I have a '71 with a 420hp 383 stroker crate engine, with an MSD Digital 6AL ignition, MSD Billet Corvette ditributor, MSD Blaster 2 Coil and MSD wires. Anyone have a suggestion as to what gap I should be using on the plugs?

Thanks!

 

I use 42 on electric dizzys
you can go smaller or bigger just get them all the same

 

I use .45 with my NGK Irridiums

 

i use .060 " with msd 6a and billet dist. i think that 's what msd recomends

 

I use .035 on all of them that includes the electronic ignitions with L88, and LS7. And the lone 426....

 

What would the gap be for a 350, brodix ik180 heads, about 370HP?

 

GM originally recommened 0.060" gap on HEI ignitions in 1975-77 but soon found out those large spark gaps created multiple problems with ignition wiring/rotor buttons and other ignition parts. Large gaps are not commonly used today a gap of 0.032-0.035" with conventional ignitions and 0.040-0.045" running a MSD type ignition are generally used by most engine builders. If you like your plug wires, run 0.040" spark plug gaps

 

Correct answer. Although like Iron Cross there, I use only .035". Larger gaps do squat.

 

MSD recommends .050 - .060:

Here's a link to MSD's installation instructions (which also contains general info on their 6 series boxes): http://www.msdignition.com/pdf/6%20series/6series.pdf

 

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 - .040" 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" - .040" range.

 

Hmmm...I have pretty much the same setup as the OP (383 w/all MSD ignition system), about 10.5:1 compression, and no smog equipment. I've got the plugs gapped at .050", which is the shortest gap that MSD recommends for that compression ratio.

Are you saying that I should ignore MSD and regap my plugs to .035" - .040"?

 

I run my MSD setup @ .050 and have no issues.

 

Run any plug gap you want that keeps your plugs firing... but here are the facts as I have personally oberved them:

At Westech Performance (they're the guys who do all the engine dyno testing for Hot Rod, Car Craft, and most of the other performance magazines), I have had the unique opportunity to play with some of their test equipment.

Amongst all the amazing test equipment they have, they have a really cool piece of equipment that they've built themselves: It's a SUN distributor machine that's been modified with a pressure chamber that you can install the spark plugs into. Each spark plug has a little viewing window so you can actually see the spark at the plug while the distributor is spinning and firing. The chamber has a pressure regulator on it, so you can adjust the pressure that the spark plug is exposed to (the spark plug is exposed to the dynamic compression ratio as the piston comes up on TDC on the compression stroke, and this varies by static compression ratio and the engine's volumetric efficiency - V/E). Their "normal" ignition configuration for this machine is an MSD system, so they have plenty of voltage potential to "light the fire" at the plug.

Here is what I have personally seen on this machine while playing with it - these are just the facts, and you can argue this with me all you want - I have personally seen the following:

As the spark plug chamber pressure gets into the 200 psi range (not an uncommon pressure to see on a high compression engine with good V/E), and rpm gets up into the usable power rpm range, spark plugs with gaps exceeding .045" will actually stop firing. Plain and simple: The plugs simply don't fire any more, and you can see the spark jumping across plug boots, wires, and off the distributor cap to engine ground. There is a big difference in the brand of plug used - some brands will fire with wider gaps than others, and some plugs will stop firing at shockingly low pressures and gaps. Some of the best plugs I tested on the machine were the plain ol' Autolite Resistor plugs, and some of the worst plugs were the "exotic metal" plugs. The "safe" plug gap range for an engine running cylinder pressures in the 200 psi range was .035 - .040". Any gap wider than .045" made the spark plug highly likely to misfire or not fire at all unless the cylinder pressure was reduced to mimic the pressure of a low-compression smogger engine. Increasing plug voltage made no difference at all - the spark simply jumped someplace other than at the plug gap.

So if you have a high compression performance engine, and you want to make sure your plugs are always firing, keep the gaps modest. The wider your gap, the greater the chances are that you're not always firing on all cylinders - I've seen it with my own eyes while testing on the Westech plug machine. If your engine runs great with .050" gaps, that's fine. But I'm running mine at .040", and I know my plugs are all firing every time. I also set up all the engines I build with .035" - .040" gaps so I know they're firing on all cylinders all the time as well.

If you are running at high altitude, or if you have a cam profile that does not produce high V/E at high rpm, you can run slightly larger gaps due to the lower cylinder pressure that the plug is exposed to. But I don't know that you really have anything to gain by doing so...

 

lars- that's some good info. i never knew that spark plug gaps played such a crucial role. i will be setting mine to .040 next time they're out

 

 

 

No you don't !

 

No you don't !

 

Lars, I'm certainly not calling BS, but I guess I don't have the same confidence in spark plug pressure vessels that your dyno aquaintance does. I've used them for decades to put a bit of load and stress on the coils and ignition controller switching transistors, but it's an artificial environment for the plugs. The compressed air in the vessel is usually room temperature, and full of ozone unless continually purged. The plug electrodes aren't hot (or carboned) as they would be in an engine, and the level of swirl/turbulence is generally much less than in a cylinder. The turbulence in a cylinder frequently blows out the arc, causing the ignition to multistrike the plugs (inductive systems do this all the time, not just CD systems) to keep the arc going, and wil continue to do this until the coil energy is exhausted, or just too low to charge up all the plug wiring capacitance to a high enough voltage to arc the plug gap.

I do agree with your comments on plug gaps with regards to compression/cylinder pressure.

As we both know, there's no free lunch in the ignition world, and the result is a bigger gap causes a shorter arc duration. There's only so much energy in the coil, and it will be used up as high voltage (large gap) and short duration, or lower voltage (small gap) and longer plug arc duration.

Just my experience with bunches of test cars, dyno time, and lab bench seat time.

 

Note that this is a 5-year-old thread that someone dug up out of the archives, so the Mods will probably shut this down pretty quick...

I agree: The static pressure tester for the plug spark test does not accurately represent a running cylinder, so the pressure data cannot be directly translated to operating cylinder conditions. It does, however, provide a comparative test of ignition system components, such as the plugs themselves, and it allows comparative testing of other system components. It also demonstrates my point that a big, wide gap may not be optimal for a high compression engine using a stock HEI or other near-stock components: Once resistance gets too high at the plug, the spark is going to go someplace else. And resistance at the plug increases with gap and pressure.

Lars