Air/Fuel ratio, is there a quick answer? Options

[malcolm2]

OK, I have been tinkering with the new-to-me AFM on my 1911cc engine all weekend. I have adjusted the spring about 20 teeth CCW to get to this point.

I have been using an A/F sensor and gauge. can someone tell me, or direct me to a chart/table of what A/F I should be shooting for on my daily driver.

The last adjustment of the CO screw has left me here:

I have always had a strange idle situation, so lets ignore that for now. any normal adjustments to reduce the idle at normal operating temp eventually stall the car. A problem for another post. (IMG:style_emoticons/default/blink.gif)

the idle is high.... at 1200 the A/F is a smooth 12.2.

I found a long hill and drove up it at about 3800 rpm and the A/F was 12.5.

thoughts?

[Mark Henry]

Yes 15:1 cruise is too lean for an aircooled 14:1 would be the max and even then I'd want a good headtemp gauge to be sure they're not getting too hot.

That chart might be OK for a watercooled, but not aircooled. WC big advantage is they can control head temps way better than air cooled.

Aircooled has a much narrower burn AFR range.
This would be my aircooled chart;

About 11:1 is the max for rich burn
WOT power between 11.5 and 13
Max performance WOT 12.3 to 13 maximum
Lean burn no higher than 13.5-13.7, nickies can run 14:1-14.3

The exception to the above is decel, AFR on decell is of no concern, of course it's a high reading, you just took away all the fuel, but no load so it's not producing heat.

You can run richer than 11:1 without really hurting the engine, but your power and tune goes to poop.
Anything over stoich you better have a damn good head temp gauge to monitor your temps, you are getting into dangerous territory.
As you head temps go over 350f you will start to lose power as a heat soak condition starts to set in.
Running 17-18:1 and you will have a hole in a piston PDQ.

Basic rule on heat temp and heat soak is you can go over 350f on a long hill, as long as the temps recover on the downside. Once the temps are not recovering on the downside of the hill you have a heat soak issue. Even a cheap VDO gauge you can easily see this heat soak/no recovery condition.
Head temps consistent 325f is about perfect, 350F is OK, but I don't want to see much more for long periods of time.
On my LN nickies I don't see much over 300F unless I'm really hammering it on a hot day.

[72hardtop]

Yes 15:1 cruise is too lean for an aircooled 14:1 would be the max and even then I'd want a good headtemp gauge to be sure they're not getting too hot.

That chart might be OK for a watercooled, but not aircooled. WC big advantage is they can control head temps way better than air cooled.

Aircooled has a much narrower burn AFR range.
This would be my aircooled chart;

About 11:1 is the max for rich burn
WOT power between 11.5 and 13
Max performance WOT 12.3 to 13 maximum
Lean burn no higher than 13.5-13.7, nickies can run 14:1-14.3

The exception to the above is decel, AFR on decell is of no concern, of course it's a high reading, you just took away all the fuel, but no load so it's not producing heat.

You can run richer than 11:1 without really hurting the engine, but your power and tune goes to poop.
Anything over stoich you better have a damn good head temp gauge to monitor your temps, you are getting into dangerous territory.
As you head temps go over 350f you will start to lose power as a heat soak condition starts to set in.
Running 17-18:1 and you will have a hole in a piston PDQ.

Basic rule on heat temp and heat soak is you can go over 350f on a long hill, as long as the temps recover on the downside. Once the temps are not recovering on the downside of the hill you have a heat soak issue. Even a cheap VDO gauge you can easily see this heat soak/no recovery condition.
Head temps consistent 325f is about perfect, 350F is OK, but I don't want to see much more for long periods of time.
On my LN nickies I don't see much over 300F unless I'm really hammering it on a hot day.

Well in a 914 one may not see much over 350F or so if everything is dialed in. But temps over 350F are not for much concern. It's when you go over 400+F then start worrying.

As for AFR's once you start pushing beyond 15:1 the EGT's go down. Why? Simple you are closing the burn (fuel off). The below chart shows the relations between EGT, CHT, HP and ICP. Peak EGT is ~14.7-15:1

During 'cruise' get into the 16-17AFR for cooler running. Part throttle cruise is NOT a load condition. Ones foot dictates load. If you're running around during cruise at 13:0 - 13:5 you are wasting fuel and running hotter EGT's and CHT's than you would be if you were running 16-17AFR at cruise (part throttle).

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