2.5 litre running 1.5-1.55 bar, it sporadically seems just like an overboost fuel cut, but overboost fuel cut is disabled. I can't think of any other limiting factors, and it isn't consistent, but happens in a high gear at around peak torque when the car feels like it is going real nice.

Plugs are only about 3000 miles old, NGK PFR 7B 0.7mm, will probably close it to 0.6 tomorrow, mixtures are 11.2-11.7 AFR typically (speed density as yet without temperature compensation).

Given that this boost on a 2.5 is not dis-similar in torque terms to about 2.1 bar on a 2.0, could the spark be the problem?

Does it feel like an overboost fuel cut?

Has anyone else had such trouble on bigger engines at such low boost?

 

HT leads, Coil pack thingy? You've got the fuel and the air! I can't imagine that you could actually block the spark with compressed air as it's still a gazzilion volts isn't it? Or perhaps too much fuel? More likely you're drowning the poor spark plug and it's earthing out over the insulator. See what the spark plug looks like when you regap it.

F

 

I had similar problems on a modded Escort RS Turbo (running 28PSI boost ). It was indeed the spark getting blown out. I did as you are suggesting by regapping the plugs to a smaller value. A performance ignition system that allowed a faster, more powerful spark was my next option.

 

It is the pressure, or more accurately gas density, at the point of ignition that determines whether or not the voltage at the electrode is sufficient to ionise the gas across the gap or not, assuming for now this is proportional to torque (almost but not quite) which in turn is created as a function of the bmep, then the ionisation limiting torque on your 2.5 will be at a similar ignition pressure as T*2/2.5 on a 2 litre.
ie if a 2.0 will run 400 lbft without issue then a 2.5 should be ok with 500 lbft

As you are reducing octane and raising boost then you are backing off the timing, this means that the spark is trying to fire at a higher pressure (later in the compression stroke) It may well be quenching if you are way back at early teens in timing ?

A smaller gap will prove this but be prepared for reduced quality idle and general lethargy off boost !

Andy

 

No problems with 2 bar and 1.1mm gaps here!

Although I do have a little help from the MSD Ignition amp! Idle quality and cruise smoothness and economy are both improved with larger gaps and good spark.

Paul

 

0.6mm gap made if misfire horribly, returning to 0.7mm is better, but still misfires over 1.1 bar. I think these plugs are shot and need to be replaced again.

Need to decide between replacing PFR7Bs every 3000-4000 miles or getting far more expensive racing 8s which may not have a longer service life?

Interesting the fuel cut sensation may well have actually been fuel cut, JECS is quite complex how it determines it so I adjusted lots of things to get rid of it properly. Had I done this first might not have ended up with a plug problem, although they look OK, my last 7Bs that went off did too.

 

the racing 8s do seem to be able to spark at higher boost then PFR7Bs. I wonder if this is due to the shrouding of the plug at all.

Paul

 

John, this is exactly what happened to my 7's part way through mapping before SSO

We were Running fine at 1.1 / 1.15bar, baring for a missfire at around 6000rpm,we then decided to go up to 1.3bar, then as soon as boost got above to 1.3 bar, really bad missfire.

Went back down to 1.1 bar, and it still missfired at/around 1bar. So plugs were goosed.

Changed to the racing 8s, and it was all fine and dandy.

 

That is also the description of what mine is doing..

which I am putting down to the old coil packs..

So another vote for blown out spark.

Time for MSD?