Individual Cylinder Trims Discussion

[spfautsch]

I'd like to think I've searched the subject exhaustively, but if there's an in-depth write-up that I haven't found please feel free to flog me (after you point me in the right direction).

What I've found on the subject is mostly geared towards setups with tube headers - I'm thinking it's safe to assume no-one running log manifolds will be concerned with tuning this, but all discussion is welcome. From what I've read the drill is: measure exit temp with IR sensor, add fuel to the hotter holes assuming they're burning lean but don't discount the possibility of nearly dead cylinders reading cold from weak combustion (need more fuel). Rinse and repeat until you brick your ecm.

A little background - my current (and probably only efi) project is a freshly rebuilt '95 LT1 with a fairly sloppy cam and headers. My understanding of the subject is that the individual cylinder trims are used to balance low-flow AFR to accommodate any cylinder to cylinder airflow differences.

So here are my observations, questions, discussion points, whatever.

1) Reading primary tube surface temps on headers in a car with the engine running seems a far cry from controlled experiment. Assuming you have a clean line of sight to some point on each tube, through experience I've observed reading different points at different distances from the flange or points around the primary bend produce vastly different results on the same cylinder.

2) eehack's cylinder balance test - disables one injector at a time and averages RPM and MAP changes to extrapolate relative cylinder strength. Obviously this is extremely platform specific, but can this be factored into the equation?

3) Is there any value in aiming for a summed balance between the banks - i.e. once a temperature balance is reached should one then strive to scale the right and left bank trim values so their sum is each roughly 4.00?

4) Short of fitting your heads / headers with individual EGT thermocouples, is there a better way? The IR thermometer seems a bit haphazard and my experiments with a single portable contact thermocouple have been problematic due to the amount of time needed to bring the sensor to equilibrium with the tube temperature.

I'm more than willing to find answers through trial and error, but if anyone would like to save my PCM a few dozen flash cycles your contributions will be greatly appreciated!

[steveo]

egt is tough. exhaust temp can be lower on leaner cylinders but if they're way rich, they will drop again, as the unburned fuel wont react and cools the pipe.. and as you noted heat transfer will screw over your results.

on race engines with individual throttle bodies, you just use vacuum per cylinder for balance. since you have shared vacuum in the manifold, the cylinder drop approach is a great method

of course perfect balance at idle isnt super critical, and as soon as you open the throttle plates, all hell breaks loose anyway.

[spfautsch]

egt is tough. exhaust temp can be lower on leaner cylinders but if they're way rich, they will drop again, as the unburned fuel wont react and cools the pipe.. and as you noted heat transfer will screw over your results.

With all the accessory equipment on the odd bank I felt like I was herding cats trying to get reliable temps with the IR gun. It was even a "fancy" one with a laser aperture indicator.

since you have shared vacuum in the manifold, the cylinder drop approach is a great method

The balance test showed that 1, 2 & 3 are overall the weakest, with 1 showing nearly 20% lower #s than the strongest holes (5 & 8). I know 1 & 2 are usually weaker on smallblocks with this type of intake plenum. But doesn't #1 also has the smallest idle air feed hole? I have to assume GMs engineers did this for a valid reason, but wonder if making them all the same size would do any good.

of course perfect balance at idle isnt super critical,

I might have a touch of OCD so I take this statement as a challenge.

and as soon as you open the throttle plates, all hell breaks loose anyway.

I know what you're saying, and I've read posts where you mention disabling the part-throttle individual trims. When I was running speed density to tune VE I noticed my cell 16 split was always lean on the even bank by +15-20, but the adjoining part-throttle cells (5,6,8 & 9) always had the odd bank showing lean. I was starting to wonder if I had accidentally plugged the post-cat O2 sensor into the primary connector.

Strangely, after I buttoned up my VE tuning and turned the MAF back on with no other changes to the tune the splits all but disappeared. Cell 16 split is averaging 1, and the worst tip-in cells (5 & 8) are 6. I'm starting to wonder if I can't use the part throttle cylinder trims to try and tweak this out. With a sloppy cam like this you can use every ounce of torque you can develop in those low airflow ranges.

Whatever the case, I'm baffled as to where the split "disappeared" to.

[Terminal_Crazy]

With all the accessory equipment on the odd bank I felt like I was herding cats trying to get reliable temps with the IR gun. It was even a "fancy" one with a laser aperture indicator.

I've tried that several times. If i hold the gun upside down i can get upto 100Degrees C higher.

The balance test showed that 1, 2 & 3 are overall the weakest, with 1 showing nearly 20% lower #s than the strongest holes (5 & 8). I know 1 & 2 are usually weaker on smallblocks with this type of intake plenum. But doesn't #1 also has the smallest idle air feed hole? I have to assume GMs engineers did this for a valid reason, but wonder if making them all the same size would do any good.
[/QUOTE}
The Balance test also seems to vary from day to day.
The front cylinders are set to have more fuel and the rear less in the cylinders in the stock tune.
Drilling the manifold idle hole out makes no difference at all as far as i've seen, although the drillings on the Edelbrock LT4 manifold is a bit crap to start with.
Splits move around & seem to come & go on a whim.
Idle timing changes, I have seen affect the timing on the injector BPW which i would expect to increase the BLM splits. As the timing increases the bpw drops but more on one side than the other.

[QOUTE]I might have a touch of OCD so I take this statement as a challenge.

Me too. I want to know it's right.

I know what you're saying, and I've read posts where you mention disabling the part-throttle individual trims. When I was running speed density to tune VE I noticed my cell 16 split was always lean on the even bank by +15-20, but the adjoining part-throttle cells (5,6,8 & 9) always had the odd bank showing lean. I was starting to wonder if I had accidentally plugged the post-cat O2 sensor into the primary connector.

I suspect the Mac midlength headers for some of it.
The 02's seem to be sat in 1 pipe rather than reading an average in the collector so I think playing with the fueling doesn't really show up on the other 3 cylinders the same.

Strangely, after I buttoned up my VE tuning and turned the MAF back on with no other changes to the tune the splits all but disappeared. Cell 16 split is averaging 1, and the worst tip-in cells (5 & 8) are 6. I'm starting to wonder if I can't use the part throttle cylinder trims to try and tweak this out. With a sloppy cam like this you can use every ounce of torque you can develop in those low airflow ranges.

Whatever the case, I'm baffled as to where the split "disappeared" to.

Haven't got around to putting the MAF back in yet.
I went SD with some success but the wideband & narrowbands don't quite match up. Wideband ran really well but i felt was too rich.
The narrow bands feel a little light.
Since I've dropped the timing down loads again the car is much more relaxed than before.

If it's happy & it runs it will be fine.

HTH
Mitch

[spfautsch]

Haven't got around to putting the MAF back in yet.

I've read a bunch about your project searching for ideas for mine - it may be worth the attempt. I was pretty frustrated with the parking lot manners in SD/CL but the difference is night and day with the MAF. I have no idea why, but I'm not going to look that gift horse in the mouth. I was on the fence as to whether it was worthwhile to finish installing the A/C piping and charge it, but I'm thinking it'll handle it now.

The only issue I can't seem to resolve is surge / bucking in gear at closed throttle. I know the fix, but don't want to take idle timing out because at 29 the lope turns into a "dead cylinder" feel that shakes the car while at 30 you can barely feel it.

I suspect the Mac midlength headers for some of it.
The 02's seem to be sat in 1 pipe rather than reading an average in the collector so I think playing with the fueling doesn't really show up on the other 3 cylinders the same.

What does your int delay table vs airflow, and corrcl vs airflow look like?

By my estimation the O2s are only around 6-8 inches further from the exhaust ports with mine (stainless works C4). I'm having trouble finding pics of your headers to compare.

[Terminal_Crazy]

I've read a bunch about your project searching for ideas for mine - it may be worth the attempt. I was pretty frustrated with the parking lot manners in SD/CL but the difference is night and day with the MAF. I have no idea why, but I'm not going to look that gift horse in the mouth. I was on the fence as to whether it was worthwhile to finish installing the A/C piping and charge it, but I'm thinking it'll handle it now.

The only issue I can't seem to resolve is surge / bucking in gear at closed throttle. I know the fix, but don't want to take idle timing out because at 29 the lope turns into a "dead cylinder" feel that shakes the car while at 30 you can barely feel it.

I might switch the MAF back of again then next time i go out.
Mine just felt uptight like itr was being held back.
Timing was around 39. Idle was really smooth but it didn't drive nice. SDOL fattened it up but it cruised nicely evreywhere else.
Increasing the timing smoothed idle, reduced Ibpw, reduced MAP etc etc everything we are looking for.... just drove a bit jerky.

I've since raised my O2 swing thresholds up. It passed the emmissions test like that and drives much better with the lowered timing.

What does your int delay table vs airflow, and corrcl vs airflow look like?

By my estimation the O2s are only around 6-8 inches further from the exhaust ports with mine (stainless works C4). I'm having trouble finding pics of your headers to compare.

[/QUOTE]
Fast 02 Rich/Lean
0 451
16 438
32 424
48 424
64 424

o2 Int Delay - Airflow
0 0.80
16 0.66
32 0.46
48 0.43
64 0.38

CorrCL - Airflow
0 08
16 26
32 26
48 26
64 32

I'll stick a pic of the headers on my website.
http://www.sand-hill.uk/TerminalCraz...Length_004.jpg

The port for the O2 sensor is basicaly at the end of the header pipe rather than in the collector.


Mitch

[spfautsch]

After finding a "smoking gun" cracked exhaust tip that was surely the cause of the goose-pitched exhaust drone and welding it up I'm sad to say not much has changed. I'm going to move my (false?) knock issue to a new thread so as not to steer this thread even further off the title.

I did however want to report that on a whim I added some individual trim to #1 as it was reporting weakest on the eehack cylinder balance test, with decent results.

since you have shared vacuum in the manifold, the cylinder drop approach is a great method

of course perfect balance at idle isnt super critical, and as soon as you open the throttle plates, all hell breaks loose anyway.

My thinking was that a possibly leaner cylinder was exacerbating a possible lean swing condition in cruising cells, causing knock. I haven't completely ruled out this possibility, but I did greatly lower the individual trims tps limit so it shouldn't be an issue in any realized load range.

Beyond that, my main concern with cylinder balance is how individual trims might affect cell 16 and any BLM split there. I've completely revised my timing tables, so most of my logging data in regards to that has become more or less invalid. However, I'd love to hear thoughts on the value of making the sum of the indvidual trim banks add up to something close to 4 and 4.

Whatever the case, if the worst idle split I see is 4-6 counts as observed yesterday I'll have no problem calling that "good enough".