dmoss69

I've searched and searched, with no luck.

If we have a big cam, with overlap, how do we stop burning rich at idle in closed loop?

Facts:

1. Big cams have overlap, which scavenges raw air and throws it out the tail pipe.

2. The scavenged air is read by the o2's as unburned air, which is a false lean condition.

3. The PCM tries to correct the lean conditionby adding fuel.

4. The extra fuel added makes an overly rich condition, which continues to get worse until the blms and integrators are at 160, and maxed out.

5. The scavenging continues. o2's still reads lean.

I know that we can't stop the scavenging, cause our love for the cam will not let us go back to smaller shafts.

I've seen people say to use the VE tables (lower the numbers), which will tell the pcm that the we aren't using all the air for combustion. This is incorrect. The o2 sensor is still going to read that scavenged air and try to get the o2mv to 450.

I've seen to modify the MAF sensor, which is going to be incorrect. The sensor is still going to read that scavenged air and try to get the o2mv to 450.

I've seen this answer, which seems to be the most logical, we can't tune our cars for idle in closed loop. Make it open loop.

(I'm thinking) If there isn't a way to change the target o2 mv or stoich ratio in closed loop, then we're just like the dog who chases his tail and never catches it.

Any inputs?

I think someone out there knows this answer, but hasn't published it.

D Moss

bunker

I know exactly how to do it & figured it out all on my own, so I don't know if I should say it, I used common sense & might tell you how to modify your tune if your really nice to me LOL.

Or Maybe it should be a Sticky & my name to stand out loud & proud for once LOL but I did it & car runs awesome at idle, been running like this for over 2 month now, kinda had to think beyond the ussual when I got a big cam that ran crappy at idle.

97Z-M6

hey are those dyno numbers at the bottom of your sig current. 405

dmoss69

I'm all ears if you say something.

I haven't begun tuning my car yet, all I did was rebuild this monster and busted it off to break in the cam (2000rpms for 10 minutes). I didn't let the car idle but for a second or two before I shut it down, I just noticed the fumes in the shop was horrible.

I was going to tackle this, and figure this thing out for everyone if hadn't already been done.

I guess the GM847 will be big enough for my wants. Now I have to finish the final touches, crank this baby up, and begin tuning.

D Moss

bunker

Yes those numbers are current through 4.10 gears, ZR1 11inch wheels & 11.7-12.1 A/F Range, I blew a header gasket on the dyno so I stopped going, next time I'm back on the dyno I"ll bring her to 12.8-13.0 A/F & I'm sertain I'll pick up some HP maybe 415-420rwhp or so my TQ & power was higher throughout the RPM range & started sooner then my 847. I'll post the fix tommorow, it's quite long to explain but its so simple & obvious just tired right now, I'll do it tommorow, you'll love me

Matt.

JSK333

You generally want to increase timing advance at idle with big overlap cams.

Try 5* more and see how it affects things.

Highlander

you should do an export of your map values and calculate an average and you will find which timing it likes best. the lower the map the merrier.

dmoss69

I busted it back off today, and I didn't have the fumes like i had, but I also had a couple of things that I had to take care of.

It sounds good, and I think it's going to be alright, but I'm still interested in this answer. It's killing me now.

D Moss

TriPinTaZ

I have a large solid roller cam. Give up now on a closed loop tune wit hstock PCM. its not going to happen. You need to make the car stay in open loop and ignore the O2 sensors.

There is one other way, you can set the PE tables to negative amounts at idle RPMs and set teh WOT table to 0% throttle at idle RPMs. However I tried this, and due to teh long term fuel trims the car eventually idled rich at an idle.

97Z-M6

bunkers right has the fix.

Dan K

VERY interested in the fix!
Let's hear it bunker....

shoebox

Must be a secret...

EDS Z28

I ran my car in open loop at idle for a period of time. It smoothed out the idle and helped low end torque. I basically made it run richer at idle, and this gave it more vacuum, like around 16 in Hg. You can do it using LT1 edit.

If I remember correctly, you set the WOT table to 0 at the desired rpms. This will force it into open loop. You will need a scanner to verify this. My motor was running in open loop from idle to around 1700 rpm. Beyond 1700 rpm, it would run in closed loop.

But, I kept on damaging or destroying cat converters because it wasn't running at the target 14.7:1 air fuel.
The only way to run a cat is in closed loop.

Highlander

The CAT also needs overtemp protection which will make the engine run a little richer... and you can always measure the mixture with a lambda meter with a wideband. It doesn't have to be in the range of 10-12:1 to use a wideband... anyways... you can set the blm boundaries a bit higher to avoid the PCM to correct the problem and run by tables.

dmoss69

I have the innovative tech's wideband o2 sensor, so I can program for open loop if I need too. I also have a CAT less car, so that doesn't bother me either.

Ok Bunker, You've gotten all the big dogs with their ears on you,,, me, tripintaz, shoebox, dan K, highlander, and I'd imagine several others looking on.....

Hey, most of the old posts that I dug up from the search had these names in them, so I know they are interested, just as much as I am.

Thanks

D Moss

Trying to piece another part of that puzzle together.