Sorry to hear about your troubles, there's a lot of good advice above. If it's a true 12:1, that's definitely a lot of compression even running 110. Your dynamic will be slightly higher with the milder cam too. I second the mentions that 30 degrees is a LOT of timing for twin-plug. I run 24 in my 11.5:1 3.0 twin plug race engine (built to HSR rules). It did make a little more power on the chassis dyno with 26-27, but I wanted to be conservative especially since I'm running carbs. The pics are of the plugs in an engine that ran lean, didn't even run it a full lap. After that I installed an AEM afr meter. With sunoco purple (110), the O2 sensor only lasts ~5 race weekends.
http://forums.pelicanparts.com/uploa...1743212901.jpg
http://forums.pelicanparts.com/uploa...1743212901.jpg

Ugly.

Yep that motor was really unhappy

Damn SV - Flame thrower. How long did that run and it wasn't misfiring like crazy? I have never seen plugs that bad even on Turbos that went kapputt.

R lane You didn't share which A/F meter you were using? Also, given what you have seen a new sensor is on order no matter what at this time even if a high quality device is on hand. In our experience the Bosch Lambda 4.9s last at least 50 hrs, often 80-100 w leaded fuel. Never buy from a cheapie internet source(goes for everything on these engines) , loads of Chinese knock offs out there.

Also check your rod bearings are okay, or you will be right back here with even greater damage...and how did ring land gaps on top ring measure?

Lastly, trusting your piston sheet is a mistake, actually CC your dome displacement volume, sometimes they are quite off from intended design as I shared.

Good luck

Kevin
GAS Motorsport

Hey Kevin, yeah it was a bad day. Rolling race start and down the front straight at summit point. It was down power, so after turn 1 i limped it back to the pits but the damage had been done. Amazingly those plugs were still firing. The HVC coil is a heck of a coil.

Burned p/c
 

svcetiquette, my condolences. You are running a 6 plug which I think might be a little warm. The motor is back together and will install today I plan. I cut the heads .015 which dropped the chamber to 83.8-84.2. Cut the top of the pistons to keep the compression at 12. I find measuring the pistons very tedias as the dome height measurement accuracy is very critical to getting the correct dome volumme. Replaced the rod bearings on #1 but the old ones looked just like the new ones. #1 piston and its ring gaps looked and measured fine. Bosche LSU 4.9 sensor. I now have a head temp sensor. Plan to get it to op temp, back timing down and check with a light and run it up and down the ave a few laps, let it cool and check the head bolt torque. If all appears well, will go up to RA for a PCA race in two weeks. will keep posted. Thanks for all the share. Bobhttp://forums.pelicanparts.com/uploa...1743426919.jpg

burned p/c
 

What did you find to be the culprit. Timing? You have backed the timing down and the problem appears solved? That is what is keeping me awake at night as there are a few things I can change, but is that my problem. I had an oiling issue with a motor 2-3 years ago that didn't manifest until I increased the power. So I wonder if this is what I am dealing with. An existing problem that didn't bring the previous motor down, but assuming that I have increased the power with this motor, is now a problem. Bob

31 degrees ignition advance at full throttle for a twin plug motor is way too advanced. Doesn't matter if you're running 110 octane, or your dynamic or static compression is as low as 10.0:1. You're causing the peak pressure of combustion to hit your pistons as it's still rising up in the cylinder, hammering on the pistons, cylinders, crank, bearings, etc. Imagine when you press down on a bicycle pedal for max torque. You want to press down in at 1:00 position of rotation, not at 11:30 - you'll hammer your knees. If you use a dyno, you can advance your timing until power levels and back down from there for margin, assuming fuel octane is not your limitation. When you advance timing on the dyno, when it levels, and goes down after that, you went too far. The suggested timing numbers given by others here are a good place to start.

svcetiquette, in the picture of your head it looks like there is a crack from the intake seat over to the spark plug hole.

An article on pre ignition vs detonation: http://forums.pelicanparts.com/porsche-911-technical-forum/250384-detonation-knocking-etc-3.html#post2232959

My last race engine (120HP per liter at the crank) had 12.5:1 compression using 110 and 112 leaded race gas. I ran no where near advance 30 degrees of timing...

Oh, NTK oxygen sensors handled leaded fuel better than Bosch oxygen sensors...

Installed a knock sensor kit but the controller appears defective. Fellow sent another by USPS and they apparently have lost it. Have a dyno appointment on Tues morning, so will make some runs without the benefit of knock control or being able to see when PI or det is occuring. I plan to set my timing at 24* and be armed with a stethoscope. Not sure for what I will be listening but will continue to raise the timing a bit at a time until HP has reached its terminous. I am thinking for my setup, 26* might be my limit. Prescribed procedures On how to know where to set ignition to avoid melt down here would be greatly appreciated. Thanks, Bob

Start at 20 deg, then keep advancing until the power drops off, then retard 2 degrees from there.

Installed a J&S knock sensor system and took it to the dyno yesterday. At 20* we had 267HP, added 2 degrees and had 287HP. 2 more degrees and 291HP. At the top of that run the low end of the LED bar flickered briefly. The LED bar has low, medium and high light intensity indicating the severity of the knock and that timing has been pulled back. Off to the track to see if we are safe. Bob

Nice result. So you ended up at 22 degrees?

My last run was at 24*. I think I am ok with that, maybe replace think with hope, but will I think bring it back to 22* as the extra 2* netted me but 4 additional HP. An interesting aside to backing off timing is that as you reduce that number, the piston finds itself further up the barrell when ignition happens. Meaning that it has less distance to travel when pushing against the force that ignition has just initiated. Bob

That’s a LOT different than the previous 30 degrees !!

Tested at a track night, 3 sessions. Engine felt good and no issues. 4 days at Summit Point and no engine issues. Have just pulled the valve covers to check valve lash and head torque, all good. Am at 22* timing and engine has good power. Don;t know if it is an improvement over the previous engine with ''my'' heads at 28* timing, but it felt good and it stayed together. Thanks again for all the counsel. Bob