Get Up And Go For A GTO
I have a ’74 GTO with a 350 engine that’s been rebuilt to stock specs with a numbers-matching Quadrajet carb and distributor. The only modifications are 0.030-over pistons and a Pertronix ignition. What can I do to give it a little more get up and go, and retain stock appearance? This car has the original 3.08 Posi rearend.
Jim Taylor responds: I personally like the ’74 GTO—I had one for about a year. My one and only complaint was the same as yours. It needed “a little more get up and go.” Two things will produce very real seat-of-the-pants performance—a supertune of the engine and a change of the rear-gear ratio.
The first thing is to change the original 3.08 ratio to at least a 3.55, or better yet, a 3.73. The 3.73 will offer a great mechanical advantage for the original 200hp engine, making the 3,400-pound-or-so car much quicker. The 3.73 will increase your cruise rpm, as will the 3.55.
You must select a rear gear based on what you want out of the car. You can find one of the many published calculators that detail rpm and mph with what tire size and gear ratio to guide you. Don’t forget to change the speedometer gear in the transmission to correct the miles per hour.
Now to the engine. First the ignition—specifically, the distributer needs to be curved. The ignition curve allows for a certain amount of timing advance at idle (initial timing). As the throttle is opened and rpm increases, timing (spark) must advance with the faster engine speed. More rpm means more mixture to burn, so the timing must occur sooner, or advance.
Eliminate the spark switch and vacuum advance line to ensure the distributer will be controlled only by GM’s original weights and spring arrangement. Also, GM used a rubber bushing in a slot to stop timing advance at a point. Most cars I’ve serviced whose owners complained that “all of a sudden it began to ping” had lost these age-old timing limiter bushings. A new bronze bushing comes in the Mr. Gasket curve kit (PN 928G). It also includes three different sets of tension springs that control the weights, which are moved out by inertia of the spinning distributor shaft. The springs control how much advance when in the rpm range. The bushing stops continued ignition advance. This very low-compression engine could need 38- degrees total timing and 18 degrees at idle. It would take 38-degrees by 3,200 rpm. If you aren’t comfortable doing the recurve yourself, call Larry Rowe [(301)739-8320; 7-9 p.m.]. Talk to him about your setup and he’ll take care of it.
With your gear and timing done, you can tweak the carb. It probably has a set of 0.053 or 0.057 secondary metering rods. On engines like yours, I’ve had a healthy response using 0.041 rods, available from Ames Performance (PN N201AFG).
If you have no issues now (rich or lean) with the primary side, leave it alone. The biggest gain to the performance of your car may be in the fine adjustment of the secondary air valve. They are the two rear plates above the rear throttle plates. These plates are screwed to a shaft that also lifts the secondary metering rods. When these plates are open, a huge volume of air rushes into the engine. This air is joined with gas as the metering rods (acting like plugs) are lifted out of the secondary metering jets. The speed at which the secondary opens is controlled by a spring-and-lock assembly on the right side of the shaft.
The service manual shows this simple screwdriver and Allen-wrench adjustment, which can be the difference between fast and slow. Too weak tension will cause the engine to load up and bog, and too heavy will have the secondary not kicking in until 4,000 rpm.
All this stuff works, and so does loosening up the valves. Get a set of rocker posi-locks to replace the original jam nuts. Set the valves at a 1⁄2 to 3⁄4 turn after zero lash. The engine will rev up higher and quicker.