Carburetor Tuning & Reader Questions - Performance Q&A

Kevin McClelland Jan 1, 2010 0 Comment(s)

Tuning In The 21st Century
Carburetor tuning has always been up to the driver and his/her senses, to determine how a car behaves and performs. We can use vacuum gauges to get the idle side of the carburetor correct, but the main circuit of the carb can be quite tricky. What does a lean bog versus a rich bog feel like? Can you easily determine which direction the jetting needs to go? Since fuel changed back in the '70s to unleaded mix, we can no longer read the spark plugs for mixture; most unleaded plugs look completely white when the fueling is correct. The best way to set up the main jet calibration has always been at the dragstrip. Tuning the jetting for maximum mph in the quarter-mile is what the engine wants, but with ever-changing weather conditions between the density altitude and wind conditions, some jetting changes will only yield small changes in mph, which can then be skewed by a shift in the weather.

1001chp_01_z Carburetor_tuning_and_reader_questions Performance_parts 1/2

For years we've been using wideband oxygen sensors on engine and chassis dynos. They have given us a window into the combustion chamber and the air/fuel ratio (AFR) of the burnt gases. These dyno-grade sensors have been expensive and require data-logging capability. This past weekend I had the pleasure of setting AFR with a very inexpensive wideband oxygen sensor for permanent installation into your vehicle. Advanced Engine Management sells a complete Universal Exhaust Gas Oxygen (UEGO) Gauge Controller featuring a Bosch UEGO "laboratory grade" sensor. This sensor has an accuracy and repeatability to 0.1 AFR. The gauge is a standard 21/16-inch with a sweeping LED readout, and a digital readout in the center of the gauge. We had one on our Stock Eliminator wagon tuning the highly modified Quadrajet, and another on an EFI-equipped Stocker. Within two runs on the wagon and two metering rod changes, we had the AFR honed right into 12.8:1. This is where the engine made peak power on the engine dyno. On the EFI car we had been chasing our tail working out other calibration issues. The EFI was commanding 12.5 AFR but was actually at 15:1 going down the track! We quickly adjusted the fuel tables in the EFI and the AFR came right in line at 12.9:1, and we picked up 0.4 in the quarter-mile. I sure wish that the simple rod change would have given me that kind of performance increase in the wagon.

The AEM Gauge-Type UEGO controller kit, PN AVM-30-4100, is available from Summit for $218.95. For the price of this sensor you can have your car dialed in quickly and accurately. After doing this for years, and using a wideband at the track for the first time, I will never tune another car without one. This is just another easy tuning tool you should have in your toolbox and install when changes have been made that require new jetting. I'm tired of working so hard!

Thumpr On
Q: Can you offer a recommendation for my project '55 Bel Air? I purchased a 400hp LS2 from an '06 Corvette with less than 10,000 miles. I want to deviate from stock a little, to give the engine a better sound and a lope, as well as gain some power, and need some help with a cam choice. I have read a little about the Thumpr from Competition Cams and wanted your opinion on which version will best suit my needs. Can I use the stock lifters and pushrods since they are still somewhat fresh? Will I have to have the computer reprogrammed to compensate for the different bumpstick? Will the injectors need to be upgraded to make the whole package work? Is there a better choice or combination to give the engine a lope with a gain in horsepower and torque? I love your articles and layout of the magazine. Thanks for the help, and keep them coming!
Mike Yore
Indian, AK

A: Having a cool, choppy idle in a '55 Chevy is just about mandatory. Doing this without killing all the slow-speed torque to pull your very nice street rod around is another thing. The Comp Cams Thumpr camshaft line does give you somewhat the best of both worlds. With the advanced inlet lobe and tight separation angles, it helps boost slow-speed torque while maintaining idle quality. We'd recommend going with the smallest of the Thumpr line for your project. The 275THR9 camshaft specs out with advertised numbers of 275/295 degrees duration, 219/233 degrees at 0.050-inch tappet lift, 0.553/0.536-inch max lift, ground on 109 centers. Anything more than this will hurt the slow-speed driveability of your very refined LS2 small-block.

The original injectors will feed this combination just fine. You will probably need to do some calibration work to clean up any driveability glitches. With the swap, you'll need to create an air ducting system to feed the air valve. Incorporating the mass air sensor correctly into this tube will make the most difference in the calibration. Whatever the factory ducting size is at the actual mass air, must be maintained in the fabricated air inlet system. This will keep your fuel trim numbers as close as you can in your calibration. This is where you will want to do any final cleanup with your engine swap and camshaft installation.

The rest of your factory LS2 components will work well with the Thumpr camshaft. Enjoy your engine swap and many years of happy cruising!

Need A Little Content
Q: Are there any magazines or catalogs out there with performance parts for Chevy Cobalts? I see little things here and there in the import magazines, and pretty much nothing from the Chevy magazines.
Jeff Anderson
via email

A: The Cobalt SSs are little rocket ships, first with their supercharged Ecotec engines, and then in '08-and-up with the turbocharged upgrade. Sorry for the lag in coverage here in CHP. However, our sister publication GM High-Tech Performance has run a few stories over the years on these models. Just lately they featured an '09 turbo direct injected tuning story. They came away with some pretty impressive horsepower gains.

As for component parts, you'll find most of the performance parts in the sport compact arena. After some digging we've come away with several one-stop shops out on the internet. Check out Cobalt Addiction and Modern Performance. Both feature a wide range of performance upgrades for Cobalts of all years. Modern Performance supports most sport compacts, and Cobalt Addiction only caters to the Cobalt enthusiast. Hopefully, these three avenues will feed your need for content.

Sources: cobalt-addiction.com
gmhightechperformance.com
modernperformance.com

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