View Full Version : Mixture theory.

Simon TR4a
03-27-2006, 12:14 PM
For as long as I can remember I have believed that a lean mixture will cause an engine to run hot, and vaguely thought this was because it had to work harder to make the same power. This, I believed, is why a lean engine will sometimes run on when the ignition is turned off.

Circle Track publishes another magazine called "Engine Masters", which foolishly I have read, causing myself confusion!

They say a lean mixture burns more slowly, exposing the hot gases to the cooling cylinder walls for longer, and therefore running cooler, (and because of the slower burn requiring more spark advance.) Seems reasonable.

I also assume that a rich mixture also burns slower than an ideal mixture, so doeds the same thing, and also has a cooling effect on the piston crowns.

Can anyone add to my understanding, please?
Thanks, Simon.

Tim Hollister
03-27-2006, 01:26 PM

I'll not pretend to know all the dynamics of lean versus rich, faster versus slower burns, etc., but I can tell you from experience in the race car (both 948s and 1275s) that a lean mixture does burn hotter. I've observed this on the exhaust gas temperature (EGT) gages (one thermocouple in each of the front and rear tubes of the header) in the race car. Richening the carbs does lower the EGTs and leaning them out raises the EGT's. BTW, we've a certain temp range that we shoot for that approximates the ideal air/fuel ratio for making the most power, thus the the EGT gages in the car.

Also, race motors that are run too lean are much more likely to burn a hole through the top of the piston -- not a good situation!



03-27-2006, 02:57 PM
Think of combustion as an exothermic (gives off heat) chemical reaction between two components (air and fuel) to form a third (exhaust gas). At stoichiometric conditions, roughly 15:1, the maximum amount of heat will be generated, if you lean it out, or richen it up, then exhaust temperature will go down.

The reason people say that a leaner mixture gets hotter is that we always operate an engine on the rich side of stoichiometric. Maximum engine power is usually somewhere around 12 to 13.5 : 1, if we lean it out to 14 or 14.5 : 1 the exhaust temp will go up, then max out at around 15:1 and begin to fall at sat 16:1. We all know you donít want to go there. I havenít seen the article you are referring to but they may be talking about being on the lean side of stoiciometric. Tim is correct that it will get hotter as it gets leaner, as long as you are still richer than 15:1. Exhaust gas temperature is useful if you know how to use it (and Iím sure Tim does). Iíve heard stories of people trying to match one engineís EGT to another, and it always fails. EGT will tell you if the front carb is matched to the rear, but it wonít tell you anything about A/F ratio unless you have dyno data to compare it to and that even needs to be at the same timing and such.

03-27-2006, 04:14 PM
It is also commonly accepted that the unburned fuel, in the rich mixture, takes some of the heat out of the engine.

(the extra gas cools the cylinders)

03-27-2006, 07:15 PM
This is a good question, Simon.

I wish I could add something, but it tends to generate more questions for me.
I know that rich engines will have a cooler exhaust at idle (due to unburnt fuel in the exhaust system). But very lean cars tend to have lean misfire at idle, placing unburnt fuel in the exhaust system and generating a cooler exhaust as well.

But when timing advances and the engine revs up, is this still true?

One thing I've seen (too often) is burnt valves due to lean mixture. I would assume this is due to high combustion temps created (somehow) due to the lean mix.

In fact, last Fall I drove another 1275 race car at an enduro practice just after we got the wide-band O2 sensor working on it. After it warmed up, the wide band meter was reading 18:1 !! Naturally, I brought it in and we played with the mix.....went out and it still ran 18:1
Then the owner took it out after some carb fiddling and still ran lean....and after a second lap it burned an exhaust valve (which is why we ran my car in the 4-Hour last Fall).

So if the EGT temps are cooler at lean mixtures (leaner than around 15:1) then why do valves burn?

For the record, my racer is running about 13:1 and valves and plugs always look nice and toasty brown.

Dave Russell
03-27-2006, 07:25 PM
EGT's are not the same as combustion temps. One burns inside the engine one doesn't. Lean mixtures burn more slowly. If the mixture burns slow enough or spark timing is retarded, the mixture is still burning when the exhaust valve opens. This can cause very high exhaust temperatures but reduced combustion temperatures & power.

Richer mixtures burn progressively faster up to the point where not all of the fuel is being burned. After this point the extra fuel just provides a bit of cooling.

03-27-2006, 08:58 PM
For the record, my racer is running about 13:1 and valves and plugs always look nice and toasty brown.

[/ QUOTE ]
My average is about 12.5 & the plugs look picture perfect.

Wish I could scan my LM-1 log & post it (RPM's & AFR-vs-sec.)

Very interesting how the AFR oscillates during gear changes.

03-27-2006, 09:14 PM
A-series engine make a bunch more HP with a rich mix. I think I could go to bout 11.5 to 1(really fat) Anyone out there blowing head gaskets might want to check fuel mix. Some needles that are suppose to be rich will lean like a big dog(15+ to 1) at 5000 plus rpms. Scary stuff to see on a dyno...

Simon TR4a
03-28-2006, 04:16 PM
Thanks for all your responses.
In the context of the particular article I read, which was quite technical, I think Dave Russell has the answer.
While in usual terms "running hot" would mean high coolant temp, and in this type of discussion would refer to exhaust gas temp, I think the article was likely referring to actual combustion temperature.
Glad I have now grasped the concept, but not much practical application in my case!
Thanks again, Simon.