351W Spark Plugs

[Bryan Wilson]

Need help,
What would be the best spark plugs for a 351W high compression engine.
I have been running NGK AP5FS plugs that colour wise looked a little dark but ran ok.
I changed the plugs the other day and the parts guy recomended Autolite #33.
The problem is now the engine runs a litttle rough and the plugs are as black as soot.
Any recomendations?
Cheers,
Bryan

[Bill V]

Sounds like you need to run with a hotter plug.

[mzhun]

I'm running the Bosch Platinums with a similar set up just fine.

[Bill V]

After my last post, I had second thoughts and figured I'd verify what I said to be sure I wasn't blowing smoke. Here is an excerp from the following website which bears out what I said originally. I'll also include a link to the page.

Carbon Fouled

Soft, black, sooty, dry-looking deposits indicate a rich air fuel mixture, weak ignition or wrong heat range spark plug (too cold). These carbon-based deposits are conductive, much like oil fouling, and will allow the voltage coming out of the center electrode of the spark plug to track down the core nose rather than jumping the gap. This will result in an engine misfire and further aggravate the carbon fouled condition. Check for correct plug heat range. On fuel injected engines, check for sticking injectors, malfunctioning cold start valves and/or circuits. Also check for correct fuel pressure specifications. On computer controlled vehicles, the "limp home" computer mode will always result in a rich condition. Therefore, it is imperative that you check the operation and condition of the on-board computer system. On carbureted vehicles, check choke and choke pulloff, high float level, and needle and seat condition. On all engines, severe vacuum leaks can decrease manifold vacuum, resulting in a rich condition. Weak and/or damaged secondary ignition systems will fail to spark across the gap lowering combustion chamber temperatures and promoting carbon deposits. This condition could also result from continuous low speed driving or poor cylinder compression.

The rest on reading plugs can be found at http://www.nightrider.com/biketech/s....htm#discussed

Cheers

[Bryan Wilson]

Thanks for the info.
Now that we have decided that I need a hotter plug what would you suggest as I have used NGK AP5FS and Autolite #33 with these being to cold.
Thanks,
Bryan

[Bill V]

That's a tough question, but I did some research. Using a 1970 Mustang, Torino etc as an example, the 351W compression ratio was 11:1 and was equipped with Autolite AF-42 with the gap set to .035". The Motorcraft AF42C is the equivalent with the gap set to .035" or .889 mm (.9 mm). These translate to an NGK UR4 or UR4GP (Platinum).

Anyway, this is where I got my info it may or may not apply to you depending on your particular config but I think there is enough info in these links to get you where you need to go. Best of luck.

http://www.thecarsource.com/fords/en...351engine.html

http://www.motorcraft.com/default.asp?T=3&PageID=0

[What'saCobra?]

If you have a hot 351W with alloy heads and want max hp, try NGK R5671A-9, V-Power, non-resistor racing plugs. They are one of the favorites of dragster fans of FORD SB's.

The V-Power means they have a split V in the tang end, like a snakes tongue.

You should learn to start the engine without flooding it if you have no choke (I'm using Holley's new HP series carb, mechanical secondaries and double pumper) with these rather cold plugs. Not so easy when cold, but you can catch it.

Mechanical secondaries are just great because the new HP series carb has a high Q sensing curcuit and meters correctly even when the total air velocity is very low, such as at WOT and low RPM. Pulls like a train with very fast response to foot.

These plugs will foul if you jerk around and insist on big accellerator pump movement when the engine is hot and needs none. Or you fail to learn how to start a cold or hot no-choke carb.

At the first sign of too much gas (smell it?) leave the throttle wide open and wait for it to lean out, clear and start up.

Catch it without too much but just enough throttle movement.

These plugs must be indexed if you use the 1.7 rocker arm ratio (suggested, as it really wakes the engine up), rather than the standard crate 1.6.

Indexing consists of marking the top outside ceramic (you know, where the plug wird slips over) with an indelible dot where the tang attaches to the threat shoulder and tightening the plug such that the dot ends facing up (towards your eyesight), away from the piston top.

Test for clearance, however, if you have anything but a standard crate cam in your 351 and you decide to use the 1.7 ratio. It is the worlds cheapest HP increase, just a set of roller rockers.