Spark Plug Selection For Boosted Application

[7-UP BILL]

Don't want to get too wordy, but I have detonation when the pedal is hit hard. Normal driving street and hwy is uneventful. A friend at work told me I should go for a colder plug. I did go one colder when I built motor. GT-40 int. Edelbrock heads, 14mm x 3/4 plugs. 3624's were called for stock. Has an Isky cam on a 114LC and stock bottom end. 46K on bottom end. 42lb inj and have 50-55 lbs of FP under boost with FMU. I went with 3623's. Can anyone make a suggestion of a plug where I can start? Autolite doesn't make a 3622 or at least their website says no. Your usual help will be appreciated. Timing is 8* and boost retard is at -3.

 

[mikestang63]

Not sure why you need to go 3 steps colder than stock. How much boost are you running. What octane gas are you running. What is your AFR.

 

[7-UP BILL]

Blower is a V-1 "S" trim. Whatever the stock boost is, that's what it is. No intercooler. I use 93 octane supposedly no ethanol. I think I misprinted the plug type. I think they are 3923's. Can you suggest a plug that is a range colder? I can start from there. This motor was built back in '94, gets about 600-800 miles a year. Don't beat it to death. I would like to every once in a while hit it and hold it. The old plugs that were there since '94 were the 3923's and were amazingly clean. A friend who is knowledgeable said they were burning real nice (gray color). Is 50lbs of FP enough under boost? I almost get the impression it may be running out of fuel. Don't know what the Air/Fuel Ratio is?

 

[mikestang63]

i highly suggest before you start swapping parts out you get your car to a tuner who can tune it on a dyno and measure A/F ratio. Without knowing that , you are experimenting. There is no need to go to colder plugs with 5-6 lbs of boost. I assume you are using an FMU? What size fuel pump are you running.

 

[7-UP BILL]

Thanks, but $500 for a tuner with a daughter in college isn't in the cards now. I'd rather experiment with $2.00 plugs. Has an FMU and I have a 255 in-tank and a T-Rex external. Can you just suggest one range colder than the 23's?

 

[mikestang63]

22's. good luck

 

[Mustang5L5]

I'd also recommend the tuner as well.

Detonation can kill motors, and a tune is cheaper than a new engine

 

[a91what]

A wide band Guage and sensor can be had very cheap. As little as 150$ can tell you what your afr is.

Not sure you need colder plugs though, you have a very low timing curve, high octane fuel, and low boost.
Your max timing (assuming an A9L) is 23* above 5k.

Clean plugs on a boosted motor =bad you want a small soot ring and a ruddy colored porcelain.... detonation shows up as shiny dots on the porcelain, or looks like "peppered" black dots.