EGT data from burnt pistons.....

If you are trying to relate EGTs to piston temps then you are comparing apples to oranges. The temps on the top of those pistons will not get to 1250* because of the heat sink effect of the rings and skirts, the oil being splashed up underneath the piston and the intake of cool fresh air when the intake valve opens. All these aid in keeping a piston cool. For an example with a nitrous motor I have burned a piston at 1300* on the EGT but then have run successfully with the EGTs at 1500+* on three stages. If you advance the ignition the EGTs will decrease and if you have a lot of retard then the EGTs will increase because you are moving the peak burn time of the mixture back and forth and when you retard it more flame and heat comes out the exhaust so it goes up. Normally when you have EGTs above 1100* then things become critical such as ignition timing or the CR of the motor and the amount of boost or the fuel mixture. Any of those items can cause detonation and that is one of the main causes of piston damage. High EGTs are a good indication that you are heading in that direction. Detonation normally pushes down the crown on the top edge of the piston and pinches the rings and will also cause the aluminum from the top edge of the piston to tranfer onto the cylinder wall then it is downhill from there real fast as more and more transfer until you get a hole for the flame to travel through. I have always been told that if the piston damage is in the center of the piston such as a hole or caved in then that is caused by pre-ignition which means for some reason or another the ignition fired off way too soon while the piston was still coming up and a tremedous amount of heat is now generated as this exploded mixture is further compressed during the cycle (like a diesel) and the internal temps now become hot enough to melt the middle of that piston. This pre-ignition can be caused by sparkplug wires that have had the insulation break down and are cross firing, or the spark jumping around inside the mag cap and firing off the wrong cylinder or from using an unshielded cranktrigger cable that causes the mag to fire wrong cylinders.

I would be interested in knowing where the damage to your piston occured.

 

That is detonation damage. Was there any damage to the sparkplug that was in that cylinder? If not look at the groundstrap of that plug with a bright light and with a magnifying glass and see if the corners or sharp edges of the plug's groundstrap have been rounded off. Compare it with a new plug and a plug from another cylinder. Let me know what you see.

One of the problems on running without EGTs on a blown alky motor is that you can't tell which cylinders are getting more boost than the others so you can't adjust the port nozzles to balance all the EGTs. That cylinder may have been getting more boost than the others. You may have also had the fuel system swallow some dirt and block that associated port nozzle jet so check it and see if it is blocked. Do you run a fuel filter or have the mesh strainer on the inlet of the barrel valve? The problem is with dirt stuck in the port nozzle is that when you let up the dirt can get sucked out of the jet so there is no evidence and you have the perfect murder.


What do the flat metal bases on the plugs look like? Like new, light gray, dark gray or frosty? Do all the plugs look identical on coloring and how much coloring?

What is the compression ratio of the motor and have you actually measured it? How much boost are you making at 8000 RPM in high gear? Or what is the peak on the boost gauge? If no gauge then what OD on what blower are you running?

 

head, egt location

Unfortunately there is no magic number that covers every blown alcohol combination. There are a number of factors at work against this, and the only way to definitively know on your combination is to creep up on egt's until you burn a piston.

The better a head flows, the lower the egt where you will get into trouble. Also, if your EGT's are on the bottom of the tube, they are about 80 deg cooler than top mount EGTs.

EGT's are about the third thing you need to be looking at. Plugs, bearings then EGTs. A hot EGT doesn't always mean your running hot, you could have retarded timing firing fuel in the pipe, thus making the egt read warmer.

I've ran cars that get into trouble at 1060 and a PSI BBC that routinely had EGT's over 1300 with no trouble.

 

Well your CR maybe just a little high but close. Your timing should be good. I believe that racers run more advance than that with that motor but maybe somebody can chime in and tell you about where the timing is best at. When you say the plugs are half to three quarters on color then what color are they?

If it was me I would keep that timing where it is and check that port jet for blockage and maybe bring it up five sizes bigger.

That aluminum on that cylinder wall sometimes can be peeled off with a razor blade and a little patience then run a ball hone up and down a few times. I have fixed worse.

Don't feel bad about that happening. I bet it has happened to all of us.

 

Sorry, totally missed that you are a BBF. That's what I get for reading it on my BlackBerry screen. What heads are you using?

 

I will get back to you