Itinerant Air-Cooled

How lean can you go on a Type4 for extended steady state cruise? I am mostly done with my fuel map, and now need to start turning for economy at 45,55,60 and 65mph.

My latest backroads highway run (45-55-60-65 cruise) run averaged 12.5mpg, with 13:1 AFR. Granted, I have a lot of racks, and there was a lot of headwind, but I feel like I should be able to go leaner...

If this image is to be believed, and various articles accompanying it, I should be able to go way leaner, as long as the engine does not buck due to load...



Thoughts?

Your Dakota Digital CHT gauge will tell you once you've gone too lean. =)

airkooledchris wrote:Your Dakota Digital CHT gauge will tell you once you've gone too lean. =)

Brilliant.

Don't run stoich.

airkooledchris wrote:Your Dakota Digital CHT gauge will tell you once you've gone too lean. =)

Little late to the party, but the graph is showing a drop in CHTs as the mixture leans.

When I was experimenting with mtcamper's AFM on the highway while he drove, we discovered the same thing, as I leaned the wiper, the CHTs would drop. It appears that the work load has as much to do with CHTs as anything. Whenever I reduced the engine's output by egregious tomfoolery at the AFM, the CHTs would drop.

It is a delicate thing, this CHT/EGT/HP/AFR map.
Colin

how lean did you guys go?

luftvagon wrote:how lean did you guys go?

No LM-1, but I was robbing the engine until he could get no acceleration at all. Full throttle at 60 MPH, I would lean it until he could only maintain speed.

I got it up to 18 AFR at 65mph at about 85 kpa manifold absolute pressure. It is possible that it could go leaner.... at what risk, I wonder.

luftvagon wrote:I got it up to 18 AFR at 65mph at about 85 kpa manifold absolute pressure. It is possible that it could go leaner.... at what risk, I wonder.

Uhm, at frying the top of your pistons, see image for example...



Not what I would call healthy by any account!

The thing is that a temp sensor won't really help you here, and here's why.
At full load a classic engine will want to hover around AFR 13.5, maybe 14.0 if you feel really froggy. This is so that the fuel helps and cools the piston surface. As you increase the AFR, you are taking away that cooling effect.
Going super lean can cause nasty things like the burnt piston that I linked, marred cylinders, fried valves, etc. You may also hit detonation issues, which means that you need a knock sensor so that you can retard the timing, which robs you of even more power.

So, essentially better fuel economy and the death of your engine walk a very fine line. As I said, look at doing 13.5, maybe 14.0 and see what that does for ya. If it's marginal, then take it back down to 13.0 with the knowledge that your engine will last longer.

luftvagon wrote:I got it up to 18 AFR at 65mph at about 85 kpa manifold absolute pressure. It is possible that it could go leaner.... at what risk, I wonder.

That is a question that begs another question. Why do you want to experiment with how lean you can get away with? Fuel economy? Sort out your needs. You want power, you want happy metallurgy, you want fuel efficiency.

You lose fuel efficiency in the name of power, you lose metallurgy in the name of fuel efficiency. Strike a balance and do not get all over-focused on one particular variable.
The BobD seemed to really like 12.5 - 13 at cruise and dive down to 11 - 12 under throttle. I had "acceptable" fuel economy that mirrored the factory claim of 17 mpg, and my CHTs were around 380-410 and my 50-60 acceleration times were around 7-8 seconds.
Colin

Here is the current AFR table:


And a beautiful matching fuel map: