|HEI Tuning Secrets|
Written by: Damon Nickles & Mike Ervin
The HEI is a great distributor for a street or street/strip car. Its simple, easy to tune, and plenty powerful to light off any naturally aspirated engine up to 7000 RPM if properly equipped.
Let's start with the coil. Its located on top of the distributor between the plug wire towers under a plastic cover. It's the "engine" that makes the sparks. Stock its capable of about 35,000 volts and so-so total spark energy. Its fine for a naturally aspirated street motor that rarely sees the high side of 5000 RPM. It will provide the energy to jump a plug gap of .040-.045 with no problem in these applications.
You can upgrade the coil with an Accel or MSD replacement coil that will jump the voltage up to about 42,000 volts and total spark energy will also jump about 10-15%. There are even hotter coils than this from Accel, MSD and others that will give you the same 42,000 volts but a LOT more total spark energy (like 50% more). Either one is a worthwhile upgrade for a hot street motor- you'll get better response and HP especially at higher RPM where the stock one hits a kind of "wall". A new coil is also a lot cheaper than a complete MSD ignition amplifier setup and at least 95% as effective in most naturally aspirated applications. With either hotter coil you can open the gap up to .045-.050 for just a smidge more HP.
For blown or nitroused applications I always recommend
an MSD ignition amplifier box setup. You can do it with a stock HEI + hi-po coil but
you'll likely have to close the gap way down (.030-.035) to make it work well.
THE ADVANCE SETUP: Centrifugal and vacuum advance.
Centrifugal advance assy. on the HEI is pretty darned good. The stock weights and advance plate are perfectly acceptable for all but the highest HP/RPM engines. ALL stock HEIs that were installed in V8s are designed to have a total centrifugal advance of 20 degrees, +-1 degree due to production line tolerances. This is as measured at the crankshaft (10 at the cam).
It is susceptible to old age, though. Typically
the centrifugal advance weights wear their pivot holes into an "oval" or eat a
trench into their pivot pins OR BOTH. This is bad and no attempt to change the
advance curve should be made on a distributor that suffers from these problems- fix it
first or get another HEI to start improvements on. Also, the centrifugal advance
plate (that pivots around the main distributor shaft as the centrifugal advance moves it)
up at the top of the distributor sometimes gets gummed up and sorta "sticky"-
slowing the advance curve and generally preventing it from working correctly. If
your centrifugal advance doesn't "snap" back to zero when you twist it with your
hand and let it go then you have this problem. You need to pull the distributor
shaft apart and clean everything out, especially up top, before you proceed with upgrades.
TUNING FOR PERFORMANCE: Changing the advance curve.
Centrifugal advance: Stock the advance mechanism is pretty good but the stock springs are usually way too strong, causing the advance curve to come in too slowly, if it ever gets fully advanced at all. All you need are the right springs and the right initial advance setting. Most Small Block Chevys like about 32-38 degrees total advance at WOT. Since we know already that the HEI has 20 built into the stock mechanism the first thing we need to do is set the initial advance correctly- that means you need an initial advance between 12 and 18 degrees (you might want to retard it 2-4 degrees for daily street use just to build in a little safety margin).
Now all we need to do is make sure the centrifugal advance comes in at the right RATE relative to engine RPM. You want it "all in" by about 2800-3200 RPM for a street motor. You do this by changing the centrifugal advance springs to lighter ones. If you use the Crane advance kit like I do you are looking to install one Blue (heavy) spring and one Silver one (medium). These springs are located directly under the rotor and are easy to remove/replace by hand or with needle nose pliers. These springs will give you an advance curve that starts at about 800 RPM and ends at 2800. Easy so far, right? If you don't have the Crane kit then install whatever springs you have and check the advance curve with a dial back timing light and a tachometer, swapping springs until you get it close to these specs. It doesn't matter if the springs are not "matched" side to side- you can install one heavy one and one light one.
Vacuum advance: Stock cans typically provide 22-24 degrees of advance. This is WAY WAY WAY too much once you have recurved the centrifugal and initial advance the way I describe above. You will get "3 rocks in a coffee can" kinda detonation. Can you just leave it unplugged? Yes. Your highway mileage will be off by about 5 MPG and your plugs will load up with crud within just a few thousand miles. For a race car or a weekend street/strip car this is probably no big deal. For a street car, forget it.
Vacuum advance for the street: You want
about 12 (crankshaft) degrees total vacuum advance if you run WITHOUT a functional EGR
system, 16 degrees if you run WITH a functional EGR system. Regardless, you want it
to come in between about 5 and 15 inches of manifold vacuum. I have found the most
expedient solution to be the Crane advance kit once more. Install their can with
about 9 turns on the adjustable advance can spring. IMPORTANT!- Also, use the little
"lockout" cam that comes with the Crane vacuum advance to lock out AS MUCH
ADVANCE AS POSSIBLE. This will still leave you with about 12 degrees of available vacuum
advance. If you set it with 2 notches LESS lockout than maximum you will end up with about
16 degrees available vacuum advance
That's about it for distributor setup. You're ready to rock and roll. Now all you have to do is install it correctly and plug everything in correctly. Here's a few tips: Number 1 plug wire should be at the front of the distributor just to the driver's side of centerline. If that's not your #1 plug wire then you have the distributor installed off by a few teeth. It won't hurt performance if timing is still set correctly but plug wire routing gets a bit messy.
The vacuum advance canister should be plugged into a "manifold" vacuum source on the carb or intake manifold. This is a vacuum port on the carb that provides full manifold vacuum at curb idle. This is best for cruising/part throttle and will give the best performance overall.
WATCH YOUR IDLE RPM WHILE YOU SET INITIAL ADVANCE TIMING!!! You note that the centrifugal advance curve that I recommended above starts at about 800 RPM. If you try to set your initial timing with the engine idling above this RPM point you will NEVER get a true initial advance reading. Set it with the idle temporarily slowed WAY down if you have to but DO IT RIGHT!
A final word about that "module:" Lotsa
mystery around this little "thingamajig." This is just the tiny little
electronic brain located on the floor of the distributor housing with 4 wires going into
it (2 per side). All it does is read the magnetic pickup signal from the
rotor/stator assy around the distributor shaft and then tell the coil when to fire and
with how much "dwell". I always recommend a good stock GM module, not a
parts store cheapie. No reason to go crazy and get that MSD $70 super high output module
here- I have tried it and it does nothing for performance. But get a good GM
one. Stock modules incorporate what they call a "variable dwell" circuit
that reduces dwell at lower RPM to keep the coil from over saturating. This is good
for sharp performance and long coil life. Some parts store modules don't have this
circuitry in them.
Once more the setup for 35 degrees of mechanical advance:
14 initial + 21 centrifugal + 12 Crane adjustable vacuum advance without EGR = 47 total.
14 initial + 21 centrifugal + 16 Crane adjustable vacuum advance with EGR = 51 total.
14 initial + 21 centrifugal + 22-24 stock vacuum advance = 57-59 total. Too much!
With the stock vacuum advance is when you start getting the "3 rocks in a coffee can" kinda detonation. That's why I say its important to get a vacuum advance can that is adjustable for both RATE AND TOTAL ADVANCE once you get the centrifugal side really dialed in for maximum performance. Some vacuum adv. cans for really hot 60s muscle car motors had as little as 8 degrees total advance in them, BTW!
Also, Its REAL important to make sure your centrifugal advance is not worn out because when the weights wear themselves "oval" and the pivot pins wear out you end up getting MORE total centrifugal advance. Like up to 26 degrees if its really shot. In that case you end up with a really over advanced timing (66 degrees if plugged into our previous example- OUCH!)
I should also point out that the best way to REALLY check your total timing is to buy an $8 "timing tape" that basically wraps around your harmonic balancer and puts 60 degrees worth of degree marks ON THE BALANCER where you can easily see them. You won't need a dial back timing light to use it (they're expensive for a really good one) and you won't have to start "guessing" once the timing mark on the balancer starts "crawling up behind the water pump", off the top of the range of stock timing marks. Really useful little piece of equipment. Make sure you get the right one for your balancer- 6", 7", 8", whatever your particular small block happened to come with.
HEI Tips Addendum by Mike Ervin
After reading the above article, I
decided to try this. After getting the kit, and installing it, I had to experiment
to get it right. I couldn't figure out which way to turn the adjustment screw, the
one that Damon said to turn 9 turns. Also the lockout plate that comes in the kit,
the instructions are pretty vague on how to install it. So after messing with it for
about an hour, I finally figured it out. The instructions below will explain it
When installing the Crane adjustable can turn the adjustment all the way clockwise and then back it out about 9 turns counterclockwise. When you install the advance lockout, the instructions are not real clear on the orientation. The best way I can describe it is, it goes under the pickup coil arm (for a better word), the part of the pickup coil with the hole in it that the vacuum can rod goes in, which means it is laying on the can mounting plate. Once you have the lockout in place, the hole in the lockout, which is offset to one side, will be toward the back of the distributor and the notches will be towards the inside. I know the instructions say mount it with notches facing the outside, this would be the same as what I have stated, but with no advance locked out. If you have the maximum amount locked out, the straight side of the lockout will be parallel with the mounting plate of the vacuum can. After the lockout is in place it should be in the same position as the picture above. As you can tell from the photo, the lockout has 9 notches. The maximum is at the bottom left hand corner, and the minimum is at the top right hand corner. With my setup, I have it installed at the seventh notch (arrow) in photo. This equals 16 degrees of total vacuum advance. If you look at the photo above you can clearly see the difference from min. to max. advance.
If you need to lower the amount of advance you have locked out, loosen the screw that holds it in place and spin it towards the outside of the distributor, or to the left. You will need to pull back on the vacuum advance canister rod as you do this. As Damon said, about 2 notches less than maximum is what you need for 16 degrees of available advance. I hope this is clear. One more thing, as the instruction sheet says, for every 1 notch (2 degrees) you move the lockout, you increase your initial timing by the same amount. Why? Well, when you add the advance lockout and limit the amount of vacuum advance you are rotating the pickup coil 2 degrees counterclockwise for every notch (2 degrees) you move the lockout, which increases the timing. You will find that by the time you are through, your distributor will be turned clockwise (around toward the passenger side) a fair amount from where it was at initially. Don't panic, this is normal.
To better understand what the Crane Adjustable Vacuum Advance does, I will try to describe it. The vacuum advance canister rod, the part that actually does the pulling, is attached to the pickup coil in the distributor. When vacuum is applied to the canister it pulls the rod, and this pulls on the pickup coil. The pickup coil rotates counterclockwise and in effect advances the timing by the amount of rotation.
What the Crane unit does is adjust the amount of vacuum and limit the total amount. When you install the lockout, it limits the effective total amount of vacuum advance. How? It limits how much you have to start with. You are shortening the distance that the vacuum rod has to pull on the pickup coil, shorten the total distance the rod can pull and you decrease the total advance. This is why you spin the lockout clockwise to raise the total vacuum advance, you are increasing the amount the rod can pull.
The adjustment you make to the spring in the hose nipple on the can adjusts when the vacuum starts. The farther out (counterclockwise) you turn it the later it starts vacuum advance. If it is adjusted all the way in (clockwise) it is actually about the same as a regular non-adjustable can. I don't really know how much each turn limits though.
I just thought I would add this to what
Damon has already said. He knows more about this than I do, but I wanted to explain
what I found out so you wouldn't have to. I hope this clears it up for you.