Quote:
Originally Posted by KoolBRZ
Here's a pic of what I'm trying now. I lowered the PI percentage and threshold even more. I think even 20% might be a bit much. Here's a pic of what I changed so you can see for yourselves what I'm talking about. I lowered it to .047 in the middle of my cruising rpm's to see if I can get a bit better mileage.
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Interesting. Do you think the drive feel could better with higher percentages of PFI?
In part load conditions PFI wins hands down especially as the speeds increase. At lower speeds OEMs use DI to have better fuel control. PFI tends to have a puddle effect and OEMs sometimes even have a puddle model (accumulation and evaporation model)
The change you are feeling could be down to the fact that as we switch from DI to PFI without changing spark we are changing the location of peak cylinder pressure (LPP) to a more advanced position. This is because the charge cooling effect of DI makes the combustion speed slower and hence to get same LPP we need a bit more spark advance. Now when we keep the same spark advance and just change the ratio we can be speeding up combustion. So how does this help in the drive feel.
Okay so in combustion there is something called Combustion variance. This is the variability in LPP cycle to cycle. Typically there is a LPP that gives best torque/efficiency at a certain speed. Say for example at 1200rpm it is 14deg ATDC (just an example). OEMs generally tune spark to get the LPP at this precise angle. But it has been observed that as you advance the LPP a bit closer towards TDC say by a degree or so the variance in LPP drops but efficiency suffers. Meaning combustion gets more consistent but less efficient. Now for drive feel this variation in LPP is critical but for fuel economy it is not.
If I were to tune a race car I would tune it differently than tuning a street car for FE.
Back to the topic. By adding more PFI at same spark timing we are advancing LPP slightly and as a result getting better feel.
As a science experiment we should feel a better idle, accel, etc when we add a degree or so spark across the part throttle region (non knock limited). This should feel much like the way it felt with more PFI.
Okay so a lot of I said is theory, yes but being a dyno/vehicle calibrator myself I have experienced most of this.