Upgrading to TFI Ignition on the AMC V8


One of the more common upgrades on the AMC I6 and V8 engines is to swap out some of the components of the factory supplied ignition for parts that will give you a hotter spark. In the 80's, AMC used Motorcraft (Ford) ignition parts on its V8 engines. This basic ignition system is generally reliable and it gets the job done. However, even as AMC was using this system in the 80's, Ford had come up with a better system. Starting in the early 80's, Ford started switching over to TFI (Thick Film Ignition). The coil is designed to produce a much higher voltage burst for each spark. This hotter spark will help the combustion process. There is no small amount of debate about how much it really helps. Many of the people who install this system report significant improvements in throttle response and even noticeable gains in horsepower. It is suggested that simply swapping out the plugs and wires would have made almost as much difference. Regardless, the upgrade is simple, quick and not expensive.

The best part about this upgrade is that you don't have to replace a lot of expensive parts with complicated installation instructions. Installing an aftermarket ignition system can be a lot of work. The TFI upgrade replaces a few easy to access parts on the distributor, the coil, spark plugs and wires. Since the factory AMC ignition was a Ford unit built for AMC, all of the Ford parts swap in with no problems.

The biggest issue with installing a higher power coil to the factory ignition system is that the cap is too small. As the rotor turns underneath the cap, it passes by the posts that the wires attach to. When the coil fires, the spark jumps to the post the rotor is closest to before going on to the spark plug. When you bump up the voltage with a hotter coil, the spark tends to jump to the neighboring posts causing more than one spark plug to fire. The solution that Ford came up with is to make an adapter that attaches to the distributor. The cap is noticeably bigger and the posts are farther apart. This prevents the spark from jumping to multiple posts. It's really that simple.

When you go to the parts store, they will inevitably ask you what vehicle the parts are for. When you tell them that you're adapting one thing to another they may not understand what you want to do. I find it's best to go in knowing a vehicle you can tell them. The parts are not for that vehicle but it's just easier than trying to explain what you're doing. For my purposes, I tell them I'm getting parts for a 1984 Ford F350 with the 460ci (7.5l) gas engine. I got my parts from Autozone but any parts store should have what you need. And this is what you need:

Before you disconnect anything, make sure you know to get the wires back on in the right order. If you pull the wires and don't have any idea how to put them back on right, you're stuck. For reference, the firing order of the AMC V8 is 1-8-4-3-6-5-7-2. The cylinders are numbered like a Chevrolet engine. The #1 cylinder is the front cylinder on the right hand side as you face the front of the vehicle with #3, #5 and #7 in a line towards the firewall. The #2 cylinder is in front on the left hand side with #4, #6 and #8 as you go back. Note where the #1 wire plugs in. That's where you start counting when you're putting the wires back on.

Once you have all the parts, it's time to start disassembly. You can pull the spark plug wires and the wire that goes to the coil. After that, you can remove the plastic cap from the top of the distributor. It's held on by a couple of clips. Once you take off the cap, you can pull off the rotor. As long as you don't pull out the distributor, you don't have to worry about adjusting anything. The rotor only goes in one way. Don't remove plugs yet. If you pull all the plugs things can get into the cylinders and cause all sorts of trouble. I wait to pull them until I'm ready to put in the replacement plug.

The cap adapter, rotor and cap all are keyed so they only go on one way. Start with the cap adapter. You may need to rotate it to get it to sit flat. Once you get it in place, the clips on the distributor should snap in place to hold it on. Next, put the rotor on the shaft and make sure it doesn't spin. It will go on facing exactly the same direction the old one did before you took it off. Finally, put the cap on and clip it in place.

tfi_bracket_1 tfi_bracket_2 tfi_bracket_3 tfi_coil Figure out where you want to put the coil. If you put your coil on the firewall or on one of the fenders, it may affect how you use the plug wires from your kit. I decided to put my coil in the factory location. I built a very simple bracket to hold the coil. The coil comes with holes in the case that are roughly the same distance apart as the attachment points for the canister bracket in the base bracket. I went to the hardware store and found some 2-1/2" L-brackets. On the end that attached to the factory base bracket, I cut them short and opened up the hole from 1/4" to 3/8" to let the factory screws fit. When I put the coil up against the brackets I noticed that the offset holes in the bracket were causing one of the brackets to rub up against the coil housing. So I broke out the grinder and shaved a bit off. It's still there to support the coil but it doesn't contact the plastic housing. Once I got that to fit I took some 1-1/4" #10 machine screws with some lock washers and attached the brackets to the coil. I tightened everything down and bolted the base bracket to the engine.

tficoil4 Wiring the coil is pretty simple. The factory put a large plastic clip that slips onto the top of the coil. I cut the wires going to this clip and crimped on some weather tight female spade connectors. They attach directly to the coil. The green wire attaches to the coil negative ("-") and the yellow goes to the coil positive ("+"). The factory uses what is called a "resistance wire" to go between the ignition box and the coil. There is quite a bit of discussion about whether or not this is necessary when using the TFI coil. The purpose of the wire is to drop the voltage going to the coil and reduce the load on the ignition controller. If you use the stock coil and no resistor wire, you'll run too much current through the controller and burn it out. Some people take the resistance wire out of the loop and have no trouble. Others burn out controllers. I figure the extra 1 or 2 horsepower I would get without the resistor wire isn't worth the hassle of dealing with a dead ignition controller so I just left it in place. Also, some people remove (or don't bother to replace the broken) capacitor that comes with the factory coil. You'll notice that I don't have it installed in my pictures. I get no ignition noise in my aftermarket radio so I figure it doesn't make any real difference.

tfi_cap When you get your spark plug wires, they may come in assorted lengths. Sort them by length, longest to shortest. If you make a mistake and cut the wire too short, you can always use it on a closer plug. If your kit comes with just one long coil of wire and you have to crimp both ends on each one, you still should start with the longest and go to shortest for the same reason. Start with the #8 cylinder, which is the farthest one from the distributor. Run the wire from the distributor to the spark plug. Leave some room for it to move around and make sure it doesn't interfere with anything. Cut it to the length you want and crimp the end on the wire. Go on to the #6, #4 and #2 wires. Then do the #7, #5, #3 and #1 wires. Finally, install the wire from the coil to the distributor. If you want, you can dress up the wires a little. There are plenty of wire organizers out there. Or you can use a couple of zip ties like I did. Before I plug them in for the final time, I put some dielectric grease on the connector to make sure I have a good solid electrical connection.

Once the coil and wires are in place, it's time to swap out the plugs. Again, they need to be properly gapped. Bouncing around in the box during shipping can cause them to bend just a little. Get a decent gapping tool and set the gap for each one. As is the case with many vehicles, getting to some of the spark plugs can be a bit challenging. I use a spark plug socket that has the insert which holds on to the plug and doesn't let it slide out easily. I bought a socket from Craftsman that is formed in the shape of a large 6 point bolt on the top so I can turn it with a box end wrench when my socket wrench just doesn't fit. In the picture of the installed coil above you can see the heater box right next to the engine. That box leaves little clearance for the #6 and #8 plugs. The brake booster on the other side makes it a challenge to get to the #5 and #7 plugs as well. Having that socket I can turn with a wrench makes getting those in and out a lot easier.

They say that spark plugs can tell the history of an engine. You can look at the condition of the spark plugs and see if you're running lean or rich, if you're getting coolant or oil into the cylinder, or if there are some other problems. I'm not going to get into that much detail. But if you're interested, it's not hard to find the information. Like I said earlier, I recommend pulling one plug at a time and replacing it with the new one. Once you have gapped the new plug, put a little anti-sieze compound on the threads. Take out the old plug. Put the new plug in and start it by hand. Make sure it's turning smoothly before you start hitting it with a wrench. If you cross the threads, you have just turned an easy job into a very difficult one. If you can, use a torque wrench when you tighten the plugs. They require 25-30 ft-lb (roughly 35-40 newton-meters). If you can't use a torque wrench, get them "good and snug" but not "super tight". If you push too hard you will strip out the threads and again turn an easy job into a difficult one. When I did mine I was able to get the torque wrench in most of them. The #6 and #8 were tricky but doing it right is worth the extra effort. Again, before I put the plug wires on, I added some dielectric grease to the contacts.

tfi At this point, you should be done. Since you didn't disconnect the distributor you don't absolutely need to set the timing. The engine should start and run at least as well as it did before, if not a little bit better. If you have the timing light available, it's never a bad thing to verify that the timing is set properly.

Of course, once you're done with everything, take a minute to go over everything again to make sure you didn't forget to tighten a bolt or leave a plug wire loose. Take the vehicle for a spin around the block and make sure everything works like it should.

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