Red69

On the 427/400hp with R43N plugs, the service manual calls for a gap of .035". When would you want to either increase it or decrease it?

... in other words, how can you use variation in gap to aide performance?

Thanks

Mike Ward

You can't. Bubba will try to tell you that it makes a difference, but I have land for sale in Florida that's more credible :lol:

Desertdawg

The only time I have ever seen anything on increasing plug gap, was with my truck when I installed a MSD ignition. they told me to open the gap up 10 to 15 thousands to allow a larger, hotter spark.
But you have to remember this is with a big increase in voltage.

Larry B.

Spark plug gap is determined by the coil. If the coil doesn't have the capacity to jump a larger gap then a larger gap will overneat the coil and the spark may not be consistant resulting in mis-fire. Now if the gap is too small the spark will jump very easy but not very hot. GM "HEI" (high energy ingnition) put out higher voltage so they use plug gaps up to .060. I think one year they used .080. In all cases condition of the plug wires is very critical. An old wire with breaks interally is a huge strain on the ingnition system.

Jvette73

if yur running stock ignition stay with the stock gap. If you run a higher output coil then you can experiment with larger gaps. but as stated above, too much gap will make it difficult for the spark to jump the gap. too much gap can cause mis-firing and hard starting

vett70

I know this doesn't answer your question but I just installed Bosch Platinum series Spark plugs in my '70 350/350 and I can't believe the difference! I used the 0.035 gap recommended in the specs. The plugs were 3 times the cost of Champions but they came with a money bacl guarantee from Bosch so I said, "What the heck."... I recommend them.
-Bill

Matt Gruber

higher compression sometimes requires less gap.
i had to drop to .022 with 22 psi boost as it would miss with wider gaps

battsup

Well when i regapped my plugs i set them to .035 because thats what the gap on the 350/350 motor was, but after reading this thread should i set them to . 042 like it says on the firewall and in the book?

Larry B.

If you have the GM HEI ignition you need the bigger gap to take advantage of the hotter spark. If you are using an aftermarket set-up, use the gap reccommended by the mfg. of the system.

battsup

Yup the ignition system is stock, could this explain my some of my bad emmisions results and why my car sometimes diesels after i turn it off.

Jvette73

dieseling aka runon is usually caused by carbon buildup in the combustion chambers and or combustion chamber temps are way too high. It creates glowing red hot particles in the chamber which continue to provide an ignition source after you turn the key off. Sometimes a worn out timing chain will cause retarded cam timing and can make the cylinder temps too hot. i had one once that ran on really bad. I did everything to try and solve it. It wasnt untill the timing chain went up totally and the car wouldnt start. After the new chain and gears were installed it never ran on again.

Red69

Thanks all... good technical commmon sence information!!

By the way... to the factory specs and it runs great!!!