Post by slingy1052 on May 9, 2015 8:55:16 GMT
So, having seen a few requests for ignition timing info, i'd thought i would start a new thread for everyone to chime in on to try and quantify what is required for forced induction ignition timing.
Ignition timing isnt unfortunately as easy to quantify as AFR.
An AFR is pretty much easy enough to say, every force induction engine should go no leaner than 12-1 with 11-1 not unreasonable when into higher boost.
Timing, though has ALOT more variables, and is dependant on the engine spec alot more.
So this is my first statement:
For Optimum Engine Efficiency with minimal mechanical stress, Peak Combustion Cylinder Pressure (PCCP) needs to be achieved @ 14* ATDC!
My brother has developed a Dyno simulator, and while working out the algerrythms to run the software he found this out.
So, if you think about its logical, any nearer TDC and the piston and rod and crank are too aligned and much of the combustion force is going into trying push the piston into the rod and rod through the crank rather than rotating it.
ANy further after TDC and piston is already on its way down the bore so energy is not being efficiently transfered to the piston.
So the key here is combustion time, or burn time. It takes time for the fuel to ignite, and fully burn, hence the requirement to ignite it BTDC in order for it to have time to fully burn and achieve peak cylinder pressure at, you guessed it, 14*ATDC.
The issue here is, that as stated the following factors are some of the main ones that we have control over that have an effect on burn time:
So, changing any one of the above will have an effect on the burn time and so when the spark event needs to occur to ensure peak pressure is being kept near that magic 14* ATDC.
So, i think alot of what people think is need to be done with turbo motors STILL stems from the past, carby and points days where it was hard to control some of the the above factors.
SO lets run through an old carby and points setup.
SO we turbo a stock engine, so the first change to the above list is to add boost-this effectively increases the CR so speeds up the burn. Without changing the timing, the faster burn brings the PCCP nearer TDC and creates knocking or cracks the pistons?crushed ring lands.
So an easy mechanical way to slow the burn back is to reduce the CR to compensate, as its too hard to change the timing curve of a mechanical advance unit.
The boost is raised more which speeds up the burn, so the next easy step to slow it down is to richen the AFR to slow it down again.
So i suppose, what i am trying to get at is that, for example, if you add boost via a blow through carby set up with factory ignition, and you lower the CR with spacer plate or Busa pistons, and aim for a 11.5-1 AFR on boost, you probably wouldnt need to retard the igntion as well.
Likewise, an engine with stock CR and EFI, which has full control of the AFR and timing could still run a Bar or more of boost.
I know i'll get alot of flack for some of these statements from people who have been there and it cant be done etc etc, but everyone , please share experiances etc.
The big question is, how can it be quantified into a bit of a table or something to allow fairly accurate predictions to be made based on the origional NA figures i suppose.
Ignition timing isnt unfortunately as easy to quantify as AFR.
An AFR is pretty much easy enough to say, every force induction engine should go no leaner than 12-1 with 11-1 not unreasonable when into higher boost.
Timing, though has ALOT more variables, and is dependant on the engine spec alot more.
So this is my first statement:
For Optimum Engine Efficiency with minimal mechanical stress, Peak Combustion Cylinder Pressure (PCCP) needs to be achieved @ 14* ATDC!
My brother has developed a Dyno simulator, and while working out the algerrythms to run the software he found this out.
So, if you think about its logical, any nearer TDC and the piston and rod and crank are too aligned and much of the combustion force is going into trying push the piston into the rod and rod through the crank rather than rotating it.
ANy further after TDC and piston is already on its way down the bore so energy is not being efficiently transfered to the piston.
So the key here is combustion time, or burn time. It takes time for the fuel to ignite, and fully burn, hence the requirement to ignite it BTDC in order for it to have time to fully burn and achieve peak cylinder pressure at, you guessed it, 14*ATDC.
The issue here is, that as stated the following factors are some of the main ones that we have control over that have an effect on burn time:
- Compression Ratio
- Air fuel Ratio
- Actual Ignition Timing
- Boost
- Octane rating of Fuel
So, changing any one of the above will have an effect on the burn time and so when the spark event needs to occur to ensure peak pressure is being kept near that magic 14* ATDC.
So, i think alot of what people think is need to be done with turbo motors STILL stems from the past, carby and points days where it was hard to control some of the the above factors.
SO lets run through an old carby and points setup.
SO we turbo a stock engine, so the first change to the above list is to add boost-this effectively increases the CR so speeds up the burn. Without changing the timing, the faster burn brings the PCCP nearer TDC and creates knocking or cracks the pistons?crushed ring lands.
So an easy mechanical way to slow the burn back is to reduce the CR to compensate, as its too hard to change the timing curve of a mechanical advance unit.
The boost is raised more which speeds up the burn, so the next easy step to slow it down is to richen the AFR to slow it down again.
So i suppose, what i am trying to get at is that, for example, if you add boost via a blow through carby set up with factory ignition, and you lower the CR with spacer plate or Busa pistons, and aim for a 11.5-1 AFR on boost, you probably wouldnt need to retard the igntion as well.
Likewise, an engine with stock CR and EFI, which has full control of the AFR and timing could still run a Bar or more of boost.
I know i'll get alot of flack for some of these statements from people who have been there and it cant be done etc etc, but everyone , please share experiances etc.
The big question is, how can it be quantified into a bit of a table or something to allow fairly accurate predictions to be made based on the origional NA figures i suppose.