One thing some tuners don’t pay much attention to is how fuel pressure affects tuning. To show this, we left the rest of the tune the same and dropped our fuel pressure to 6 psi. By dropping pressure 1 psi, our A/F reading went to an average of 12.8:1, and also showed a loss in power throughout the pull.
We put the fuel pressure back to the previous 7 psi and made a change to the air bleeds, changing them from number 30’s to number 33’s. When tuning with air bleeds, it is important to note that a change in air bleed sizing can often change the shape of the fuel curve and cause an engine to be too rich or lean in a different rpm range than just changing a jet. With the air bleed change, our average A/F reading was now 13.2:1, which is where most people will tell you to run. In our case, our power was down even further than before. Knowing that a dyno loads the engine differently than how it will run at the dragstrip, we looked for a car we could next test our theories on.
04] The easy to navigate on screen display makes for an easy setup. The F.A.S.T. Air/Fuel
To test our theory at the track, we had an opportunity to work with a street/strip car that runs on premium pump fuel. The ‘69 Satellite belonging to Scott Dunnuck has an engine based on a stock 440 bottom end , and is equipped with Edelbrock Performer RPM heads, a Holley Street Dominator intake, a Holley-based 830 carburetor with a Proform main body, and a custom solid flat-tappet camshaft. We brought the Satellite to our local ¼-mile test track filled with 93 octane E10 Fuel, and again hooked up the FAST air/fuel me.ter to both sides of the exhaust.
A baseline run showed an average reading of 12.0:1, and the Satellite turned in a ¼-mile e.t. of 12.28 at 109.78 mph. We then jetted the carburetor down from 80’s in the front (with a 6.5 power valve) and 88’s in the rear, to 76’s in the front and 84’s in the rear. This leaned our A/F readings to an average of 13.0:1, and also slowed the car down to the tune of 12.47 at 107.59 mph.
We decided to go back up on our jetting to just slightly leaner than our baseline. We installed 78 jets in the front and 86’s in the rear. This brought our average A/F reading to 12.4:1, and resulted in a ¼-mile e.t of 12.33 at 109.23 mph.
There are many reasons why your engine might want a different A/F ratio than someone else’s. The type of fuel you are using, how efficient your engine is, even air quality can affect it. Our best advice is to try adding a little more fuel and see how your engine responds.
We have noticed through testing that an engine that is slightly lean will lose power faster than the same engine that is slightly rich. We also need to remember that most enthusiasts are measuring these A/F reading in one or two exhaust pipes, after all the cylinders have merged via a set of headers at the collector or exhaust manifolds. Due to fuel distribution issues, you may need to add fuel to the engine to keep one or more cylinders from being too lean. Unless you are measuring each cylinder and making changes that fix these distribution issues, you have to keep the leanest cylinder happy. This can lead to a richer than “ideal” A/F reading.
Although our testing was aimed at maximum horsepower and/or minimum e.t., an A/F meter can also be used to dial in better fuel economy by tuning for a more efficient ratio while at cruising rpm. We can also can it to monitor accelerator pump shot effectiveness, and the point at which fuel enrichment circuits start to come in. Even though both of our combinations wanted similar A/F numbers for best performance, remember that every engine and fuel combination is different and your resulting A/F ratio may be different.
Using an A/F meter and testing is the best way to find out what your combination likes. We have tried the old “jet it for best mph” routine before just to find out our car was running extremely rich through a good portion of the ¼-mile. Achieving an A/F ratio that is correct throughout the entire range may take more than just a simple jet change. Changing the jet/air bleed combination can get you the extra fuel you need at a certain point in the run. A/F readings can definitely help you figure out which direction to go if you let it be your guide and don’t get hung up on the numbers.
If you use one on the street, it is probably best to take a passenger with you to take notes while watching the meter so you can pay attention to the road. Taking readings while going up a steep grade can show where an enrichment circuit adds too much extra fuel.
Cost for F.A.S.T. Dual sensor A/F meter: $481.95
05] Holley carburetors should have their fuel pressure set around 6 psi for a gasoline car
06] Most new HP carburetors have jets for adjusting the air bleeds. If you are running an
07] While at the track, we jetted the carburetor in our test car down from 80’s in the fro