How lean can I go for part throttle (safely)? I have heard
LT1's are tuned real rich and would like to improve fuel economy and performance (I am willing to take a tiny loss of performance for a few more mpg).

Someone told me LT1's are tuned around 11:1 A/F factory.
I have heard the ideal A/F for WOT is 13.2:1. I know 14.7:1 is stoic but does that mean that the car actually is tuned at that ratio?

I have an OBD2 97 LT1 Camaro (stock engine/cai/headers/catback) and will be getting a tuner (probably madz28) to tune it. I have heard his tunes are a little on the rich side. I would like for him to lean it out more than usual (would he know how to do this or do I have to tell him the values to put in?). If anyone has any info or values to input that would be very much appreciated.

Also what is the leanest you have heard of someone running in an LT1? What do you guys run at? My car is a DD and will probably never see redline or the track. Any help/opinions/or thoughts on this will be greatly appreciated !!!

thanks - Pete

 

The stock tuning in closed loop (part throttle/low to moderate load) is 14.7:1. Its hard coded and I don't think you can change it. You'd probably get better fuel economy with something in the 15.3-15.8 range, without exposing the engine to excessive detonation. But the factory selected 14.7:1 because that's the A/F ratio that produces the lowest total emissions when considering HC, CO and NOx..... any richer and HC and CO start to climb rapidly.... any leaner and the NOx starts to climb.

The "LT1's are programmed rich" comment applies to power enrichment (PE) mode - essentially WOT with no feedback from the O2 sensors. In PE mode, the PCM calculates a "target" A/F ratio, based on several sensor inputs. It is that target A/F ratio that typically ends up at 11.7:1, a bit on the rich side. Its easy enough to change with the PE tables. But it isn't going to effect fuel economy - you don't spend that much time in PE mode.... its just going to pick up a few more HP. Typically, 12.8-13.2:1 will provide peak power/torque, but each engine is a bit different.

 

Even if you did change it, as long as it is running closed loop, it'll go right back to stoichiometric. Provided you don't run into the correction limits. O2 sensors and closed loop operation ensure that the engine is always running around stoichiometric.


Most mail order tunes run a bit rich at WOT because it's much safer for them to do so. There's only so much you can do without working directly with the car and collecting data. If you want the best tune, it must be done in person. The same can be said for all tuning parameters, not just fueling.

On an essentially stock car such as yours PCM tuning will only net you a little more power at WOT. The factory tune is fairly aggressive in most respects. When you start diving into more aggressive modifications PCM work becomes more and more essential.

There's a lot of information out there on EFI operation you might want to read to make the picture more clear. EFI tuning is a deep dark hole with no bottom in sight

 

The only way I can think of to do that is to run LC-2 widebands in place of your stock O2's. You can program their analog "simulated narrow band" output they'll send to the computer such that 450 mv = 15.5 or whatever AFR you want instead of 14.7.

You'll have to drive a ways to save the $400 it will cost in fuel...but you're also getting a wideband for each bank. If you do this I'd suggest setting PE mode to activate at lower throttle openings than stock programing for safety.

 

Its very easy to change your desired closed loop AFR with some careful manipulation. Using a Twin Tech wideband and a Narrowband O2 ran a few datalogs and determine exactly at which AFR my car drove best then tuned the narrowbands to chase that AFR automatically. I did this by plotting a 3 set logrithmic fit to what the NB's and WB's where doing in the same collector at the same time.

i get flamed every time I suggest that to others on the TGO tuning boards but oh well.. to each thier own. They can continue to tune open loop and re-tune everytime the season changes.


Stock cars are typically tuned high 11's low 12's to protect the cat. They never know whos going to try to set some land speed record with the car and spend a solid half minute or more at 13:1 on a cat at wide open throttle.

 

Would it be possible to have a wideband O2 sensor in each of the primaries on a set of LT headers? Of course then you would have to lean out each of the injectors individually aswell.

 

Without question! We are waaay beyond Horriba here, but actually for a noticeably less $$. It is still on the other side of $1500 however! Interesting you should say that. A high profile aviation outfit known as GAMA out of OK., did just that to equal the AFR to allow safe and effective operation lean of stoich. In their case however, they were dealing with a can-a-worms scenario, in regards to manifold refinement. For the LTx crowd, it's not quite as complicated, however, easier said than done. So yes, the more logical (read economical) approach would be individualized injectors. What complicates the approach, is the difference between part throttle and WOT variation. This is where the testing ($$) comes in.

A good opportunity presenting itself comes via the modified manifold community, or better yet, the small group that markets replacements such as the popular single planes or built-from-scratch purpose built manifolds. While relatively new and thus not commonplace, if anyone would be implementing individual cylinder testing, it be those that sell this type of manifold. You don't have to believe me, ask those people yourself, whether or not their product fabrication has been verified with this type of testing. A more appropriate question would be, are there ANY, that this forum is familiar with, that actually conduct individual cylinder testing on the modified/replacement manifolds they sell? If they don't, how can they possibly be sure their product has acceptable minimal cylinder to cylinder variation.

 

Probably because the LT1 manifold isn't setting the bar too high. The stock computer throws 15% more fuel at the cylinders next to the TB at idle than the ones at the back and around 5% more under other conditions. To me that means it was pretty obvious to the GM Engineers the LT1 manifold had more than a "minimal" cylinder to cylinder variation and were compensating for it.

 

Just to clarify, 'other' does not include high RPM or WOT operation. They (GM) were focusing on general operation in the closed loop mode.

 

Interesting, I didn't know that. How did you determine that to be the case?

 

very interesting and greatly appreciated. Looks like I am stuck at stoic unless I get widebands. I don't suppose playing around with the timing will improve mpg? Theoretically it could help but I don't know? Would upping the rpm at which PE kicks in have dangerous consequences? (shift point rpm etc to ensure I can drive the car normally without going into PE unless I want to.) at what rpm (or is it throttle) does pe kick in at?

I know some people with a stock car claim better mpg from a tune. Could it be fan temps or timing?

 

PE is a function of both throttle postion and RPM:

<2,800rpm - TPP=66%
4,000rpm - TPP=38%
5,600rpm -TPP=28%

My experience with a highly ported LT4 intake has been very good with regard to uniformity of air flow from cylinder-to-cylinder. Spraying a 300HP dry-shot about 10" in front of the throttle body, repeated engine dyno pulls, each followed by examination of the plugs, showed extremely uniform results. Even though my MoTeC ECU has the ability to adjust both fuel and spark advance by individual cylinder, we didn't find it necessary to make any adjustments to compensate for cylinder-to-cylinder differences.