Ignition timing trimming - forced induction

[phoenix]

Hi

Can anyone give me some guidance.

Before mapping properly I am trying to get some base maps ready for a first dyno run.

I know I will need to pull some ignition advance out as air charge temperature increases. But starting from what temperature and at what rate?

The only information I have found was on a Megasquirt write up. It suggested 60 degrees C ACT as the threshold and then pull one degree of advance for every 5 degrees C increase in ACT above that. Does that sound like a good starting point?

Thanks in anticipation

[racedna]

Hello, Just some thoughts, I think the basic idea of pulling timing on IAT is not the right way to sort the problem out, on zero boost map the engine as if it is NA, this will net you the best IGN curve and a safe AFR range, this will give you some idea what sort of VE your engine and get a nice stable map.

When you add boost you at lest can work in the ballpark of the engines and the turbo , efficiency and limits the error amount to the per PSI mapping of the trubos fuel need.

Do the Boost maps per PSI next till you have a nice stable map looking at the IAT range the motor is stable with.

High IAT's will lower power and charge density 5c of Intake temp is about 1% power, and so will alter burn speed, and may become less stable, but heat soaking the engine will be worse for stability
than just charge temps.

When I think of timing I think “advancing” as start burn more early, and retarding as starting the burn later towards tdc or atdc even.

So if IAT goes up retarding timing starts the burn later, and may net a more stable burn, but lose power because the Burn-shape and time is not ideal for the engine geometry, the other side effects can be poor burn and later burning that burns all the the way out the ex port, and heats up the head even more and makes the motor even less stable than the IAT increase would have.

So find the range engine is stable at, note the IAT stability range, (it is linked the the turbo and the engine combo as well) and pull boost down at a slope thats tracking the IAT change, this will keep you in a happy place from the IGN / AFR point, of view as well as drop CHT (head temps) down and keep the motor happy.

You could combine a small IGN modifier based on IAT as well but keep in mind the above.

Hope this helps

Perry

[phoenix]

Thanks for that Perry.

I have absolutely zero problem with fuelling - i.e. I cant keep the AFR I want at any boost/rpm/temperature/load, because we have feedback on that from the wideband lambda sensor.

With ignition there is no such feedback and I don't run knock sensor. Avoiding detonation from a hot charge is what I think is behind the 'retard at high ACT' strategy, though of course you are right in that power will be lost and EGTs may rise. I only want something in the maps to deal with out-of-normal operating conditions (ACT in this case) to save the engine.

I am not sure that retarding at 1 degree per 5 degrees C would a) lose too much power or b) produce an increase in EGT sufficient to give bring the other problems you suggest.

Pulling boost on ACT increase is of course an option, but in this case I wanted to put something sensible into the ignition/ACT trim map - it is in the ECU for a reason after all!