Went to our local Bosch service because of a severe technical issue I have with my 1975 Longchamp and thought BTW about buying new spark plugs. Ever since the motor runs with the ACDELCO R45TS. Now I have read that the perfect plugs are the ACDELCOs CR43TS. Was I using all those years the wrong spark plugs ? What is the perfect combination for my 351 C 4V ? Someone in the forum recommended the NGK 4 V-Power plugs (but can`t find them here in Germany).
Thanks in advance.
Original Post
I have always used Autolite 25 and seen them recommended many times as the best plug for the 351C.

Walt, other than the obvious embarrassment of running AC Delco plugs in a Ford engine, follow the old "if it isn't broken, don't fix it" adage. The R45 is hotter than the R43 so if it's running well now, why change? Are your plugs indicating a problem? Is the engine pinging? The Autolite 25 may be a close match heat-wise but I don't know (George?).

I would trust Julian's recommendations here as he has a lot of experience with Ford engines and Panteras.

Originally posted by Mark Charlton:

... The Autolite 25 may be a close match heat-wise but I don't know (George?) ...

Try this ...


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Thanks a lot for the answers. Mark, I once in a while realized that one plug refused to work, so motor working on 7 cylinders was shaking a little bit. I replaced that one plug and everything was right. So, I just thought maybe the ACDelco plugs are not supposed to work in a 351 C. So, following your advices, I ordered the Autolite 25. Bosch, by the way, sold me the crossreferenced HR10BC (ACDelco R45 TS). Those plugs are much smaller in length, so I will return them today. They definetely will not fit onto the cable shaft-contact of the wires. Anyway thanks a lot for support and for the "plug list" George. Walt
there should difference using another brand but each manufacturer always claims best to use theirs. 25+ years ago we used to change to new plugs and filters and call it a tune-up. Nowadays plugs in new cars have a "80,000 miles life"

The correct science is to read the plugs.


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Consider the advanced science where the funny cars have multiple style plugs as the immense power and heat needs to be optimized.

Ideally use the original plug but then newer plugs have resistors to reduce radio static and now you can get a platinum electrode for a longer plug life. Engine builders will often recommend using a different heat range plug when you have a hotter cam.

Again assuming you start with a ARF42 time the motor and carb properly, then examine your plugs after a few hundred miles and decide if you need to change to a plug with a different heat range.


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I wouldn't use platinum plugs in a carb hi perf engine. Copper is more forgiving for incorrect heat range and varying rich/lean conditions. Platinum extends period between replacing them, but requires a computer controlled injection engine.
I grabbed some RV15YC4 Champions for my (sometimes painfully) stock 351c/'73 Pantera L (dual points/external res.)
I'm also replacing the coil while I'm in there, and checking for a good signal from that resistor.
They all came out of the box with a gap of .046, and is that the recommended gap for these plugs?
Frowner Sorry if I'm among the wrong DT models forum, but it's because I was drawn like a moth to George's spark plug chart!)


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Spark plugs are gapped based upon the "spark energy" of the ignition, not the type of plug. Use whatever gap Ford specified for a particular ignition. Folks with aftermarket ignitions should follow the manufacturers recommendation for plug gap. People who mix and match parts are left to experiment.

Spark Plug Gaps (Ford Ignitions)
  • Breaker points (1956 - 1974): 0.035"
  • Breakerless Ignition (1974 - 1976): 0.044"
  • Duraspark II (1977 - 1986): 0.050"
  • Duraspark I (1977 - 1979): 0.060"

I can't let you get away that quickly, gotta tell at least one story. The goal of an ignition is to have a nice "fat" blue spark, strong enough to ignite the mixture in the combustion chamber, but not too strong. Yes it is possible for a spark to be so strong that it blows the fuel air molecules away from itself, rather than igniting them. Big gaps generally create "fatter" sparks, and often make more horsepower. I know drag racers that run their gaps so wide that it cooks their ignition wires. I've heard the wires sold by Ford Racing are some of the best at enduring wide gaps and resisting being cooked.

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