[JasonL]

Quick question, Looking for spark plug advise and gap.

Motor: 355, 9:1 cr, 492 heads with 2.02/1.60, performer intake, holley 670, comp cams 268HE cam.

Thanks in advance

[327NUT]

You forgot the most important item......what kind of ignition are you using... points, HEI, magneto....stock, MSD, Mallory, Accel (hope not)? What type of wires?

[JasonL]

Sorry, HEI Dist with a 65k volt coil, spiral core wires.

[F-BIRD'88]

I cannot remember if those specific 492 casting heads use a .460' reach tapered seat plug
or the older style plug with a 3/8" reach.

but when you figure that out gap the plugs at .035"

In as NGK you would use a 6 or 7 heat range.

in a champion you would use a 9 -10 or 12 heat range.

[JasonL]

trying to figure this out, the heads are also straight plug which would have come on a 70 camaro. I am showing autolite 86............ but not sure of dizzy that came on that car.

There are autolite 26's in there now, the cam I took out was bigger, elgin 1213 with 500+ lift. So still wondering what plug to use.

[F-BIRD'88]

look at the threads in the head..tapered seat is new style. .460" "V" style ( Champion)
flat seat is the old "J" style 3/8" they are not the same.
Look at the spark plug hole in the head , not the plug.

[66GMC]

Once you get the seat style and reach figured out ... you'll want to determine the correct heat range.

The NGK rep was out educating us counter people on what "heat range" really means.

He likened it to a thermostat in that a plug has the ability to draw heat AWAY from the combustion chamber.



Don't be afraid to make a call to the NGK tech line (1-877-473-6767, ext #2) with your setup. Those guys have valuable insight, and are there to help.

(I love NGK plugs, BTW)

[ChevroletSS]

Have to say thats cool. I never knew that spark plugs had heat ranges. Ill have to find out what my truck needs.

[66GMC]

What I took away from the rep's advice was that you should close up the gap and go colder than the recommended plug on a "modified" engine.

Modification was described as ANY performance-oriented upgrade ... even a cold-air kit for example.

How much colde heat ranger? How much to shorten the gap?
I'll leave that to the tech guys for their input, but I suppose the real-life answer would lie in proper "reading" of your plugs?

[cdminter59]

Originally Posted by JasonL

Quick question, Looking for spark plug advise and gap.

Motor: 355, 9:1 cr, 492 heads with 2.02/1.60, performer intake, holley 670, comp cams 268HE cam.

Thanks in advance

The 492 heads are the 1970 LT1 heads. You can use AC R45 for street driving. These heads oridinally used AC R44 but they are no longer made. The AC R43 is used for a colder plug when racing. A Champion RJ12YC can be used also. Gap is .035 for these plugs. BTW do not use any AC R45TS plugs if the pistons have domes. The S stands for extended tip and will hit the top of the domed pistons on 11.0+ compression motors.

[Jimbo17]

Which way the open end of the gap faces is another interesting subject.

We used to look at the open end of the plug and put a magic marker line on the white part of the plug and then insert different sizes of copper washers under the plugs to get the open end to line up in between the intake value and the exhaust value.

After doing many dyno tests trying different plugs we found that the job of a spark plug was to fire at the correct time very time.

Now how long the spark fires for is a whole different story which leads to a much cleaner burn of the fuel.

Jimbo

[JasonL]

Originally Posted by cdminter59

The 492 heads are the 1970 LT1 heads. You can use AC R45 for street driving. These heads oridinally used AC R44 but they are no longer made. The AC R43 is used for a colder plug when racing. A Champion RJ12YC can be used also. Gap is .035 for these plugs. BTW do not use any AC R45TS plugs if the pistons have domes. The S stands for extended tip and will hit the top of the domed pistons on 11.0+ compression motors.

Thanks for the response. I pulled the plug out and there is a Tapered plug in there and the AC r45 look to be flat plugs. Is there a pic of how to tell the difference of heads? I am not sure what my heads need.

[cdminter59]

Jason, The easiest way I know is to pull a valve cover to look at the casting # on the head. Also there will be a date casting # starting with a letter, then three numbers. Example: C 15 0= March 15, 1970. The 1970 head uses a gasketed 13/16 spark plug and 1971 head uses a 5/8 tapered plug. The gap should be set at .035 on either spark plug. Once you have the number, check it out on this site http://outintheshop.com/faq/casting/heads.html. to find out what you have.

[Richiehd]

There sure are many different theories here. I was always taught that .035 gap for points ignition and bigger for Electronic. That was one of the advantages of HEI. I have always used a .040-.045 gap when using a HEI ignition, thus more spark, more burn time. Back in the day of smog motors we had gaps of .060 and even .080 with HEI and stock coils capable of 65,000 volts or more. Im just saying....

[F-BIRD'88]

The higher the cylinder pressure (like a hi perf motor @WOT) the smaller the spark plug gap.
Large gaps promote arcing crossfire, shorter cap/rotor life etc. You don't want or need large gaps with a HEI
.035" is fine.