Which is the correct A/F ratio ?

[Stephan427SC]

Hi guys,
I want to set the correct A/F ratio or lamda for my engine ( Ford FE390 stroked to 445cui with 48 IDA Webers). I installed a oxygen sensor in exhaust on passenger side.....

I was told to set the A/F ratio while idling at about 14.9....what do you think ?

Is the engine with a 12 A/F ratio too lean ?

Can you get me some more information or tricks ?

Thanks,

Stephan

[Tom Wells]

Stephan,

If you are serious about learning A/F ratios for various load/speed combinations for an engine, you're embarking on a real "learning experience." Use internet searching to the fullest and don't make any changes until you really understand the entire concept.

Idle mixtures are one small part of the tuning process, and whatever you do can and will affect the whole spectrum of engine performance.

There's a kind of old saying about tuning that goes something like this: the engine can run rich for quite a while at almost all loads and speeds, but only once if it's too lean.

Factor in today's liquid that some laughingly call gasoline and you'll find tuning tweaks are needed to compensate for the changes there, too.

Have fun learning!

Tom

[Gaz64]

Originally Posted by Stephan427SC

Hi guys,
I want to set the correct A/F ratio or lamda for my engine ( Ford FE390 stroked to 445cui with 48 IDA Webers). I installed a oxygen sensor in exhaust on passenger side.....

I was told to set the A/F ratio while idling at about 14.9....what do you think ?

Is the engine with a 12 A/F ratio too lean ?

Can you get me some more information or tricks ?

Thanks,

Stephan

You won't achieve a satisfactory idle at 14.9:1.

12:1 is rich not lean.

12 parts air, 1 part fuel. Both by weight.

[RallySnake]

I have spent a lot of time tuning Holley Carbs. The most recent one is an Avenger 670. It is SO much easier with an A/F Ratio meter. My primary goal was to get rid of the hesitation as I started up from a standstill. This is the avenger's biggest fault. I am also trying to keep the plugs clean longer and cut down the unburnt gasoline blowing out of the sidepipes at idle that make you smell exactly like exhaust fumes. The third goal was to improve the gas mileage for the long distance rallies that we run. What I have learned from much reading on the internet is that you want 14.7 (parts air) to 1 (part gas) in idle and low load (cruising) situations. You want 12.5 to 1 in full throttle acceleration. And, it does not matter if the ratio goes as high as 15 or 16 to 1 in throttle off, deceleration.

I started with the jets. I reduced the sizes of the two jets in the primary side (front bowl) several times until I got close to 14.7 at steady 50 mph cruise (3rd gear). Then, I concentrated on swapping the rear jets until I got 12.5 under full throttle (testing this is dangerous)! Next, I swapped the plastic cams until I found that a blue one on the primary side would get rid of most of the delay leaving the line. However, it ran out of juice just before the vacuum secondary kicked in. So, I changed the spring to the yellow one and that helped the secondary kick in faster. Then, I reduced the squirter size in the primary to make it last longer. Now, there is hardly any delay in the changeover. The car runs about 250 miles on a 22 gallon tank of gas on fairly fast rally legs. It does a lot better cruising on the freeway. The plugs stay very clean.

You can get the jets and cam sets very quickly on Summit Racing's website and you can buy individual items like the squirters on eBay really cheap (but it usually takes much longer).

RS

[RICK LAKE]

Stephan427SC Steve There is a idea A/F ratio for making the most torque from a motor and making the most HP from a motor. Under full throttle you are looking for 12.7-13.2 for the max power range on most motors. At Cruise a 14.0- 14.3 is a safe ratio. You can run leaner if you want but may start to burn down the motor over time. The best thing would be to find a good dyno shop and let them setup the car. If the motor has forged pistons you can run a little leaner, if cast you are looking for trouble. Reading the spark plugs will help. Have you air balanced all 8 webers for flow? What camshaft are you running. Does it have a 113-115 LSA to stop reversion of the motor? You might want to setup a base line and record each change. start at idle and check the A/F ratio every 250 rpms and record. Do 1 change at a time and retest. Will take time. If you get lost go back to last step. Timing will also effect power the motor produces. Some motors like alot of timing, others like a little, this also depends on the gas and qualitity. Good luck on your learning curve. Rick

[olddog]

Afr at idle typically was a bit rich 14.2ish with a carb. However you need to understand that the cam matters. Large valve overlap causes unburned air fuel to be pulled into the exhaust. A wide band O2 will see this air and report a false lean afr. So the O2 is working correctly, but you have to understand the what is happening or it will lead you astray.


So the person who told you to set it too lean, either understands this and knows your engine, or doesn't know know didly. I'll let you figure out which.

[olddog]

I have read some top notch efi tuners who say on performance cams you should adjust the idle afr with a vacuum gage. Find the afr that gives the highest vacuum. With efi it is a bit easier, because it will hold the idle at the same rpm. With carb you will have to watch the rpm and correct it. You have to keep the rpm and timing constant or you are letting multiple things that affect vacuum change at the same time.

[twobjshelbys]

The perfect AFR is 14.7:1. 15.0 would be lean, 14 would be rich. The problem is that where you measure makes all the difference in the world, and so an O2 sensor measurement taken in a Cobra side pipe by a sniffer cannot be taken as absolute. You can use it to se how much you are changing things though.

P.S. I got to know way more about this than I needed or wanted to when I was trying to get my Accel DFI system working. As an engineer it was an interesting exercise. Now I just either use new cars with really good computers (like the Ford GT) or tune the carb until it sounds right (like my grandpa taught me).