Jim Smart
February 4, 2011

Last month we took a '65 Mustang with a 225hp, 289 4V and did some minor engine tuning to see what kind of performance improvements we could make. After some work on the ignition, the spark plugs, and the air cleaner, we only added about 4 hp and 5 lb-ft of torque, nothing outstanding, but still significant. This month we'll take it a step further with this question: What can you do if you want more power from your vintage small-block?

We called the folks at Edelbrock and asked them what they would recommend to warm up a street small-block. They sent us an Edelbrock 600-cfm carburetor and Performer RPM intake manifold to get us started. We drove the Mustang back across the Valley to West Coast Classic Mustang for the manifold and carburetor swap. They also recurved the distributor timing and managed to get the vacuum advance working to improve low-end torque.

We have to admit that our first dyno run with the Edelbrock Performer RPM and carburetor was disappointing, as we achieved only a modest increase in power. After West Coast Classic Mustang performed the induction swap, we took the Mustang directly to Morgan Motorsports for the fifth dyno run. Upon checking the timing, Derek Real concluded that he needed to reinspect the initial timing. He checked the vacuum advance for proper operation using a vacuum gauge at the carburetor in off-idle conditions, then he set the initial timing at 15 degrees BTDC (before top dead-center). He brought up the revs to 3,500 rpm to check total advance. The engine's performance just wasn't crisp with the rapid response you'd expect. We wanted to see more from our 289. Real took to the driver seat for the sixth pass.

Setting the initial spark timing to 15 degrees yielded some improvement, but not what we expected from the Edelbrock Performer RPM and a 600-cfm carburetor. Real looked at the fuel mixture, which was decidedly rich. This is where we learned a lot about induction. Small-block Fords with stock heads and camshafts don't need 600-cfm carburetors. The optimum for our 289 would be 500 cfm. With a 500-cfm carburetor, we would gain horsepower and torque. The Performer RPM manifold is the best dual-plane manifold we have ever tested. However, our mild-mannered stock 289 needed the Performer 289 manifold, which is more in line with the stock four-barrel intake with the lightweight advantage of aluminum, and Edelbrock's engineering benefits.

To see what we could do about an overly rich mixture, Real swapped the Edelbrock carburetor's 0.098-inch out-of-the-box jetting for smaller 0.095-inch jets.

Rejetting the carburetor yielded modest results. Torque is up and horsepower is about the same; however, we need less carburetor and manifold atop our stock 289. Real gives the ignition system another look and retards the initial timing by 4 degrees for dyno run No. 8. Retarding the timing a few degrees netted 6 hp and 7 lb-ft of torque. See what you can do with a simple ignition timing adjustment?

The dyno teaches us something important about power: An engine's state of health and tune directly affect power. The two most important issues you can address in engine tuning are spark curve and fuel mixture. Too much or too little of either rob power and harm the engine. Too much ignition timing (too much spark advance) can cause detonation and engine damage. The vacuum advance and centrifugal advance must work in harmony. We can achieve this by tuning and retuning until they're working together. The vacuum advance must work smoothly in unison with the rate of engine acceleration. As you tip in the throttle, the vacuum advance should advance the spark enough to help the engine make torque when it's needed most (coming off idle into acceleration).

As engine speed increases as we head toward cruise, the centrifugal advance takes over, with the vacuum advance working only during moments of acceleration. This is a tricky balance, getting the vacuum and centrifugal advance units to work as a team. We curve the vacuum advance with shims (stock advance unit) or an Allen wrench in the vacuum port on aftermarket units. Adjustment controls how much spring pressure we have against the vacuum advance diaphragm. When we increase spring pressure by adding shims or turning the adjustment clockwise, we slow the rate of advance. When we take away shims or turn the adjustment counterclockwise, we speed up the rate of advance.

When speeding up the rate of advance, you know you've gone too far if the engine misfires. You should become attuned to your engine's personality. Rev the engine to 3,000-3,500 rpm and hold it there. Does the engine run smoothly, or does it misfire? Goose the throttle and rev the engine to 3,500 rpm, immediately returning to idle. Is it a smooth rev or does the engine stumble? If it stumbles, then you have too much timing (not enough spring pressure), and vacuum advance timing is activating too quickly. When the vacuum advance is properly adjusted, revving the engine should be seamless with no missing or stumble. You have to keep at it, though, trying different numbers of shim or adjustments until the engine responds smoothly.

Using a timing light (assuming your harmonic balancer is accurate), try the above and watch the timing mark. A small-block Ford should achieve 36-39 degrees of total advance at 3,000-3,500 rpm. Anything beyond 40 degrees is asking for piston damage. Goose the throttle and watch the timing mark. It should smoothly and smartly advance neck and neck with rpm. Take the Mustang for a spin and see how it feels. If it falls flat and lacks torque, then there's not enough timing. If it pings and misses, then you have too much timing.

Fuel mixture is still trickier. Proper carburetor sizing and jetting is vital because the wrong carburetor can rob power and even damage the engine. If the mixture is too lean, then your pistons will burn. If the mixture is too rich, then you will foul the air we all have to breathe. A good rule of thumb with small-block Fords is, the 260, the 289, and the 302 don't need 600-cfm carburetors unless they're running aftermarket heads and a hot camshaft. For a stock mill, 500 cfm is sufficient, and is available from Edelbrock. For 351 engines, a 600-cfm carburetor will work fine.

Our first dyno tests have taught us that we should modify engines as a package, not as a bolt-on frenzy. Unlike us, start with a good exhaust system. Jerry Choate's '65 Mustang GT had the original factory dual exhaust system, which is too limited in terms of flow. With a free-breathing exhaust system, you can step up to an aftermarket manifold and carburetor, and then check the results. Next month, we're going to install headers and a new dual exhaust system, and then go back to the dyno for another blast.

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