Detonation

Hi guys!
I cracked a piston last races (for the first time!) which forces me to investigate possible detonation. The piston lasted 2 hours and I checked it after 1 hour without any signs of detonation or cracks. As I have understood (at least) the following parameters are crucial:
-pressure (compression ratio, cam/ignition-timing, etc)
-geometry
-fuel
-temperature

I have surely missed something. If having problems with detonations, what is the right things to start to look at?

Engine spec: 2003-model, 2004-piston, #53-cam, Shell V-Power

RE: Re: Detonation

Hello Sparks!
I have never noticed any signs of detonation, even when running 13,5-14,0 compression ratio and 99 octane Shell V-Power (normal trade fuel). I´m aware of how those damages look and where they normally appear. Let´s say I building up an argumentation with my supplier... With this actual piston I have measured the geometric compression ratio to 12,7:1. Seems like I´m failing to enclose pictures. I´ll try to send you one directly.

RE: Re: Detonation

NSMAN,
I do not monitor either CHT or EGT. Since I fitted the short exhaust system, the muffler insulation have turned into glass rapidly, but that has more to do with the shorter primaries. I think I managed to enclose a photo. Sparks put his money on detonation. There are no other signs of detonation and the hole has sharp edges without any signs of wear around it. :?:

RE: Re: Detonation

I will investigate a little bit further. As far as my racing experience goes: You must find the primary cause of the failure and make a solid action to prevent it from happening again, OR it will, without any doubt, jump up and bite you again.

RE: Re: Detonation

I'll get on it next week. I´ve allways been using NGK-plug heat #8 or #9. Before I shut off the engine the plug got very oily, so any investigations will be difficult. I had a close look at the piston this morning and there are absolutely no signs of detonation anywhere.

Today is "Midsummers Eve", one of the most important holidays in Sweden. So I'm occupied with eating marinated herring and drinking vast quantities of alcohol and rounding off with strawberries. If I survive I'll get back to you guys!

RE: Re: RE: Re: RE: Re: Detonation

Hi again! I'm writing to you from the "other side".

Yes, a colder plug is preferrable, but I´ve run #8/#9 since 2002 without any problems. For the moment I'm leaning against the piston definitively having a defect in the material and a slight possibility of detonation or overheating. Here are my arguments:

Detonation - "Myth plausible"
-The engine was used in the second last races by another racer for app. 1 hour. He mounted the engine with his own ignition, carb and exhaust. It might very well have been detonating with his setup. After I got the engine back I runned it, in track speed, for less than 2 minutes then the piston cracked.
-Too hot plug
-Ignition advance is set high. An ex SEM engineer has programmed the module to my wishes. With less advance the carb wasn't possible to jet properly.
-The #53 cam gives high combustion pressure.

Detonation - "Myth busted"
-There are no signs of detonation damages anywhere on the piston, not even in the coldest sections on the intake side. The area close to the hole has no detonation damages. As the hole is only 2 mm wide, signs of detonation should have been visual in the area close to the hole.
-I've been testing out setups after hours of dynoing and been using "same" setup for several races. Both my engines have their individual setup on carburetion. I've been using compression ratios up to 14 without any signs of detonation. This piston gives only 12,7.
-I have raced this piston, when it was new, with this exact setup for 1 hour, without any visual detonation damages.

Defect in the piston - "Myth plausible"
-The direction of the hole and crack is square to the normal deformations in the piston during load. If detonation was the only problem, the crack would have grown in the front-rear direction instead.
-The edges of the hole are clean and sharp. It looks like material was lost along a defined precise geometry, instead of worn out through an undefined force (posssibly detonation or overheating). The hole has the exact shape and dimension from underneith the piston roof.

Defect in the piston - "Myth busted"
I cannot find anything that woud outrule this...

These are my theories (only theories) and I apprecheate all your comments!

Thanks!

RE: Re: RE: Re: RE: Re: Detonation

I'm away from home on a project (work), but I'll take some better photos for you guys when I get back.

Underneith the piston roof, on the piston pin or the rod, there are no signs of overheating. Just clean metal as it should look. There was some damage on the top of the rod, from combustion/material blowing from/through the hole.

I'm not sure wether to call our roadracing tracks long or short. During a race (10-15 minutes), rpm never drops below 5000rpm and the only time we're not giving full throttle is when braking (figurly speaking). No straights are so long you will run on constant rpm (final top speed is usually between 180-220 km/h).

RE: Re: RE: Re: RE: Re: Detonation

Hi again, guys!
I´ve uploaded some pics from the piston in my gallery. After I cut the piston in 2, it was easier to see that the roof actually have sunk a little bit (up to 0,5 mm). That points in the direction of overheating (too hot plug, lean mixture and perhaps bad ignition timing). Still the piston roof looks OK from underneith and there are no signs of overheating on the piston pin and upper rod bushing. There are still no evidence of detonation. The shape of the hole/crack makes me think that this may be a combination of slight overheating and a flaw in the material. As the engine was run 1h in another unoptimized environment, I trust my hours at the dyno are still an investment intact. Are you with me on my conclusions?

I've just ordered a new special piston from Wössner, with 14:1 and 1,1 mm squish with large pockets for my billet head.