Been working on my '66 vette with 327/300 HP.

The plugs were fouling something fierce. It appeared to be a combination of gas and oil.

Dick Olson and I have just finished replacing ALL of the valve seals... they were mostly trashed (twisted, or split).

Now I'm looking at plugs. The car had AC43 installed. AC44 is the factory recommendation, but aren't available any longer. Some folks have suggested going UP a number (AC45) instead of down.

Here's what I have available to me. Any suggestions?




AC R43 44 44S 45 R45S

A tell-tale sign of leaking valve seals is blue smoke when first starting the engine.

The heat range of the spark plugs depends on how the car is driven. A lot of cold starts and idling and low speed driving would call for a hotter plug. That would be a 45 or 46.
Normal mixed driving would be a 44 or 45. All highspeed driving (over 80 MPH) would call for the #43. The "S" plug has the extended tip. This was to burn off lead deposits. Not necessary. If you use the #45 and it is too hot the insulator (at the tip) will burn very white and the center eletrode burn away a little faster.
I would go for the 45 if it were mine and they should last at least 10,000 miles with unleaded gas.
The old AC plugs were good. The newer Delco plugs are questionable.

Thanks Gene. Only element missing was the "R".... Since it is a Corvette, the radio reception already is poor - at best. So, I"m leaning towards an "R" plug. So that would make the decision lean towards the R45S plug. Sound okay?

And, unfortunately, except for the trip to California, the car is used for fairly short stop-and-go trips. That supports your comments for the hotter plug (ie. 45).

the S suffix denotes an extended tip. some piston intererence could be experienced depending on piston top design.

the higher compression ratios of that era were hard on plug electrodes. a couple of thousand miles of "spirited" driving with 10.5 cr and the center electrode becomes rounded and less efficient at jumping a spark.

in 1970, my 454 SS required points and plugs every 2,000 miles. I became very proficient at the procedure. In contrast my Sierra has 94,000 miles on the original plugs. Electronic engine controls, unleaded gas, and plug design has come a long way.

for short, every day drives the R45 should serve you well

The R plug would be the best....either the R45 or R45S.

Hi Bill,

You can run the R45S in your Corvette.
We used to install the "s" version in many of these cars when the customer's were driving them easy. It helps prevent fuel fouling at low engine speeds. No worry on any clearance problems with these plugs in that engine.

As Gene mentions, AC plugs today are a far cry in quality from the AC we grew up with. For that reason, I only use NGK spark plugs and have never had a problem with any.

The equivalent number in NGK would be:
AC 45 NGK B6S
AC R45S NGK XR5

You will also have the option with iridium with the XR5. That would change the number to XR5IX. You can install these plugs and just about forget about them. They will most likely outlast the car.

Good source for NGK plugs is www.rockauto.com
You can use this code for an additional 5% off the bill 32549011810745
Cut and paste it on the where did you hear about us box during checkout

Best regards,

I also heard from racer types that the NGK XR5 is one of the best all around plugs for high performance small blocks.

Thanks Ollie for the link and discount code.

RayG

Bill,
For what it's worth, I'm running ACR43 in my 66 Vette 327/300hp. Have had them in for a couple years and no problems.
Tim

I also run the a/c R-43 in my 66 327/300 with no problems...

The R45S plugs are safe to use in that engine. The extended
tip spark plug OK to use in most overhead valve engines. I
run them in my 283 small block and even in my '35 Chevy with
a stock 206 cube engine. I have done this for several years
and many miles with no problems. Never use the extended tip
plugs in a flathead engine! That could be a recipe for
disaster if the piston top contacts the plug!
Phil Lipton

In "their day" we had 235 engines with aluminum pistons burn holes through the top of the pistons. This only happened with the "S" spark plugs.
The S or extended tip plug was originally made back when gas had a high lead content. The long tip helped to burn off the lead deposits that caused a high speed miss due to the lead shorting out the plug under load. I had better luck back then with the Champion J-12Y extended tip plug There is no real need for an extended tip plug today.

I remember using UJ12Y in my 69 Vette w/350 300+HP. They ran great. However I heard that Champs are not what they used to be since they are now made in China.