air/fuel ratios

[kifried]

I've got an Olds 350 with a edelbrock performer intake and 750CFM performer carb. My question is, in general is there a correct air/fuel ratio that one should try and achieve? If so, is the best way to tune in such a ratio by installing a air/fuel ratio monitor and adjusting your jets till you acheive the desired ratio?

thanks, ken

 

[jerrycad472]

I believe the magic number is 13:1, but check with a better source than me. I have been reading a book on Fuel Injection and that number sticks in my head. If you install a Wide Band O2 sensor and a data logger, you can see where your tuning is off. You will probably never achieve exactly 13:1 but you can tune to a reasonable range.

 

[megaladon6]

for performance AND emissions 14.7:1 is considered ideal. for street performace only, 12:1 is generally optimum (that's measured at the point you're operating at--ie WOT throttle for the drag strip). i'd just tune the carb with a vacuum gauge for the best idle vacuum. that will usually put you really close to the best power efficiency.

 

[Geesie]

14.7:1 by mass is what you get if you do the chemistry, it's the ideal ration for perfect combustion of gasoline and air.

 

[megaladon6]

that assumes a perfect mix though. that's why richer is better. hey, ain't it always?

 

[kifried]

From what I've been reading today, (while bored at my job) I've found that the correct answer is, there is no optimum ratio. This seems like a pretty good reference.

http://www.bristoldyno.com/tech/airfuel.htm


Also, related to carbs. On the edelbrock website they have a calculations section. I calculated for my olds 350, 30 over, 100% Volumetric Efficiency, at my highest RPM of 6000 needs 585CFM.

I have a 750CFM carb, does this 165CFM difference affect performance? Or is it just a buffer for if I ever were to go to say, forced injection, raising that 100% VE to 110%. VE, thus increasing the necessary CFM...

 

[megaladon6]

if you have a q-jet i wouldn't worry about the extra CFM as it is self metering. if you have a straigght forward vacuum secondary carb them it can hurt performance because it won't have enough air flow or velocity to function at 100%. especially since the engine has to be built just right to attain 100%VE. stock is usually considered to be 75-85 and an average build up to 95. however the nice thing about a vacuum secondary is that the secondary will only open as far as necessary for the delivered airflow.

 

[kifried]

This is the carb I have, what is your analysis?.....

750 cfm, Manual Choke
CALIBRATED FOR PERFORMANCE
Designed and calibrated for optimum street performance in small-block and big-block engines with Performer RPM, RPM Air-Gap and Torker II manifolds and with Performer manifolds on large cubic-inch engines. Includes both timed and full vacuum ports for ignition advance. Electric choke #1478 can be added if needed. Comes with: Metering Jets – Primary .113, Secondary .107; Metering Rods – .071 x .047; Step-Up Spring – orange (5" Hg). Use our Carb Stud Kits #8008 or #8024 if needed, see Installation Items. For auto trans throttle lever adapters and more, see Tuning Accessories.

 

[twisted]

I have always tried to be in the neighborhood of 14:1.

 

[megaladon6]

that sounds like an edelbrock. in terms of keeping from overcarbing the engine they are second to the q-jet. if you go to how to's/knowledge base and read the sticky at the top--holley vs q-jet we had a discussion about the plusses and minusses of holley, q-jet and edelbrock which will explain alot.