26 replies to this topic

[Steve Scheifler]

Don’t want to start the old pre-ignition vs detonation debate that tends to ignite on car forums, but a colder range plug is typically used in higher performance engines to reduce the risk of pre-ignition. Go too cold and plugs may foul more easily especially during warmup and idling around at low loads, so there are limits both ways. Pre-ignition is just what it sounds like, the air/fuel ignites before the spark plug actually fires, before the piston gets where it belongs on the compression stroke (your base timing BTDC plus whatever the ECU is mapped for at that RPM and load). It’s particularly easy to light the mixture at that point (gets tougher as it is compressed) so a hot plug tip or glowing carbon deposits can start it burning. That’s bad. As it burns of course it expands (the whole point of the exercise) but too soon, so as the piston gets to TDC the flame front is nearly complete and the volume of gasses being squeezed has greatly increased, which means combustion chamber pressure spikes way above normal and the hot gasses are in there longer than they should be. You lose power (wasted trying to compress those premature hot combustion gasses) and the head absorbs more heat, and that cycle can escalate. So, we never run stock heat range plugs. You might get away with it forever, but there’s no point risking it. One, even two ranges cooler has never been a problem for us.

[Ron Alan]

And does detonation(pre-ignition? not the same?) in fact cause early "explosion" which in theory beats on the rod bearing causing failure(early)? If the piston is beating down before the crank(rod bearing)is at or beyond the 12 oclock position...will/does this create a damaged bearing(larger clearance) which then leads to the oils inability to fill the gap/lubricate properly leading to an overheated bearing and an eventual seize??

 

Can detonation cause a motor to fail very quickly...like less than 30 minutes of run time?

[Steve Scheifler]

Detonation is different than pre-ignition though the two can exacerbate one another and both can generate/transfer the heat that James describes. Ideally the spark lights the fuel which then burns very quickly but smoothly across the chamber. Detonation occurs after the spark fires and the burn begins but is an uncontrolled spontaneous “explosion” of the remaining fuel. I put that in quotes because it’s probably not technically accurate and what you hear, the ping/rattle/knock, is actually the engine metal ringing like a bell rather than the “explosion”. Anyway, the instantaneous pressure and heat transfer can be very high. An old low compression Pinto like my sister drove can lug around doing that for years, a turbo engine can crush a piston in seconds, so what fails when depends on a lot of factors. Our cars are somewhere in the upper middle. You may see tiny aluminum deposits on your spark plugs (get a jeweler’s loop), that’s from the piston. Open it up and you will see graining of the piston surface near the exhaust side where it’s getting burned away and scoring of the cylinder. I’ve posted pictures from a Sunbelt which had the piston oilers (actually “coolers”) plugged which shows it very clearly.

Yes, detonation can beat up on bearings too but I expect that’s not the likely failure point in our engines. If you are experiencing excessive bearing wear it may be something else. If you have early/sudden bearing loss look for an over-rev that caused rod stretch, piston touches head, rod bearing get compressed (slightly crushed) and then begins to fail. The damage to the piston can be almost invisible but the force transmitted to the very soft bearing material is enough, and it takes only once.

[Danny Steyn]

High combustion chamber temps can cause valve issues even if they aren’t bouncing, and the way we tend to tune lean with quite a bit of advance I expect things are marginal to begin with. If things lean out a little more at the track due to fuel used, environment, a weak injector, or whatever then sure things can stretch or burn. I haven’t checked EGTs, perhaps someone has, but I saw indication of too much heat on the same cylinder or two I’d probably get my injectors rechecked. If they all seem to be getting too hot I’d question my timing and AFR. Any sign of detonation? What plugs do you run?

 

One of the smartest guys in this class used to tune his car's fuel injectors using thermocouples measuring exhaust gas temps. IIRC, the temp we was chasing was 1350F, but my memory might be failing so don't hold me to this

[Steve Scheifler]

EGT still rules for diagnostics and some kinds of tuning. There’s a reason that prop plane pilots still use EGT for adjusting mixture even long after wideband technology became common. Their lives depend on it. You can learn a lot with one probe per exhaust primary tube.

[BNaumann]

One of the smartest guys in this class used to tune his car's fuel injectors using thermocouples measuring exhaust gas temps. IIRC, the temp we was chasing was 1350F, but my memory might be failing so don't hold me to this

Like every number people want the answer to on the internet (camber, fuel pressure, tire pressure, hp, etc), the actual value only means something if you have a repeatable way to measure it. EGT measurements are pretty sensitive to the location of the thermocouple.

[Steve Scheifler]

True, and the type of engine and fuel. What works in my peripheral port 13B Wankel is not what I want for my SM.

Example of SM engine that had prolonged detonation:

http://forum.specmia...2439;p=0#000001