SIMON HOLTBY

Should the timing curves (main spark table) be the same for an 'ideally' tunned car if you were to switch between a 7730 and 808 speed density system?
Its just when you look at a few bins of each that people have posted they look a whole lot different. Especially at low MAP values. 7730 seems to have more advance.
You want quite a lot of advance for low MAP values dont you to reduce backfires and reduce overrun engine brakeing?
I'm finding that if I let off the throttle at 90 mph in overdrive it backfires slightly and the same in drive at a lower speed. ie when there is a lot of vacume in the inlet. Am I correct in thinking that backfires are caused b the ignition being too retarded? Worn valves or piston rings can cause it too cant they?

Simon

Grumpy

With everything done the same they will be the same. I've run several different codes, and they all turn out the same, when everything is done the same.
That said there is some difference from C3 to P4 ecms, the 165/808 is kind of a step child in that league thou, so the difference in fueling might show in timing.

Too lean, too rich, too much advance, too retarded can all cause crackling in the pipes at overrun.

Poorly seating exhaust valve can do it, but gaads, that's rare. I don't see off hand where oil consumption would cause it on overrun.

SIMON HOLTBY

Thanks Grumpy. Interesting to read that spark table can be interchanged between codes, maybe VE table also.
I'm finding that if I have timing values of much more than 30 deg at MAP values of 40 and below I get some backfiring. I think the spark must be firing the previous cylinder which may have the exhaust valve open.

Yours,

Simon

Grumpy

During overrun, I turn the spark way down, ie to like idle settings.

JohnL

This should be possible if the VE actually meant something in real volumetric efficiency terms, but I'm not sure it does. I say this only as a guess, based on the fact that the Injector Constant algorithms (litres/gm per sec) that most often get posted here don't seem to give the actual number that is used as a parameter in the .bins (and no, I'm not mixing up hex and dec values ). So you can move your VE numbers up and down the scale depending on the value you put on the IC.

From what it looks like to me, the VE table values are set to allow the maximum resolution across fuel and spark, rather than to really represent how much of the cylinder volume is filled as a proportion of the cylinder volume.

I'd be interested to get some feedback on this.

John