Q&A

Ask Away! with Jeff Smith: Improving Fuel Economy in a Quadrajet-Equipped 350 Chevy Camaro Z28

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I have a 1980 Chevy Camaro Z28 that’s in really good shape with the stock 350 and the original Quadrajet carburetor. The car is all stock. Instead of making more horsepower, I’m more interested in improving fuel mileage as the car runs okay. Lately it seems to be rather sluggish and the fuel mileage is now poor. A couple of friends have said to get rid of the original Q-jet – they called it a Quadra-bog. Any suggestions?

L.M.

First thing I did was do a little research on your engine combination. While Chevy called this car a Z28, it is a long way from a performance engine. The stock LM1 specs include 190 hp and 8.2:1 compression ratio. More than likely, the compression is even lower than claimed.

Cylinder Heads

What would really give this engine a shot in the arm would be a boost in compression. It appears the engine came with 76cc combustion chambers and dished pistons. A swap to a Vortec head with 64cc chambers would bump the compression to somewhere near 9.2:1 which would really help both the power and fuel mileage. I know this may be beyond what you were asking so we also have some tuning advice later in this article.

The Vortec heads are not the only heads you could use, but they do offer a small chamber size to increase compression and they are very affordable. Summit Racing offers a pair of Vortec heads for less than $800 and are ready to bolt on. These heads do require a Vortec-specific intake manifold as well, which will also add to the cost.

The benefit however would be the single greatest thing you can do to improve both power and mileage because the heads will flow much better than the existing stock cylinder heads while also increasing the compression. A full point in compression alone is worth roughly four percent power. Since your engine is only rated at 190 hp, this will only be worth about seven to eight horsepower. However, the increased airflow will also be worth power so we’d be confident the head swap alone would be worth at least 15 to 20 horsepower and even more torque.  

Again, the compression will also improve fuel mileage because the engine will now be making more torque with the same throttle opening. This means that light acceleration and cruise will require less throttle opening and therefore less fuel.

Understandably, if you don’t want to go with the major expense and work of swapping cylinder heads there are still some tuning changes that can offer some minor improvements. Let’s start with the ignition.

Ignition System

Before you begin adding new spark plugs, wires, distributor cap, and rotor, we’d first suggest some diagnostic work. Start by checking to make sure the vacuum advance diaphragm in the HEI distributor is working properly. Make sure the vacuum hose to the diaphragm is not split or cracked. A quick check would be to use a timing light to check to see vacuum advance works by first disconnecting the vacuum line and reading the timing as you open the throttle. This is also a great time to verify the mechanical advance is working.

After checking the mechanical advance, reconnect the vacuum advance line and see if the engine responds with more timing. If not, with the engine shut off, apply vacuum to canister. If it does not hold vacuum, the vacuum advance can is bad and should be replaced. This small change will make a huge difference in both throttle response and fuel mileage.

If the vacuum advance works, next remove the distributor cap and rotor and inspect the mechanical advance mechanism. HEI distributors are famous for becoming corroded over time and the mechanical advance mechanism may be only partly effective. The mechanism will probably be corroded and, if so, remove the springs and weights, then clean and re-install the components so that they move freely.

The stock tuning specs for your engine are eight degrees of initial timing with 22 degrees of mechanical advance and 18 degrees of vacuum advance. This produces only 30 degrees of total timing at wide-open-throttle (WOT), which is often not sufficient for maximum power. The easiest change here is to add roughly four degrees to the initial timing. This will give you 12 degrees of initial timing and 34 degrees of total timing.

Of course, all of these recommendations should accompany a new set of spark plugs, new HEI cap, rotor, and spark plug wires. Summit Racing makes an HEI upgrade kit that includes a cap, rotor, new springs, weights, a new vacuum advance canister, and an upgraded module for a hotter spark. Be sure to purchase a quality set of plug wires that are spiral-wound with relatively low resistance. A typical good set of plug wires would be from Accel, MSD, Taylor, or Summit Racing.

In our testing, we’ve not seen any power or mileage improvements from those specialty spark plugs so we are not a big fan of investing the extra cash. Fine wire plugs like platinum or iridium plugs are good, yet only offer increased durability over normal copper core spark plugs and no performance advantage.

Quadrajet Carburetor

As for the carburetor, many people consider the Quadrajet to be among the best four barrel carburetors ever designed but the Q-jet is often criticized by those who don’t understand how it works. Your carburetor may be suffering from a common ailment of early Q-jets that used a plastic float. This material, called nitrophyl, often becomes fuel-soaked which creates a higher float level. Other issues include leaking fixtures at the bottom of the carb that leak fuel into manifold causing poor fuel mileage and hard starting.

The easiest way to repair this would be to purchase a Remanufactured Quadrajet Carburetor from Summit Racing that is a direct replacement for your existing carburetor.

However, if you would rather have your original Q-jet restored, there are several companies that we have personal experience with that can do the job. Two in particular are Jet Performance and Sean Murphy Induction (SMI) both in Huntington Beach, California.

These companies can not only rebuild your carburetor but also recolor it to make it look brand new. This rebuild can also extend to replacing worn throttle shaft bushings and also offer mild performance upgrades if desired based on your existing engine combination.

Replacing Old Parts & Proper Tuning

Hope this may help you improve the performance and mileage of your Camaro and maybe after it runs better with the stock components, it might lead you to consider increasing the power slightly with better aftermarket parts. Either way, we can all but guarantee that a little tuning effort will make your Camaro run much better.

Parts List

  • Autolite Copper Core Spark PlugsATL-26
  • Summit Racing GM HEI Ignition Tune Up Kit – SUM-850032
  • Summit Racing 8mm Ignition Wires – SUM-G813
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7 Comments

  1. Andrew Craig says:

    I did all these things on my 1980 Z28. I then did headers and intake and it was fantastic improvement over original.

    Then I rebuilt it with better heads (used), increased compression, and improved upon the factory induction.

    Now I don’t save any money on gas because it’s so much more fun to just hammer in it. BUT everything Jeff said is 100% correct, and exactly what I did before the rebuild and heads. It didn’t disappoint.

  2. Fred D Franklin says:

    Glad your back sir

  3. Robert Moser Mr AC Delco says:

    Can’t believe you recommended putting Ford plugs in a GM engine

    • Ford doesn’t own Autolite. And even if it did, so what? What kind of idiot discriminates against something that makes their car perform better?

  4. AC Delco hasn’t made a spark plug in 30 years.
    Last I looked they were NGK made.

  5. Another source for Q-jet rebuilds is Mountain Man Fuel Systems in Arkansas. The husband and wife will rebuild your original carb, make it look like new, ask you what your engine setup is (cam, engine, rear, etc) then will set jets and idle so it should need no tweaking once you drop it in. All for under $300, many happy customers on our vehicle forum.

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