GTA matt

iTrader: (1)

I'll start by saying that I'm posting in this section rather then the DFI/ECM section since many here are familiar with short runner intakes (miniram in this case) and its fuel/cylinder distribution problems. This will also be a fair amount of reading and lots of technical data, but I'm hoping some questions I have will get answered. Its pretty well documented that the rear cylinders run richer on this intake. Mine suffers the same problem. My engine setup is a 383, 11:1 compression, comp xfi292 cam, AFR195 heads and the miniram. I'm running EFI connections 24x setup on it, using Fitechs ultimate LS ecu to control it (if the terminator x was out when I bought this, I would have gone that route, as holleys tuning software is more streamlined). On my engine, cylinder 7 seems to be taking the brunt of the rich condition. Front 6 plugs look absolutely perfect, cylinder 8 is a little darker, but bearable, however, 7 comes out black if I let it idle for a while. The plug cleans up after a WOT hit, which leads me to believe the problem is caused by low engine speeds. Compression is also the same on cylinder 7, and had 5% leakdown, so I don't suspect a mechanical problem. With the factory batch fire ecu's most are using with this intake, I can see fuel reversion being a problem with bigger cams, as both the intake valve and exhaust valve are open at the same time as the piston is traveling up towards TDC on the exhaust stroke. In my case, there is 70 degrees of overlap, with the intake valve opening 37* before TDC and the exhaust valve closing 33* after TDC. In batch fire systems, fuel is sprayed against the back of a closed intake valve, and as its opened, some fuel is being blown out the open exhaust valve, and I suspect some is being blown back up the manifold, and being collected at the rear. For whatever reason, cylinder 7 seems to be the hotspot, as before I installed a catch can on my pcv, that runner was covered in oil. A whole 'nother story, I had to remove the valve cover baffles to clear the rocker stud girdles. GM's sequential fire strategy is to also spray fuel on the back of a closed intake valve, as supposedly it helps evaporate the fuel for atomization. Problem is, with big cams, the result is the same as batch fire. That brings me to the point of all this, the fitech ecu allows injection timing to be altered, and you can change it based off MAP and rpm. Ideally, I'd like to fire the injector after the exhaust valve is closed. I wanted to see when the injectors were firing, in relation to valve events and piston location, and alter it to fire when I wanted. However, it won't allow me to move it as much as I need, more on that later. I realize most of you have never seen this before, so I'll do my best to explain. I put an in cylinder pressure transducer in a cylinder, in place of the spark plug (blue trace). I probed the coil turn on signal on that cylinder, as you can measure spark timing with the tools provided in the labscope software (red trace), and used an amp clamp to monitor injector current. The ecu won't fire the injector if its unplugged, so I just used the clamp, rather then unplugging and probing it like I did the coil (green trace).

In this picture, I have rulers set up between TDC events, 0 to 720 crank degrees, showing all 4 strokes. peaks on the blue trace are compression peaks, this is at idle, with the default injector timing settings. Starting at 0 degrees on the first peak, the piston begins traveling down to BDC, which is 180*. Just before the 180 marker, the exhaust valve opens, which is where you see presure come back up to zero. The wavy line between 180 and 360 is essentially pressure in the exhaust, as the exhaust valve is open during this period and the piston is moving upward on the exhaust stroke. The exhaust valve remains open for 33* after the 360 marker which is tdc on the exhaust stroke. The intake valve opens 37* before the 360 marker, you can see when the exhaust valve closes and the cylinder goes into vacuum, with the piston moving back down on the intake stroke. The intake valve closes 75* after BDC, which is the 540 marker, and the piston continues up on the compression stroke. The injector (green) fires as the exhaust valve is opening, and fuel sits on the intake valve until it opens. Coil fires when it turns off, 20* before tdc in this shot.



In this shot, I've changed the injector timing, its now firing just as the intake valve has closed and the piston is coming up on tdc. It may look like the injection event has advanced from the exhaust stroke to the end of the intake stroke, but its actually retarded, moved back from the exhaust stroke. This is as far as the ecu will let me move it in that direction. Not good at all.



In this capture, I've changed the injection timing the max it can go the other direction, you can see its moved a little towards the middle of the exhaust event, compared to the first picture. I've also included markers, showing when each cylinders injectors are firing. What I WANT to try, is re-wiring the injectors. in this case, I'm on cylinder 7, if I could have it fire when cylinder 8 fires its injector, It would accomplish my original intent. So basically, I need to wire the cylinder 8 injector to cylinder 7, cylinder 7 would get wired to 3, and so on down the line. A fair amount of work for a 'what if'....



This is a shot of fitechs default injector timing, I had already changed some of the high rpm, high MAP numbers to 0, but the default in the 5000-7000 range was already at 0. The software allows you to change it from 0 to 20. So my main question is, has anyone tried changing the injection timing (and verified its timing) to fire on the open intake valve, and what was the result? I can still move the timing back at higher rpm and greater pulse widths so the injector its completing its event between compression events, where fuel reversion in the intake won't be an issue. My main intent is to clean up the rear plugs, any power gain is a bonus. I have zero intent on ditching this manifold, it makes great power and its compact size and air gap design help clean up a busy engine bay. This is also a street car that sees a fair amount of mixed use.

gta324

I have done some testing mostly at idle to clean up the fuel smell.

Injectors fireing just after EX valve closed cleaned up the fuel smell alot and AFR dropped around 1 at idle.

I have some small issues with the transition of the injector fireing but haved tried to fix it, going from 390 degrees back to 0 in 1 big jump so I have to smooth it out.
Its winter now so will tuen some more next year.

I'm running maxxecu so I no limits on what values to enter, actually there are no limits at all with maxxecu you can almost do whatever you want....

GTA matt

iTrader: (1)

That's encouraging to know that it helped. I have my idle AFR set to 15.3 to counteract the effects of cam overlap and fresh air going past the AF sensor. 14.7 will make your eyes water.

I stuck a bore scope inside the manifold with it running. There is a LOT going on in there at idle. Still see a small drip coming in from the pcv, but the amount of fuel spray going past the camera was unreal. It's very hard to tell what is going on in the pictures since the camera got misted up so fast, but I could see fuel basically puddling on the plenum floor, and then due to the casting of the manifold, was running into the #7 runner. I'm taking the car on a 600 mile road trip this weekend, so I don't want to jump into modifying the harness until I get back, in case something goes sideways...

First picture is the pcv port, I put my camera in one of the vacuum ports on the opposing side of the plenum. The other pictures are of the plenum floor, and you may be able to make out the runners, but it got messy quick. On the camera, it looked as if I was filming in a rain storm. I'm interested in doing a comparison once I get the timing where I want it.

Orr89RocZ

iTrader: (20)

Guess you dont have the ability to offset injector flow since its sequential? Factory gm lt1 did it for those reasons you see

i do it with holley hp efi on my bbc. I havent played with it much and have to look to see how i had it set up, i think closed intake at ilde areas and transition to all in before bdc at wot, but there is alot of available reading on that subject. For some guys closed intake doesnt work, closed exhaust does, or as close to closed exhaust as you can get. Improved afr readings and drivability improvements

however to ideally fix your situation i would do individual fuel control per cyl but understand if thats not possible

ULTM8Z

Yeah, this is likely the dreaded "Split BLM" phenomenon you're up against.... with batch fire, the only way I was able to solve it was to ensure equal air distribution since I couldn't modify fuel on a per-cylinder basis...

https://www.thirdgen.org/forums/diy-...ram-split.html

I"ve changed around the air intake set up quite a bit from when I posted this, but the distribution scheme is the same. The LT1's had idle air distribution holes cast into the intake manifold, which is what gave me the idea.


I used to have BLMs of >140 on one side and <120 on the other... now I get the same on both sides. Moving my WB side to side also shows similar AFR's on both sides.

Several guys I've worked with that had Minirams and HSR's were finding their WB's showing pig rich on one side, while the ECM showing high BLM's (lean) on the other... or vise versa...

yeah this is a tough nut to crack...

GTA matt

iTrader: (1)

Yeah, I have no way of changing individual cylinder offset with this ecu. I may eventually change systems down the road, but for now I'm going to stick with what I have. I like a good challenge anyhow. At part throttle and full throttle, it runs amazing, zero complaints, so if I can clean the idle up, maybe I'll stop screwing around with it.

gta324

With maxxecu I can change fuel on each cylinder, this helped my "split BLM" issues at low rpm.

And the injector opening angle helped the fuel smell at low rpms.....

now I'm running closed loop with dual WB one for LH and one for RH, I use 8 EGT to trim the fuel om each cylinder.

antman89iroc

My combo is a bit different with more CI, less cam and a different ECM. However, the rich in the rear seems common to front TB intakes. I found a lot of improvement at idle giving the motor a LOT more air at idle and hotter plugs for the street. I run basically a controlled vacuum leak at the rear of the plenum with very little throttle body opening and very low IAC counts. Makes it idle like a kitten. Maybe an angry kitten but lots better than before.

Orr89RocZ

iTrader: (20)

For mine, i checked the other day, i have end of injection around 30 deg after exhaust valve close, and my idle injection event in degrees crank rotation is only like 10 i think. Its pretty stable and smooth running. Part throttle bit higher rpm i ramp closer to bdc intake stroke. Wot is near bdc so part throttle ends anywhere from 50-100 degs after exhaust valve close. So in your pictures, between 360 and 540 deg. My Cam is decent sized but so far all plugs look fairly similar and both banks are close in afr.

You look like you were able to test moving it quite far right and left so can you try moving it towards evc? 400-500 deg?

GTA matt

iTrader: (1)

The pictures can be a little deceiving, I can only move the injection event back and forth across TDC on the compression stroke. In the screenshot where I have the cylinders numbered, it would be from 450 degrees to 720 degrees as shown in the small labels at the bottom of the rulers. The software won't allow me to move it any further. I tried advancing and retarding the cam sensor to see if it would move the injection timing, and it remained the same. I then installed the cam sensor 180 out in hopes that the events would move 360 degrees, but that resulted in a no start.. I'm going to go ahead with my plan of rewiring the injectors so I can get them to fire after EVC at idle, and then advance the injection timing at higher rpms so the injection even is complete before the intake valve closes. The car runs beautiful, idles great, low rpm driveability is wonderful, I just want to clean up that #7 plug, and in doing so, I figure it can only run even better.

I took it on a 600 mile trip last weekend, before leaving, I swapped an older plug into cylinder 7, since the plug that was in there was in pretty bad shape, mostly oil fouled from before I had a catch can installed. The plug had 6000 miles on it, and was in 'ok' condition prior to installing. These are plugs after returning from my trip, which was all low rpm driving. The plug on the left is out of cylinder 1, has 1500 miles on it. Plug on the right is out of 7. The front 6 plugs looked like #1, cylinder 8 was about half as bad as 7.

Orr89RocZ

iTrader: (20)

Yeah thats quite a difference. Its almost like the iac valve needs plumbed directly to those last holes like others have mentioned.

Your planned injection strategy is correct. Dont want fuel injected after intake close or before exhaust close so that gap between the two events is ideal for idle and part throttles