recommended plug gaps for MSD box and distributor??

I am changing the oil today and replacing the plugs with a set of the AC Delco Rapid fires. The old autolites I think I had set to .55 or .56 but wanted to see if you guys recommend something different. I am right in the 10.5-11.0 CR range with Edelbrock heads. Demon 850 carb and a big ISKY solid flat tappet.

Thanks
Wade

I am using .050 gap with pretty much the same set up you have,except i have Brodix heads.

.050 -.055 should be fine

What should the gap be for the HEI ignition? I am running .45 as that is what was posted sometime back.

I set mine at .035

We run all of our race engines and high performance street engines equipped with MSD at .035". Reason for this is as follows:

Plug gap is not just a function of how much voltage you have available from the ignition source (MSD, HEI, or your points/coil). It's a function of how much resistance there is across the plug gap: As cylinder pressures get higher (from improved volumetric efficiency and the compression ratio of a performance engine), the resistance across the plug gap gets higher. Once resistance across the gap goes above a certain point, the plug will stop firing and the spark will jump the next lower available resistance gap, which is usually inside the distributor cap or off the plug wires/boots. It doesn't matter how good your ignition system is - the spark will quit firing the plug and start firing your plug wires.

I saw an excellent demonstration of this while doing dyno tuning at the Westech Performance facility in LA - they have a distrubutor machine set up with a pressurized spark plug tester so you can actually see the plugs firing (...and NOT firing) with varying cylinder pressure. Using an MSD distributor with MSD box, I ran the machine while cranking up the pressure on the spark plugs. At .060" gap, the plugs stopped firing at 180 psi cylinder pressure and the wires started arcing from the available MSD voltage. By decreasing plug gap to .035", cylinder pressure could be increased to over 250 psi without the plugs misfiring.

If you have a real performance engine, run plug gaps at .035". If your engine has only modest compression and does not have good volumetric effeciency, you can run wider gaps since you don't have enough cylinder pressure to cause a problem. GM ran wide plugs gaps on their HEI cars because the compression ratio on those cars was down in the 7.5:1 range. Just because you have HEI or MSD does not mean you can run .050-.060 gaps on a 10.5:1 engine with much success...

Lars

Lars has some good info above. :thumbs:
One additional item I often mention is that, given that there are no free lunches in this world, there is another potential downside to wider gaps. The energy (Voltage x Current x Time) stored in the coil is primarily used up as either a lot of gap voltage, or a longer arc duration. Increasing either one of these necessitates a reduction in the other (for the same amount of stored energy in the coil).

Wow thanks guys this is great info. I actually have not fired the motor since changing the plugs so I will make sure they are set to .035.

Lars,
Out of curiosity what plugs do you run on your HP motors?


Thanks
Wade

We run Autolites in everything. Plain ol' resistor plugs. The new "exotic metal" plugs actually run so clean that you cannot get an accurate plug "reading" off them, but the old resistor plugs can be interpreted easily, and will show signs of problems that can be masked by the new style plugs.

Lars

I bought a set of Autolite 5224 for my ProComps190CCs. Will they do? I needed plugs to install during measuring chambers so I just took a change. The static comp ratio is approx 10:1.The engine is a 383 with flat tapped XE268H from CompCams. No abuses, just ordinary street use. Please discuss the heat range to be used for these Autolites. Suggest also other brands, I'll convert to Eyquem or NGK locally, when I know what to use. Also recomended plug gap for 50kV HEI is also wanted. Thanks in advance!
Arne

Aluminum heads tend to run a little cooler than cast iron, so you can run a stock heat range plug for high performance street use, and the plug will actually run a little cooler than the same plug in an iron head. If you get slight sooting, you can go one hotter.

Plug gap has been discussed above in detail.

Lars