Mailbag: Adding Performance to a Stock 305 in a G-Body Monte Carlo SS

85 monte carlo 305 engine bay

(Image/Chevy Hardcore)

You’ve got questions, we’ve got answers. We work with the Summit Racing tech department to help you tackle your auto-related conundrums. In this week’s Mailbag, we’re discussing removal of emissions controls on, and rebuilding a carburetor for, an ’85 Monte Carlo SS.

Q: Two questions:

I have a stock 1985 Monte Carlo SS with a 305. I have taken the A/C and air pump off. What emissions stuff can I remove without harming performance, and what electronics can I remove?

The second question is about my carburetor. It’s the stock four-barrel Rochester. What can I do to upgrade it? Would bigger jets help without affecting the computer?

A: You might want to hold off on removing emissions controls in your Monte Carlo. Depending on where you live, it might be illegal. But legal stuff aside, it doesn’t actually improve performance like you think it might. By removing your air pump, you may have actually decreased performance a little by making the engine less efficient than before.

Considering the age of your Monte Carlo, we assume it’s a high-mileage vehicle. Instead of removing the emission controls, we recommend concentrating on the condition of the engine.

In other words, if the engine is worn out, performance is going to suffer.

On low-mileage engines, upgrading your camshaft or cylinder heads are good upgrades to increase performance.

A carburetor rebuild may also bring back lost power. If you’re uncomfortable doing a rebuild yourself, ask around. It’s a common job, and most competent shops can do it for you without much hassle.

If you don’t want to deal with a rebuild, there are several fully prepped aftermarket Quadrajet carburetors available.

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  1. You must be careful in removing ANY of the electronics from a computer controlled system. While very simple by today’s standards, the 80’s “CCC” systems still require input from all of the sensors for the ECM (Electronic Control Module) to work properly. Both the carburetor and the distributor are controlled by the ECM. If the ECM is not receiving the proper inputs, It has no basis on which to accurately control the carburetor for the “idle circuits” (which affect performance and drivability up to 30% throttle) and distributor timing (affecting the entire power band). As far as the air pump, it IS a part of the emissions system, and its removal from the system can fall into an “illegal modification” category. The air pump affects the emissions during cold-start and warm-up cycles. Its removal from the system will not adversely affect performance and technically, the removal of drag on the system required to turn the pump will INCREASE performance (granted, a negligible amount, most likely indistinguishable to all but the most sensitive of butt-dynos).

    A carburetor of this vintage (if original) would most likely need to be rebuilt as the main throttle shafts had a tendency to wear the base plate where they make contact. It causes a pretty good vacuum leak that affects drivability. As far as jetting it can be changed, but it is different than the older style carb in that the flow is part of the computer controlled portion of the carburetor. On a stock set-up, without major changes to the package, it should NOT be required on the primary side. You can easily make adjustments on the secondary side which I’ll address below. One thing to keep in mind, all of the computer controlled carburetors were based off the 800 CFM design of previous generation of carburetor, so there is PLENTY of performance capability there. It’s just a matter of making it work correctly for YOUR application.

    There are some things that can be done to improve the performance of a stock 305 of that era:

    1) Lower temperature thermostat. The factory thermostat was almost always 195°. Changing to a 180° thermostat allows your entire engine/transmission assembly to run cooler. This is good in multiple ways: 1) Cooler air (cooler air = denser air. Denser air = more oxygen/volume. More oxygen = more potential for power) 2) Better longevity of parts (heat is a killer).

    Some people can get away with a 160° thermostat, but some applications will see a “SERVICE ENGINE SOON” light if the ECM does not see sufficient temperature.

    2) Open element air cleaner. While the “SS” dual snorkel factory air cleaner housing looks cool and does offer the ability to duct cooler air into the engine, it’s still physically restrictive (not to mention blocking the 4bbl sound when you jump on it). When you’re selecting an air cleaner, I would suggest adding a 1/2” air cleaner spacer to give a little extra clearance for the electrical connectors.

    3) Adjust the secondary air door limiter. On a Quadrajet carburetor, the upper air door was set to limit the air flow from factory. In fact, the upper air door was usually limited to less than 50% of its travel and flow as the Quadrajets were originally designed. There is a small tab on the passenger side of the carburetor the limits the travel. You can bend the tab to increase the travel limit. By bending (instead of removing) you can go back if you’d need to. After opening up the air filter as noted in step 2, allowing the upper air door its full travel will offer a BIG improvement in air flow. This step MUST be done in conjunction with step 4 (below).

    4) Secondary Metering Rod change: On the “performance” (H.O.) applications, the secondary metering rods were EXTREMELY rich (…very narrow to allow a lot of fuel flow…) I believe this was because the upper air door (which controls the position of the metering rods) was limited and in turn, physically limited the fuel flow. Now that you’ve adjusted the upper air doors (point 3) to allow more travel (and air flow), the metering rods will need to be replaced with leaner (thicker) rods. The Edelbrock 1953 (EDL-1953) or 1954 (EDL-1954) metering rods are a good place to start. I always suggest testing drivability with the leanest rods first, moving richer from there.

    5) Timing: Now that you’ve made the improvements above, you need to increase the ignition timing to better take advantage of these changes. The factory timing was 6° BTDC if I remember correctly. With these changes, you should be able to increase the base timing by 5° or 6° (sometimes more, but always increase in SMALL steps**) and feel a distinguishable increase in power. **IMPORTANT: If you hear ANY pre-ignition knock (ping), you’ll need to back off the timing increase until there isn’t any knock or engine damage could occur.

    6) Catalytic converter: The original converters were nowhere near as efficient as newer converters. Based on the age of these cars, it is highly doubtful that the original converter is still in good working condition and most likely has become a restriction in itself. Replacing it with a newer efficient converter offers a big improvement in flow and keeps you emissions compliant. Keep in mind: replacing or removing a good converter is illegal just about everywhere, so replacing is not suggested unless it’s bad. Removing it entirely is NEVER suggested.

    I hope these pointers can help you get the most out of your car.

  2. Edelbrock and Hooker used to make emission legal shortie headers that were direct fit. Hedman still makes longtube emission legal headers. A new true roller timing chain will regain a lot of power. Stick with a 195 t-stant, it extends engine life, as engines wear faster when cold. If you are running a 87 of later ECM, you can use a ZZ4 PROM chip which adds power.

  3. Looking get a little more power out 86 monte carlo ss I have the 305 with a 600 holley street warrior carburetor, what else can I do to increase power in my engine

  4. Larry Van Brunt says:

    I have changed the air filter housing on my 1987 Monte Carlo SS. There are provisions for relocating the tube, but not the wires from the side of the stock aircleaner insert on the left facing the front of vehicle. The question is do
    I need to somehow attach the connection to the new air cleaner assembly?

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