Does anyone know what the spark plug should be on 350/350 using a Petronix ignition system and the FlameThrower coil? I had it gapped at .035 and a friend said that I should change the gap to maybe .044 0r .045. Thanks

 

Start with a .045 and see how it runs. You maybe able to go a little wider than that.

 

You should have a look at this recent post, with the same subject of debate :

http://forums.corvetteforum.com/c3-t...-plug-gap.html

 

Thanks 73Streetrace. Took the vette out for a cruise. I set the gap at .045 and it seems to run stronger. Not sure if it's because everyone is saying that it will run stronger. Feels good though. I'll have to regap my brother's gap too.

 

I am running Bosch Platinum Plugs with a .060 Gap and run great!

 

You can open the gap to as large as you want, as long as you retain a reliable spark production on every plug. The higher the coil voltage, the larger gap you can select. But it still has to fire every time; if not, you have too large a gap.

Theory says that the largest reliable gap is the best, as it provides a 'bigger' spark to set off the fuel charge. Others disagree and say there is no advantage over a smaller spark. If in doubt, set it at .045" and leave it alone. That's what the GM HEI system plugs get set to, and that system has worked pretty well over the years.

 

Would it be any different on a 327? I was told to set it at .035...and all electronics are stock. However, when I pulled the old ones off a few weeks ago, the gaps were not even, most were .025-.031.

 

ive got a 68 327 300hp with a mallory ign. ran good at the 35 gap,but heard about the 60 or lower gap,,then put it 60 and it would breakup under load in higher gears,,tried 45,it didnt run good either,,went back to 35,runs good,,i guess its an individual thing,good luck.

 

The plug gap issue has been discussed before, with the same amount of opinions. I've had the opportunity to run the "Plug Gap Testing Machine" at Westech Performance, and it's shocking to see how often a plug will misfire when run with gaps over .045" when the compression is raised.

Here are the facts on plug gaps as posted before:

The guys quoted above have the correct answer. A few points regarding HEI, MSD and plug gap:

The HEI was developed and released as part of the emissions control package in 1975. The engines at that time were low compression engines, and they were set up to run very lean. A lean mixture is harder to "light off" than a "correct" mixture, so the HEI system was used so that a much wider plug gap could be used. This would ensure that the lean mixture would ignite and burn.

When a wider plug gap is used, there is more resistance across the gap, and more energy is required to jump the gap. Increased compression will also increase the resistance across the gap. The wide plug gap could be used on the smogger engines because the compression was low. If you use a wide gap with high compression, you can get the resistance across the plug gap so high that the plug simply won't fire: the HEI spark or MSD spark will jump at the point of lowest resistance. This can become the plug wire boots, the inside of the distributor cap, or right through the plug wires if plug gap resistance (from the combined effects of gap and compression) is high enough.

Keeping this in mind, it is important to note that you cannot run an "HEI Plug Gap" (some gaps in the mid-70's were in the .060" range) on a high compression engine, even though you have the HEI system or an MSD. You will, in fact, get plug misfires from the high energy spark jumping any point of lower resistance rather than across the plug. For this reason, most high performance applications with HEI or MSD use plug gaps in the .035 - .045" range. This assures that the spark stays at the plug and does not move to the inside of the cap or goes through the wire insulation... If you have a modest compression engine, you can run a wider gap, but you have little, if anything, to gain. If you want to assure that your high perfromance engine is firing at the spark plugs, run plug gaps in the .035" - .045" range.

 

And there you have it! Thanks!

 

Thanks Lars, I had it at .035 and changed it to .045. Seems to run stronger and the exhaust sound better. Was going to go to .060 but Lars saved me from wasted time.