Quote:
Originally Posted by moto-mike
Also, as a comparison the DI equipped FA20 in the 15 WRX...we can't run half the timing on those! DI only we hit the knock threshold at a much lower HP using DI only. Can't explain it and it's still a very new system, but it made me wish for the BRZ setup having tuned it.
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You can't really see this accurately without drilling the heads for combustion transducers, but keep in mind the difference between spark timing, combustion phasing, and combustion speed.
I'll tell you exactly why you can't put much timing into a FA20DIT besides some situational knock tendency. It burns WAAAAY faster with the high tumble port and combustion chamber shape. Put it into a flow bench and you'll see it flows like crap compares to a BRZ engine.
So I divide my combustion into 4 basic parameters:
1) the time of spark, which you're all familiar with
2) the burn delay in crank angle degrees between 0-10% burn
3) the location of 50% burn - this is the combustion phasing!
4) the bulk burn duration - crank angle between 10-90% burn
with a modern DI turbo engines you'll see high tumble ports. This makes the 10-90% very fast and helps mixing, but they flow pretty bad.
So if I fire the spark at 20 degrees BTDC on a slow burning combustion system it may still burn 50% of the mixture at 8 degrees ATDC (considered to be MBT). If I fire the spark at 5 degrees BTDC on a fast burning combustion chamber it may still burn burn at 8 degree ATDC.
The difference is the combustion speed and the spark, not the phasing.
The reason why an FA20DIT doesn't take a lot of spark is in part because it doesn't need a lot of spark. It burns fast, but the heads don't flow like a BRZ's head. If they didn't have the fast burning head, it would run like crap at part load (poor combustion stability), have high CO and O2 emissions, and probably knock a lot more at full load.