Wrong spark plugs helping performance?

[QuantumMotorsports]

Okay, so I've had a Non HO 5.0 in my cougar since it came from the factory. I recently put on HO heads (as well as other HO parts) and used the same spark plugs. I didn't even think that they might be different. Recently I went to get new plugs and got a set for a 91 Mustang since that is the year of car the heads came off of (I realize that they are an 87 casting and are the same, but anyway) the plugs for the mustang had about a half inch less threads. So basically the plugs I've been using stick a half inch farther into the cylinder. I was worried about clearance probs with the pistons or valves, but since I've been running them for a while, and the plugs look fine, I don't think there are any probs. Anyway, I went ahead and bought the Mustang plugs, put them in, ran the car (without changing anything else) and if felt like it had lost a step, throttle response wasn't as fast and it felt like especially on the low end it was not as strong. I went and bought a set of cougar plugs (longer) of the same brand, same temperature rating and all and put them in, it runs like it did before, much better than the stang plugs. I'm curious as to whether anyone else has done this. My theory is that when the spark is farther out in the chamber it spreads easier when the fuel ignites. If someone wants to experiment on a dyno. You might try running the cougar plugs. If you want I'll get a part # for you.

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Michael Black
Quantum Motorsports
Norman, Oklahoma

1988 Merc Cougar 5.0 HO, P&Ped heads, 2.25" custom mandrel bent dual pipes, T5 five speed tranny
15.43 @ 91.08mph (not shabby for a 3600 pound car)
60' 2.453 Street tires suck!!!

[Jeff Chambers]

What were the two spark plugs? There are full threaded and half-threaded (like the Autolite 10x and 76x series, respectively). Both protrude into the cylinder the same amount since the tapered seat is the same distance from the electrode. Other than the thread, there is absolutely no difference between the plugs. They do say that the half threaded plugs are a tad bit colder since they have a better conduction path from the electrode to the seat without the threads to disrupt the conduction.

Could this be true for the plugs you tried? Look at the electrod to seat distance for both.


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Jeff Chambers
Trophy Stock #3
11.97 Seconds / 114.5 MPH
Chambers Racing Team

[QuantumMotorsports]

The cougar plug was about 3/4 of an inch from electrode to seat, and if I'm not mistaken the mustang plugs are about 3/8 ? At least, that's the plugs they gave me for the mustang. Maybe their computer showed the wrong plugs for the mustang (not a first).

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Michael Black
Quantum Motorsports
Norman, Oklahoma

1988 Merc Cougar 5.0 HO, P&Ped heads, 2.25" custom mandrel bent dual pipes, T5 five speed tranny
15.43 @ 91.08mph (not shabby for a 3600 pound car)
60' 2.453 Street tires suck!!!

[Capri306]

You're experiencing what spark plug junkies have known for years: stick the spark farther into the chamber, get a bigger boom. Instead of the flame path going across the chamber like it's engineered to do, you're creating a fast-burn situation. This can hurt in some engines, but in most cases, like yours, it really helps. This is also why they make projected tip plugs and the like. I'm supposed to be running RV17YC's in my '87 notch, but I like to run RV15YC4's just because they're a little colder, designed for larger gaps, and they stick in a drab more. I'm not brave enough yet to try some RV12YC6's!

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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch <--- New Timeslips

[Moxie]

Yep, that's been a favorite trick of racers for years. One thing to remember: it works well for naturally aspirated engines, however, DO NOT do that with power adders. Very bad.

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Moxie Racing