cb75

I have a 1975 cutlass. When i got it, it had all the smog stuff (cat, single pipes, smog pump etc) 396021Fa with C heads...ya C heads under all that crap...i have no idea how this combination occured. I got the car at an auction.

THIS IS A FACTORY 455 CAR.......

I took everything off and put an X pipe, crossmember from Summitt, edelbrock 750, accell super coil (HEI). It has factory limited slip..both tires spin and believe it or not ive had my plugs gapped @ .080. It runs really well and smokes the tires with no problem.

What should i actually be running for spark plugs?? Am i over carbed?
cb
this car runs very fast and im assuming its the heads..gotta be

cb75

joe_padavano

Maybe (the VIN will prove that), but the heads did not come on that motor from the factory. California-sold cars from the 1967 model year got C heads with A.I.R., so that's what the heads must be off of if they have the ports and manifolds for the smog pump. The engine isn't stock, so you don't know what was done internally.

MDchanic

As far as plugs go, from cold to hot: R43S, R44S, R45S, R46S, depending on what the engine needs.

- Eric

cb75

Yes I was sure to check the VIN and it is a 455 car. I knew the heads were only made from 67-69'. I guess its safe to say I have had the wrong plugs in it? The gap for those is real small and a vast contrast to what I have in there now (0.080). It runs fine... yeah I have no clue what was done internally. I just want an opinion from someone other than the parts store that thinks im crazy.

Thanks for your replys
cb

frankr442

For a 75 Cutlass 455, the AC Delco catalog calls for R46SZ plugs with an 0.080 gap. Granted, who knows what else besides the heads may have been modified, but with HEI , that gap is OK.

MDchanic

Well, I'm not about to argue with you about whether you're crazy, because your parts store guy probably knows you better than your shrink does.

As for the gap, 0.080" was specified briefly for HEI in the late '70s, but that number was later dialed back a bit because it occasionally led to firing problems.

It seems most people with HEI run their plugs between 0.040" and 0.055", though if 0.080" is working for you, there's no reason to change it.

The original spec. for points was 0.030", and they ran fine with that, too.

- Eric

svnt442

Personally I would (and have) get the plugs for something points related (before 1974) and gap them at .045 as that has always worked well for me. I was never a fan of the .080 gap as it can lead to misfires and over stressing the coil. The heads in the case of an Oldsmobile won't dictate the spark plug as they all use the same type and reach.

Ozzie

Here's a bit more which may help clarify things or may muddy it further. My comments are based on the AC Delco spark plug "standard" line of numbers. As has been said the catalog shows the R46SZ with a 0.080" gap. This is when everything is original on the vehicle. And also as has been said, the specs seem to have been adjusted downward on the gap from the 0.080" value. This may be due to the fact that generally a worn plug will require more voltage than a new plug to fire consistently all else being equal. When vehicle modifications have been made, a variation in spark plug may also be in order. The R probably should be kept for suppression reasons. Any devices using the airwaves will appreciate it. The 4 is for the thread size. 14mm is correct. The 6 is for the heat range. This could be different depending on your engine's modifications, amount of wear, and driving conditions. You will be able to tell after some use. If the plug is too hot you could get pre-ignition. If it's too cold you might get fouling. Adjust as needed. The S is for the extended tip. Most BBO & SBO users consider it advantageous regardless of the cylinder head. The Z, which can also be an X or a digit indicates which gap the plug is intended to operate with due to the attachment of the side electrode. They try to make it as close to paralleling the bottom of the center electrode as possible. Choose this depending on whatever gap you settle on. Some number designation combinations are not available. Some which have been available in the past are now discontinued. Choose from those available which is closest to your needs. I am attaching a chart which helps to clarify the situation (I hope).

nsnarsk65cutlass

Just to add here the .080 plug gap was needed to fire the lean burning air fuel mixtures of the mid to late 70s for emission requirements, prior to c3.That gap will work just fine with one lets say isn't equipped anymore.I ran my 75 with 72 455 at .045 with hei.

tru-blue 442

Interesting thread.
Has anyone used the R-47 S plugs
in the big blocks? With the crap they call
gasoline today, I was thinking a good hot plug
would help some. Thoughts?

Also, thanks for the chart Ozzie.
Cleared a lot up for me as far as
the A/C plugs are concerened.

MDchanic

Why? Do you routinely foul plugs now?

If not, then hotter plugs will only increase the chance of preignition, which IS a problem with modern gas and older high compression engines.

Personally, I use the coldest plug that won't foul.

If I recall, the AutoLites I've got in my car now are the equivalent of R43S to R44S.

- Eric

tru-blue 442

Eric, thanks for the response.
I do have light fouling in the 400
with B heads. If I recall I used some
Autolight 46's..Does that sound right? In
my 69 442 400, C head when it was on the road and
they seemed to burn very well. I always run premium
gasoline.

MDchanic

Yes, if you're fouling a little, you should go up a number.
I don't recall the AutoLite numbers now.

I'm running 10.25:1, using tetraethyl lead additive for octane, and so try to keep the heat as low as possible,

- Eric

tru-blue 442

I will try that. I found the Autolight #
they are 85's. They are still in the engine
sitting in the back of the shop. I am going
to try a .035 gap with them. I have a new
Sorensen coil that bit me pretty good last weekend, Youch!

cb75

This seems to be a good topic...thank you for your help! Im going to try running something different and see how it goes.

-cb

cb75

I put R45S. Ive never had her runnuing this well! I went with 0.045. (incredible performance diff.) I was running Autolite 847(?), at 0.080......

Thank you for all of the help form all of you.

-CB