How lean during cruising using Mega Squirt II

[speedracing944]

For those who have rigged up thier 944 with MS ummmmm....ae1969 How lean can you go during low MAP and around 2600 RPM? I am thinking cruising speed of about 65 MPH. What AFR are you running about there? What are the EGT around there with your given AFR?

For the past year I have been putting in $600 worth of gas a month in my 944 and am getting between 22 and 26 MPG. Can I use the MS and a wideband to get leaner mixtures and squeeze a little for fuel mileage out of my '83?

I'm thinking cost savings here with the added benefit of MS at the top end

Speedy

[SoCal Driver]

Lean it till there is a small hole in each piston then richen it a half turn.

Actually you need a knock sensor or two to be accurate. Kind of like the 'S' model.

[speedracing944]

You can use a knock sensor as well but it will pick up knocking (detonation) but that is after it starts knocking which is what I don't want to do. With the knock sensor it tells me 2 things. Knock or no knock. Using the EGT sensor will give me a gauge to tell me how close I am to the line without going over.

IIRC based off my old dyno sheet at cruising the AFR on my '83 was about 16:1

Speedy

[JustinL]

I run mine at 14.7 with 4% ego correction so it oscillates up and down a bit.


**I'm not an engineer and they can feel free to correct me**

Detonation should not be a concern at cruising loads. The cylinder pressure is relatively low and unless you get insane heat build up should not be a factor. I've heard of guys leaning out the mixture as much as possible to improve fuel economy, though I don't subscribe to that theory. I've found that as I get leaner (above 16:1) on deceleration I get lots of popping from the exhaust. This is because there is not enough fuel to get good flame propagation in the cylinder and you end up pumping fuel into the exhaust system where it ignites and pops. Because of this I feel that if the fuel isn't burning in the cylinder, it must not be as efficient.

[SoCal Driver]

Shut off the fuel on deceleration.

[JustinL]

I shut it off if I'm trying to conserve fuel, run it lean if I'm trying to maximize spool in between shifts, and stoich when I'm on the track to reduce engine braking on lift off.

[speedracing944]

I am looking at my dyno sheet for my '83 and with the stock engine computer the car is running at 16.2:1 ratio from 2500 rpm - 4300 rpm.

After 4300 rpm it drops to 13.7:1 and continues to drop to 13.2:1 at 6400 rpm.

So I guess I am already running pretty lean at the lower end of the rpm range.

I guess I want to play the AFR Limbo. How lean can you go???


Speedy

[legion]

Originally posted by JustinL
I've found that as I get leaner (above 16:1) on deceleration I get lots of popping from the exhaust. This is because there is not enough fuel to get good flame propagation in the cylinder and you end up pumping fuel into the exhaust system where it ignites and pops. Because of this I feel that if the fuel isn't burning in the cylinder, it must not be as efficient.

This is exactly why running lean has a tendency to burn up exhaust valves.