injection timing vs exhaust popping

[Yves]

I still have the following question about an exhaust high rpm popping on my car during decel :
So far I've established that the engine pops in the exhaust mainly due to the fuel that clings to the intake walls and intake port and subsequently that fuel is sucked into the cylinder, doesn't burn and accumulates in the exhaust and then burns the first time I open the throttle after a decel.

Now my intake as far as injector angle to the inlet air stream is far from optimal, which led me to introduce an injection timing that was well in advance of the intake valve opening. I've tried several setups in injection timing that led to the conclusion that an injection timing well in advance was functioning better at low rpm (< 4000 rpm). However I'm not so sure at high rpm. Lately I have reduced the advanced injection timing which seems not to have a great effect. I'm now leaning towards setting the injection timing close to intake valve opening at anything above 4000 rpm. Any ideas about this ?

[BigBlockMopar]

Any chance you're not removing enough fuel on deceleration?
Also timing could be earlier when mixtures are lean, like during decel. Maybe increase timing a bit more when engine loads are low?

[Yves]

I tried fuel cutoff during decel, but it made it pop even more.

[BigBlockMopar]

Too much initial fuel shot, which doesn't completely burn, when you're accelerating again after decel?

[Yves]

Too much initial fuel shot, which doesn't completely burn, when you're accelerating again after decel?

Is possible. But wouldn't that show at all rpm's ?

[BigBlockMopar]

Exhaust leak somewhere?
Any logs of this happening? msq?

I have no experience with FI at all.
Just a number of years of carb-tuning.

[turbo conversion]

I tried fuel cutoff during decel, but it made it pop even more.

That should not happen if working correctly as fuel is shut off completely when in over run.

David

[BigBlockMopar]

Unless, there is still fuel on the intake port walls as he suspected which get sucked in by the really high vacuum on Decel.
Personally I think wet intake walls could only happen with a cold engine and rich mixture. Or a plain overly rich mixture all together.

Another thought,
if the injection timing is really early and during a closed intake valve, it would mean portflow is low/absent and fuel may just be sprayed against the intake port wall and 'puddle up' there until the valve opens and fuel enters the cylinders as a crude combination of air and droplets again.

[kaeman]

I found that my fuel injector timing worked best when I had end of squirt at 265 degrees. This made the mixture better and the engine idled smoother, with less fuel. That setting worked on my hilborns and on the new Accufab/rhs setup. If you have the settings to advanced then you might as well just run batch fire, because your fuel is just going to fall out of suspension on the back of closed valves like the batch fire, which is one of the reasons that the sequential is supposed to be better.
I know a couple of car builders using the holly injection systems and they are trying to end their fuel pulse during the intake stroke just before intake valve closure and the start of the compression stroke, They said it made a big difference in the engine... after playing around with my timing I found the sweet spot was at 265 degrees.

[Yves]

A couple of notes to this : I run a crossram ITB. The problem with this manifold is that there is limited room to place the injectors as the air horns are sitting just above and on the inside of my valve covers. The maker of the manifold therefore placed the injectors 90° to the airstream (well in fact a little less than 90° but in any case not spraying at the valve).
I have found that the engine idles better when the fuel is allowed to vaporize due to the manifold heat.
I also have a tech article here about injection timing in which it was explained that best results were gotten when the injected fuel was allowed a 8 ms timing interval to vaporize. This turned out to work very well at idle. I then converted the 8 ms to crankdegrees taking into account the speed of the engine. So in effect injection timing increases with rpm.
Only side effect seems to be the popping, which as explained above seems to have some correlation with wall wetting, which is then sucked off the walls when the engine is under decel and strong vacuum.

Another note is that I tried injecting the fuel at the time the intake valve opened. This made for same rough running throughout the rpm and deterred me from using intake valve opening as an injection timing at high rpm/decel to combat the problem.

[turbo conversion]

I understand wall wetting but once over run is activated the puddled fuel should burn off very quickly.

I have the fuel cut delay set to 1.2 seconds and as soon as it activates the exhaust is smooth with out any popping.

With fuel cut turned off I have exhaust popping until over run is eliminated by rpm and load.

Not sure why the popping continues once fuel cut is activated on your set up

David

[whittlebeast]

What David said.

[Yves]

I understand wall wetting but once over run is activated the puddled fuel should burn off very quickly.

I have the fuel cut delay set to 1.2 seconds and as soon as it activates the exhaust is smooth with out any popping.

With fuel cut turned off I have exhaust popping until over run is eliminated by rpm and load.

Not sure why the popping continues once fuel cut is activated on your set up

David

I think the fuel is not burned due to not enough O2 being present on decel. The fuel vapours coming from the wall are then pumped into the exhaust, where they burn upon the next time the engine fires because of more air being present in the exhaust. At least this is what I think happens.

[turbo conversion]

I understand what you are saying but at some point once over run is activated all residual fuel should be burned.

If you have over run activated and then decel for say 10 seconds or so does the popping stop then?

David

[Yves]

I understand what you are saying but at some point once over run is activated all residual fuel should be burned.

If you have over run activated and then decel for say 10 seconds or so does the popping stop then?

David

Based on my memory, it pops when going into decel and then again when opening the throttles again.

[Moody]

I have played around with this on itb motorcycles and have had good success with increasing ignition timing in the decel areas. My highest ignition timing is during the decel on my ignition tables. I don't even use the fuel cut feature due to the abrupt on and off of the feature.

The attached pics of my ignition table is on my personal boosted bike where I'm using %baro as primary and alpha-n secondary guel loading and %baro ignition loading
rather than ITB loading.

Dustin

Sent from my SM-G930R4 using Tapatalk

[Yves]

Yesterday I was thinking that maybe my VE is too high at the high rpm decel areas. I never really tuned these areas as I used the mlv veal to correct them. Possibly it could have been rich misfiring and pushing the mixtures in the exhaust. Set them a lot lower now. Still have to test this in driving situations.