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-   -   Myth? Lean mixtures will cause engine damage (http://www.fuelly.com/forums/f10/myth-lean-mixtures-will-cause-engine-damage-9677.html)

GasSavers_Erik 08-08-2008 02:15 PM

Myth? Lean mixtures will cause engine damage
Doing a bit of research, it looks like lean mixes alone do no harm, but at low rpms and high load, lean mixes can lead to detonation and its the detonation that can damage pistons, burn valves etc.

This is why the Civic VX lean burn does not damage the engine and why the Honda insight can go up to 40:1 under cruising conditions and do no damage.

Here are a few good reads:
Aircraft engines and leaning/richening mixtures (pilots do this all the time when they change altitude)

A discussion amongst engineers on the subject "Running lean and melting pistons"

Combustion temps actually decrease as you get leaner and leaner


When I first heard about the EFIE devices and clogged fuel injectors, I wondered why ppl weren't killing their engines since these would cause one or all cylinders to go lean. The above links explain the lack of damage. If the above info is correct- as long as you have a functional knock sensor (so the ECU can retard timing as needed), leaning an engine a moderate amount should be a safe way to gain MPG as long as you do not ignore the sounds of detonation.

These are my thoughts assuming that the above webpages are accurate.

GasSavers_Erik 08-08-2008 02:20 PM

On a related note- here is a lean burn engine design which claims to be very efficient.

Bourke engine
Another Bourke site

trautotuning 08-08-2008 04:58 PM

Wow I have been saying this forever... THANKS!

As long as the engine is under very little to no load there should be NO problem!

For example, at idle! Or cruising!

I plan to modify an EFIE to work only when I am under (let say, I still have to do more testing) under 20% throttle.

So it would not be active when under load, acceleration, going up hills, etc. But would in constant driving, or simply idling.

And yes, it would increase MPG A WHOLE LOT and it does burn cooler, think about it, there is a lot more air in the cylinder...

suspendedhatch 08-08-2008 05:33 PM


Originally Posted by Erik (Post 114551)
This is why the Civic VX lean burn does not damage the engine

The Civic VX/HX can run lean because it was specifically designed and tuned to run lean. You can't take a Civic DX or EX motor and run it as lean as the VX. The fuel wont spread evenly, hence it wont burn evenly. The fuel fails to ignite and you get lean misfires if you go any leaner than 16.5. At medium load 15.5 is the limit. And yet the Civic VX/HX can run 18:1 and still accelerate.

This is why:


Closing one intake valve causes the air to spiral in to the combustion chamber, in essence stirring up the sparse fuel evenly. You also advance the ignition timing quite a bit and decrease the load placed on the engine by eliminating weight and accessories that drag on the crank.

It's difficult to damage an engine at light load. Why? Because it's at light load. It's self evident. You can get pre-ignition, detonation, misfires etc at light load just as you can under heavy load.

Those websites make generalizations but that info doesn't apply to all engines. Engineers have been successful at creating engines that run at both extremes of the parameters of gasoline.

trautotuning 08-08-2008 05:35 PM

So what is so special about the vx engine that makes it be able to ignite so well?

Piston design? Ignition Design?

GasSavers_Erik 08-08-2008 05:45 PM

Its my understanding that there are several conditions that must be met before the VX will go into and stay in lean burn including throttle position, temperature, vacuum etc.

I thought part of the VX lean burn system has to do with the swirl of the incoming charge by the position of the two intake valves, but suspended hatch will have to help us out.

The old Honda CVCC system (1987 was the last year for it) is ideal for being leaned out because the rich mixture in the precombustion chamber is easy for the spark plug to ignite and its combustion can more easily ignite a cylinder full of an extremely lean mixture.

GasSavers_Pete 08-08-2008 06:00 PM

Some modern cars use the ECU to achieve basically the same result.
My car runs in lean mode when the cruise is active and after 15 seconds of steady state input.
Not sure how lean exactly but the FE is impressive for what it is and does.

Sort of related there is an item on the Autospeed site (www.autospeed.com)
at the moment.
The article address is:

Cheers , Pete.

GasSavers_Erik 08-08-2008 06:09 PM

Thanks Pete- that was a good article

trautotuning 08-08-2008 06:30 PM

Oh and if anybody is planning on doing this (like me), it is a very wise idea to go with an aftermarket ignition module like MSD or the such.

These devices can multifire below a certain rpm, which is what you need to be able to light those lean mixtures (and it is exactly what the Honda insight does, multi fires the plug).

Just a heads up on what I have learned through my research in order to get my engine to lean burn safely :)

GasSavers_maxc 08-09-2008 04:32 AM


Originally Posted by trautotuning (Post 114565)
So what is so special about the vx engine that makes it be able to ignite so well?

Piston design? Ignition Design?

Fuel system design is the most important!

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