I've poured through more threads here and HP Tuners regarding EOIT than I'd like to count and thought someone may have some additional insight. I have a cam with 32* overlap ( at 0.050) and in an attempt to decrease short circuiting and eliminate some of the fuel smell at idle I'm going to try moving injection timing in the lower RPM ranges.
I've read where some have had success with moving the EOIT according to the difference in exhaust valve events from the stock to their aftermarket cam.

For my application:
Cam Specs: Duration @ .050 = 248/260, valve lift = .657/.655, LSA = 111
This puts the EVC at 19* ATDC.
I believe the stock LS7 EVC is at 2* BTDC. (Or -2* ATDC)

19* - (-2*) = 21* difference in these valve events.
I planned on adjusting the EOIT in the idle RPM range and up to 2000 RPMs.

If my understanding is correct then, EOIT = Boundary - ECT - RPM
Stock EOIT is 520 - 110 - 20 = 390* (at my idle of 875).

Where I continue to confuse myself is which way to move the EOIT... Using my example, wouldn't lowering EOIT to 369* cause the injection to end earlier and also start earlier, therefor dumping more fuel out the open exhaust and increasing short circuiting?
And increasing EOIT to 411* would cause injection to end later and start later, so the exhaust valve would be closed/nearly closed, reducing the amount of fuel dumping out the exhaust?

So to try to reduce short circuiting: new cam EOIT > Stock cam EOIT ?? Is this right or the other way around? It seems right in my head, but in other threads, guys with bigger cams are increasing the normal ECT therefor lowering the EOIT.

 

520 deg boundary is after BDC of the intake stroke. So when the piston is basically done sucking air into the cyl almost ready to close the intake valve.

You will want to spray it later once the exhaust is closed to prevent fuel reversion.

So, 520 - 110 normal - 0 rpm is firing the injector later (what you want) than say 520 - 110 normal - 30 rpm (stock file).

So yes - you want a higher number after your calcuation instead of a lower number.

 

Man, I have an engineering degree and I'm still having a hard time wrapping my head around this...Confusing the **** out of me. I'll play with the numbers but I'm just trying to understand without blindly doing something.

I think I see though: 520 - 110 - 0 = 410* EOIT, 520 - 110 - 30 = 380* EOIT. At idle with injector pulse width of 2.8 ms at rpm of 875, I found the crank duration to be 14.7*, which would put the first SOI at 395* and the second at 365*, starting to spray later for the overlap on the first example.
Where I keep confusing myself is looking at a graph for the cycle in relation to crank angle. Is there a GM graph for this? I couldn't find one. The graph I'm looking at just doesn't make sense with the crank angle in relation to overlap region as to how vaporized fuel would escape from the exhaust.

 

Hopefully TurboLX (a forum member and master calibrator) will chime in so I don't have to dig through all his material that I have. Over simplified, he suggests changing EOIT to match overlap.

 

Fuel doesn't go out the exhaust. Spraying later prevents the fuel from being sprayed and then going backwards into the intake manifold due to the EGR effect cam overlap has at low throttle %. Your exhaust will be higher pressure than the intake valve so airflow will initially go that way. You want to avoid spraying fuel at that time and spray once airflow is finally flowing the right direction through your intake valve.

 

Interesting. I'll have to look into that. I increased the Boundary by 21* last night and tested it out. The idle was a little rough starting out, then leveled itself. Spark was all over the place for the first minute and wideband was reading a little rich. Before this, I had my cold idle perfect so I'll have to revisit that.

Once warmed up idle was great and the drive was absolutely amazing! I idled through my subdivision in 1st gear at 1100 RPM with barely any surging. On the highway I was able to cruise in 6th gear at 1300-1500 RPM no surging/bucking at all with 27 mpg.

Fuel smell has decreased as well. I don't smell it any more while idling at a stop light, although idling in my garage it'll still make you cry.

 

Hmm. I've read some threads from HP Tuners where Banish discusses short circuiting from large overlap in cams. Like I said, I've been studying valve events through the engine cycles and it didn't make sense to me how this was happening based off the valve events I was calculating and the graph I was using for crank angle vs. engine cycle. Here is a quote from Banish over at HP Tuners forum located here: https://forum.hptuners.com/showthrea...njector-Timing

"Bigger cams can exhibit a phenomenon known as "short circuiting" of the fuel where the vaporized fuel cloud gets blown right out the exhaust valve during overlap. To reduce the chance of this happening, one can DELAY the end of injection event so that there is less chance of this happening. To accomplish this, you would effectively INCREASE the values.

The downside to this is that you don't give the fuel as much time to evaporate, so combustion quality may suffer. It's still better than the misfire associated with dumping the fuel out the exhaust instead of trapping it in the cylinder. Also, since the fuel is now being injected across an open intake valve (it's normally early enough that it hits the closed valve), there is an opportunity for bore wash at low speeds until port velocity is sufficient to carry the fuel charge dominantly. For this reason, we only try to delay as little as possible to avoid the short circuiting.

I bet you don't find info like that in your average class or internet post "

 

Yeah - I have read that as well, however the physics of it doesn't hold up. What will happen (and does happen) is the fuel that has been blown backwards into the manifold will go into a different cylinder and cause uneven / poor combustion. That lovely chop chop old timers get hard-ons over. Some cyls will be starved of fuel and others get it shoved in.

As you found out, delaying the eoit caused your fuel to go rich - this means more of the injection is now being used and burned properly vs mixed with other cylinders. You are on the right track. I have 18 deg overlap, I run 520-110-0 at idle. I then added 20 deg to my RPM (at 4000 rpm, and interpolated from 0 at 1000rpm) to put the fuel eoit on intake valve opening when at wot as stock does.

 

Ok, I think I have a much better understanding of this now. I graphed out my cam to crankshaft angle and see where delaying by the degrees of overlap would be best at idle. In this case, delaying by this amount at idle puts SOI immediately after EVC. I've logged my IPW at idle and get 2.8 ms. This puts the IPW duration at 875 RPM at 14.7* in crankshaft duration. Using this, my EOIT at idle should be about 420* . Right now I have it delayed to 411* from the 390* stock setting and noticed significant positive results. So where it sits now the exhaust is still slightly open while spraying. I'll add another 9* to the delay and log some more data. Graph attached for what I estimated SOI and EOIT.

Apocolipse, now on the other end, at WOT you ADVANCED the EOIT to spray on a closed valve right before the valve opens, the same as stock. The thought of this is that the fuel on the hot valve is better atomized? So in theory, at WOT EOIT should occur immediately as the valve begins to open?

 

There's a really good sheet on HPTuners forum that lets you put in cam specs and all of the injection timing data and will spit out graphs, etc. You may want to look for that.. it basically spits out the same graph you just did. Here is the old version that I mostly did, someone else made this and another one much nicer though. If you can't find it let me know and I'll try to dig it up

 

Hey man, this is good stuff. Thanks!