Re: What say you, Graham?
 

Lance,

My point was that Graham did not experience significant divergence of BLMs bank to bank perhaps because the OEM injectors were better matched. Or since it was in the design phase, they may not have had the normal variance found on production pieces. More than 20 years later, various injectors are being used, w a difference in electrical and mechanical properties, and this could be aggravating the differential side to side. Throw mods into the mix and voila!
I'm no code writer, but it's my understanding that due to SPFI, it would not be hard to program individual L/R fuel trims to help even things out.
What's interesting is that it is almost always the left bank which runs leaner than the right. So it may not be injectors. This "split BLM" is not as pronounced in stock motors as it is in modded ones. And idle or low rpm operation has a greater degree of it than high speed cruising from my observation of it.

Re: What say you, Graham?
 

My experience has been that, given injectors that are well-matched–and not all OE injectors were well-matched--the bank/bank difference in long-term fuel trim (BLM) was not greatly different unless there was a problem somewhere.

To me, from the perspective of the LT5 being a very high-performance engine, well-matched means a flow difference of no more than 1.5% across 16 injectors.

As for Graham B....now that he's finally arrived here at the ZR1 Net forum, we need to start a thread about country western singing when consuming large quantities of beer.

Re: What say you, Graham?
 

This is good food for thought but how many engines have you looked at for left vs right running more lean?
Seems counter intuitive. I wonder what would possibly contribute to this? Injector variances left to right would normalize out over many engines. Placement of the IAC on the right side? ???? nahhh

Re: What say you, Graham?
 

Dom, have you tried left-right swapping of the O2s to see if the BLM characteristics follow the sensors? I'd be curious to see what effect that might have on BOTH BLMs and injector dwell time; before and after the swap.

P.

Re: What say you, Graham?
 

The "split BLM" issue is not unique to the LT-5. The LT-1/4s exhibit similar bank to bank differential. My suspicion, and that's all it is, is based on my experience w the Xfire motor. There you have a much simpler example of individual bank fueling. The fueling for that motor used twin TBIs in series. The "rear" TBI (left bank) had the regulator built in. I measured fuel pressure differential between the two TBI was .5psi. This was enough for GM to use staggered injectors of
63/65 # F/R to equalize fuel delivery. No other TBI injectors were available that would retain the stagger and deliver more fuel needed for higher hp. It became necessary to run individual fuel lines to each TBI so that equal size injectors could be used. So I am speculating that there may be a slight pressure drop at the left hand fuel rail due to the added distance fuel travels to reach it. Perhaps the use of injectors with different mechanical and electrical properties could also magnify the differential.

Re: What say you, Graham?
 

A way to isolate on this theory would be to re-route the fuel to the left side first. Painful to do but would be conclusive.

Re: What say you, Graham?
 

It could be cylinder to cylinder airflow difference, especially at idle speeds. Perhaps the rear cylinders are a little hotter as well.

I don't see it as being a very big deal so long as your blm map is close enough to center to be able to allow adjustment. As I recall, the LT1 engine has calibrated vac leak holes to each intake port to allow fine tuning of individual cylinders rich/lean from some mechanical design issues that can't be overcome.

Re: What say you, Graham?
 

LT1's also experience a cylinder to cylinder to cylinder variance, hence the individual cylinder fuel trimming tables in the calibration.

peace
Hog

Re: What say you, Graham?
 

be cool to put a wideband or egt sensor on each header tube to see the AFT variance

maybe a really nicely flow balanced set of injectors would help.

the chevy motors (especially BB chevy) have "good ports and bad ports" that flow pretty differently. I don't suspect that's an issue but I'd be willing to bet that there is a difference in flow from front to back on each side due to the manifold (air doesn't like to make sharp turns)

the LT5 probably isn't nearly as bad as the older stuff in that respect.

when was data logging with EASE to have marc burn me some chips I noticed the difference in the bank to bank BLM too and thought something was wrong.

Re: What say you, Graham?
 

Ron,

I use Ease as well. Look at your scans and tell me if the left bank wasn't running leaner than the right.
BTW, I used a laser thermo to measure heat from exhaust gases of each cylinder.