How to Gap the Spark Plugs in Your S197 Mustang
Posted 7.9.2008 in Tech
When you take the step towards supercharging your 4.6L V8, you’ll find that some minor adjustments to the engine’s ignition system may be necessary. Most aftermarket supercharger systems recommend swapping out the factory spark plugs, to a colder plug. This is done to help prevent any detonation, which takes away ignition timing and results in a loss of horsepower. Needless to say, any detonation is bad, since it can eventually wear away at your pistons, rings and valves, costing you much more than a new set of plugs.
Most avid Mustang owners, and professional engine builders of supercharged Mustang engines, recommend a variety of spark plugs that are one step colder for street applications. Supercharger manufacturers such as Kenne Bell, Roush, Whipple and others supply or offer a colder spark plug with their supercharger kits. “Most any supercharged 4.6L application should use a colder plug,” says Ken Christley of Kenne Bell Superchargers. “These engines have very sensitive knock sensors that can detect the smallest amount of detonation. The result is a drop in horsepower because the computer takes away lots of ignition timing to prevent detonation from happening again.”
Many Mustang owners and engine tuners have experimented with several spark plugs, including NGK and a new Brisk spark plug. “The Brisk spark plugs worked great in our 800rwhp Mustang GT 4.6L 3V,” says supercharger expert Jim Bell of Kenne Bell. “With the Brisk plugs the engine fired flawlessly all the way to 28 psi boost.”
Although there are many spark plugs for supercharged and turbocharged applications, the most popular is the Autolite Revolution HTO spark plug. These plugs are relatively inexpensive and cost around $90 -$100 for a set of eight, compared to $136.00 for a set of the Brisk plugs. The Autolite plugs feature a 10mm high-thread design to seat better in the cylinder head, and are available at most auto stores.
Out of the box, the Autolite HTO comes with a .045 gap. In many instances however, owners of supercharged Mustangs often claim that installing these spark plugs causes some misfires, poor idle and they resort back to using the hotter factory plugs in their supercharged, turbocharged or nitrous equipped street engine.
The problem, according to engine experts, is that when these plugs are used in supercharged 4.6-L 3V applications, they should be gapped to .032 or .035. JR Granatelli of Granatelli Motorspors, builds supercharged and turbocharged 4.6L engines for late model S197 Mustangs. He says that the Autolite HTO plugs should work fine, as they are an emissions legal plug. “If some Mustang owners are experiencing fouling at low rpm ranges, then there is probably too much gap or not enough coil energy,” says Granatelli.
Despite the fact that spark plugs like the Autolite HTO have a solid electrode that doesn’t look like it can be gapped, there are tools and methods to do this properly. The easiest is to use one of several gapping tools specifically made for this purpose. We found one from Modular Mustang Racing and another from Steeda. Both work like a vice, which threads the spark plug into the tool and uses a small screw to gradually push in the electrode to the desired gap. These retail anywhere from $25-$35 and are excellent ways to do the job correctly.
Another method that we found to be just as good, but may be a little scary to some, was using a simple 3/32 punch, a 9/16 deep socket and a hammer. Although the gapping tools are much cooler, lightly tapping the electrode with the punch and hammer will get the job done as well. The trick is to insert the spark plug into the socket so that it can provide a solid platform without harming the plug. Then gently tap the punch onto the electrode with a hammer; just enough so that you don’t go past your desired gap. Once you get the first one done, you’ll know how much force to use, where a couple of taps will reach your desired gap.
It is also a good idea to purchase a 9/16 spark plug socket. These sockets are not readily available at your local hardware store, but you can order them from Sears online under the SK brand. Aside from the socket a narrow feeler gauge also works to ensure that you have set your plug’s gap correctly.
Once all of your HTO spark plugs have been properly gapped, use a dab of anti-seize on the spark plug threads. Many engine builders recommend using anti-seize on all spark plugs that are being inserted into aluminum cylinder heads. This prevents any galling or stripping of the threads on the cylinder head.
Most Mustang owners who have properly gapped their spark plugs notice a huge difference in the idle quality, and can fully enjoy the power produced by their supercharger.
Sources:
Granatelli Motorsports
1000 Yarnell Place
Oxnard, CA 93033
804-466-6644
www.granatellimotorsports.com
Kenne Bell Superchargers
10743 Bell Court
Rancho Cucamonga, CA 91730
909-941-6646
www.kennebell.net
The Autolite Revolution HTO is commonly used in supercharged, turbocharged and nitrous applications as a colder spark plug to prevent detonation.
Gap tools like this one from Modular Mustang Racing can be used to properly gap these plugs to reduce misfiring.
Shown here is a gap tool from Steeda which also works well to gap the Autolite plugs.
You can also use a 9/16 socket, 3/32 punch, hammer and feeler gauge to accomplish the same task.
By sticking the spark plug into the deep socket, you can use the punch and hammer to lightly tap the electrode into position.
This is what the Autolite HTO spark plug looks like when it is properly gapped. Notice the slight dimple in the middle of the electrode.
If you’ve never changed your 4.6L’s spark plugs before, simply unplug the harness connection to the coil.
Remove the bolt that secures the coil to the valve cover.
Pull up on the coil and remove it to expose the spark plug underneath.
Use a 9/16 spark plug socket to remove the spark plug. It may be difficult at first so you may also need to use a little penetrating lubricant to help you remove them. Reverse the installation to install the new plugs.
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