Downside to lean idle?

[KGB Pilot125]

What are the downsides to having a lean idle?

I know that 14.7 :1 is ideal for an AFR, but is it ideal for idle? and most syty stuff wants to idle at or around 11-13:1 or so I think from the other threads. One thing that I noticed is if you lean the idle way out like say 18 to 19:1 you get a little dead spot on tip in but i did get it to idle at 800 rpm with no real bounce on the tach for 20-30 minutes at 17.5:1. With that being said I could richen it up to say 15 or 16:1 and the dead spot would go away and I am not wasting twice the amount of fuel at idle as I am at say 10-11:1.

Any thoughts?

Btw This is on my DSM but I trust your guys opnions more than that of a bunch of guys with SAFC's and narrow band sensors. (There are some really good guys on the DSM board but it seems you have to sort through way to much BS.)

 

[MikeMcNair]

Re: Downside to lean idle?

in my experience, lean is not good, no matter the load.

adjust it to 14.7 at idle, and test it at different loads. you are better running rich then lean.

 

[4C FED]

Re: Downside to lean idle?

I could never get a good idle with anything leaner than 13-13.5:1.
I know my 02 Civic, which is a lean burn motor, has that same dead spot off idle.

 

[turbodig]

Re: Downside to lean idle?

What are the downsides to having a lean idle?

I know that 14.7 :1 is ideal for an AFR, but is it ideal for idle? and most syty stuff wants to idle at or around 11-13:1 or so I think from the other threads. One thing that I noticed is if you lean the idle way out like say 18 to 19:1 you get a little dead spot on tip in but i did get it to idle at 800 rpm with no real bounce on the tach for 20-30 minutes at 17.5:1. With that being said I could richen it up to say 15 or 16:1 and the dead spot would go away and I am not wasting twice the amount of fuel at idle as I am at say 10-11:1.

Any thoughts?

There is no such thing as an "ideal" number. What your truck wants, is what's correct.

For idle, the least amount of fuel you can use that doesn't cause deto or excessive exhaust temps, and runs smoothly (ie, no stutters or dead spots), is the one you should use.

That being said, I'd question whether you're 100% positive that your wideband sensor or display is working right. Not that it's impossible that you're running 17.5, but it's rare. It's rare to get 16:1 to run smoothly.

If you can get it to happen, though, great. I'm assuming you have no catcon... you'll smoke it in short order at that AFR if you do.

The 13:1 idle is, for a lot of them, the leanest they'll go while running smoothly. A lot of this is cylinder imbalance, and improper idle timing.

In a motor that was truly set up for cylinder balance (even air and fuel distribution), it wouldn't surprise me to see 18-19:1 that idled well.

 

[KGB Pilot125]

Re: Downside to lean idle?

When I fired the car up I was suprised at the super lean AFR and that was before I touched anything in the fuel trims,I thought it was a little wierd but it idled fine. A little high but target idle was 1000 rpms, I fattened it up to 14:1 but decided to lean it back out since I was going to be idleing for quite a while as I was tuning in the rev-limiter, idle target, screwing with the boost control and pretty much just getting famililar with the AEM software.

The wideband appears to be functioning properly. If I fatten up a few cells on the fuel map then it goes rich, it still idles great at 10.5:1. 13:1 and 17:1 I think I may have a couple of intake leaks around the throttle body and BOV so i will have to get those fixed, vaccum at idle is pretty crappy.

I think I will bring it back down to 14:1 until I get an EGT, I didnt even think about excesive temps at idle but thinking about it the manifiold gets hot fast and with out any heat shielding on the manifold, turbo or downpipe I will want to watch the temps. and for reference there is no cat in there right now, just a test pipe.

 

[turbodig]

Re: Downside to lean idle?

When I fired the car up I was suprised at the super lean AFR and that was before I touched anything in the fuel trims,I thought it was a little wierd but it idled fine. A little high but target idle was 1000 rpms, I fattened it up to 14:1 but decided to lean it back out since I was going to be idleing for quite a while as I was tuning in the rev-limiter, idle target, screwing with the boost control and pretty much just getting famililar with the AEM software.

The wideband appears to be functioning properly. If I fatten up a few cells on the fuel map then it goes rich, it still idles great at 10.5:1. 13:1 and 17:1 I think I may have a couple of intake leaks around the throttle body and BOV so i will have to get those fixed, vaccum at idle is pretty crappy.

I think I will bring it back down to 14:1 until I get an EGT, I didnt even think about excesive temps at idle but thinking about it the manifiold gets hot fast and with out any heat shielding on the manifold, turbo or downpipe I will want to watch the temps. and for reference there is no cat in there right now, just a test pipe.

I wouldn't mess with it. If it's happy at 15-16, use that. Don't tolerate a stumble, though, if it still does it, fix it. I would look for a method to verify your wideband readings... that might bite you later.

Your flat spot is likely a leanness at higher MAP (50-70) and lower RPM (<1800).

Borrow a infrared temp gun from somebody, and point it to each cyl as you step leaner. If you see a major increase, you went a step too far.

It might sounds strange, but after a certain point, going leaner actually cools EGT. You're not even close to it yet, but it's something people don't expect.

There's some old NACA papers on this... they were playing with ultra lean 50-60 years ago. And, in an aircraft environment, so they were *kinda* concerned about safety.

Makes more sense, on a DSM.. different head setup.

 

[KGB Pilot125]

Re: Downside to lean idle?

I wouldn't mess with it. If it's happy at 15-16, use that. Don't tolerate a stumble, though, if it still does it, fix it. I would look for a method to verify your wideband readings... that might bite you later.

Your flat spot is likely a leanness at higher MAP (50-70) and lower RPM (<1800).

Borrow a infrared temp gun from somebody, and point it to each cyl as you step leaner. If you see a major increase, you went a step too far.

It might sounds strange, but after a certain point, going leaner actually cools EGT. You're not even close to it yet, but it's something people don't expect.

There's some old NACA papers on this... they were playing with ultra lean 50-60 years ago. And, in an aircraft environment, so they were *kinda* concerned about safety.

Makes more sense, on a DSM.. different head setup.

Thanks for your help, I will try the temp gun out. Just bought one a few months ago and havnt really even used it yet. From my experience with EGT's it seems that timing really has a huge effect on them is this still true at idle? i wish cars were as easy to tune as 2 strokes!

 

[turbodig]

Re: Downside to lean idle?

Thanks for your help, I will try the temp gun out. Just bought one a few months ago and havnt really even used it yet. From my experience with EGT's it seems that timing really has a huge effect on them is this still true at idle? i wish cars were as easy to tune as 2 strokes!

That is sorta the "gotcha" of watching EGT... it'll move temps more than fuel will, in most cases. If you keep your timing stable, though, it should provide you a decent reference.

You don't want heat in the manifold, obviously, which is what happens when timing is retarded too much. Too early, and the motor is working against itself. Getting the pressure peak "perfect" minimizes the EGTs, and gets the maximum power out of the fuel you're burning. (Which is a good thing, even at idle)

It would have been trivial for engineering to do this; they have the instrumentation to measure the peaks and where they occur in terms of crank degrees. They have to have the tradeoffs of emissions, and a really variable gas quality. You can control your environment a bit better, by keeping consistent gas quality, and if you're not a slave to the sniffer, emissions goes out the door.

 

[Aeroking]

Re: Downside to lean idle?

i idle mine at 850rpms / 41kpa, with no deadspots, 14.7 AFR. and .580 lift in the cam.

play with it a little and it'll come to you.

tryin putting your Park/Neutral idle speed where you want, and your Drive idle speed up around 1000rpms, I tried setting my Drive idle for 650, and the truck died everytime i would slow to a stop.

 

[Don W.]

Re: Downside to lean idle?

I would check, recheck?, timing also. You can run allot of advance at idle that will get you trouble at any other RPM.