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Old 11-10-2008, 05:44 AM   #11
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Originally Posted by Mark_alot View Post
thank you alot. what are the consequences of long term leaning out if your not hooking it up to a hho system.
Some folks will say that running lean will melt pistons, but this isn't entirely accurate. Preignition/detonation (spark knock) can melt pistons, and it can occur more easily at high loads and low rpms on a lean mixture, but otherwise lean mixes do not melt down engines. In fact, a really lean mix will burn cooler than a 14.7:1 mix.

Lean it out and listen for the tell tale rattle of spark knock at high loads and low rpms. A tiny bit during hot weather is Ok, but moderate/heavy spark knock is a no-no.

There can be an issue with stumbling/misfires when you get too lean and misfires will lead to more unburned gas out the exhaust - which will reduce your mpg.
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Old 11-10-2008, 06:31 AM   #12
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I don't know how lean you have to run or under what conditions, but if the consequences do happen, they apparently look like this:
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Old 11-10-2008, 07:48 AM   #13
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I don't know how lean you have to run or under what conditions, but if the consequences do happen, they apparently look like this:
Yep- that's classic detonation damage. Slightly lean, low rpm, high load= detonation risk.

Turbo'd engines also have a habit of detonation damage if they don't get enough fuel (extreme compression increases risk of detonation).

If you were able to keep a Civic VX in lean burn for 4 hours straight on a flat Kansas highway, I believe that the pistons would not melt down.

It's the knock sensor (detonation sensor) that protects engines from detonation when they get partly clogged fuel injectors or intake vacuum leaks- the ECU retards timing to stop the pinging.

Here's some other info on the subject:

From- http://www.foxvalleykart.com/egt.html

"So how can you stay on top of the effect that changing air conditions is having without bringing your own weatherman with you to the track? With a EGT gauge you can take alot of the guesswork out of carb tuning. 'Remember we said that it was generally agreed that a leaner fuel/air ratio was always hotter. And when we asked if hotter was always better? Well, you guessed it, neither one is true. If you get the fuel/air ratio too lean, the combustion temperature will actually go down! Let's look at another example of this, one that you can actually see with the naked eye. An Oxy-Acetylene torch will burn with a wide variety of fuel/air ratios. Generally when you light the torch the mixture will have too much fuel (acetylene) for the amount of oxygen that's flowing. The flame will be yellow and produce alot of smoke, and not be very hot, relatively speaking. But as you turn up the oxygen valve, the yellow flame and smoke disappear, the flame turns bright blue, and the flame temperature goes up dramatically. So leaner here is definitely hotter. But as you continue to turn up the oxygen, the flame begins to shrink, and the flame temperature actually goes down, even though it's leaner! Eventually, if you keep turning up the oxygen, the flame will just go out! Believe it or not, the same thing happens inside your engine.

"Wait a minute," you say. "I know that when I lean the engine out it just keeps getting hotter until it sticks!" If all you have to go by is CHT you're absolutely right. When your engine gets too lean, the skyrocketing temperature you see on the CHT is probably not really an indication of hotter combustion. Most likely it's a warning sign of DETONATION. Detonation is the collision of two flame fronts inside the combustion chamber, where there should be just one, and it's the single biggest cause of heat related engine failures. Savvy drivers can often sense that an engine is slowing down and richen up the mixture to control the detonation. But you don't need decades of experience to spot detonation before it puts you on the trailer for the day. Just like with the Oxy-Acetylene torch, when the mixture gets too lean, the flame temperature goes down! Detonation floods the combustion chamber with heat, so the CHT goes up, but with CHT and EGT readings, if you see CHT rising and EGT going down, it's a sure sign of detonation."

Graph showing EGT's for an aircraft engine:




Summary of my point- if you are going to lean your mix for better mpg, it will likely work, but do it cautiously and listen for detonation. If detonation is happening and you don't hear it, your ECU may just retard your timing and thus negate any gains from running lean.
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Old 11-29-2008, 08:52 PM   #14
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As an aircraft mechanic I've seen this graph and gone over it with some of my customers to help explain it to them. By using this graph (along with the proper fuel injectors) they can make massive savings in fuel burn. Some engines can be destroyed by trying to follow this graph... If you have an engine which requires a little more fuel on one or two cylinders because they typically run hotter than the rest, than those two cylinders will either burn up or just start to pop and sputter first. Most pilots pay attention to things like this, but others are completely clueless and careless, and all they want to do is play games with their new avionics, radios, etc. +

I had a guy who went through three sets of cylinders on both engines on his twin when he should have still been on his first set, and it was because he leaned it to peak instead of going past peak temp and back down to a little cooler temp. He thought he was being safe by not pulling the mixture further back LOL. We realized what he was doing when I flew with him, and since then he hasn't burned a cylinder yet. On his engines, it cost $15,000 each engine to replace all the cylinders. He thinks he broke even by saving fuel, whatever.

Moral is, pay attention to your engine. Turn your radio off, know your car's normal noises, and then make adjustments. Detonation only takes seconds to destroy a piston/engine.
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