Fuel Cut On Deceleration?

[benl1981]

Since fitting a PFC I notice that the injectors don't pulse on decel. Or as soon as the throttle is closed (assuming still ingear). This is to save fuel, just wondering how the ignition still carries on. The engine keeps going because of the inertia etc but I assume the spark plugs still spark but there is nothing to ignite but the engine keeps going I guess.

Cheers

[siksII]

how do u know that the injectors dont pulse? i really doubt it...

im assuming u are talking about how the injector duty drops to zero(powerfc) on no throttle sometimes, it does with my car too and believe the injectors are still injecting, just too small an amount to register.

[Cubes]

The PFC and stock ECU does cut fuel on deceleration to conserve fuel.

One good reason to use lots of time decelerating, also saves your brakes.

You will notice in the rev/idle menu of the pfc there is 2 options that allow you to set the fuel cut return rpm. The RB20det's are typically set of 850rpm a/c off 950rpm a/c on.

Edited October 9, 2005 by Cubes

[siksII]

so you are saying there is physically NO fuel being injected when they injector duty shows zero? that would mean the equivalent of turning the ignition off would it not?

[Cubes]

Exactly.

All modern ecu's contain this requirement as its required for emissions.

[siksII]

so in theory if this is the case, there would be NO exhaust gases coming out of the exhaust when injectors are at 0%, just air?

In that case the exhaust would all of a sudden go very quiet would it not, and i dont think that happens....???

[Cubes]

lol.

Roll down a hill and turn the ignition off, be sure not to have your steering lock up.

Tell me if the exhaust still has noise.

On deceleration, a car that has deceleration fuel cut sounds different to a car that doesn't have it.

Edited October 9, 2005 by Cubes

[benl1981]

My car is quite a bit louder as soon as I lift of the throttle. The smallest throtttle brings the injectors on again and it is quieter.

Just wondering if the spark plugs still spark while on fuel cut....I didnt think it would be good as it is running lean - i.e. no fuel - but I guess there is no load..

Since I found out this if I'm coming up to a set of red lights I'll just leave it in gear and hit brake... I used to sometimes knock it into neutral, this would be using more fuel and also there is no engine braking.

[siksII]

lol.

Roll down a hill and turn the ignition off, be sure not to have your steering lock up.

Tell me if the exhaust still has noise.

On deceleration, a car that has deceleration fuel cut sounds different to a car that doesn't have it.

<{POST_SNAPBACK}>

i will try it next time i drive, although my exhaust is nearly silent most of the time anyway so it will be hard to tell...

i always leave it in gear until im nearly at a standstill, i dont trust the car to stay on when dropping to idle especially with the random turning off problems that people get..

edit: and how do u explain bens exhaust noise when injectors are at 0? surely just the air is not making the exhaust sound loud?

Edited October 9, 2005 by siksII

[benl1981]

Are you running standard cat back siks? I wouldnt mind putting a stock cat back on mine so it doest hang as low and is a bit quieter. Not that mine is really loud though.. its a kakimoto cat back.

Would I lose much putting standard cat back on? I have 3" dump/front/3"cat

The engine is under huge vacuum on decel - not sure if this contributes to the louder noise. Its more noticeable below 3500rpm. My exhaust gets quieter as you rev it

Edited October 9, 2005 by benl1981

[siksII]

i have a full 3 inch exhaust from turbo back, and the muffler has 4 inch tip. not sure about the cat though.

mine is one of the quietest i have heard, not much noise on or off throttle it doesnt even have a proper muffler in the middle so it must be a good mufller, i think its an apexi super megaphone.

although, above around 5500 it changes tone and goes quite loud, people driving behind me say it sounds like i have an external gate at high revs

Edited October 9, 2005 by siksII

[Cubes]

Check the internal design of the muffler, some jap mufflers I've seen tend to taper down to a smaller size than the entering and exiting pipe, as a result its a quiet muffler.

It may also be quiet as it may have a center muffler.

[siksII]

Check the internal design of the muffler, some jap mufflers I've seen tend to taper down to a smaller size than the entering and exiting pipe, as a result its a quiet muffler.

It may also be quiet as it may have a center muffler.

<{POST_SNAPBACK}>

the muffler is definetly straight through with no tapering at all..

and it does not have a center muffler, only one of those skinny hot dog things that is about half an inch thick around the pipe.

in my car the intake and engine noise is way louder than the exhaust, which is the opposite of most others ive experienced.

[Cubes]

Thats all good then..

Just a well designed muffler.

I've got a rather larger (as big as will fit) aussy straight through muffler on the end of mine with a resy in the middle.

It was fairly quiet with the stock turbo on it, since I bolted up the new turbo with a slightly larger turbine housing it has got a little louder.

Should also mention mildsteel is quieter and less drony than s/steel.

Offtopic lol

Edited October 9, 2005 by Cubes

[siksII]

yeah more gas going through it i guess u cant help the extra noise...

i have stock turbo and 10psi so im sure that helps with the quiet-ness

the cat back came with the car and im sure its at least 5 or six years old so if anything its probably louder now then when new!