SBC Spark Plugs

[836dstr]

This seems pretty basic, but could use some advice. I want to try some different spark plugs.

The engine is a 301 ci Chevy with iron heads, 12 1/2 compression ratio, Carb on gas. MSD-6Al box and MSD 85501 Distributor. I have been running NGK R5670-7 plugs, 14mm, 3/8" reach, gasket seat with "J" ground electrode.

I want to try "cut back ground" plugs, using Autolite/Champion/AC Delco. It's kind of a buy USA thing.

Guys running a similar engine combo, what plugs are you using and what gaps? I think I'm using too small a gap @ .035".

I plan on having the Roadster on a Chassis Dyno before Bonneville and might try several different types of plugs.

Any comments would be greatly appreciated!

Tom

[doug odom]

You say Iron Heads. What iron heads? What CC's? Big dome on the piston? Where you getting good readings from the NGK?

[maguromic]

When we use to road race we spent days on the dyno spark plug testing,and found that the Autolite gap-less plugs made HP for us.  But the same plugs in my friends motor lost HP.  The best thing you can do is get  different plugs and different heat ranges and try them till you find one that the engine likes. There is no substitutes for the Dyno.

This may sound crazy but for my motor I have a set to warm the engine and another set for racing.  Now all I have do is finish the car. LOL

[thundersalt]

I  heard that you could run up to .060 gap with a MSD.Hotter spark, longer spark. We gap at .045 with a MSD.

[836dstr]

Dave,

Heads are Chevy double hump with accessory bolt holes. I don't know the numbers. Circa 1970 with Mondello port job. Probably 64 cc with dome. Bought assembled long block with only a few runs from another racer, so haven't had the heads off yet. The most consistant readings on the plugs are on the top and sides of the ground electrode. I haven't indexed the plugs so the coloration on the insulators varies around the circumference but roughly the same color.

Maguromic,

Not crazy at all. We warm up the engine on a set of plugs a few heat ranges hotter then switch to the race plugs.

Thundersalt ,

I'll give .045" a try, I think it should work better. Also the old racer trick of grinding back the ground electrode to be adjacent to the side of the center electrode to expose more air-fuel mixture.

Thanks all!
Tom

[ddahlgren]

Indexing the plug is a big deal in that cylinder head and you probably find more with that than anything else as far as different brands of plugs. If you have to run warm up and race plugs the engine is wildly too rich and or has retarded timing at light loads. There is zero to gain running a big gap that I have ever found compared to the odds of getting into a missfire situation if the plugs are not fresh every run. The idea might be to get within a couple miles an hour of the record before you spend money on the right side of the decimal point gains. You might also note that opening the gap artificially raises the secondary voltage output to some degree so now you are testing the dielectric quality of everything else in the ignition system as well. This is all for a hp gain that is probably the same percetage as the air temp moving 10 degrees one way or the other and adding in a list of failure points that are new. BTW what makes you so sure all the others are made in the USA. I used to spec a lot of AC Delco parts for ignitions and relays.. Now they all come in a box that syas made in China...
Dave

[836dstr]

Thanks for the info Dave,

I assume that you index the ground electrode away from the valves.

I know what you mean about good old USA brands where all fabrication is done in China. I come from a manufacturing background and it astounds me that the US can't economically produce alot of the stuff we import when you consider shipping costs, but that may change.

Back to plugs, time to order some indexing washers.

Tom

[jimmy six]

Tom, It seems like there are not as many 3/8" reach plugs as in the old days. I use what I can find and find most of them at www.sparkplugs.com right up the way from you in Menafee. I have used 1/2" reach plugs with a spacer (maybe Dave can comment on this practice) The GMC uses the same plug. A HEI on higher voltage can ue a wider gap but not sure on the MSD. We used .035" on Patrick's dyno runs. ...Good luck

[sanger351]

I saw a couple dyno runs on a iron headed 355 sbc late model engine in which the autolite racing plug ARXXXX made about 7 hp over the next best plug, Champion racing plug if I remember right.  The run was backed up.  Gapping was around .035   Am I saying that the Autolite is the better plug?  No, just a better plug/heat range for this engine combo.  That last 7hp might be easier found some place else unless you have unlimited dyno time.  But then a change in weather could throw it all out the window.

[836dstr]

JD / Sanger351,

Thanks for info. I found the Autolite equivalent for the NGK is the AR72 which already has the cut back ground and available from Summit ( hey, the Internet is great for looking up stuff). Looks like I'll buy some and gap'em @ .035"

Tom

[interested bystander]

Pretty sure .035 has become a "standard " for modern electronic ignitions-Old-style magneto users beware.

Washers DO change heat range but when Lodge was the ONLY plug to run (just after the wheel came into vogue) seems like we had to use washers w/3/8 reach heads to get what we wanted - not a no-no.

[Cajun Kid]

My lil SBC  "E" motor at 12.6 to 1 uses  AR3933  at a .035 gap 
with MSD 6 box...   It turns 8400

[ddahlgren]

Be careful with dyno data most are accurate to no better than 1% plus or minus. Better to average several runs and see what you have. Best is to do a test where A is the new setup and do an A B A type test to verify the baseline setup B... I have found that champion also has more heat ranges so the difference smaller between changing 1 heat range and when you go from one brand to another they can fall at top or bottom of the other brands heat range so there is at times no direct cross over from one brand to another.
Dave