Fuel Air Spark Technology - Better Breathing Exercise
Tuning Carburetors By Analyzing Air/Fuel Ratios
From the August, 2009 issue of Rod & Custom
By Kev Elliott
By now most people are aware that modern fuel -injected engines benefit greatly from the use of electronics, whether through the use of an on-board computer which constantly alters the tune depending on driving style, altitude or a number of other variables, or through the ability to plug in a laptop computer to make tuning changes. But in our little world the majority is still running carburetors and have in the past done our tuning with the combination of a screwdriver, seat-of-the-pants instinct and real-world testing on the street. Wouldn't it be nice to be able to know exactly what results you are getting from any tuning changes you make? It probably won't come as any great surprise when we tell you that you now can, thanks to the Fuel Air Spark Technology's (FAST) dual sensor air/fuel meter.
While there are a number of such meters available that allow you to see results from your tune-up while driving, the FAST meter has a couple of features that appeal to us. Its on-screen data logging means it doesn't require downloading the information to a laptop and its "Dual Sensor" capability means it can read data from both sides of a V-8, or any V-formation engine. This allows for more accurate carburetor tuning. An added bonus is that the kit is easy to use and simple to read--definitely a plus for the electronically challenged among us! The FAST meter can be used with computer-controlled engines and EFI, but it's carbureted motors we're interested in for the purposes of this article.
The FAST dual sensor O2 air/fuel...
The FAST dual sensor O2 air/fuel meter kit comes with a pair of O2 sensors, a lighted graphic display screen with built-in data logging facility, outputs for external data loggers, digital gauges, and a narrow-band simulator function that allows you to feed your engine's computer a factory signal while using the wide-band sensor for air/fuel measurement (not that we needed this on our carbureted engine). The meter takes its power from the vehicle's cigarette lighter to make it truly mobile.
So just how does the FAST meter work? It measures the air/fuel ratio, vital information not just for performance or economy tuning, but for anti-pollution reasons too. An optimum air/fuel ratio ensures a cleaner running engine as it offers the opportunity to achieve that perfect balance between being too rich or too lean. Modern factory vehicles with oxygen sensors automatically compensate for changes in the air/fuel ratio by analyzing exhaust gasses and adjusting accordingly, and the FAST meter works on this principle, letting the operator make changes to achieve the desired ratio, such as by changing jet sizes in the carburetor. This does mean installing a couple of threaded bungs in the headers or exhausts on cars not so equipped, in order to connect the O2 sensors. Hardly a major job, and they can be blanked off when not in use, but it does involve a little workshop time before the meter can be used.
Keen to put the FAST meter to the test, we hooked up with Westech Performance in Mira Loma, California to spend a morning at their facility doing just that. Our test mule for the session would be this author's '46 Ford roadster pickup. Hardly a performance combination, its 305ci Chevy small-block was rebuilt internally stock some 2,500 miles ago, but with headers, Pertronix ignition, and an Edelbrock Performer manifold. Running 87-octane gas, it's never going to win any drag races, but is perfect for what it was intended to be--a fun and useable pickup.
We were also keen to test Holley's new Street Avenger carburetor, as the truck had always seemed a little "flat" in performance, even given its mild motor. The aluminum Street Avenger is lighter than Holley's previous offerings, and lighter than the zinc Street Avengers too (it also stays brighter longer and is easier to polish!), has clear sight windows in the float chambers for easy float level adjustment, and comes already calibrated, as Holley's press blurb says, "Street ready out of the box."
So it was that we showed up at Westech one morning in March with the truck, a new Holley 670 Street Avenger and the FAST air/fuel meter kit. While the meter is designed to enable you to tune the engine at a cruise speed on the street, we strapped the pickup to Westech's dyno to simulate driving and make life easier on ourselves. Here's how the morning panned out.
In order to fit the O2 sensors...
In order to fit the O2 sensors we had to weld threaded bungs into the headers. Ernie drilled holes in each header and then MIG welded the bungs into the header collectors. We then installed the dual wide-band O2 sensors from the kit which measure the air/fuel ratio in both engine banks simultaneously or can average them together for precise tuning of carburetor jetting (or EFI profiles)
Holley's Street Avenger vacuum...
Holley's Street Avenger vacuum secondary series of carburetors are available with manual or electric choke, and in 570, 670, 770 and 870cfm applications. However, the aluminum versions, which weigh less than 8 pounds, are only available in 670 and 770cfm. All feature quick change adjustable vacuum secondaries and the Street Avenger fuel metering system, which is calibrated for "monster low-end torque, awesome acceleration, and unmatched top-end horsepower" according to Holley. Included with the carburetor are an installation and tuning DVD as well as a printed manual, gaskets, fuel line kit and two vacuum secondary springs, giving three different rates when you include the spring that comes in the carburetor.
The Holley Street Avenger carburetors are designed to be calibrated correctly straight out of the box if they're used for their intended application (and there's a chart on the Holley website to help determine this), though admittedly the 670cfm carburetor is slightly large for a 305ci engine and we'd probably have been better off starting with a 570, but then we probably wouldn't have been able to improve on the stock settings! Given the engine's stock state of tune and its stock cam, it wasn't going to respond to tuning much beyond what we did, but we did pick up 4 lb-ft of torque and almost three horsepower by installing smaller secondary jets. The motor also ran slightly leaner above 3,500 rpm, though admittedly its cruising speed in third gear is around 2,300 rpm, so it's a somewhat moot point on this vehicle.
Out on the road after the test however, the pickup was more responsive, with better acceleration, though I haven't driven it far enough yet to report any change in gas mileage. While the FAST meter is of more use on a performance engine, you can see how it is useful for tuning fuel delivery. 14 parts of air to fuel is where you'd expect to see a carbureted engine in the lower rpm range, so we were in the right area to start with, and managed to improve the ratio at higher revs while increasing both peak horsepower and torque. Sounds like a successful morning's work to us!
The Holley has a square flange...
The Holley has a square flange bolt pattern, meaning we had to move our carburetor studs to the outer holes. Two nuts locked together make it easier to remove stubborn studs, using the wrench on the lower nut. Beats mangling them with Vice-Grips!
We used the fuel lines that...
We used the fuel lines that were supplied with the carburetor to hook up the fuel supply as this was the easiest way to connect to our existing gas line. The electric choke wires were in the same place and the Heim-jointed throttle linkage also needed no modification. The only change we had to make was to lengthen the vacuum line to the PCV.
Though Holley supplies a base...
Though Holley supplies a base gasket we had to use a thicker insulated gasket to raise the Street Avenger enough to enable the fast idle set screw to clear the intake manifold at wide-open throttle. Weiand manifolds have a groove machined into them to eliminate this problem, but on stock intakes and some other aftermarket manifolds, including the Edelbrock Performer, there can be a slight clearance issue.
With the sensors installed...
With the sensors installed we strapped the pickup to the dyno to simulate a real world road test (this also allowed us to get some horsepower and torque figures to illustrate the results of the tests). This offered all the benefits of a test with the drivetrain under load at cruise speed without actually having to drive the truck at a steady speed on a long stretch of road. Call us lazy but it enabled more data to be collected in less time, though we should stress the FAST kit is designed for mobile use. The FAST meter is ideal for tuning to achieve the optimum air/fuel ratio, but it doesn't give horsepower figures.
The FAST meter can be mounted...
The FAST meter can be mounted to the dashboard using Velcro strips, but for our purposes, as we were not actually driving the truck on the street, it was simply hand-held while Ernie "drove" the pickup. I have very low gearing in the rearend, so our tests were run in second gear in order to reach 4,250 rpm without reaching speeds of way over 100 mph!
In most cases with normal...
In most cases with normal gasoline, an air/fuel ratio of approximately 14:1 (14 parts air to 1 part fuel) is considered ideal. A mixture less than that (12:1) is considered rich and higher (16:1) is lean. Our first dyno run revealed a maximum torque reading of 211lb-ft at 3,350 rpm and maximum power of 154.1 hp at 4,200 rpm. These corresponded to road speeds of 64 and 84 mph respectively, in second gear. More importantly, the FAST meter showed a ratio of over 14:1 below 2,800 rpm, 13.1:1 at just under 3,000 rpm, dropping to under 12:1 at 3,600. 154 horsepower doesn't sound like much, though that was measured at the wheels, and the guys at Westech said they'd seen power losses of up to 80 hp through TH350 transmissions. Okay, so I'm disappointed, but it's only a stock 305.
In a bid to improve the air/fuel...
In a bid to improve the air/fuel ratio by making the secondary jets smaller, and hence leaner, Ernie decided to replace the 68 jets with a pair of 65s, which is as easy as removing the rear float bowl. It was full of gas so Ernie removed one of the lower retaining bolts and drained the gas through its hole into a receptacle first.
With the float chamber removed,...
With the float chamber removed, the secondary metering block was gently pried from the main body in order for the jets to be swapped out.
With everything buttoned back...
With everything buttoned back up, Ernie took the pickup for another "drive," resulting in a leaner air/fuel ratio for most of the rpm range and slightly better peak torque and horsepower figures. This time we saw max torque of 215 lb-ft at 3,400 rpm and peak power of 156.9 hp at 4,269 rpm. The FAST meter indicated an identical 14:1 ratio below 2,800 rpm, a slightly richer 12.4:1 at 3,000 where previously it had been 13.1:1, 12.7:1 at 3,300 and never dropping below 12.4:1 again throughout the entire range up to 4,300 rpm.
Overlaying our two runs in...
Overlaying our two runs in graph form, it illustrates our slight gains in horsepower and torque