| Re: [GMCnet] 4.8 MPG [message #324210 is a reply to message #324188] |
Fri, 22 September 2017 14:17   |
rvanwin
Messages: 325
Registered: April 2007
Location: Battlefield, MO
Karma:
|
Senior Member |
|
|
James Hupy wrote on Fri, 22 September 2017 09:40Hmmmm. I seem to remember Randy and George replacing cylinder heads
somewhere in the middle of a cross country trip because of a "lean cruise
experiment". I for one, will back the timing down a bit, and go with a rich
fuel mixture on my coach. A couple of miles per gallon is not worth a
beside the road overhaul to me. But, what the heck, America is still a free
country, spend your retirement income any way you choose, before the
Government figures out a way to extract more money from you by taxing that
too.
I'm on my way home from Elkhart and have limited time here but since my name has come up several times I want to clarify a few things.
First, Jim, neither George and I have replace cylinder heads on any trip and neither of us has experienced any issues with running with a lean mixture. I DO NOT want the wrong information to get out or the inference that lean cruise has or is causing issues with our engines. I did pull one head to replace a valve but the final diagnosis of that was a failed spark plug that caused some mis-firing heating the back side of the intake valve causing it to extrude (tulip) then that cylinder went dead. Replaced the valve and now after 25,000 miles not a bit trouble. When the head was open, a close inspection revealed no evidence of excessive heat or even the hint of any damage. I have put over 100,000 miles on this engine running lean cruise most of the time.
Everyone has their opinions on what AFR is best to run. I use the EBL upgrade on the 7747 GM ECM. That provides the ability to run under lean cruise during light cruise conditions. I run as high as 16.5 but the ECM monitors several conditions as Emery stated and will richen the mixture based on load. As load increases, if you don't richen the mixture then temperatures can climb at a rate that is scary. I would not advocate that a carbureted engine go very lean as you do not have a computer that is reacting in nano-seconds to make AFR changes. If you jet a carb to run at 16.2, then based on what George and I have observed, it will start getting really hot (hot enough to melt something) and will do it quickly unless you are on it enough to kick in the secondaries. Once you start dumping in fuel the temps will stabilize. What we learned is that once additional fuel is introduced, temps stop climbing but it will not go back down during the loaded interval. In our case, the computer monitors load and starts riching the mixture as load increases until it gets back to 14.7 AFR. As load continues to increase, the computer goes into Power Enrichment (Secondaries) going to around 13.8 initially, then continues to richen mixture as load continues over time. I have seen the resulting mixture get down to around 12 to 1 and a couple of times in the high 11s. With a carburetor you do not have this kind of control so if you are going to play with AFR, I suggest a WB O2 sensor as well as Exhaust Gas Temperature (EGT) probes. That way you can monitor and possibly avoid engine damage. One other point: When we change to lean cruise we limit the ratio to 16.5 to 1. I would never go this high with a carburetor or some of the aftermarket EFI units. Unless you are also changing Spark Advance (SA) as you go into lean cruise you will start losing the ability to make sufficient power at around 16.2 to 1 in our coaches (might get away will slightly more in a car). By advancing the timing, you can continue to make good power to up around 17 to 1. After that, the mixture is too lean and you cannot keep our heavy coaches moving and the engine starts to struggle. At 16.5 to 1 AFR, I advance the timing by an additional 5 degrees. This EBL computer has the ability to do this. So, as AFR gets leaner, the SA advances more in parallel and goes the other way as load increases. All done automatically - I never have to think about whether a problem is lurking or not.
Bottom line, don't mess with lean cruise unless you have a fuel injection system that is set up with the logic to manage that environment. If you still want to go lean during cruise, be sure to get a WB O2 sensor and EGT gauges so you can monitor the results. I do state in my fuel injections sessions at rallies that I am running lean cruise but try to qualify the fact that it is under computer control and done after a lot of testing.
And, I agree with what Jerry Work has said except that I believe that lean cruise is possible (if done correctly) without damaging our engines. Look at Hal Kading's post. Aviation guys have been doing this for years - even doing it manually but they get a lot of training on how to monitor and adjust properly.
Randy & Margie
'77 Eleganza II '403'
Battlefield, MO
|
|
|
|
GMCforum
Search
Help
Register
Login
Home
