Yeah, chasing the perfect numbers in an imperfect situation can be an endless chore. Been there, spent too many hours of my life doing it.

Since we've got a single manifold intake plenum and short runners cylinder distribution will always be a problem. Simesed ports and our firing order doesn't help either, but hey considering the LT1 is just a late 80's version of a regular small block chevy it does a surprising good job. notice the LS1's corrected a lot of the issued addressed, but remember we easy make the same power with just a couple mods on our LT1s.

Just on a note, I've noticed over the years dynotuning that as the car gets closer to ideal timing the AFR split between the two cylinder banks is minimalized (notice my AFR above.) I assume it's no small coincedence, but thought I'd share that. Most places don't bother to install dual widebands, which is what's necessary, but I consider myself lucky to tune at Woodbridge Dynotech.

 

Wow cool I didn't know this, I'm going to look at a bunch of my logs & see what the timing was LOL

Matt.

 

Wow, I've been gone a week and all kind of things are happening in here.

So, If you're running a big cam and you want to run closed loop, then you're gonna be rich at idle.

Open loop is the only way to really correct the overrich problem with the big cam.

???

D Moss

 

i tried what chris said about enabling pe mode at idle and tuning that way and works like a champ. killed all the horible gas smell and idles better. and the pcm still works fine. once the car goes above 1200 rpms its back to normal.

 

In not so many words, yes. The second critical thing is spark advance. Bump up your closed throttle spark advance at idle rpm range from stock (20 degrees) to 26 degrees, should really clean the idle up a bit. I find vacuum will continue to increase with advance, but beyond 30 degrees the NOx smell will get to you, eyes may burn as well. Varies with size of cam of course.

Hope this helps,
-Christian

 

I have 36* of advance at idle & she seems fine don't know how I ended up with so much advance but I did, helps keep the headers cool incase I run lean at idle with open loop aswell, just trying to smooth things out, I think I'm going to go with closed PE once again & see.

 

BTW just wondering, what do you guys with big cams end up for numbers in the PE table for idle for reference?

Matt.

 

well I cant say much for the PE tables other than when you tell the PCM that closed throttle IDLE is in PE mode, the PCM assumes 12.6 as the target fuel ratio so its almost a given you will have to set the table to about -10% or so to get the PCM target AFR to 14.7 at idle. If you have a scan tool this can be verified by looking at the TARGET AFR line in your scan tool, just keep subtracting till you see it show 14.7 when you are idling. From this point you will want to either add or remove fuel depending how its running, but 1st get the PCM to make the target AFR 14.7 at idle.
Another note, if at 14.7 target AFR the car is too lean try backing the timing down a little at closed throttle idle, it all depends on your setup what works best.

I use 38 degrees of timing at closed throttle idle

 

Wouldnt you want it leaner than 14.7 reading off of the O2's? I thought this was the whole problem, that what the PCM saw at 14.7 was lean when it really isn't. If the PCM shoots for 14.7, o2 hits the sensor, and it throws more gas too the equation, resulting in rich mixture again. I thought it was more like 18.1. (I saw that somewhere)

BTW....I have the LM-1, so I can see what is really happening in the tailpipe, just have to know where to fix this.

Plus I thought that the PCM didn't use the o2 sensors in PE mode. If this is correct, then PE is in open loop, and there should be some tables that I can find to use to subtract the fuel during PE. (not the MAF) It's the O2's that are killing us during this time, we just have to get them out of the equation and be ignored during idle.

D Moss

 

Im not talking about reading the O2 sensors. or what the O2's are doing, they are worthless.

What Im talking about is the PCM's TARGET air fuel ratio. When the PCM goes into PE mode it ignores the O2 sensors and uses the MAF, Injector size, displacement, etc.... Information to TARGET to 12.5 Air Fuel Ratio. There is a reading on scan tools that have the F BODY specific codes that tells you what the PCM's target AFR is. Altho this has nothing to do with what the real AFR is, just what the PCM is trying to get by way of its sensors. In closed loop mode the target AFR is 14.7 and the PCM uses the O2's, MAF, etc. to maintain a 14.7:1 AFR. When in PE mode the PCM has a RICHER target AFR and I was simply stating that you would have better luck getting the PCM to target 14.7:1 at idle as a STARTING point.

 

Thanks for the clarity.

D Moss

 

The best way would be to use an EGT.

 

Not sure how much it's going to help you since every car is going to be different, and that static fuel pressure, injector constant, maf table values, and ve table values are going to affect it.
But on the Joe O. big-cammed car I did this spring, I want to say I'm right around 0 in the idle range and 12 for the entire rpm range to get a 13:1 a/f ratio at wot.
Idle is still a bit stinky, but once the car is road worth again I'm thinking that I'm going to need to pull quite a bit more fuel out to lean it out. Probably somewhere around -10 to -15. I think that should get me around 16:1 at idle. Only thing is the wideband isn't helping me much at idle. I might need to use an EGT gauge...I just need to find out what temp range I should shoot for.

 

Hmm.. I"m thinking of pulling -19 or so at idle since PE is just that, by putting in "0" you are adding 16 points to the 128 setting, this is because in your PE vs. TEMP field you have the thing adding 16 points, this is 16 points ontop of your closed loop parameters say being 128, so to get 128 blm & not have the PCM add anything past 128 blm you need to set idle at -16 in the pe vs.rpm range to not add anything. I have the big Joe O' cam in my car & in open loop at idle I have the o2's reading NOTHING as in "4" for o2 readings LOL & I have a laser temp reader & my primaries are showing around 110-130* C which is not too hot at all so go figure, at this point the o2's are thinking the car is completely turned off LOL & the car idles real smooth so I guess the duration/overlap is throwing A LOT of oxygen because on a stock cammed car if the o2's read 300mv at idle you start to see headers glowing at that point while my cam is reaiding "nothing" at the o2s & the primaries are still only 110-130*C which I think is perfectly cool. Also note that closed throttle timing makes a difference, by running more advance you will run cooler, so if you end up running on the lean side at idle, by upping the timing advance you will cool the primaries off, I noticed that aswell, less timing = hotter primaries.

I think to be on the safe side I'm going to shoot for 100-150mv on the o2's at idle which is about 17:1 A/F at idle with a wideband, the last time I hooked up a wideband to my car it read 18:1 A/F at idle, & I think its cuz the wideband couldn't read anything past 18:1 A/F LOL.

I think a very good way to check would be to put a new spark plug in say cylinder #1, run the car at idle for about 5 mins & take the plug out & see if your running rich/lean, then you'll know what to shoot for on the o2s again every cam is different, thats why I was looking for a base figure of what to put into the PE table at idle, I personally put in -19 points, which is taking out 3 points from the 128blm, I'll see what that yields at idle for me.

Also, every car is different because Heck I ended up with -9 almost throught my whole rpm range in the PE table to get 12.6 A/F, but i bet if I put a screened MAF back in that would be too lean.

My highest AFG readings to date were 398 AFG's at WOT & 6300+rpm.

 

WIDEBAND cant measure at idle, the LM1 unit has the ability to adjust pulse width monitoring but I havent messed with it yet to see if it works for idle. the exhaust pulses are too slow to be correctly monitiored at regular monitoring speeds. EGT is the best way to do get the idle tune down. But even if you run a tad lean on the idle its not going to damage anything.

AS far as timing goes, INCREASING timing will make the car run HOTTER because it has a LEANING effect on the AFR.

DECREASING timing has a RICHENING effect on AFR and therefore the exhaust temps will be lower