At what point do you need to retard the ign timing as ACTs increase?

I'm aware of the issues of air density and tempreture... but what about the effect of ACTs on detonation?

Any thoughts peeps?

Alex

 

60 degrees iis around the norm

 

so I should retard mine on the mini...

they get to about 100 degrees c pmsl


alex

 

Its hard to say without more details as if the system your using isnt true speed density or mass Airflow mapped then you will be getting richer and richer as the temps go up which can offset the dangerous effects of the temperature to quite a large degree...

More details required please Alex

 

ahhh something i have never thought about

 

more details?

Such as?

The question is thought I had from reading someones website about their mini turbo (t3 on carb) running mega squirtn edis...

Alex

 

Well, for example, are you running the same AFR at 20 deg C as you are at 100? If so, you have compenation working correctly, suggesting some electronic intervention... Or is it getting very rich?

worse still.. is it a CARB

etc etc

 

It i`snt a carb is a mega squirt setup.

It will compensate for air density, if i remember correctly it is about 3% per 10c.
Darren

 

In my opinion, it's the actual *mixture temperature* that has the real effect on timing, the air charge temp alone will rise rapidly as soon as it enters the combustion chamber. And the mixture temps will depend on the fuel, the cylinder wall temps and countless other factors.

 

Sorry, whilst that is an interesting theory, it is largely incorrect.

The temperature of the air PRIOR to the inlet valve closing dictates quite substantially the actual amount of oxygen present in the cylinder and thus affects both the Peak Cylinder Pressure & thus the burn time / advance requirement. This is of course one of the main reasons we fit intercoolers.

 

That's a bit too rule of thumb...

Some more detail needed here..

400 c is the threshold where auto-ignitive species will develop, whether your inlet air charge is 20 c or 40 c. The colder the air charge, the faster it will rise in temperature, fuel will slow this rise as it soaks its fair share from cylinder wall heat transfer. The air and fuel mixture together will rise in relation to cylinder wall temperatures, and compression from the piston. A rise in temps untill equlibrium is reached that is.

Intercoolers aren't solely used to prevent knock, they were fitted to remove the heat added by the process of compression whilst maintaining the pressure differential between the cylinder vacuum and the pressurised air supplied by the compressor. All this means is that air mass is maximised at the set boost pressure.

 

I dissagree about it being rule of thumb, It is fact, theres a difference. Also, no one suggested intercoolers are there to prevent knock. They are there to maximise the available oxygen at any given pressure for our oxygen consuming machine. Remember that in this thread we are talking about the difference between 20deg C and 100+... not nominal 20 - 40s. As the intake air gets hotter we have to lean off to maintain acceptable AFR. This is a fact, and one that the speed density and Mass systems both take into account and utilise accordingly and correctly, which takes me right back to my first point.....

 

Well, the difference in intake temperatures can affect the egts, which in it's own right affect the likleyhood of knock.

A cooler charge unaccounted for can create a lean or even worse a stoich mixture, raising the egts which then raises the mixture temperature as the cylinder conditions would have directly changed. There is too much of a complex cycle by cycle relationship here.

You are just directly correlating charge temps before it has reached the combustion chamber with the risk of knock. That assumption is the rule of thumb that is just not right.

 

Really?
And show me where i have mentioned knock

I think that YOU are the one making assumptions there, as im generally talking about the fuel mixture and asking for more input

 

Alright, no explicit mention of it, but isnt the question of air charge temperatures and ignition timing implicates the subject of knock/det? Why are we worried about timing retard if it isn't?

 

intercoolers are used for one simple job,, too increase air density