green.)
https://cimg2.ibsrv.net/gimg/www.mia...ae7f43388b.jpg A bad tune would exacerbate the issue, but I'd say the root cause was ring gap that was too tight. |
The other factor that few seem to talk about is that we are tuning with a single 02 sensor. But each cylinder has its own EGT and O2 values at a given RPM. So you are basically tuning an average. OEM intake manifold already generates 4 different curves. Put an aftermarket exhaust manifold on there and things get more wack. Thus why we need to tune F/I B series engines so conservatively.
Years ago when we were working on developing a header and intake manifold, we built a special exhaust manifold to monitor each shoulder independently. Project got set aside and never completed. Would have been interesting to see the data. Long story short, that's why number 4 gets toasty. EGR valve forced Mazda to put a kink in the number 4 runner so it's a bit longer. So it resonates at a different frequency than the other cylinders. So basically when the others are rich it's lean and vice versa. We're only talking a few tenths of a point AFR but that's enough to to melt things long term. The B6 has no EGR, equal length intake runners with the same number of bends, thus can be tuned a skosh leaner safely. |
Cast pistons are weak and sensitive to temperature. The thrust side of the piston has to contend with several types of loads and the ring imparts a large force on the perimeter of the land. It could be ring butting, but my intuition tells me that the crown temp was too high and the load just exceeded the material. If I were building with new cast pistons I would clean up the crowns, deburr the valve reliefs and have them coated. The upper ring gap can be fairly loose to be safe, .020" etc. I would just run .020" on both upper and lower. Turbo engines don't typically run at high speeds and the upper ring runs substantially hotter which will net a smaller functional gap then the second in operation.
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who tuned this car and let's see the tune/log. it takes talent or serious part failure to melt pistons on a BP
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