When on closed thottle you set the injectors to switch off below a certain rev point.

Is this purely to save some fuel as im aware the extra unburnt fuel will increase hydocarbons, but what im wondering is where the sometimes pop and flames come into it?

I realsie that the fuel hitting the hot exhaust can be enough to ignite the fuel and cause the flames and/or pops but is this mapped in???

I'd guess there is a point where by injector cut off will save the most amount of fuel without affecting the engine run down and conversely a point where the amount of fuel ignites and causes the flames/pops???

But is there any adverse affects to letting it flame/pop???

I know my old heavily tuned X-flow many many years ago used to flame on overrun running 40 dcoe webers, yet my Cossie engine with 26psi Boost never did, it didn't even pop on over run.

Just wondered what input any of you experts have on the decel timing???

 

I "think" cos I am no expert that a little extra fuel is sometimes allowed on overrun to help cool the pistons, but I am really not 100% sure about this

Good question anyway, both my cossies pop and bang from time to time when really pushing on, and decelerating hard, but neither has been live mapped.

 

Frog, thats spot on, quite common for tuners to set the zero position on throttle over (for example) 4000rpm to fuel a lot in order to cool the piston crowns down, and this leads to the fireworks.

 

Another reason for pops and bangs is a lean condition on over run. There may be not be enough fuel in the charge for ignittion in the chamber but after a few cycles there is enough in the exhaust to ignite and create the bang.

 

Thanks for the replies......

Begs the question, how does a mapper know where to set the cut off level???

 

Yep, mine has "over-run fueling" if I push it VERY hard, I then lose shite loads of petrol and see a huge fireball out the back of my car for a good 1/4mile+ if just crusing on over-run say in 4th or 5th!

 

his experance,,,

 

Come on Gareth, you can do better than that

Based on what been said so far, its good to have some fuel, but what determines how much and at what point?

Is it really just a calculated guess or is there a measure taken to determine it???

 

experance,,, so a calculated guess

 

The only measure thats likely to be relevant is EGT as thats the only hint you have at piston crown temps, but there isnt a direct correleation between the two.

 

So will an engine setup that doesn't pop/bang or flame on overrun not benefit from holding the injectors open slightly more if it s going to help the cool the pistons?

Or is there a likelyhood of the pop/bang/flame happening on any engine??? when certain conditions are met irrespective of the mapping???

And ive just never had those conditions met......???

 

The heat in the exhaust is absolutely key, so you need to hold the engine flat out under load for a long time to get the heat there to cause the fuel to do anything interesting even if it is present in the right amounts in the exhaust.

I bet its quite hard to keep the engine under hard load in your car for very long at a time.

 

Thats true, it is difficult.

I suspect that the operation of my old dcoe's must have allowed more fuel on overun than my current fuel injected setup, as apart from the much hotter turbo on my current setup the exhausts are essentially the same.

 

will the amount of over-run fuelling have a noticeable effect on the amount of engine braking?

 

nope

 

agreed

 

@28 psi ive got egts ov 740C when i back 0ff instantly down into the 600s, my mate was following me and phoned me said fuck me that was a flame couldnt see the back of the car

 

Is that 740 before or after the turbo?

 

after chipster

 

bet you in the high 10's or very very low 11's then to keep it like that.

 

13.0 up to 5 psi 12s up to 18 11.4 20 to 28 psi

As recomended by chip and stu

 

i always get flames on the overrun if giving it lots of right foot. mapped by kenny@msd 30psi boost

 

Nice