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DFI and ECMDiscuss all aspects of DFI (Digital Fuel Injection), ECMs (Electronic Control Module), scanners, and diagnostic equipment. Fine tune your Third Gen computer system for top performance.
Been studying the issue, lots of reading here. Also see how gm dealt with it in the 94-97 lt1 by using individual cylinder fuel multiplier tables for idle, and off idle up to around 14.8 %tps.
With sequential (Holley hp)efi I can do the offsets. But with a ported miniram, afr 195, and non stock camshaft, 58mm t-body, headers , and a lack of 8 egt or 8 widebands I am hesitant to use the stock offsets. I have it idling at 850, lowest kpa i can get with 24* timing is 53-55 and it isnt very steady. the fueling, and timing are consistant, all cylinders crank @ 195-202* it runs strong, pulls hard with safe timing to 6300, would like to get the idle smoother. the zf with aluminum flywheel is noisy as hell, ive tried it down at 800, and as high as 1000, and the box of rocks noise is the same. I need to get the idle smoother
the best I have is a temp gun. I let it heat soak idling, and the richer cylinders/leaner cylinders are following the factory offsets pretty closely as far as lean cylinders being cooler, richer being hotter. Have any of you played with using these individual cylinder multiplier offsets on dfi, bs3, holley, haltech etc? I do not want to burn a piston without accurate data and these tables are pulling as much as 6% per hole. do any of you experienced gents have advise, or experience that will help me proceed in a safe manner? or should i just put o2 bungs in each primary and take it to a dyno? im at 10.3 scr, dcr 8.3-8.4, has .041 head gaskets, zero decked. have driven it and allowed it to learn, multiple data logs. closed loop correction % everwhere with hot engine is 3-5% but it is just an hp, so single O2 on passenger side collector.
Are you getting split BLM's significantly above idle speeds? On mine, I was only getting it up to about 1000 rpm if I recall. After that, the BLM's equalized from side to side.
This is what I ended up doing to solve the idle distribution issue... (can't believe that was 15 year ago... lol)
But It's interesting... I didn't even know the $EE mask had an off-idle fuel trim per cylinder.
Yeah, the front cylinders appear to be about 10% richer than the rear cylinders. But it seems like if you wouldn't be in danger of burning holes in the piston until you got pretty lean (like into the 16 or higher range?). If I tune the engine to say 12.5:1 in the aggregate, ±10% would up to 13.75:1 or down to 11.25:1. Neither extreme seems like it'd be damaging. If you had individual cylinders varying more than 10% from one another... I don't know... I'd probably suspect something else going on besides air distribution in the manifold.
Seems like the only way you'd be able tune that differently from the factory (but also accurately) would be to actually measure each cylinder exhaust as you hinted at. Otherwise you're relegated to measuring the aggregate on each side.
After I typed all that it occurred to me even having a stable afr bank to bank with dual O2 would help. Wish I'd spent the other 1200 and bought the dominator back in 2019, cost a lot more now. Over the winter I'll pull the headers and get egt fittings welded on. Spent an hour reading your old posts about slaying the split blm beast. I can try that as well. Maybe it won't need 8 O2 to get a smooth idle. I love the way it performs, but the rough idle is driving me nuts. I can't begin to tune timing with knock sensors until I get rid of at least half of the gear rollover noise. I am probably leaving significant power, and acceleration on the table with the mild low rpm timing I have to run.
Are you getting split BLM's significantly above idle speeds? On mine, I was only getting it up to about 1000 rpm if I recall. After that, the BLM's equalized from side to side.
This is what I ended up doing to solve the idle distribution issue... (can't believe that was 15 year ago... lol)
But It's interesting... I didn't even know the $EE mask had an off-idle fuel trim per cylinder.
.
The 94 & up lt1 had these they are "individual cylinder fuel trim" multiplier, but remember the ltx has an iac "plenum" with runners feeding each cylinder, so in the case of a Speier ported miniram, the idle, and off idle could well be worse and were sacrificed in the name of stable higher rpm airflow. Factory the idle table offset was only at closed throttle, the off idle was to 14.8% TPS, they do not list any rpm, just TPS. Thinking the maf location, t-body size, air snorkel size, bends all come into play here. Modify any piece of it, or idle rpm, heads, exhaust and it all changes.
One thing you could potentially try as well would be to put a NPT air inlet hole at the back of the plenum as well. That way you get another "IAC" port feeding the rear of the intake. Ostensibly that should help feed the rear cylinders at low TPS and RPM.
it'd be a much simpler approach than mine. If it doesn't work simply plug the hole and then you could replicate my set up.
One thing you could potentially try as well would be to put a NPT air inlet hole at the back of the plenum as well. That way you get another "IAC" port feeding the rear of the intake. Ostensibly that should help feed the rear cylinders at low TPS and RPM.
it'd be a much simpler approach than mine. If it doesn't work simply plug the hole and then you could replicate my set up.
Hmmm... it's sizeable, but doesn't seem outrageous. I'm assuming it's idling rougher than you'd expect for a cam of that size? What was your displacement again?
I recall there was a thread on tuning the O2 constants for a large cam. Has to do with excessive overlap and how much unburnt O2 gets dumped into the exhaust. There could be some nuggets of info that could help there.
On mine, forgot to mention... I idle in OL. Requires S_AUJP to do it on $8D. After 1200 rpm, it goes into CL.
My cam is signifcantly smaller (215@.050 when counting the 1.6 rockers, on a 114 LSA), and on a 383. But I still idle in OL to get the desired smoothness. I idle at about 13.75:1 as measured by the WB on the passenger side with NB controlling on the driver side. I've swapped sides on the O2 sensor and it's pretty consistent side-to-side. But yeah, the Miniram seems to prefer a slightly richer idle and it also seems like idling in OL is best way to achieve that. I"ve played around with the O2 thresholds trying to idle in CL and it's Ok I guess, but I get better results with the OL idle (and the throttle tip-in is better with OL too).
Hmmm... it's sizeable, but doesn't seem outrageous. I'm assuming it's idling rougher than you'd expect for a cam of that size? What was your displacement again?
I recall there was a thread on tuning the O2 constants for a large cam. Has to do with excessive overlap and how much unburnt O2 gets dumped into the exhaust. There could be some nuggets of info that could help there.
On mine, forgot to mention... I idle in OL. Requires S_AUJP to do it on $8D. After 1200 rpm, it goes into CL.
My cam is signifcantly smaller (215@.050 when counting the 1.6 rockers, on a 114 LSA), and on a 383. But I still idle in OL to get the desired smoothness. I idle at about 13.75:1 as measured by the WB on the passenger side with NB controlling on the driver side. I've swapped sides on the O2 sensor and it's pretty consistent side-to-side. But yeah, the Miniram seems to prefer a slightly richer idle and it also seems like idling in OL is best way to achieve that. I"ve played around with the O2 thresholds trying to idle in CL and it's Ok I guess, but I get better results with the OL idle (and the throttle tip-in is better with OL too).
its a 385. over 10:1 scr, dcr over 8:1 the seat to seat duration is low, 58* of overlap i can throw 32* at it and get kpa to drop to 52-51 kpa, but thats not a long term fix. ive leaned it out (14:1), fattened it up as much as 12.5 afr and everything in between, turned off all closed loop correction. even tried alpha n idle and the best i can get is 850 56-54 kpa and its shakey, idle fluctuates from 920-800. FUNNY THING IS IF I CHANGE IDLE TO 950-1000 IT DOES THE SAME THING. it has to be a distribution issue. ive played with injector end angle. its not stock ecu, it is holley hp, 36 pph injectors with accurate data. and i just cant get it to smooth out anymore than that unless i set it to 1150-1200, and this cam is small, there is no reason it shouldnt idle smoothly at 800-850 ive done compression and leak down, even smoke tested the intake and exhaust for leaks, changed plugs, tried different gaps. triple checked valve lash, plug wires, ignition etc. its an air distribution problem and the plugs show it. it cleans up ok around 11-1200 rpm. The iac mods may not fix it, but wont hurt to try and wont hurt the intakes value either, if it doesnt work ill just put some nitrous fittings in it. ( why i wanted to use 3/16 compression fittings, If it doesnt work I have a holley efi single plane as well as first tpi. but im not giving up on it yet it runs strong. based on fuel flow, map. and other stuff its safe to say its making a solid 485-+ hp. if i could just get smoother idle/off idle. other option is having the lt1 intake i have converted to gen1 bolt pattern, distributor, and coolant lines with a remote t-stat.
Last edited by Bill Chase; 08-29-2022 at 11:59 PM.
228@.050 on a 110 LSA in a 385 isn't that small, IMO. I guess I don't know what you'd consider smooth or rough. The fluctuating idle would get annoying though for sure.
Is there something in calibration that alters timing to correct for idle speed? I know in $8D there's idle speed correction via spark advance/retard (ECM will add or subtract 5 deg to correct for idle speed). That can sometimes cause fluctuations on the idle speed on a modified car, so it's often zeroed out.
I'm assuming you degreed in the cam when you installed it? 100% sure it's not retarded by some amount? Alternatively, you could advance the cam by say, 4 deg. That should help smooth the idle out a bit.
If you're convinced it's a fuel distribution issue, you could try putting progressively smaller injectors toward the rear of the car and see if the idle improves. It'd be mimicking what GM did in the LT1/LT4 calibration. It'd only be temporary for troubleshooting purposes. But if you see that you get a marked improvement, that would be the impetus to move forward with the manifold modifications. It'd suck to go through the effort to copy what I did and then not see an improvement.
When the split BLM subject came up years ago I read every thing I could and worried about how to fix it. I could not get my idle where I wanted it. I originally had a 355, hotcam, and vortec heads with a modified lt4 intake. I ended up with GM fastburns with 10.25 cr and pro-floXT with no change in idle. I finally went with open loop idle at 750rpm's with 28 degrees advance and 13.5-13.8 AFR and have 50kpa. Now it idles great with only a slight lope and with my manual trans I can leave a stoplight without touching the gas pedal. I'm so happy with the tune I haven't touched it in several years. Lesson learned , give it what it wants. The spark plugs will tell you, Read them.
I don't know about the split blm but at one time I was convinced that was my problem. I had an o2 sensor in each bank and I could switch between them but not see both at the same time. If the motor is not smooth the intermittent misfires will mess with the o2 sensor readings so they will not be accurate anyway. My opinion, on the later cars they separated the left and right bank for better emissions but with a good tune the difference is so insignificant it's not worth worrying about. I see no difference when reading the plugs. It's mostly about the cam and the tune. I went from "set the paint cans in the back and let it idle in closed loop, that'll mix it good " to "I can't believe how smooth it idles" in open loop with my tune. This is open loop idle at 700 rpm's. I normally set it at 750.
228@.050 on a 110 LSA in a 385 isn't that small, IMO. I guess I don't know what you'd consider smooth or rough. The fluctuating idle would get annoying though for sure.
Is there something in calibration that alters timing to correct for idle speed? I know in $8D there's idle speed correction via spark advance/retard (ECM will add or subtract 5 deg to correct for idle speed). That can sometimes cause fluctuations on the idle speed on a modified car, so it's often zeroed out.
I'm assuming you degreed in the cam when you installed it? 100% sure it's not retarded by some amount? Alternatively, you could advance the cam by say, 4 deg. That should help smooth the idle out a bit.
If you're convinced it's a fuel distribution issue, you could try putting progressively smaller injectors toward the rear of the car and see if the idle improves. It'd be mimicking what GM did in the LT1/LT4 calibration. It'd only be temporary for troubleshooting purposes. But if you see that you get a marked improvement, that would be the impetus to move forward with the manifold modifications. It'd suck to go through the effort to copy what I did and then not see an improvement.
It's actually stable, steady and I'm not expecting OEM like idle. The real issue is an aluminum flywheel that weighes less than 14 pounds, no dual mass flywheel, and Corvette zf6 transmission. The gear rollover noise at idle clutch out is pretty obnoxious. I was warned this flywheel would magnify the "box of rocks" syndrome these transmissions have when converting to a conventional flywheel. It runs great, very difficult to use.more than 50% throttle in 1-2 gear without a total smoke show. So anything I can do to get the idle to be smoother will help. It's more of a noise vibration harshness concern. The entire flywheel, clutch, pressure plate and throw out Bearing isn't 25 pounds, compared to the stock dual mass flywheel weight of 46 pounds. I'm not expecting miracles, but if I can get it even slightly smoother at idle the noise will not be as severe. Tempting to put the 4l60e and 2200 stall converter I have in it. But I suspect the power it makes the auto wouldn't last long. Car is a beast other than that no complaints at all.
By the way, based on the numbers I posted I added fuel at idle in 1 and 2 @ closed throttle and it did make a noticeable difference. It didn't drop kpa, still at 54-56 but is more stable, and iac % numbers dropped a couple points. Turned off learning and closed loop compensation, allowed it to cool off and it even started quicker. Enough evidence for me to proceed over the winter with the iac feed to each runner. This intake is max ported from Speier, so I suspect it aggravated the idle airflow distribution issues slightly.
Last edited by Bill Chase; 08-31-2022 at 03:48 PM.
08-28-2022, 02:30 PM
