Hi - Decided to take advantage of a nice Sunday afternoon and change my plugs.
1969 350/300 original engine and heads. 63,000 miles. Cam is a question (rumored to be "upgraded" by a prior owner, but unverified).

While I was at it, I also checked hot/idle temperature at each cylinder (laser measured at the header about 1" away from the head) before and after the plug swap.
Also checked the compression at each cylinder.
First time doing this, so I wanted make sure I did it right.
Measured compression with all plugs out, engine warm, with carb fully open and ignition disconnected.
I measured compression at both "first crank", and then again continuing to crank about 2-3 seconds after first reading.
Got pretty consistent measurements as you can see ~90psi at first pop, and then ~135psi after several cranks.
Seem good? When do you know when you've cranked enough? Do you wait for a peak reading on the gauge? I feel I could have gotten higher than 135psi if I would have leaned on the starter longer, but I didn't want to kill the battery - so I stayed consistent at the 2-3 seconds for the 2nd reading.

On the temperature, there seemed to be more variation from cylinder to cylinder.
Especially on #5 and #7. #5 ended up with a mis-gapped plug, so I was feeling good that I caught an issue.
But #7 plug gap was bang-on at the .045" I was targeting, so not sure why that temp was also reading lower.
Would a tight plug gap mean a lower temp (less efficient combustion?)
Temps were written down for both before and after plug replacement and gap verification.

No issue on the engine drove this exercise - just time to change the plugs, and I wanted to learn this procedure (I had the tools/equipment, so why not?)
Also - any need to readjust timing or fuel/air mixture after replacing the plugs?

Thanks!

 

1. Why are you gaping the plugs to .045, do you have the stock ignition? Stock ignition is .035.
2. Why are you running a cold plug, R43S?

 

Sorry - Forgot to list that the ignition is updated to HEI, which called for .045" gap.
Not sure what you mean about "cold plug". Am I running wrong plugs all along?

 

An R43S is a very cold plug. The 44 heat range was stock but I believe it is discontinued. Most people have substituted a R45S plug for a typical driver. The plugs will run cleaner than the R43S. You didnt post pictures of the plugs but how would you describe the color of the center ceramic electrode, more towards a dark brown or a lighter brown?

 

Thanks Mel.

I can check the old plugs again tonight, but if I recall, they were pretty dark tan after only 2 years and about 3k miles on them.
Those plugs were installed by me, but can't remember why I picked the 43's originally - maybe that was what was on the engine when I bought it?
Is it worth swapping the new ones to 45's now?
Is there any difference in performance that I'd see? It's not the $25 that's an issue, just my time.
Again - original '69 350/300. HEI upgrade. Possible swapped Cam.
And I typically use a mix of 5 gals of 110 leaded + 10 gals of 90 octane ethanol free when I do a full fill-up.

 

My plugs were on the brownish side, so I'm guessing they weren't reacting well to the heat over the last 2+ years and 3000 miles..
I went ahead and pulled the 43s out and installed 45s that they happened to have in stock at my parts store.
Started up and idled fine, but I didn't have a chance for a test drive yet.
Let me know if there would be any performance differences I could expect right off the bat (between the 43 and 45s), or if it really is mainly a matter of longer term durability of the plugs.
Also - would there be any need to adjust timing and/or carb fuel/air mixture based on the plug change?
Thanks

 

When was the last time you checked timing and engine vacuum at idle? Maybe it's time?

 

Hi Resdoggie - thanks,
Yes - checked vacuum earlier in the summer and was getting a good 17psi on my MightyVac gauge.
I haven't checked timing since last year.
But in both cases, I wasn't sure if changing to different plugs would affect either, leading to a need to re-adjust.

 

It was always recommended to check timing/dwell after changing out the points and plugs. Just a plug change probably not but other wear components can effect timing so it should be checked every so often much like checking the oil level at every fill up.

 

Res, he has an HEI.

There will be zero performance difference between the two heat ranges on a well tuned engine....people just run hotter plugs anymore to keep the strap clean. A hotter plug sticks out in the cylinder further and is exposed to higher temps, the opposite for a colder plug. You want to run as hot of plug as you can without melting the strap or pre-ignition......trucks that pull heavy loads or blower/nitrous engines use a colder plug due to higher cylinder temps. You will be fine with the 45 heat range.

Jebby