fresh tune-up , msd coil and accel 8mm wires , should i stay with stock .035 or do i have a lil gap space to play with maybe .040? and what will happen with a wider gap?

FireBirdKnight,

For a stock or slightly modified motor, a gap of 0.035 will do the job just fine. For an after market ignition with a higher out put and a built motor, it would be justified to open them up a bit more to 0.040-0.045.

It is relative to your setup. If you open it up with the stock motor/ignition the spark will not be as strong as it has the same voltage but now has to cover more of a region. If I am wrong please some one do step in.

Rabi C.

Leave the Spark plug Gap at 0.035. You really won't see any gain by making the gap bigger. At the tech school I went to we did a test with 3 LT1 motors that were all stock. We kept one motor stock but but on an msd ignition setup on it. No difference on engine dyno was noticed. The second engine had headers and an intake and a cam put on it with the msd ignition setup. Still no really big difference that justified the amount of money spent. The last motor had an intake, headers a cam, larger fuel injectors burned custom chip, forged pistions and new rods and a new performance crankshaft put in it. As well as the msd ignition on it. Only this engine showed any performance difference with and without the msd ignition on it. And it wasn't much. All the engines were tested with and without the msd setup. This given you really don't need to go all out with your ignition if you only have a slightly modded car. Only when you do major upgrades to an engine will you see a difference. I find it really dumb that people go out and buy a msd ignition coil when it doesn't do anything. If it only takes 12,000 volts to jump the gap on a spark plug then why would you buy an msd coil and ignition setup with a coil that puts out 60,000 volts when all the other voltage is wasted.

Run it as big as you can. I gap them till you have a high rpm studder.
Then either get a msd box and coil or tighten the gap a touch.
bigger gap more burn/more power.
I have picked up a tenth or so by playn with the gap.

Couple of things to consider.... stock replacement plugs were designed to work at a certain gap (.035" in this case). If you open them up much further the side electrode stops being parallel with the center electrode and you defeat any gains you get from openign the gap, plus the plugs wear out faster.

Now, there is probably some smidge of power and economy to be gained from a wider gap IF you use the right plugs and IF your ignition system packs enough of a wallop to reliably jump that gap for a meaningful period of time. Fortunately, even the stock computer-controlled HEI ignition system in good operating condition is perfectly capable of workign with a considerably wider plug gap, at least up to 5500 RPMs or so in a N/A motor- the HEI packs a heluva wollop at low and mid RPMs. You just need the right plugs for that much gap, which would be a set of Autolite #24 plugs (or equivalent) for stock cast iron heads. They're similar to stock but designed for a .045" gap. Up to about 5500 RPMs there's really no downside. I use them all the time with HEIs and factory cast iron heads of that vintage. They're one of my go-to plug choices.

Where the limits of the stock HEI system arise are in the upper RPMs. Like I said above, the stock HEI system packs a heluva wollop. But the fly in the ointment with the HEI is that it takes a LONG time for the coil to recharge to full power between firings. And as RPMs rise there is less and less time to recharge the coil between firings. When you get up over 5000 RPMs or so the coil is firing so often that it can't fully recharge between those firings. That's when the spark energy start to fall off quickly as RPMs rise. So if your combo is designed to still make good power over ~5500 RPMs or so, you might just start to run into those limitations of the HEI and lose horsepower in the upper RPMs. This is why guys who build motors that run up to 6500-7000 RPMs don't put stock HEI ignitions on them.

You might think you can solve this and get higher RPMs by just changing the coil to something better, since the coil is the "engine" that makes the sparks. And that's true to a certain extent- I have used the MSD coil in stock ignition systems many times with good results and it definitely has a faster recharge rate than the stock coil (it can recharge almost twice as fast). However, there's other limitations. One is that there is a trade-off between recharge time and total spark energy. The MSD coil actually has a WEAKER spark than a stock coil at low and mid RPMs, only surpassing it in the upper RPMs where the stock coil can't fully recharge fast enough! This is becuase the ignition module limits the maximum amoutn of current that can pass through the coil- you only have so much energy to work with and you can either make a heluva spark in a modest RPM range or a weaker one over a wider RPM range. All aftermarket coils play this game of trading maximum spark energy against recharge rate (some more successfully than others). There are other limitation inside the ignition module with how much dwell it can give in the upper RPMs that also affect things at high RPMs. Add those things up and you get the basic reason why guys with motors that make power at high RPMs don't run stock HEIs.

For a near stock TPI that doesn't often see the high side of 5500 RPMs the stock (or near stock) HEI ignition, in good working order, is a very well matched ignition system, however.