top of page
Search

The Effect of Timing on Air/Fuel Ratios




This morning I saw a thread about a guy having a 2x4 FE setup with a medium riser intake. #1 and #2 headers were glowing red. He found turning a mixture screw out 2.5 turns cooled that exhaust.

That was literally all the info given. Car owner later posted a photo of I think the #1 plug. It was jet black.

Last I saw there were 157 comments and most were not positive, mostly bickering. Many people have sent me the link or asked me to comment. I would sooner put my hand into a running blender full of salted lemon juice. If I comment in that thread it would be a waste of time and only cause frustration.

Yet, this is a good teachable event and maybe worth talking about.

So. With hardly any info, here is what I think about the situation:

-a mildly lean or rich situation rarely turns a header red. It can, 100% it can. But typically the engine will die before it gets to that point. Most of the time retarded timing causes this.

-Why just two cylinders? Most likely they were the lean ones to begin with. Retarding the timing makes it worse.

-Why did turning out the primary carb, passenger side mix screw help cool the exhaust? It flooded that cylinder with fuel which generally cools it some. It also makes for a quicker burn.


So lets a have a talk about it all. First, understand, I am sharing what I have seen. This is not something I have solely read or heard about, this is all based on experience. Also understand that I am only discussing carb and ignition settings…. We are assuming the engine itself is sound and all valves function correctly.

Retarded timing will 100% of the time mess up metering of fuel, and how a carb affects an engine. My view is to never, ever, touch any carb settings until ignition is perfect. Plugs in range? All wiring secure and making good connections? Initial timing appropriate for the engine and camshaft? Has the timing been checked with a timing gun AND by reading vacuum and rpm? This matters if a balancer is off 10 degrees it can completely and totally ruin the tune. Retarding initial timing on an untuned engine can and will make for some really bright red exhaust tubes.

There was a lot of infighting about red pipe means hot exhaust, does this mean lean or rich??? several people were saying it was lean, several saying it was rich. so lets chat about that. “Who was correct?” You may ask me. My answer is that they both were. Why is that?

Ok. So here is my view: unburnt fuel is unburnt fuel. At idle, most engines with no load can typically run somewhat decently with a wide range of a/f ratio. Are some better than others? Yes, I prefer to err to the fat side of things. For me this helps when changing load or temperature. Truth of the matter is that most engines like the one in question can idle as fat as 12:1 all the way to the lean 15.5:1 without a ton of drama. When going leaner the exhaust rapidly becomes VERY hot. What about fatter can it do it to? Yes. If the engine is so rich (lets say 10:1) it begins to misfire. If it drifts richer still the engine will misfire even more as it is outside of the explosive range for gasoline.

To complicate this situation, other factors are involved. One reason lean mixes tend to show excess heat so rapidly is due to burn times. The molecules of fuel are further apart in a lean mix. Imagine putting 125 marbles in a bag. You use 115 white marbles and 10 black ones. The black are fuel. When the bag gets mixed up, we assume the black marbles are somewhat fairly spaced out. When the black marbles carch on fire they catch the other black ones on fire so the flame spreads. It takes time for the flame to jump.

Now…. Take the same 125 marbles and use 25 black instead of ten. The flame takes less time to travel to each other. Partially because they are closer, partially because with more closer, the heat rises faster and flame front travels faster.

In the scenario above, with a slow burning lean mixture, fuel is still burning when exiting the exhaust valve and into the header pipes. In this case, mildly retarded timing does what? It delays the spark. This makes the situation considerably worse.

In the case of an excessively rich mixture we do indeed have a faster burn. But some fuel exits the exhaust valve unburnt. Typically this is not a huge issue but if the timing is delayed, you may find a situation where a substantial amount of fuel is burning in the exhaust.

Throwing a wrench in the works is the fact that as many tuners have likely noticed, when you go far enough in one direction, many signs appear the same. What I mean by this. If you are used to seeing lean=hot you may richen the mixture sufficiently to the point of misfire. This results in a hot burn. You are used to your brain saying it is too lean, you pull a plug and it is jet black. Conflicting information. Hence my view that a misfire is a misfire and the cause needs more investigation before a conclusion can be made.


If starting with an engine that has not run. A few tricks I have noted: -first, the goal is to get the engine running so we can tune it. -engine will be cold, so set mixture rich. As the engine begins to build heat it will naturally need to be leaned out as fuel vaporizes easier. -On first fire, there is no load. With no load you can advance the timing a huge amount with no fear. As such, I often will start a fresh engine with the timing statically set at 25 degrees. Being a cold engine it will crank fine. When I say statically, I mean get#1 set to 0 degree tdc on the balancer. Turn engine so timing mark is at 25 advance. 25 degrees before tdc the spark will happen very early and will provide plenty of time for the flame front to consume itself before the exhaust opens.

Once the engine is running, there is plenty of time to retard timing to desired initial and to lean the engine. Cold engine is inherently lean due to bad vaporization characteristics. Advanced timing allows the slow burning mix to burn well. As intake becomes hot, less fuel, and less advance is required. You find the happy place for the engine to idle.

Conclusions: -In the original discussion, both parties were correct, or at least had the potential to be correct. -lean and rich mixtures can both act similar if sufficiently out of range. -Lean mixtures due to their slow burning nature are impacted in a much more dramatic fashion by retarded timing. This is also why vacuum advance works so dang well at a lean cruise. -Order of operations for tuning always begins with correct timing and ends with the last carb adjustments. If you set the carburetor correctly at 20degree btdc spark, the carb settings will not be correct any more at 10 or 15degree btdc. -If an expert recurves your distributor prior to first fire, you may be very close to correct with their suggestions. If you start with a generic off the shelf carb, YOU will have to run engine for the first fire and determine best practical initial timing. Once it is found the curve needs to be adjusted to limit maximum mechanical advance.


Disclaimer: I wrote this in ten minutes so I am certain I forgot something. Feel free to ask me to clarify something above.

Thanks! Drew

73 views0 comments

Recent Posts

See All
bottom of page