I’m in the process of tuning up the HEI distributor in my 350 small-block Chevy. The engine has a mild cam. My understanding is that to re-curve my distributor it needs to be tested on a distributor machine. Where I live this isn’t an option. So what other options do I have other than guesswork? — J.D.
Jeff Smith: While using a distributor machine is one way to set up an advance curve, it isn’t necessary. This task can be accomplished without removing the distributor from the engine.
Several kits are available for the HEI distributor. Most contain both weights and springs. We’ll focus on how you can create your own custom curve and all you need is a timing light, an accurate tachometer, a pen and paper, and perhaps a friend to help record the information.
This task is much easier with a dial-back timing light. The dial-back models allow you to dial the timing to indicate zero on the timing tab while the light displays the actual advance. Plus, the light that we use, an electronic Innova timing light, will also display the rpm which is very convenient.
If your timing light is the more traditional style, that can still work. For this process to work, it’s important to be able to clearly read timing marks on the balancer. If your balancer isn’t degreed, MSD sells a white, adhesive timing tape sized for several diameter balancers. If you’re in a hurry—you can make your own with a length of masking tape.
Here’s how to make the tape. We’ll use an 8-inch balancer for our example. First, we must determine the circumference of the balancer.
The formula for circumference is Pi x diameter so for an 8-inch balancer, this would be 3.1416 x 8= 25.13 inches. To get the equivalent of 10 degrees we would divide that 25.13 by 36 (360 degrees in a circle), which would be 0.698— call that 0.700-inch. Every two degrees would be 25.13/180 = 0.1396—we’ll round that to 0.140 inches.
Next, lay a 12-inch long length of tape on a piece of clean, flat metal stock. Make a heavy TDC mark that will line up with the balancer zero line and then use a dial caliper to set up 10-, 20-, and 30-degree timing marks. Then use the caliper to make 2-degree tics in between those larger 10-degree increments and you have just constructed a homemade timing tape.
Next, hook up your timing light and tachometer and create an rpm curve like the one we’ve included on a sheet of paper. You can change the rpm increments if you desire. Disconnect the vacuum advance line to the distributor and plug the hose from the carburetor.
Start the engine and make sure the trans is in Park (or neutral with a manual trans) with the parking brake on. Remove the air cleaner so you can operate the throttle linkage while watching the timing light. You will rev the engine to one of the rpm numbers while you watch for the amount of timing indicated on the tape.
Have a friend write the numbers on the chart as you rev the engine and call out the rpm and advance. What you are doing is using your engine as a distributor machine.
Keep in mind that the number you read at a given engine speed will be the combination of initial timing at idle and the mechanical advance. As an example, if the initial timing is 10 degrees Before Top Dead Center (BTDC) and the total timing at 2,000 rpm is 20 degrees, then you have 10 degrees of mechanical advance at 2,000 rpm.
We’ve plotted a typical mechanical advance curve in the accompanying chart.
Each engine will want a specific advance curve but for a typical street engine with 9.0:1 compression and pump gas, this will be pretty close and would certainly work to begin your effort. Don’t just automatically choose the two lightest springs in the kit. Our experience has shown that mixing a light spring with a heavier spring tends to work best, but ultimately you will have to experiment to come up with a suitable curve.
A common mistake is to use the lightest springs and bring the curve in very quickly. Unfortunately, light springs often struggle to pull the weights back into the base (zero advance) position. This allows the engine to idle higher than normal because it has more timing at idle. That’s one of many reasons why slightly heavier springs tend to work better.
If the engine wants more timing down low, consider adding more initial advance. This, of course, adds to the total timing at maximum advance so it’s a balancing act.
Another crucial point is that up until now, all these changes have only addressed the rate of advance, not the total.
The total mechanical advance is limited by the length of the slot that contains a pin. The travel of the pin in the slot determines the total mechanical advance. Changing the amount of advance requires modifying the slot.
For factory distributors like your HEI, if more mechanical advance is required, the slot will need to be lengthened by grinding the metal with a die grinder. If less mechanical advance is demanded, the slot needs to be shortened with a weld or possibly a heavy-duty epoxy like JB Weld.
MSD distributors operate similarly but use bushings of various diameters that determine the travel in the slot. This makes it easy to customize the total mechanical advance.
So far we haven’t talked about the vacuum advance. It’s a completely separate advance determined by engine load as dictated by engine vacuum. We’ve addressed that issue before in this column and you can read that here. These two systems must work together. Many tuners merely disconnect vacuum advance thinking it isn’t necessary, but that article will outline why it’s important as a load-based timing curve.
Don’t be afraid to try several sets of springs to come up with the mechanical curve your engine prefers. Pay attention to what the engine is telling you. Rely less on a pre-determined magic number and just give the engine what it wants. It will tell you when it’s happy.