Spark gap with Pertronix ll

[markafss]

what spark plug gap should I use with my Q code 351c with the addition of the Pertronix ll Ignitor and Flamethrower ll coil?

 

[71ProjectJunk]

I would still use the .035 factory gap. If you went the a CD ignition system, like an MSD 6 of something or the sort, then you can go to .045. The gap increase does not make much of a difference anyways.
When I was in the speed shop business, and I kind of still am, I always told customers that the worst place to spend their money, to get power, was in the ignition system. You can spend over $1,000 on the latest wiz-band ignition box, coil, wires, distributor etc..., and not see much of a gain, if any, over a well set up stock system that has a distributor that has been recurved correctly.
Here is a good dyno test on Points VS HEI VS DUI HEI
https://www.motortrend.com/how-to/ccrp-9812-ignition-performance-test/
There was a test Hot Rod (maybe it was Car Craft or Popular Hot Rodding) did years ago, probably late 80's early 90's of points VS MSD ignition. Where they used an MSD 6A box and an MSD coil, can't recall if they also used an MSD distributor. But all they got on the 400+ HP engine was 6 HP over a single point distributor. You will never notice 6HP.

Not saying not to upgrade to electronic ignition, as there are advantages in reliability and no maintenance, but there is little if any power to be found there.

 

[Ryunker]

what spark plug gap should I use with my Q code 351c with the addition of the Pertronix ll Ignitor and Flamethrower ll coil?

Great thread, I will be doing the same thing in the next few weeks

 

[Omie01]

I tried an .040 gap after P2 install and my car actually ran worse. Been at .035 ever since. Good luck!!

 

[Stanglover]

From my own experience learning and teaching myself how to time MY engine (71 M code) using a (modified to a 10L slot) Autolite distributor, P II and coil, 8mm spiral wound wire set, Autolite 24 plugs (book calls for 25's in a stock 351C) gap set at .045", it pulls like a train.
I agree with 71ProjectJunk, a properly set up and curved stock distributor should be all most will ever need unless you're into racing of some sort or just want something that looks "pretty". The problem I think, is 90% of stock distributors were and still are, set up for 1971-73 requirements and many of us these days are way beyond that now. Most basic initial timing settings were about 6 degrees for 351's, except the Boss 351, which was 16 degrees initial. That is pretty much were we are all now aiming for, so a stock dizzy set for 6 degrees (15L slot),i.e. 36 degrees total mechanical, will never give good timing if the initial timing is set higher, no matter what the plug gap is. It all has to work together.
That's my thoughts on it anyway, but then I'm no expert either.

 

[tony-muscle]

I have a MSD CD ignition and run the car with .032" and then upped to .045" and did not notice a difference. Larger gap will need more power through the system so you need a good coil and cables to keep the electrons from finding ground else where. Theory tells you that more electrical power through the spark plug should lead to better combustion but in practice I don't know how much of that translates into power. Could it reduce pinging, could it change chamber temperature, could it make a difference at very high RPMs? I don't know.

 

[73inNH]

That is pretty much were we are all now aiming for, so a stock dizzy set for 6 degrees (15L slot),i.e. 36 degrees total mechanical, will never give good timing if the initial timing is set higher, no matter what the plug gap is.

Stanglover,

Can you elaborate a bit? What does 15L mean?

 

[c9zx]

A well written and informative article on plugs and gaps. Chuck
https://www.briskusa.com/spark_plug_gap_spark_plug_gap_chartI use their plugs in a 4.6 3V with a 10 PSI supercharger. They work great, have one piece steel body, and gas mileage went up 1.3 MPG over Ford Racing plugs. Must be fewer misfires in my mind.

 

[Stanglover]

Stanglover,

Can you elaborate a bit? What does 15L mean?

15L refers to the crank advance degrees. it actually doubles, so 15 = 30 degrees crank advance. On my car, the factory initial timing was set at 6 degrees and the distributor limit slot was a 15L (or L15) The equated to a total of 36 degrees total mechanical timing without added vacuum advance.
Here is a pic of what I mean by the 15L limit, but actually, this one is set at 13L or 26 degrees crank advance. In this instance, 26 + 6 = 32 total mechanical which would be ok to set the initial to 10 degrees to equal 36 degrees total. One 'trick' that could be done as seen, would be to add a sleeve of Nylon to the post. This would in effect cut the limited advance and allow possibly 12-14 degrees of initial timing without trying to find, or in my case weld up and recut the slot to equal a 10L slot. On top of all this, the springs used and the tension they are set at make for how the "curve works". It gets to be fun for sure.
Hope that explains it a bit better.

 

[73inNH]

15L refers to the crank advance degrees. it actually doubles, so 15 = 30 degrees crank advance. On my car, the factory initial timing was set at 6 degrees and the distributor limit slot was a 15L (or L15) The equated to a total of 36 degrees total mechanical timing without added vacuum advance.
Here is a pic of what I mean by the 15L limit, but actually, this one is set at 13L or 26 degrees crank advance. In this instance, 26 + 6 = 32 total mechanical which would be ok to set the initial to 10 degrees to equal 36 degrees total. One 'trick' that could be done as seen, would be to add a sleeve of Nylon to the post. This would in effect cut the limited advance and allow possibly 12-14 degrees of initial timing without trying to find, or in my case weld up and recut the slot to equal a 10L slot. On top of all this, the springs used and the tension they are set at make for how the "curve works". It gets to be fun for sure.
Hope that explains it a bit better.

Got it, thanks for elaborating.

My Pertronix disty included the proper springs etc. the set timing and curve. I have 36 all in around 2,700rpm (I think, I may have to check again)!