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Mike - 74 Nova Baltimore, MD
1,658 Posts
Discussion Starter #1 (Edited)
Dialing-in the Timing Advance Curve of my GM Style HEI - Part 1
Last year, I wanting to “dial in” the advance curve of my aftermarket GM Style HEI that is used on my SBC (I have a Pertronix D1000).

What started me on this project was I could not get a steady initial timing reading at low RPM from my Pertronix distributor using a regular timing light. With the vacuum advance disconnected and plugged, the timing mark was jumping all over the place at idle. I could get a steady mechanical advance timing reading once the engine was revved higher than 1500 RPM, and I set my total mechanical advance to 36° @ 3500 rpm. After I set my mechanical timing as stated above, I reconnected my vacuum advance to a “full time” vacuum source and my timing was way too high at idle (over 40° at 850 RPM and still jumpy). I also noticed that at intermittent times, my engine seemed harder to start. Something was not right.

I spent a couple weeks reading hundreds of posts about this subject on several automotive forums and learned a lot of information. This post is a little lengthy with a lot of details, but I also provided several photos to help illustrate what I was trying to achieve.

So I began to look into different aftermarket advance curve kits. There seems to be a couple advance kits for the GM Style HEI that are of high quality, made of steel (either chrome moly, or melonized steel… it depends on who you talk to), and produced in the USA (as printed on each box). These kits are the MSD 8428 and the Moroso 72300. Both kits looked identical, but I decided to try the MSD kit in my HEI. I also ordered the MSD Vacuum Advance Limiter Plate – Part #84281.

MSD 8428 HEI Advance Curve Kit

NOTE: There seems to be some confusion about the bushings that come with these kits…. as most of the major online speed part venders described the bushings as “Advance Limit Bushings”. THESE ARE NOT ADVANCE LIMIT BUSHINGS. These bushings are installed into the holes within the advance weights. There are different inside diameters of these bushings that fit onto the various mechanical advance weight pins that are available on GM type HEI distributors (small pin, large pin). These bushings are color coded based on I.D. of bushing hole. If your advance weights were held originally in place with small metal clips, you will not need to reinstall them when using the bushings that are provided with these kits.

I received the MSD 8428 advance kit and compared it to the Pertronix weights and centerplate that were originally installed in my distributor. From what I can tell, the weights and the centerplate from both brands seemed exactly the same (size, shape, weight, material). But I did see something strange… As I reviewed the installation instructions provided with the MSD kit, I noticed that my Pertronix's original centerplate was installed upside down within the distributor, but my weights were installed correctly. I purchased the Pertronix distributor brand new (about 10 years ago) and this was how it arrived sealed in the manufacture’s box. I never messed with the weights or centerplate.

Below are photos of the correct and incorrect installation of the MSD HEI weights and centerplate.
NOTE: I know the module is missing from the photos shown below. This is a spare aftermarket HEI that I used for photography purposes.

Correct Install of MSD Advance Kit - Clockwise rotation of distributor – Chevy Engine (opposite install for counter-clockwise rotation – Pontiac Engine)

No Advance
When the advance kit is installed correctly, the weights and centerplate control the start/stop points of the advance curve (just as GM designed). The centerplate pins and the advance weight pins are also slightly further apart at the “No Advance” position vs when the centerplate is incorrectly installed. I will explain the significance to this below.

Incorrect Installation of Centerplate – Clockwise rotation

No Advance

When the advance kit is installed incorrectly, the centerplate pins hit the slot underneath the centerplate when at the “No Advance” position, and the weights do not fully contact the centerplate when the springs are installed (sloppy fit). The slots underneath the centerplate become the “start” position of the advance curve.
The centerplate pins and the advance weight pins are also closer together at the “No Advance” position vs when the kit is correctly installed. When these pins are closer together at the “No Advance” position, there is a good chance that when the advance springs are installed… they will be under little / to no tension. I also noticed that if I advanced my rotor slowly by hand, it would not retract completely… causing my intermittent “hard starting” issues due to additional advance in ignition timing. I believe this is what was causing the issues of erratic initial timing and intermittent hard starting, because they were all corrected by just flipping the centerplate to the correct install position (I also needed to readjust the timing).

Since I purchased the MSD advance kit, I decided to install it per the manufacturer’s instructions.
NOTE: Be very careful while removing the metal “C” clips that hold the centerplate (and the weights… if equipped) to the pins. Most advance kits do not include these clips. I used a strong magnet placed next to the clips while they were carefully removed. I also placed some paper towels around the distributor, so if the clips went “flying off” during removal, they would be caught in the towel instead of MIA.

Below is a photo of how I wrapped my distributor with paper towels while removing and installing the centerplate pin clips. Even with a magnet in place, these clips will sometimes go flying out when removed. It is also a good idea to cover the carburetor during this procedure.

After the correct install of the MSD Advance Curve Kit, my erratic timing at idle and sporadic hard starting issues seemed to be solved (I’m sure I also would have achieved the same results if I had just flipped the Pertronix centerplate over to the correct position).

Don’t get me wrong, I like my Pertronix HEI… but don’t assume that your new aftermarket “GM Style” HEI has the advance weights and/or centerplate installed correctly. I have also seen images of the “Silver Colored” centerplates that come with several of the aftermarket GM Style HEI’s installed upside down.

Now that my HEI’s centerplate and advance weights were installed correctly, I wanted to confirm how much total mechanical advance the MSD advance curve kit was providing. I started by disconnecting my vacuum advance and installing the two heaviest advance springs (MSD Kit) onto the HEI’s spring pins. I did this to ensure that I was not getting any mechanical advance coming in while setting my initial timing. I then set the initial mechanical timing to 16° @ 850 RPM by.

Next, I changed out the distributer’s heavy advance springs with the lightest tension pair so I could ensure that I was getting all mechanical advance by 3500rpm. This is where I noticed something funny. I was only getting about 16° of mechanical only advance using the MSD advance curve kit (not the advertised 24° of mechanical advance). With an initial mechanical only timing set at 16°, I was only getting about 32° of total mechanical advance (see image below). I changed back to the weights and centerplate that originally came with my Pertronix HEI (except this time the centerplate was installed with the correct orientation) and I noticed the same results as shown above…. only 16° of mechanical advance (16.25° to be exact).

Freeze Frame of Video Clip at Full Mechanical Advance

I was shooting for 36° of total mechanical advance and needed 20° of mechanical advance for my set-up. So with some research, I figured out how to modify the MSD’s (and Pertronix’s) centerplate to gain the additional 3.75° of mechanical advance.

For those that are interested, the next post will provide the details of how I modified the MSD 8428 centerplate to achieve my goal of 20° of mechanical advance at the crankshaft.

Mike - 74 Nova Baltimore, MD
1,658 Posts
Discussion Starter #2 (Edited)
Dialing-in the Timing Advance Curve of my GM Style HEI – Part 2

Below are the details of how I modified the MSD 8428 centerplate to achieve my goal of 20° of mechanical advance at the crankshaft. I approached this project a little differently. I wanted to see if I could apply some logic and math to try and achieve the desired results without several attempts of modifying the centerplate, followed by “in vehicle” testing. I am not saying that this is the correct way to do this, but it worked for me with the tools I had (or made). I was able to get the adjustment I wanted on the first try… Maybe I just got lucky!

I placed the distributor in a vise, and locked the distributor gear from moving. I installed the degree template as shown (image of a protractor copied from the internet onto a word doc, proportioned to the correct size, printed, and taped to a piece of cardboard), and taped a brake cleaner spray can tube to one of the rotor arms as a pointer. I also inserted a push pin into the end of the red tube and cut the head off for a more precise pointer.

I started by correctly installing the unmodified MSD centerplate and weights into the distributor (placed at the start position of the advance curve) and set the pointer to 90° and locked the template in place to keep it from moving.
From this point forward, it is extremely important that you do not move the pointer (as attached to the rotor arm) or the degree template once you begin to modify the centerplate.

As you can see in the photo below, I placed some masking tape onto both ends of the centerplate, so I could mark the contact points between the centerplate and the arms on the weights at the “start position” of the advance curve. These marks would be used as a guide when removing material from the centerplate to change the start position.

Since I do not own an upright belt sander or a bench grinder, I made my own version using a drill with a small grinding wheel… plus some flooring tiles to use as a work surface to modify the centerplate.

Once the correct work surface height was determined by stacking flooring tiles, the tiles were held in place on the workbench with a “C” clamp. A clamp light also helps brighten to work area.

Grind one side at a time and check movement on the distributor set-up. Since I needed 3.75° of additional advance at the crankshaft… that would equate to changing the start position of the distributor’s advance curve to about 1.9° (1° at the distributor = 2° at the crankshaft). Grind slowly, as very little material will need to be removed from the centerplate. Do not hold the centerplate in one position while grinding, as it is better to use an “arching” movement that follows the shape of the advance curve on the centerplate. After you remove a little of the centerplate’s material as shown, reinstall the centerplate… and just the one advance weight to contact the modified portion of the centerplate and return the weight to the “start” position and check your pointer movement on the degree template. Repeat the above steps until you get close to the desired results. As you get close to the final adjustment to the centerplate, finish removing the material with a metal file, and final smoothing of the adjusted portion should be done with a piece of emery cloth.
NOTE: You do not install the advance springs during the adjustment and checking process. You will be using your fingers to push the advance weight against the centerplate.

Final adjustment to one side of centerplate.

Once I finished adjusting one side of the centerplate, I attached some tan masking tape to the cover the compete surface of the centerplate and trimmed the masking tape to fit around the centerplate. I then removed the masking tape and turned the masking tape 180 degree and carefully attached the tape to the opposite side of the centerplate, so I was able to duplicate the adjustment to the other side of the centerplate. I used masking tape, but I think any method of “template transfer” would work.

As you can see in the above photo, the portion of the centerplate that is not covered is the amount that will need to be removed from the unmodified side for about a 2° change in the distributor’s advance curve starting position (which is equal to 4° additional advance at the crankshaft). Notice the minimal amount of material that needs to be removed from the centerplate

Once you are satisfied with the position of the masking tape, remove a portion of the tape so that you are able to R&R the centerplate from the distributor pins as you are checking the amount of movement on the distributor set-up. When you are modifying the second side to the centerplate, you will be checking your progress with both advance weights installed (but no springs installed yet). As you are finalizing the adjustment of the second side of the centerplate, use a file to remove enough material so that all play is removed between the weight arms and the centerplate with both weights installed and positioned at the start of the advance curve. When you are “very close” to removing all play between weight arms and centerplate with a file, finish the modified edge with emery cloth to achieve a smooth surface for final fitment with no play.

Below is a photo of the finished adjustment to both sides of the MSD centerplate, and checked on the distributor set-up. Also note that the centerplate and weights are still controlling the starting and stopping points of the mechanical advance. The centerplate pins still move freely within the slots underneath the centerplate.

IMPORTANT NOTE: While this technique worked for determining the amount of adjustment needed to change the starting point of the advance curve, it DOES NOT work to determine the amount of total mechanical advance. When I used this method and moved the weights to their full advance position, I noticed 12° of distributor mechanical advance prior to modifying the MSD centerplate. That should have equaled 24° of crankshaft mechanical advance. As stated above, I only got 16.25° when testing in the car… so there must be some physics involved with the movement of the spinning mechanical advance mechanism that is way beyond my comprehension level. My guess is that this is why the MSD 8428 advance kit is stated to achieve 24° of mechanical advance (as shown in the instructions enclosed with the MSD kit).

I also installed the MSD Vacuum Advance Limiter Plate, and used the “D” setting on the MSD plate to limit the Petronix Adjustable Vacuum Advance Canister to 11° of vacuum advance.
- I did not have to install the bushing that is used to raise the MSD Limiter Plate for the Pertronix HEI, and I used the Phillips head screw that came with the MSD Limiter Plate.

MSD 84281 HEI Vacuum Advance Limiter Plate

HEI Vacuum Advance Limiter Plate Installed

NOTE: You can also make your own Vacuum Advance Limiter Plate.
Just google “Images of Vacuum Advance Limiter Plate” for some ideas.

The results are detailed in the next post.

Mike - 74 Nova Baltimore, MD
1,658 Posts
Discussion Starter #3 (Edited)
Dialing-in the Timing Advance Curve of my GM Style HEI - The Results

Below is my timing curve with the MSD Advance Kit using the modified centerplate along with the Light (Silver) and Medium (Blue) tension springs that originally came with the Pertronix distributor. The vacuum advance canister was disconnected and the vacuum ports were plugged. The results were achieved using a regular timing light (no advance wheel), and a degreed balancer. TDC confirmed during engine build.
NOTE: I would have probably achieved the same results by modifying the Pertronix’s HEI centerplate.

After the MSD weights and modified centerplate was reinstalled, I readjusted my initial timing for 16° @ 850 RPM (no vacuum advance)… and my new total mechanical advance was 36°@ 3300-3500 RPM. I also did not experience any “idle hunting” while in drive (850 RPM), or in park (1000 RPM) since my mechanical timing did not start to advance until 1200-1300 RPM.

I chose to use the 36° total mechanical advance because I read that many people using the AFR 195 aluminum heads (with similar engine builds) liked this setting.

I videoed the timing curve of my engine after the MSD centerplate modification was completed. By viewing the “Freeze Frames” from the video, I am able to see the timing marks much easier and more accurately. The photos also verify the results achieved.

Initial Timing at 850 RPM (16 degrees)

Full Mechanical at 3500 RPM (36 degrees)

Just a quick note about the advance springs I used. As previously mentioned, I used the light and medium tension springs that originally came with the Pertronix HEI. I did not use any of the advance springs that came with the MSD 8428 advance kit. The wire used on the MSD copper springs (lightest tension) was so thin, that the circular loops at the ends of the springs started to open up just by installing these springs onto the centerplate and advance weight pins. My guess is that these copper color springs would begin to stretch and loose tension over time. I also did not use the MSD silver springs (medium tension) because they seemed to provide more tension then the medium weight springs that came with the Pertonix distributor. And the MSD strongest tension advance springs (blue) were ridiculously tight. I only used these springs to confirm my initial mechanical advance setting.

Below are the detailed results of my timing curve with the MSD 8428 weights and modified centerplate with the light weight + medium weight springs installed (with no vacuum advance).

SBC 400 Stroker (Dart SHP Block) 10 to 1 Compression
Comp Roller Cam 242 degree @ .050 (Intake) / 248 degree @ .050 (Exhaust)

Mechanical Advance Only
850 RPM (Idle) - 16 degree Initial
1000 RPM – 16 degrees
1500 RPM – 28 degrees
2000 RPM – 31 degrees
2500 RPM – 33 degrees
3000 RPM – 35 degrees
3500 RPM – 36 degrees
4000 RPM – 36 degrees
4500 RPM – 36 degrees
5000 RPM – 36 degrees

My mechanical advance is coming in quickly at the lower RPMs, which works pretty good for my set-up (411 gears with a 2004R Trans with a 2800 stall converter).

With the distributor's vacuum advance reconnected to a “Full Time” manifold vacuum port, my initial advance was now 27°… so my vacuum advance was adding 11° of advance with the MSD stop plate installed in the “D” position.

Initial Timing plus vacuum advance connected at 850 RPM (27 degrees)

My “All in” Total Advance (Initial + Mechanical + Vacuum @3500 rpm) was now 47°.

I also went from a jumpy 9 Hg - 11 Hg vacuum at idle to a steady 14 Hg vacuum at idle after the centerplate was modified and installed correctly. No engine pinging during WOT…or during light acceleration while cruising.

I learned a ton of information about the GM HEI while undertaking this project, and just wanted to share my experience.

Mike - 74 Nova Baltimore, MD
1,658 Posts
Discussion Starter #6 (Edited)
Thanks for the positive feedback on my original posts :thumbsup:

I just wanted to provide some additional information to cover some of the other aftermarket HEI distributors that are sold by different brands. A lot of these aftermarket HEI's come with silver colored metal centerplates and weights installed.

I have seen examples of the centerplates being mounted upside down on these HEI distributors. Below are examples of the correct and incorrect installations of the centerplates. The springs have been removed to provide an unobstructed view.

Correct Install of Advance Weights and Centerplate - Clockwise rotation of distributor – Chevy Engine (opposite install for counter-clockwise rotation – Pontiac Engine)

When these weights and centerplates are installed correctly, there is no play between the weights and the centerplate at the "no advance" position. These weights and centerplates generally provide about 22° of mechanical advance, so there is really no need to modify them.

Incorrect Install of Centerplate - Clockwise rotation of distributor (The advance weights are installed correctly)

Very sloppy fit of the weights and centerplate while in the "no advance" position.

Below is a Youtube video that represents a perfect example of what I am discussing. Notice what happens when the HEI centerplate is not installed correctly. The distributor with the incorrect installation of it's centerplate is a new MSD Streetfire. The original poster is comparing different HEI distributors, but the first half of the video shows what I am trying to explain.

The main message I was trying to relay is that you should not assume that advance weights and/or centerplate are installed correctly in your HEI distributor... even if you are the original purchaser of a brand new unit. The incorrect installation of these pieces will cause you all kinds of headaches and it is not easy to find unless you know what to look for.

Just a note if you find your centerplate is installed upside down. After you remove the centerplate and reinstall in the correct position, you may find that your initial timing has increased by about 20 degrees…. making your engine difficult to start.

For a clockwise rotating distributor, be sure to retard your timing by turning the distributor clockwise about ½ the distance between two of the cap’s spark plug wire terminals (Opposite steps for a counterclockwise rotating distributor). Start your engine and reset your initial timing.

Another option is to pull your distributor and reinstall it by moving the rotor position counterclockwise by 1 tooth on your distributor gear. This will position your distributor close to its original orientation once you reset the initial timing. I had to do this because of the tight space between my HEI distributor cap and the firewall.
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