Trying to dial in the tune on the LT5. Just when I seem to be close to finalizing based on BLMs, I pull plugs and they don’t indicate what the BLMs are saying. Specifically, my plugs are a very dark brown suggesting the tune is rich but the BLMs are reading in the 133-135 range. I’m tuning to within a +/- of 3 to 128. The heads and top end have been ported, larger cams, and 3” exhaust w headers.
I’ve ordered a pair of Delco O2s to see if that changes things. Thoughts?

Well the plugs are telling me there’s more than enough fuel. Maybe I should be lowering the O2 xover so the ECM does not continue to add fuel.

I’ll see if the new O2s make any difference. I have had that issue before where new O2s make a significant change in the tune.

New O2s arrived. I throw them on and we’ll see if that makes a diff. I kniw its a “throwin parts at it” but its a fairly quick inexpensive way of eliminating a possible issue. Besides, I’ll have fresh O2s in the motor which is not a bad thing.

AC Delco. Have used them now for the laste couple of years and they give me a consistent signal when logging whereas the Bosch O2s would drop signal randomly.

Have you verified with a wideband? I dont like tuning without one

what do you mean lean rich swings? What does the wb say for actual air fuel ratio?

Did you try open loop with wideband only to dial in first before going to closed loop?

When in C/L, I can see the AFR on the WB moving across the swingpoint of the O2s. It alternates between say 14.5-15.3.
Currently, the car idles at about 900-950rpm

If wideband is that air fuel ratio and seems to match blm then it shouldnt be fouling plugs unless your plugs are too cold. Or you are getting false readings from ignition issues

Well part of it is that the LT5 suffers from a similar “split blm” issue as other LT motors. Maybe a bit worse. I have been recently “tuning” out KR in mainly cruise area of the SA table which is the 1800-2400rpm/45-70kPa range. The motor has heads, top end ported w larger 63mm TB, intake and exhaust cams. Exhaust is 1 7/8” headers into 3” ID SS exhaust.
Ground strap seems to indicate the right heat range for the plugs. I’ll see what results, if any, I get w the O2 sensor swap out. If nothing, I will put back the next step hotter plug, NGK BKR5E heat range instead of the 6Es.

A 6 ngk usually works well for most performance builds. So i wouldnt think that be too cold. Plug threads will also tell heat. Whats the compression?

I have run turbo cars with 7-8 range plugs just fine which are very cold for low compression cruising. They do get dirty rich lookin in time even at stoich. May want to try touch leaner esp at idle since overlap may skew o2 sensors

Unfortunately no such capability in the LT-5. We have 4 VE tables due to the port throttle system, 16 injectors and 32 runners. The O2s trim the individual banks. But they work off the same VE cell. The LT5 engine management is a bit of a bastardization of the L98. The LT1 came later and incorporates what they learned from the LT5.

So the latest is that I lowered the O2 thresholds and that helped a but I don’t want to lower them further because they were already lower in the stock cal. The motor feels that it likes the BKR5E plug better than the 6E. I tried raising SA again but started to pick up some KC at a steady cruise as I had in a previous thread. The Knock occurs during steady cruise at about 1800-2000rpm and shows upwhen the road begins an incline. In the LT5, AE is based solely on MAP differential not TPS%, so I can maintain a steady foot on the throttle but MAP will change say from 45-60kPa or a delta of 15kPa, and that’s when I will see the KC increment. So this is why I dropped timing. It was to eliminate the KC at steady speeds. Otherwise I don’t get KCs.
So my question is, did I drop timing to eliminate KC but in the meantime lower timing is giving me a more incomplete burn?
Am I creating s problem trying to solve a lesser problem?

Well I can’t say enough about what a “treasure” RBob is when it comes to this stuff. Sent him a couple of pics of the plugs I had been using, NGK BKR5E-11. He took a look and recommended I go w equivalent Autolite because he thought the NGKs were suspect and possibly could even be COUNTERFEIT. Holy smoke he was right. I went out and got a set of Autolite 3924. My BLMs snapped into place w just a couple of tweaks and the rich looking plugs aren’t rich any longer. I’ve attached pics of what I sent Bob before and one of the result.

NGK #5

NGK from 2&8

NGK top, Autolite bottom

I’m curious as to what would make you think the ngk were counterfeit? That seems really odd to me

The autolite cross references with ngk 5 and 6 so not sure how the heat range compares exactly but seems odd it made that much of a difference unless the ngks were indeed counterfeit and not truly heat range 5 or 6

I was not the one to suspect counterfeit. It was RBob. Perhaps he’d like to chime in but basically I sent him pics of my plugs and he noted how the center electrode edges were rounded and that the thread cut was badly done. He also saw that the ground strap edge was not sharp and clean but beveled. You’ll note also in the bottom pic how much taller the Autolite center electrode and porcelain is compared to the NGK. I would think this allows a spark to be produced further into the center of the AF mixture in the chamber. Bottom line is that I threw in a set of the 3924 Autolites gapped at .044”, and I drove the car today 350 miles to Detroit. Wow! It was amazing and my gas mileage increased by 3-4mpg. The motor as not audible and the exhaust note was just a monotone vvvvvvvvvvvvvvvvv. You could not really distinguish the individual exhaust pulses. I also did a datalog yesterday, and there were only 3 VE cells that needed minor adjustment. There were just a couple of KCs requiring a 1* adjustment in SA.

The odd thing is that I actually used 3 different sets of NGKs. 2 of which were BKR5E and one BKR6E. One set of the 5s was gapped at .035”, while the other was gapped at .044”. The 6s were also gapped at .044”. They all showed similar combustion. I am going to check the other sets of the NGKs to see if the ceramic cone portion is shorter than the Autolite as it appeared it is with the one set of BKR5E.

Where did youget them from

I cant imagine them being counterfeit

in any case strange results imo. Little bit of projected tip i would not expect to make a big difference like that. Plug gap and heat range changes in the ngk didnt help, maybe the autolite is hotter? Idk

Well, the change was pretty significant. I just finished a 700+ mi round trip to MI. Once the motor cools off, I’ll pull a couple of the plugs and see what they look like. I do know already that my gas mileage was noticably better. The engine “note” was very consistent. I didn’t “hearr” individual cylinders. Just a consistent rrrrrrrrrrrrrrrrrr. I’ll also measure the differenc ein the nose projection between the two plugs

So here’s the latest. Drove the ZR round trip to MI for business. Put 700mi on Autolites. Posting pics. Trying to figure why the discrepancy in the cylinders. BTW, the car felt great the entire drive and got 22-23 mpg averaging 75+ mph.

Looks alot richer than expected for cruise. It drives better? But doesnt look any different than the ngk?

The tailpipes on the other hand are pretty clean w little soot, much less than previous. After ~800mi., you can still see the chrome under a light film of exhaust. It idles nicely for a cammed motor, very responsive.
The plugs look darker in the pics. When u put light on them, they are more of a lighter coffee brown. They aren’t black.
But again, the variance in the coloring is odd. Clearly looks like uneven fuel supply for some reason. A bad fuel regulator would affect all injectors equally I would imagine.

Its not uncommon for intake manifolds to have cyl to cyl distribution differences. The induction system may need some balancing or do cyl to cyl fueling and 8 o2’s lol.

Plenum was recently ported along w cylinder heads and injector housings. Done by someone who’s done them many times before.
Don’t I wish I could trim the fuel w a modifier for wach cylinder LOL!

Yeah ported doesnt always mean balanced. Some ports may flow different than others and especially while engine is running and during the overlap cycle

Yes the LT5 plenum/injector housing combo has 3 different direction changes before the air gets to the chamber. Having said that, these results are a bit confusing.

I beginning to suspect oil in the number 2&4 cylinders. There’s a crankcase vent located next to the intake for those 2 cylinders. And there’s been cases of oil getting sucked past the gasket into the cylinder. They needed to do a better job of keeping fluids away from each other in this motor.

Dominic- I have NGKs and I’m not I impressed with them either.

I have a set of AC Delco Rapid Fires that I will replace them with. I haven’t run Autolites before - but the AC plugs have never given me an issue.

The issues you mention sound a lot like what I have seen on my NGKs as well. Mine were gapped at .035” too. I gapped them to .045” and am running and MSD A6 unit. Still, I am seeing the same thing with plugs, AFRs, and BLMs that you are seeing.

Has nobody modified the LT5 bin to have VE tables for the different cylinders?
Or a multiplier per cylinder?
Seems like if it’s already sequential throwing in a multiplier that references the cylinder could easily be done. Id have already done it for the $8d code if it was.

Short answer is no. No such mod to the cal. There are 2injectors/cyl depending on whether the secondary runners are open or closed which I suppose would add a level of complexity. Perhaps a modifier to the VE table for left and right banks would be a start.

I don’t think that would complicate it too much but I’ve never looked trough the bin.

maybe 2 multiplier tables. One for each set of injectors per cylinder.
the code is the same, you’d just add an extra jump to the same subroutine. An extra 5 minutes of work if that.
of course that is if the coding is as basic as $8d. Which I think it probably is close.

The LT-5 was a modification of the L98 code. They started working on it in 86, so its pretty much a derivative. There are only 8 injector drivers. The 2ndarys are driven off separate relays but take their PW signal from the associated primary injector. When secondarys are energized, the cal cuts PW by .51 so now both injectors effectively deliver the fuel of one injector.

Sounds doable for sure. Is there a well documented disassembly somewhere?