1985 Monte Carlo SS known as Barney

[mfain]

Lance,

Do you see any signs of detonation?

Pappy

[SSLance]

Originally Posted by mfain

Lance,

Do you see any signs of detonation?

Pappy

A lot on #4, the one that cracked and burned and just a couple small pits on #1. Tops of all the rest of the pistons look great, including #8 with broken ring land.

[z28cp]

Originally Posted by SSLance

I'm still very happy with the power this combo made so I'll not be making any drastic changes to the recipe, just fortifying some of the weaker parts inside and putting it all back together once again. It'll just take a bit longer and cost a bit more is all.

Same situation with mine. Just upgraded the weak parts. Rods, crank. One thing we did was replace all 8 sleeves (Brodix aluminum block). We needed to replace 4. Then we would have needed to bore them .030" over, to match the other 4. Or, replace all 8, and now have a standard bore block, which will allow for some freedom in the future.

Only money...

[mfain]

Originally Posted by SSLance

A lot on #4, the one that cracked and burned and just a couple small pits on #1. Tops of all the rest of the pistons look great, including #8 with broken ring land.

Might want to investigate the source of the detonation - usually an octane, timing, or AFR problem. Cast pistons tend to give up early (broken ring lands or cracks at the wrist pin), but even good forged pistons don't like detonation. Were you running an AFR gage or monitoring EGTs? With a nominal compression ratio, octane should not have been a problem, barring excessive timing advance. Sounds like there might have been something in the tune at some point that was causing problems - just my opinion.

Pappy

[SSLance]

I was not data logging at the time, but I have plenty of WOT logs showing the tune performing as desired.

I'm still leaning more toward something happening in that cylinder that raised compression or lowered octane enough to cause the melting of the edge of the piston. There were several mechanical failures there and all of the rest of the cylinders were fine.

We may have been close to the edge and a minor variance put it over, not discounting that...but if it truly was only tune related, it would have shown more damage across the board and most likely would have happened earlier.

It had been pushing antifreeze out recovery tank for two events prior, indicating the start of the real problem. The burnt piston is more of a result of that problem than the tune.

But yes, I probably will back it back away from the edge next go ahead.

[Tinker]

Too bad you had to tear it apart again Lance, but addressing the possible shortcomings will continue to make your engine more bulletproof. I enjoy following your journey because except for a minor setback such as this your car is a contender amongst much higher dollar builds out there. Keep it up and let us all learn and enjoy with you as you go.

[CamaroAJ]

Just throwing this out there, what were the ring gaps set to?

[SSLance]

Originally Posted by CamaroAJ

Just throwing this out there, what were the ring gaps set to?

That is on my list of things to check.

[Vegas69]

Originally Posted by SSLance

More tuning on the Holley. Found and corrected several issues which created the touchy throttle and have it MUCH better now. Super easy to be very smooth on throttle now. Also added some enrichment on MAP rate of change which it didn't have before, helped tremendously with tip in power. Can't wait to race it again Sunday.

Video for attention...



Had a pretty scary hung throttle while testing yesterday. Had a green light with no traffic in front so made a pretty hard hit in first and second gear, then the throttle hung wide open going to third. Pedal was on the floor so I immediately hit the key to switch it off. After I caught my breath while still coasting, I started it again with same result. Once stopped I pulled air cleaner but didn't see the culprit, fiddled with throttle cable and all seemed good so I started it again and it was fine. Once I got it back home, I discovered this...



The screws holding the throttle position sensor had come loose, one completely gone and the other finally loose enough that the shaft jumped the slot in the sensor and the sensor held the throttle open. Yes, the replacement screws got a dose of loc-tite upon reinstall and yes, throttle application is much smoother now.

I was running a datalog during the hung throttle...finally got up the nerve to look at it.

The green line is RPM... At least I know my rev limiter works now...

This may be the answer you are looking for. A long rev limiter hit is tough on your bottom end, especially with cast pistons. When I had my ZL-1 built, my engine builder preferred I raise the rev limiter vs. bounce it off repeatedly.

[SSLance]

OMG Todd, I had forgotten all about that incident.

You very well may be onto something there.