In a time not long past there was that guy down the street fromyou--that special car-crafting guru who could tune an engine by ear. Hewas revered by the local gearheads for his uncanny knack for creatingnot just impressive power, but also for transforming chugging streetslugs into razor-sharp sweetheart engines. He was like the PinballWizard--the deaf, dumb, and blind kid who could play those flipperfingers like he was part of the machine. We were all in awe of the CarbWizard.
Float level can be used as a tuning tweaker for subtle changes to air/fuel ratio. Lowe is leaner and higher is richer.
While that guy still exists, we've discovered a little piece ofelectronic technology that can turn almost any knowledgeable car crafterin a Carb Wizard. This latest development in handheld electronic-powerknowledge is the affordable air/fuel-ratio meter. While there are manyon the market, we first told you about a favorite, the InnovateMotorsports digital unit, in "Tune In, Turn On, and Make Power" (Feb.'04) where we explain how it works alongside a few basics concerningair/fuel ratio and horsepower. This is such a potentially great toolthat we thought we'd get into how to tune your engine using this toolnot just for max power, but also to raise the bar for betterpart-throttle response and highway cruising. While the Innovate meter iscapable of logging up to 44 minutes of brain-numbing data, we'llapproach this tuning session assuming you're going to read the Innovatemeter in real time. Ideally, data logging is better because you canstudy the information more closely.
The best place for wide-open throttle (WOT) air/fuel testing is at thedragstrip. Short of that, you can do Second-gear bursts of 2 to 3seconds each and have a passenger watch the meter. For your initialwork, you should shoot for WOT air/fuel ratios between 12.5:1 to 13.0:1.Remember to make only one change at a time and keep using the same testprocedure. If you're at the dragstrip, use mph numbers to help withtuning trends. As long as your changes improve trap speed, continue tomove in that direction. In many cases, the combination will berich--like 11.8:1 at WOT. That means you should start by leaning out thesecondary side of the carburetor. With Holleys and Demons, minimum jetchanges of two sizes per step are a good idea. For example, if you wantto run leaner, go from 80 rear jets to 78s. It's also a good idea tostart your tuning with the carb in its stock jetting configuration. Ifyou are five or six jet sizes (or more) away from the box-stockconfiguration, it's possible there's something else wrong with the carbor your engine.
Contrary to what you may think, street engines spend a majority of timeat idle and at very low throttle openings. In addition, the idle circuitcontinues to deliver fuel even when the carb is delivering fuel throughthe main metering circuit. Given this, the best place to start improvinghighway and in-town fuel mileage is with the idle circuit.
A high-speed lean condition can often be traced to low fuel pressureresulting from an underperforming fuel pump.
Most general-purpose aftermarket performance carburetors are designed todeliver around a 12.5:1 air/fuel ratio to avoid lean surge conditions.Most mild street engines can tolerate part-throttle air/fuel ratios of13.5:1 all the way up to as high as 15.0:1. Keep in mind that allproduction EFI engines operate at 14.7:1 air/fuel ratio and thedriveability is excellent. It's more of a challenge to tune a carburetorto achieve a lean 14.7:1 air/fuel at part-throttle and still deliverexcellent and immediate WOT power, but it can be done. What this meansis the power-valve and accelerator-pump circuits become much morecritical. This is where a Q-jet shines, using its small, highlyresponsive primary side to achieve excellent throttle response forpart-throttle driving. But other carbs can be tuned to also work verywell. Don't be afraid of 14.0:1 or even 14.5-15.0:1 air/fuel ratios forpart-throttle highway cruising. While these are lean ratios, they don'thurt the engine. There is very little load at highway cruise speedsbecause the engine is only making about 15 to 25 hp under theseconditions.
Throwing a jet swap at the side of the road has been a car craftingtradition ever since Tw
The Innovate LM-1 is a self-contained unit that is powered by a 9-voltbattery or from the
The Innovate meter generates a digital readout of the air/fuel ratio. Ifyou're really seri
The best approach is to weld the supplied bung into the exhaust pipenear or in the header
Part of the the Aux Box software allows you to custom configure up tofive gauges on your l
WOT tuning is relatively simple. If the meter reads richer than 12.5:1,remove the secondar
This is definitely cut-and-try type work. If you go too lean on theidle-feed restrictors, the engine will surge at cruise and hesitateunder light acceleration, since most engines don't like to accelerate atlean air/fuel ratios. If you tune the idle-feed restrictors too lean,the engine will most likely suffer from an off-idle lean stumble as anearly indicator that you've gone too far. It's the transition circuitsthat are more seriously challenged and the ones that will falter firstwhen you begin to lean out the idle and primary main circuits.
An ideal accelerator-pump shot is just enough to maximize acceleration.Additional fuel only kills power.
Tuning Challenge 1
Our first real-world example started out with a 9.0:1-compression 455ciOlds with a mild hydraulic cam, dual-plane Edelbrock, the stock 455Q-jet, headers, and an HEI ignition. The Innovate meter reported an idlemixture of around 12.0:1 that went lean at around 13.8:1 as the throttleopened up in mild acceleration. At steady-state highway cruise speeds,the air/ fuel ratio went back rich at around 12.5:1. At WOT, the enginewas actually a bit lean.
The first thing we did was to adjust WOT tuning by swapping the stock CHsecondary rods to thinner rods (eventually a pair of aluminum rods)until we had a WOT air/fuel that came in around 12.8:1. Because of thecam, intake, and header swap, part-throttle transition fuel was alsoslightly lean, so we changed to a slightly weaker power-valve spring topull the metering rods out of the primary jets sooner as load increased.The engine responded with a slightly richer mixture in lightacceleration of around 13.2:1, but we still had a slightly rich cruiseair/fuel ratio. When we tried larger (leaner 52B) primary metering rods,it hurt light-throttle acceleration. Some late-model Q-jets are set upto allow you to adjust the position or depth of the primary meteringrods in the jets. If our carb had been equipped with this feature, wecould have adjusted the metering rods deeper in the primary jets to leanout the part-throttle metering.
Tuning Challenge 2
We also tried a 350 Chevy equipped with a long-duration cam and a Holley750-cfm 0-3310 vacuum-secondary carb that suffered from a bad off-idlestumble and a pig-rich 10.5:1 ratio at part-throttle cruise. The WOTair/fuel ratio was only slightly rich at 12.2:1. The challenge was toimprove the driveability and mileage without sacrificing WOT power.After properly adjusting the primary accelerator-pump linkage, thestumble disappeared. We also replaced the large 0.035-inchaccelerator-pump nozzle to a 0.028 to further improve the throttleresponse.
Setting idle mixture will change when you bolt on the air cleaner. Ifpossible, set your idle-mixture ratios with the air cleaner in place.
The LM-1 meter told us the engine was way rich at just off idle, so wefirst tried leaning out the idle-mixture screws, but it didn't help.Next we disassembled the carb and found that a previous hacker haddrilled out the idle-feed restrictors to 0.052 inch. Stock 750 Holleycarbs come with idle-feed restriction of around 0.032 to 0.035 inch.Some quick math revealed that the 0.052-inch orifice increased the areaby more than 100 percent. As a temporary fix, we tried a0.020-inch-diameter wire stuffed into the idle-fuel jet to reduce theflow area. The larger wire reduced the area by roughly 15 percent, whichleaned out the part-throttle air/fuel ratio to roughly 11.8:1, which wasstill very rich. Further experimenting with reducing the main jets from72 to 69 and using a slightly larger 0.025-inch wire in the idle-feedjet finally got the part-throttle air/fuel ratio close to 12.8:1. Wecould not tune any leaner without creating an off-idle stumble andpart-throttle lean-surge condition because of the cam. The WOT air/fuelalso improved with the leaner primary jetting to 12.5:1. These simplechanges improved the fuel mileage by over 30 percent.
|DESCRIPTION ||PN ||SOURCE ||PRICE |
|Air/fuel meter, LM-1 ||3723 ||Jegs ||$349.99 |
|Oxygen sensor ||3737 ||Jegs ||79.99 |
|RPM inductive clamp ||3727 ||Jegs ||21.99 |
|Exhaust bung/plug ||3735 ||Jegs ||15.99 |
|Lambda cable assm. ||3744 ||Jegs ||249.99 |
|Analog gauge ||3731 ||Jegs ||59.99 |
|Aux Box, multi-sensor ||3742 ||Jegs ||249.99 |
CC Tuning Tip
Poor ignition performance can be confusing when using the LM-1air/fuel-ratio meter. When a cylinder misfires, the air and fuel do notcombust, meaning the oxygen in the air moves into the exhaust where theO2 sensor will immediately pick up the additional air and indicate alean condition. Simply put, this means a dead cylinder or occasionalmisfire will show up on the Innovate meter as a lean condition, not arich mixture as you might think. Fix the misfiring cylinder(s) and theair/fuel ratio will read as a richer mixture.
Rochester Q-jet WOT is the easiest to tune. All you have to do is changethe metering rods.
The Edelbrock carb primary metering rods and springs are incredibly easyto change. Merely
You'll have to remove the lid to change the primary metering rod andjetting on a Q-jet, bu
The Q-jet uses one power-valve piston with a hanger to operate bothmetering rods in their
Holley and Demon carbs use a diaphragm-style primary power valve. Theopening point is usua
While the idle circuit is very basic, it has a huge impact ondriveability. The idle-mixtur
Big cams with low idle manifold vacuum require a larger opening to flowsufficient air to m
5 Jenner, Ste. 100