[DonWG]

Originally Posted by lazaris

"Something else to think about in the scenario that the waste spark is igniting anything at the top of the exhaust stroke. Let's say the a/f mixture went through the ignition cycle without lighting off. Now the piston drops and uncompresses the mixture.

When this occurs, isn't some of the heat from the previous compression now drawn out from the decompression?

When an a/f mixture is decompressed, does some of the mixture condense into larger droplets from the previous vaporized state, due to the drop in pressure and temperature?

Isn't most of the charge expelled from the cylinder way before the 24 or so degrees before TDC of the exhaust stroke? In fact, most likely past the turbo by this point with a short runner exhaust system?"

Don,

What I think you maybe missing in regards to left over exhaust to ignite, is the overlap , especially with bigger duration cams. The new fresh charge being pumped into the chamber during this overlap, some of it goes out with the exhaust. Cant help it. Lobe separation angle also plays a part here. When this happens I can see why it may raise EGT's. There may be enough fresh A/F to burn.

I'm following your reasoning. But is this the sort of thing usually taken into consideration when tuning a big naturally aspirated V8 with monster cams on an engine dyno? I don't believe so. When watching a F1 engine on a dyno, do you see exhaust flame out the exhaust during a power pull? No. You do see it during coastdown overrun. So fuel can definitely be ignited by a hot exhaust system, but why do you only witness it on a dyno during overrun? Or during special turbo spooling techniques? If fuel was being burned in the exhaust or during the exhaust stroke, wouldn't exhaust valve durability become a problem? We've all heard of how an exhaust gasket leak can cause burned valves.

I wasn't missing the big overlap deal. It's just that most TR cams in use are designed with zero or negative overlap numbers.