I know the standard gap for my 327/350 is .035". The previous owner installed a transistorized ignition. Does that affect where I should gap the plugs? (AC45R)

 

no..........

 

I just realized that I was confusing the idea of HEI for transistorized. An HEI would use a larger gap.

 

It can use a larger gap but there's no advantage.

 

I recently upgraded the ignition system on my L-79 to a Pertronix III & flame thrower coil. Most seem to recommend to go to a gap of .040, so I'm trying that. Big improvement in startup & idle is what I have noticed so far.

 

Correct, as well as cleaner plugs and NO misfiring.

You can easily gap them at .045", especially since you're probably using a 45kV coil. A milder engine, say, the 327/300 will like .050" even better. My engine is somewhat more radical than your L79, and is using a 40 kV coil. I gap mine at .045".

If you are using standard resistor type 7mm wires, then be careful with large gaps, as the insulation isn't heavy enough to hold the voltage required to jump a gap much larger than .035". You should be using 8 or 8.5mm wires.

 

On my 327 I am using R45S plugs gapped at 43 with 8MM wires, Pertronix II and
Flame-thrower II coil.

 

What characteristics of less "mild" engines push you toward smaller plug gaps? Higher compression? More valve overlap? I'm just trying to learn stuff here.

Steve

 

Gapped mine at 40 when I went to the Petronix III set up

 

That's a great question! Valve overlap, yes. Higher compression, not exactly! When people say "higher compression" they are generally talking about static compression ratio, but it's dynamic compression ratio that is the determining factor, because DYNAMIC compression ratio determines cylinder pressure. Cylinder pressure is always highest at the RPM where maximum torque is developed.

A "milder" engine may have the same dynamic compression ratio, or higher, than a more "radical" engine (ie: one with higher volumetric efficiency). At lower RPMs and smaller throttle opening, a "milder" engine will usually develop higher cylinder pressure, and more torque, than an engine with higher VE. In the case of NORMALLY ASPIRATED engines, an engine with less restriction in the inlet and exhaust (bigger ports, bigger carb, bigger valves, higher valve lift, etc) along with longer cam duration and valve overlap will cause much higher cylinder pressures, and consequently more torque as the RPMs are increased. By the time the point of max torque is reached, the engine with higher VE develops quite a bit more cyl press than a milder engine. More valve overlap causes exhaust pulses to actually suck exhaust gas out of the chamber before the next intake stroke, and a properly tuned intake runner causes a slight pressure pulse which can be tuned to each cylinder for a very slight "ram" effect. The result of this could be called a small "virtual" supercharger, and continues to develop cylinder pressure past the torque peak of a milder engine. The net result of all of this is sustaining a higher torque output into higher RPMs and consequently, developing more horsepower (power = torque x RPM/5252). Looking at this equation, it's easier to see that a "mild" engine which develops about the same max torque as a high VE engine is "all done" before 5252 RPM, and an engine with higher VE is still making power well beyond that.

All of that being said, an engine with more valve overlap (not to be confused with a higher VE engine, although not exclusive of it) needs richer mixtures than an engine with less overlap because of exhaust dilution at lower RPMs. At a given available secondary voltage and spark plug gap in a hypothetical engine, it is HARDER to get a spark to jump that gap as charge density increases.

A cooler, richer cylinder charge at higher pressure, as normally found in a high VE engine WITH MORE VALVE OVERLAP running near and past it's torque peak is denser than that of a milder engine and it's therefore harder for a spark to jump the same gap using the same available secondary voltage as the same setup installed in a engine with lower VE and/or less valve overlap.

 

You just described how my ram inducted 413 works! Thanks for the tutorial on gappage, you answered a few questions of my own. happy new year.

Dan

 

THAT looks like spark plug changing nightmare.

Doug

 

Maybe a spark plug changing nightmare, but it is a BEAUTIFUL SIGHT to behold. I just about "drool' whenever I see one of these 413 or 426 MAX WEDGE engines.

Larry

 

Yeah, the good old days before every engine combo had to be Emissions certified, and auto companies could offer low volume options.

Doug

 

Noooo, not really - THIS is a plug-changing nightmare; 28 cylinders (4 rows of 7), 56 spark plugs. The red plastic plugs are in the exhaust ports - when the exhaust pipes are in place, it's even worse (Pratt & Whitney R-4360, 3500hp).

 

One of those P&W, I think they are called Super Wasps, or something, came up for sale on e-bay several years ago.

it wasn't that expensive. Looked like a nice garage conversation piece, if you had a big enough garage.

I under stand those motors had a cooling problem on the rear two banks of cylinders. Even a -60F, 400 knot slipstream wasn't enough to keep the back jugs cool with that many banks stacked up

Doug

 

Spark plug gaps were increased in the 1970's to better fire the leaner fuel mixtures. The wider gaps also meant higher powered spark and that's why you got HEI.

In the '60's, it was said that the plug gap opened up .001 for every thousand miles on the plugs. That meant at the normal replacement interval back the of 10,000 miles, the gap would be at .045 when the plugs were gapped at .035 when new.

I remember they opened up some but I don't remember seeing .001/1000 miles.

The spark plug gaps opened up as millage accumulated and the points close their gap. When the point gap closes, it give the coil more time to built a hotter spark so it seems one would/could cancel the other and you'l get no misfire.

 

What ever the ignition, fuel or HP, .035 is the best performing plug gap......And no one plug is better than the other whatever price or brand.....:

 

When I finally was working and had some $$ to spend, my car usually got a "tune-up" and new plugs every few months…..even more frequently if a Saturday night race was planned. So most of the time, I never saw a change in the plug gap.

If you went 10,000 to 12,000 miles before changing plugs, you usually found the electrode covered with salts from the fuel TEL lead additive.
No good way to accurately measure the gap. You either blasted, gapped, and re-used them, or installed a new set.

Larry

 

Both are debatable.

Larry