RomRaider

I've read that when adjusting the injector timing table one should start with a flat table of 360*. The one exception seems to be 08+ STIs where the only thing I have dug up says to leave the injection table base values alone and to adjust off of those.

My question is - why can't one flatten the entire injection timing table on the 08+ STI. What is changing in those load/rpm cells that requires an earlier end of injection?



Here's the injection table for the 17 sti.

oh darn, you picked one of my special interests. looks like you get a long post.

exhaust cam/intake cam overlap.

if you inject too early while there is overlap, you get fuel out the exhaust, and no amount of fuel trim or maf/ve table correction is going to "fix" that, since the system is end of injection. if it's lean from fuel being thrown out the cylinder, adding more fuel just adds it to the front of injection, so you get more fuel out the exhaust and the effect of getting the cylinder fueled to look like the correct lambda will be difficult to come in line. saying it needs more fuel is also saying it's using more airmass, since the effect is proportional in change one to the other - so then as you try to add more fuel for the lost fuel, you're adding ve% or MAF g/s and the load value is going higher than it would technically be for the cylinder fill itself. Since everything is airmass axis related, you get drift on any table using that axis, which is basically everything.

whereas if you finish your injection after overlap has occurred, then you're not losing fuel out the exhaust but retaining it in the cylinder, and you're not trying to compensate in any airmass tables for fueling error.

oem single avcs and non-avcs cams don't have enough overlap to make a flat 360 table not work. ots and stock avcs tables aren't wonky enough to make injector timing super critical, which is why subaru gets away with flat 360 degrees for so long, as they did. and why cobb's table is not super aggressive.

This table is what came to be as finished on the 08 STi that I worked with. I already had a table and values i was working with when I had the opportunity to use Cobb on this 08 sti, and i referenced the oem table versus their stage 3 map's injector timing, and really only confirmed what i was already working on. this was my first dual avcs experience, so i was knee deep in being very analytical about what the relationships all were. Hard to imagine it was a few years ago, I remember everything about this car like I was around it last month.

What i did with exhaust avcs was to run 17 degrees flat, and then I used a known/good intake avcs table (such as what i ran on the owner's two prior 07 STi limiteds) and checked injector timing against datalogs, and then changed both intake/exhaust avcs until happy, watching for dramatic airmass metering/fueling concerns and revisiting the injector timing table when i felt it was necessary. yes, very early at high vacuum. evaporation is always easier at low absolute pressure.

I favor as early an injection as possible when in vacuum, and as late an injection as possible, when on boost. If you inject real early, the valve will transfer more heat away, the head cooling is going to be a little bit better, and the pooling/evaporating effect is what helps there. Injecting as late as possible means the velocity of the fuel near the end of the cylinder fill will be really good, hopefully.

on this GSC 272 non-avcs cammed ported B25 heads / 9:1 ej257 (04 sti block/heads done up by SoCal Porting) i made the minimum higher, and raised the lower y axis limit. also it's in MAP axis instead of load axis, and ATDC instead of BTDC, but i'm literally just inferring this table onto MAP axis by using the VE table for load reference at 86F manifold air temp. In this case, it is as though, roughly, i'm only using the table from about 0.55 g/rev up, being that the values lower than that area are causing some issues at lower MAP/load and this was enough of an increase in timing in that area to compensate for the cams/port work.

this is THE table I've been using for a while now to start with for everything. Being that I'm mostly tuning in MAP axis, this table is inferred by way of VE table for each engine/rom. So in that regard, each vehicle has it's own customized injector timing table, but if you reverse calculated it from MAP axis to load axis (by again inferring load from the VE table specific for each vehicle) you would see that all of my work over the last two years or so is essentially going to follow this table. Unless you counted the couple of CCF Accesstuner tuned vehicles, and then because of stock cams/heads/flowy type things, the tables are actually this table, identically.

The more stock (and functional) the cam setup, the more of the lower range you can use. the more overlap friendly your cams are at lower load, the higher in the load axis you'll use to start your table at, and then you can blend for more resolution between. or you can raise the lower load axis values to the higher load values in the table data, but then you're wasting table resolution.

because nobody asked

here's how that table plays out on different motors in MAP axis.

top left is the non-avcs 272 cams 2.5L
top right is a s20 heads/cams, JE 8.5:1 conversion 2.5L
bottom middle is stock s20 heads/cams ej205.

subtract 360, negative inverse, add 360, and you've got your conversion from BTDC to ATDC or the other way around.

so in the cammed example, i started with the same MAP range as the other 2.5L but didn't like the really early injection rates below 7 MAP. So i shortened and gave more resolution to that axis, instead of raising all the lower range below that map area up to it's timing value.

on the other motors with stock things, i used down to as low of a map range as reasonably possible, while still trying to get up to ~2.5 g/rev at least as highest equivalent load column range.

Thanks for taking the time to type up that explanation, I found it very helpful!

Thanks for taking the time Jon. This info is great! I will be spending some time on this in an effort to understand.

In the mean time I have one thing I can't wrap my head around and I think it's holding me back from understanding a good portion of the rest of this.

What determines how the (for example) injection timing in degrees is expressed? I see open source is expressed in ATDC while Cobb is expressed in BTDC.

I've been looking at videos and the most helpful ones seem to be drawing a 4 cycle degree wheel out and marking the four cycles. Intake, compression, power, exhaust. Rotates in a clockwise manner.

Another example is I was looking at cam specs for my 17 sti and my bc 280 cams. The intake on the 17 OEM cams opens 5* ATDC, and on the BC 280s opens at 13* ATDC.

If you advance the intake cam 20 degrees via AVCS, wouldn't this put the intake valve opening now at 345* BTDC, not 375* ATDC? But yet in Cobb (expressed as BTDC) you add AVCS intake advance to the 360* EOI which, wouldn't that then make it ATDC?

Hopefully my struggle makes sense. If not let me know where I can clarify.

360 degrees before top dead center of compression stroke

is also the same thing as

360 degrees after top dead center of compression stroke

because all four strokes happen within 720 degrees total

crank degrees, not cam degrees

which is why to convert between ATDC (of compression stroke) to BTDC (of compression stroke) you would subtract 360, negative inverse, and add 360. do the same thing to convert back. frame of reference/inverted frame of reference. third from the left, is also third from the right, when you're talking about five things in a row.

420 degrees ATDC is 300 degrees BTDC
320 degrees BTDC is 400 degrees ATDC
719 degrees BTDC would be 1 degree ATDC.

Thank you. I will now continue to draw to understand. lol. I like to understand what I'm changing when I make changes. I'll circle back when I think I have a more complete understanding of valves, degrees, overlap, etc.

How do your numbers tie into the AVCS table, if at all? I was of the understanding that adding the AVCS intake advance to the injection timing table values would dial in the injection timing..... on an oem type setup (cams/heads).

I don't base anything about injector timing off of intake cam timing. I base it off air airmass, and engine speed.

if you're adding from intake advance to injector timing, which way does the injector timing move if it's ATDC? what if it's BTDC? are you trying to get it to finish fueling just before the valve closes? if you're adding x degrees for y degrees, doesn't the cam move at a different speed to the crank?

BOLD! This is the exact thing I am trying to figure out! In my case the OEM will open at 5* ATDC with avcs in the ecu set to 0 advance. My BC 280s open 13*ATDC.

If injector timing is at 360* and the intake valve opens at 13* ATDC with 0 intake advance - which way does the injector timing move if expressed in BTDC.

I have chalked this up to lack of understanding which is why I refer to drawing things out above. The sticking point in my head is how BTDC and ATDC are used interchangeably. It seems like it should be one or the other. In any case when I can wrap my head around that I should have my answer to your question above.

That's my logic. I'm sure it's flawed. That's what confused me to the point of asking here. I didn't find any correlation or reason for the lower numbers higher up in the load on the inj timing table.

The last graphic you posted is super helpful with the BTDC/ATDC questions I have. But, an engine rotates so again I'm hung up. For me it's a mental block sometimes to get to understanding something. Once I get it thought I tent to do well.

I've read the entire threads three times and each time another thing clicks. It sounds like you put a good amount of time in that table you posted up. I just want to understand it all before I go popping into my tables - not because I'm as much worried about the values but I think it's just good tuning practice to understand a bit of what one is doing before just slapping in numbers.

the part you put in bold is me asking the question instead of answering it, with the graph in tow, and both BTDC and ATDC for reference. like a worksheet example. the relationships are shown, and the question has enough information present in the image to work it out.

every 90 degrees is dead center of a stroke. 0 and 720 degrees for both BTDC and ATDC refer to the same spot - top dead center of compression stroke. it's like saying 360 degrees is a glass half full and half empty, and then someone who says it's 10% or 20% less than half full of water versus someone who says 10 to 20% more than half full of air. but our reference (of compression stroke) is the ends of the proverbial glass, not the middle. it's not about half, but rather the air or water. an empty glass is both 100% air and 0% water (the ends), and neither of the guys talking about it being "slightly off from half empty/half full" were wrong in their terminology but with a different frame of reference in direction. 350 degrees BTDC of a given stroke is also 370 degrees ATDC of the same stroke. (atdc and btdc being the ends) and 360 degrees it is the middle for both value types.

advancing the intake camshaft brings it to the left on the image, retarding the exhaust cam brings it to the right. if injector timing is relative to ATDC, then the value corresponding the injector timing would decrease with advancing the intake cam, and if it were BTDC for injector timing, then the value would increase with advancing the intake cam. basically, if you move the intake cam to the left and the injector end point to the left, is there enough velocity/time to shove a bunch of that fuel out of the cylinder through the exhaust overlap? and look at group N/dual avcs injector timing tables and you see the relationship is opposite. injector timing is later when intake cam is advanced more at higher load. the intake cam moves to the left, but injector timing moves to the right.

seeing that injector timing is changing, and seeing that avcs is changing, in relationship to load and rpm change, is "correlation is not causation."

while saying "change injector timing relative to avcs timing change of intake cam" seems fairly natural of a conclusion to come to... that's not the relationship to injector timing that i see as having an effect for why injector timing should or might change and if you applied, would be reverse of the value difference you see between 08+ and prior flat 360 tables. but here's the kicker - if you do that on carberry, and you add 30 degrees to the injector timing table where you have 30 degrees of avcs (because one is BTDC and one is ATDC lol), you're pushing the injector timing later. start 360 degrees, mod by avcs amount, and if you're running aggressive avcs, you're almost mimicking the above atmospheric range of the group N table if you knew what the load was at it's MAP and RPM flavor. that's why i asked the question, which way does "adding value" in the table move the timing? the oem 08+ table puts the injection timing later, not earlier, so it's easily also not intake avcs related (in and of itself) if the injector timing relationship is opposite of where the intake camshaft is now opening up the valves.

restating it again, but in a different way... if you started with flat 360 but then moved injector timing to be advanced, reflecting increase in advance like the cam, you would not come up with anything like the oem table for 08+. because of the frame of reference type, lower values means later end of injection point. on carberry however, it's more natural. more cell value is later, less cell value is earlier. ATDC for the win, I guess?

but it becomes very apparent that centerline of 360 degrees at top dead center between exhaust stroke and intake stroke at all times (stock 16-bit/old 32-bit) is not going to be super good with overlap when it has overlap from exhaust cam retard. to answer, why 08+ sti and dual avcs engines don't have flat injector timing... the exhaust cam moves to the right on the image and overlaps the intake valve too much to have a centerline end of injection at all times, reasonably like older roms at flat 360 degrees.

it's safe to say 360 degrees +/- 60 degrees is the overall range you'll play around in, high airmass per revolution/low engine speed maybe going up to 80 degrees from center of 360 (440 ATDC, 260 BTDC) if you're looking at the group N rom for reference. i played around at 80 degrees before center at really low map, but idk, maybe a waste of time versus 60ish prior as minimum.

in any case, the injector timing i use is based off the idea that engine speed and airmass relate more to port speed and injection time/angle than matching the end of injection angle to the exact same "relative to centerline" that the camshaft moves with commanded from solenoid duty and oil pressure. "the four stroke motor itself moves x displacement of air every two revolutions regardless of throttle angle, where throttle angle changes air density more than air volume." - paraphrasing engines as air pumps. I see airmass over time (work, per revolution, load) as more a determining factor than actual intake camshaft advance, for why later injection timing works on all intake avcs/non-avcs stock cams. sure, i want different intake avcs at different times which shows up as a change in avcs over map or load, and I naturally want a different injection angle at a different load, which shows up as different map axis to different engines, but they do not move in relationship to each-other, injection angle and cam advance angle, but rather because of load as airmass/rev and time (engine speed).

injection with my values at load/boost/wot ends while the valve is open with or without advancing the intake cam. that is to say, the same end of injection angle i use on pretty much everything at a given rpm and load will be while the intake valve is open regardless of intake avcs degrees between all motors i supply this with. the relationship is airmass over time, like density/velocity/time considerations. displacement doesn't change injection angle to me, because displacement is already a factor in airmass calculations.

to reiterate: when doing "load based" airmass axis injector timing, i use the same table for pretty much everything except overlap happy/big cam stuff. same heads/cams but change in displacement is no change on injector timing, on load axis. same overall motor and intake/exhaust paths and mods, but now has intake avcs, no load axis injector timing change either. if overlap of cams and injection angle promotes wasted fuel out the exhaust, change it. basically, very little overlap, and you can finish injection at low airmass really really early, even before the intake valve opens. lots of overlap, and you should push the injection angle to later. too early at higher load even if you don't have overlap, and you might put too much heat into the fuel from spending so much time in the air-charge mixture while being heated and compressed and transitioned through the compression and power stroke.

when doing pressure based axis, I infer my main load axis based injector timing table and from the VE table/displacement at given MAP and RPM, I interpolate the load axis table into map axis base. injector timing isn't changing BECAUSE of pressure axis, it's changing because different pressure ranges are different airmass on different motors.

Thanks ! I learned more about how an engine works than I did about injector timing but i also learned about injector timing

Amazing

now that being said "which thanks for the knowledge bomb" does anyone have the address for this in ecuflash 2011 sti ae5I910v