August 2010 Chevy High Performance Q & A - Performance Q & A

Kevin McClelland Jun 17, 2010 0 Comment(s)
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A: You're right on track with getting your timing off vacuum control. As you open the throttle and the manifold vacuum drops, you are losing ignition timing. Once you get your mechanical advance dialed in, you won't believe how much power you will pick up.

The MSD distributor gives you the ability to adjust the amount of mechanical advance by swapping different diameter bushings that adjust the amount of advance. The larger the bushing diameter the less mechanical advance you have. You can achieve the same result by welding up the slots in the mechanical advance plate and limit the amount of advance. Dissemble your HEI by driving out the roll pin in the distributor gear. Make note of the dot machined into the side of the distributor gear by the roll pinhole. Take a good look at the mechanical advance top plate where the rotor attaches. Record where the rotor locating tag is and its relationship to the dot on the distributor gear. You must reassemble it in the same location. Once you have dissembled the mechanical advance mechanism, you'll see where the pins on the advance plate slide through the slotted holes in the distributor shaft plate. These slots are what you want to weld up to reduce the amount that the mechanical advance can travel. We'd start by reducing the travel by a third. This should reduce the distributor's mechanical advance to around 8 degrees. When you double this at the crankshaft, it will give you 16 degrees. If you go with 20 degrees initial advance and add the 16 degrees from the mechanical advance, you have a total of 36 degrees. This should work very well with the large camshaft you're running.

Next, tailor the rate that the advance comes in by using different tension springs. If you have someone local with a distributor machine (a lost art), you can spin up your distributor and set it up on the bench. However, there aren't many machines around anymore. You can do the same thing with your engine, a timing light, and a tachometer. Check out the HEI Performance Advance Curve kit from ACCEL. The complete kit comes with weights, an advance cam plate, and three sets of springs to custom-tailor your advance curve. This kit, with your limited mechanical advance travel, will get your spark right where you want it.

You have blown right past the design limits of your Torker II manifold. Also, the manifold really doesn't like any type of carb spacer. It really messes with the mixture distribution. For your application, you're right between an Edelbrock Performer RPM (PN 7101) and a Victor Jr. (PN 2975). In a buggy you want as much torque as you can get. We'd swap out for an RPM for the best all-around performance. You may lose a few ponies up at 7,000 rpm with the dual-plane design, but what you gain everywhere else will make up for it.

Enjoy your new power. If you've been using the vacuum advance to tailor your advance curve we'd estimate that you've been losing around 10 percent of your engine's output. Be careful when this thing gets dialed in.

Sources: accel-ignition.com, edelbrock.com

Leaky Runners
Q: I saved up about $3,000 in two and a half years; it all went into rebuilding a 5.7 TPI engine with a Comp camshaft and roller rocker arms, a geardrive, a 10-inch stall converter for a 700-R4, and a transmission cooler for my '92 Z28 convertible.

I picked up the car and after driving about a block I knew right away it was not running like it should. My mechanic adjusted the timing, but he didn't really know what else to do. From what I can hear, see, and feel from driving it around, it just seems like it's completely underpowered. The strange thing is, it's burning a lot of fuel and I can hear air leaking from the engine; it's idling high, and the "check engine" light stays on. For the time being, I'm not driving it because I have too much invested and I'm scared of burning it up. Please help! I really need your expertise on this one. Tigs Pacoima, CA

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