deibit

Mine pops at deceleration and also at acceleration (but much much much less than in deceleration of course), in my case it's not because I "dump a lot of fuel" but because I run slightly on the lean side (for performance purposes).

I have intake+header+HFC+exhaust+EMU, the only downside is that the HFC gets more hot than normal, but the extra power/torque is worth it

H22toF20

The only reason it'll "pop" during deceleration is if you're dumping fuel in. Mine did it when I had my AEM EMS in. I then realized that the ECU didn't think the throttle was closed when it was. Fixed that and no more wasted fuel was being dumped in.

deibit

I think it's the other way round.. in decel (foot out of the pedal) the mixture is *extremely* lean. The A/F goes totally out of range to the lean side, you can see that clearly when looking to the A/F gauge. This extremely lean mixture is not able to ignite in the ignition chamber and ignites somewhere between the header and the exhaust (in my case in the HFC since it's the hottest spot).

In acceleration, a tuned engine runs with a "safe" mixture but normally a bit on the lean side, additionally I run advanced ignition timings.. so again some of the fuel is not able to ignite in the ignition chamber, and ignites again in the HFC, that's why i have pops when upshifting, but of course much less than in decel (the mixture is not that extremely lean)

Of course you can also have pops because of extremely rich mixtures.. but that should not happen if the car has a good/safe tune.

RichwiththeS2000

I always thought it popped due to excess fuel making its way into the exhaust and igniting as it went down, when running rich. My previous turbo'd cars all did this and they all ran very rich.

deibit

Well I just know that in my case the injectors are closed in decel.. so there's absolutely no way that the mixture gets more fuel, and the added vacuum provides more air than normal -> Lean mixture.

Actually if I would add more fuel, then the mixture would probable ignite in the ignition chamber and not under my ass

Gernby

I think you have some things backwards. First, an HFC runs hotter when the engine is running rich. The HFC actually burns the unburned fuel that is in the exhaust, so it gets much hotter. You can actually test the effectiveness of an HFC by installing EGT sensors before and after the HFC to see how much it increases the exhaust temp.

Second, I don't think running lean is what causes the pops. If you are running rich durning a WOT pull, then lift the throttle to upshift or decel, then the injectors do shut off. However, there is still a bunch of rich exhaust gas in the exhaust pipe that has almost come to a stop due to the throttle lift. The throttle is closed, but there is still some fresh air flowing through the motor with no fuel. As soon as that fresh air begins to mix with the unburned fuel in the exhaust pipe, it re-ignites.

deibit

Lean = Hotter engine, that's an axiom in engine combustion

Actually, too lean mixtures is the main cause for seized/scored pistons, the temperatures of the gases increase a lot in these situations and it might cause the burnt of the oil that protect the pistons and therefore cause seizures and scoring. Conservative engines run rich just to avoid this problem.

Thats the axiom, but now lets see the my special case.

My car runs lean, that's a fact.. is not dangerously lean but leaner than a stock engine, widebands sensors do not lie (when calibrated correctly of course) additionally my ignition timimg is advanced. The combination of both facts lead to the following:

1- My combustion chamber has trouble to ignite all the mixture (too much air + advanced ignition time=mixture does not ignite well)
2- Engine runs much hotter (that's why i run colder plugs in first place).
3- Because the engine runs much hotter, the exhaust gases are much hotter, therefore my whole exhaust is hotter, including header, and obviously HFC
4- Because of the problems in point 1, there's a slight amount non combusted fuel mixture that goes out of the combustion chamber and (in the best case) it detonates somewhere in the exhaust line, obviously where it's hotter, for example in the HFC (because of that, the HFC gets even more hotter)

I know that my car runs leaner because of the datalogs, and I know that my gases are hotter because my Innovate Wideband (plugged in the header) shuts down when the exhaust gases are over 900 degrees Fahrenheit, and this happens to me (i'm in N/A, should not be that high), I had to install a heatsink (normally used in turbo setups) to avoid that.

My reasoning sounds plausible to me

Gernby

Again, you have several things backwards. A lean mixture and timing advance does NOT lead to unburned fuel in the exhaust. The leaner the mixture is, the more likely the mixture will pre-ignite with a hot / fast flame front. A rich mixture will have a slower / cooler flame front. Obviously, a rich mixture will produce lower exhaust gas temps, but that doesn't help the cat, but it isn't the EGT's that damage a cat. What damages the cat is the heat generated internally by the reaction between the catalyst and unburned fuel. If you are running too rich, the catalyst will literally burn itself up.

FWIW, a wideband sensor measures the amount of free oxygen in the exhaust. The fact that the sensor reads lean indicates that there was more than enough oxygen to burn all the fuel. When the sensor reads rich, it indicates that there is very little free oxygen in the exhaust, which means that there is probably unburned fuel. If you run a test pipe, that unburned fuel will sometimes ignite as soon as it finds some fresh air, which causes cool flames at the exhaust tips.

deibit

I think that the main discrepance in our points of view is that you consider that a lean mixture is easier to ignite (and therefore does not produce unburned fuel), and I think that a leaner mixture is more difficult to ignite.

I guess that we need a engine engineer to clarify this

spectacle

Lot of back and forth here but here's a fact: flames from the exhaust originating off a shift or decel come from three factors:

1. rich A/F's
2. not enough timing to ignite all fuel injected
3. very little fresh oxygen due to throttle closure

Exhaust gases get VERY hot when these conditions are met. Once the above happens, the hot & rich mixture sees oxygen at the tail pipe and boom, we see a flame.

Cool looking, but I wouldn't go out of my way to do it. Especially on a N/A car.