PI : DI Ratio Discussion

[Kodename47]

Seems to be plenty of discussion of PI : DI ratio about but I thought this would be a good place to chime in on people's thoughts and if they have tried anything successfully. I think both NA and FI applications are worth a shot as there may be different benefits for each. I think there may be a more optimal map over the OEM for power, but I also get the impression that there are benefits to running PI at various stages of the rev range. I guess this will focus on WOT application as I doubt many will change the lower load regions but any discussion is welcome.


Below is a picture showing the OEM map in WOT load regions between 2-7.4k. Yellow is the % of Direct Injection, blue is Port Injection. As standard it is quite heavily biased toward DI but IMO there is a reason that the OEM decided to re-introduce the PI at higher RPM. There is definitely capacity to run more DI than that. It may be due to DI firing angles and open time requirements that a percentage of PI is required, as I have had previously hinted to me.



So is a higher % of PI at the very top end beneficial and allows you to run more timing? If you're running boost and aren't running as much timing, is more DI better at controlling knock? Discuss.....

[Turdinator]

Subscribed.
Injection ratios and cam timing are two areas I've heard very little about on this forum. It will be good to hear some ideas and experiences from those that have played with it.

[Kodename47]

It's definitely something I'd like to understand more.


On a log I've compared recently, I've had very slight FLKC appear at 6.8k after the PI ratio was decreased despite running the same timing. However, the IATs are also 10C more on the most recent log so I'm not willing to put this down to injection ratio alone.


It may also be worth noting that @Shiv@Openflash uses the OEM ratios for the OTS maps. I've also heard that one tuner relies almost solely on DI for boosted applications and only introduces PI when the DI capacity is near it's limitations. It'd be nice to see some results from actual testing, and I recommend that if you do fancy testing that (a) you know what you're doing and (b) that you do it in similar conditions.


I am running a non-standard ratio map on my car, but as I've not had time to change and test for comparison sake I can't even be sure there's much/any benefit to having changed it.


This is as it currently stands:



My line of thinking is that the DI will be used to help prevent knock/cool cylinders in the mid range and at low RPM with high load (20% rather than 50% OEM). When the RPMs get higher and the DI capacity is getting near a "safe" limit then I re-introduce PI at a slowly increasing rate, but hit near stock levels at peak RPM as the ignition timing is increasing.

[Reaper]

@nelsmar might be the most knowledgeable about leaning heavy on the di until people start tuning the di only fa20det.

[s2d4]

Originally Posted by Reaper

@nelsmar might be the most knowledgeable about leaning heavy on the di until people start tuning the di only fa20det.

The DI on the FA20DIT differs from ours?

[Reaper]

It doesn't have pi at all. so tuners will be getting better at utilizing di effectively since well...they have to on the fa20dit.

[steve99]

Originally Posted by s2d4

The DI on the FA20DIT differs from ours?

I believe the DI system on the WRX FA20DIT is subaru design where as ours was toyota design DI/PI system.

[Reaper]

Yes. No pi like the pi/di 4ds system we have.

[s2d4]

Originally Posted by Reaper

Yes. No pi like the pi/di 4ds system we have.

Yeah, we know.
I was wondering if it is even applicable since different DI setup.

[s2d4]

Originally Posted by Kodename47

So is a higher % of PI at the very top end beneficial and allows you to run more timing? If you're running boost and aren't running as much timing, is more DI better at controlling knock? Discuss.....

An SAE research had shown that DI is much better at E85 than PI.
Wouldn't it be the same for Pump fuel as well?

[BadBRZ]

@jamesm

... ohh wait

[steve99]

some good info here

http://www.ft86club.com/forums/showp...8&postcount=22

whole thread

http://www.ft86club.com/forums/showthread.php?t=3172

[arghx7]

Originally Posted by Kodename47

So is a higher % of PI at the very top end beneficial and allows you to run more timing? If you're running boost and aren't running as much timing, is more DI better at controlling knock? Discuss.....

We know the DI start of injection, assuming those are reverse engineered correctly (the numbers really represent the physical degrees), but we don't know the PFI end of injection or the DI end of injection timing.

We don't know the combustion parameters at those speeds. We don't know the combustion speed, the combustion phasing, the combustion stability, etc.

We don't know the smoke & particulate emission, or the wall wetting.

I'll speculate anyway. As I've said before, this stuff was mapped on an engine dyno in mostly steady-state conditions. DI is pretty much always better for cooling the intake charge, but the problem is forming the mixture within a smaller crank angle window, especially with side injection like this.

Here's what I mean by that.



All you've got is part of the intake and compression stroke to spray the fuel with DI. That's why people are like "omfg duty cycle so low!!1! on DI engines" But if you spray during the expansion or exhaust stroke you are wasting the fuel, because it just leaves the engine unburned. With PFI, you can spray really the entire 720 degree cycle (100% duty). The mixture can vaporize in the intake port and doesn't rely on charge motion in the combustion chamber as much.



Now if you look at that above DI firing angle map, you can see it really ramping earlier and earlier at high speed, just as the PFI is coming on (higher number = earlier, and 360=TDC intake). Normally your SOI gets earlier to give more time for mixture formation to improve combustion stability, but also to limit how late the injection sprays in the cycle.

Well if I go too early I impinge the piston down causing smoke, and if I go too late I wet the liner causing oil dilution, at least on a side injection engine. At high speed though the pulsewidth is so long that you could be "trapped" and can't move the injection event later or earlier without impinging on something. That's where you dial in the port fuel. We can't see the end of injection timing for the PFI, but I suspect it is closed valve injection. So the port fuel has a long time to mix, helping combustion stability, and the charge cooling is done with the DI. Normally you can get a little charge cooling at high load with open valve PFI injection, but then you're throwing more liquid droplets into the cylinder that don't vaporize as well.

Personally, if you want the most power I recommend you run as much DI fueling as you can get away with, until the high pressure fuel pump can't flow anymore. This is due to the knock benefit. Most people on this subforum don't care about oil change intervals and particulate emissions.

Summary:

DI= more knock relief, potentially poorer mixture formation, more impingement on the piston and cylinder walls, flow limited due to injector flow rate and high pressure fuel pump flow capacity

PFI= more knock prone, more likely to form a more homogenuous mixture with better combustion stability, easier to flow at higher rates (just buy aftermarket PFI injectors and low side pump), not nearly as sensitive to fuel pressure or injection timing

[Turdinator]

Originally Posted by arghx7

Summary:

DI= more knock relief, potentially poorer mixture formation, more impingement on the piston and cylinder walls, flow limited due to injector flow rate and high pressure fuel pump flow capacity

PFI= more knock prone, more likely to form a more homogenuous mixture with better combustion stability, easier to flow at higher rates (just buy aftermarket PFI injectors and low side pump), not nearly as sensitive to fuel pressure or injection timing

First of all, great post. Thank you for your input. I am trying to get my head around all the info here and the other threads linked earlier. One thing I was curious about is, what exactly does "better combustion stability" mean?