TWO YOOTS

I have a 600 hp carbed 502 with alum afr heads just tested the a/f ratio which is 13.7:1 and my comp ratio is only about 9.8:1.After a short time #5 and#6 plugs are fouled and 7&8 are starting also to turn black. I am using NGK v power R5671A-9 plugs with a .035 gap and Crane Fireball ignition box these plugs are the deep thread design and are wondering what other plugs and gap I may want to try. Thanks.

jeffswav

Is that 13.7 at idle or throughout the rpm?

FIXX

iTrader: (2)

open the plug gap to .045 and see how it does. dont know about the crane ignition but msd reads to open the plug gap +.010..

ThisIsLivin

I would check the a/f meter, check for arcing in the cap. If you really are at 13.7 and plugs are fouling, you must have a significant ignition problem. I've run my plugs at .035" with an MSD 6M with a Blaster 2 coil and I have idled for 4 hours taking long river cruises with no fouling and I am running NGK 5674-8's. I'm running 9:1 compression with a pretty big cam.

Young Performance

The 9's you are running are WAY to cold. You should be running 7's (5671A-7). An afr of 13.7 is fine at idle and part throttle (low load) but at higher loads and wot, you need to shoot for about 13, or possibly high 12's.
I use the 9's in engines making almost 1200 hp and beyond. They are very cold for your setup. That's why they are fouling.
Eddie

TWO YOOTS

thanks for the comments... I have to recheck the rpms vs the a/f ratio numbers.I think I'll try the -7 plugs per Youngs perf but what gap with the crane box? Also what is the effect of colder vs hotter plugs/ Thanks.

Unlimited jd

iTrader: (5)

run them .035-.038, the heat range of the plug in a sense acts like advancing the timing, with the 9s you'd have to run quite a bit of timing to get the heat in the chamber during combustion but foul them at idle when advance is low, hotter plugs you can run a few degrees less timing and still have the heat at lower loads and advance, hope i explained that well enough

Philm

Too cold of a plug. Like Eddie said, go with the -7's. I run the 7's with 10.25:1 compression in a carbed 598ci and never foul the plugs at idle. When i had -8's in it, they wouldnt foul but would need a little bump every few minutes to keep from loading up.

OldSchool

I run the NGK R5671A-7's in my 800HP N/A engines gapped @ .045. They work just fine.