AFR (air to fuel ratio) On Dyno Info?

zuluracerx · 08-17-2007, 09:28 AM

Can someone explain what the correct AFR should be for a high performance motor. I am trying to read a dyno sheet and do not know what is a safe range. I assume the values relate tohow lean/rich the motor is running and would seem to be critical for good performance and longevity? I know tuners really try to hit a certain range to get all the power available. Thanks to anyone that can shed some light on this subject.

Daze · 08-17-2007, 10:26 AM

As the first number gets lower the mix gets richer

14:1 is optimal for emissions because most of the fuel is burned but hard on components because unburned fuel helps reduce heat, and 14:1 is not the best ratio for power because unused oxigen is exiting with exhaust gases that could have been used to burn more fuel.

11:1 is the threshold where extra fuel begins to hurt performance

12.5:1 is the best ratio for maximum power, because at this ratio even though some fuel is being wasted all or nearly all the oxygen is being used which maximizes power.

Other performance upgrades like ignition, compression and timing will allow a little more or a little less on these ratios, but they are a good basic reference points.

Hope this helps

zuluracerx · 08-17-2007, 11:24 AM

Thanks. Good, simple explanation of the basics. How would a Vortec blower change the magic 12.5 number? Reason I ask, is the motor is laying down 527 to the wheels. It is a Eagle stroker 331 running 12 pounds of boost. The AFR is at 11.8 at max power.

Daze · 08-17-2007, 12:14 PM

That all depends on your motors running temperature. If with the blower the motor is not running hot than shoot for the 12.5 keeping a close watch on the temp. If such a change causes the temperature to rise higher than desired, than richen up the mix to cool things off. Running a blower or supercharger does not effect the ratio as it relates to combustion however more fuel and air in a cyl than that cyl was originally designed for (the result of using a supercharger or blower) will result in increased heat per detonation, and if your block can not dissipate that heat effectively than you will be required to improve your cooling system or richen up the mix to compensate making sure you donâ€™t cross that 11:1 threshold

zuluracerx · 08-17-2007, 12:48 PM

Thanks Daze. Great info. I actually misread the max power dyno print. The AR is 12.6 at max rwhp of 527, 13.95 PSI, 5645 RPM. Engine temp is right at 180 (big Ron Davis aluminum radiator, with 18" Spal fan, electric high flow water pump). Sounds ridiculously strong! The AR actually hits 11.3 at 4446 RPM, with 395 rwhp. Sounds like I should be okay?

67mustang302 · 08-17-2007, 01:55 PM

That sounds about right. Peak torque is where hte highest combustion pressure is occuring, and in a blown motor you want to run a bit fatter to help control detonation from the extra heat produced. AFR really depends on the engine, what fuel it's using, blown or not, nitrous or not. Under part throttle cruise typical fuel injection engines may run as lean as 17:1 in order to maximize economy and reduce HC emissions, though it increases NOX emissions. If you're at 12.6:1 at peak hp and 11.3:1 and peak torque on a supercharged engine, that sounds about right, though thr peak torque AFR might be a bit on the rich side. It depends on your setup though, that may be where it makes best power without detonating. Have you had it dyno tuned, or just dynod to see what the power is?

67 evil eleanor · 08-17-2007, 02:42 PM

Sounds like you are getting the most bang for the buck where you are.Don't worry, be happy.

zuluracerx · 08-17-2007, 04:45 PM

I was worried about running lean, over cooking things and blammo! It runs great, but just to be safe, I think I run 100 octane in it. I have a local 76 station that has it at the pump. Kind of pricey...but cheap insurance. Thanks for the explanations and help.

Soaring · 08-17-2007, 04:52 PM

I haven't seen 100 octane gas at a filling station in many, many years now. You must be just lucky you don't have to go to the track to fill up.

zuluracerx · 08-17-2007, 05:02 PM

Have not had any reason to buy any lately, but about 2 years ago it was $7 bucks at the pump. Orange County, CA has a ton of 'em to keep all the high end turbos running beyond well. Most of those guys run C14, not sure where they get that but you can see the white ash look on there tail pipes and know what's up!