Can new spark plugs make a noticeable difference in performance?

Yes, depending on how cruddy and worn the old ones were.

No, but noticeable if things were overdue.

Do note that when plugs are changed, the timing is usually checked as well so there’s that.


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I have not noticed a difference between new and “old/worn, but functioning” plugs.
They are either ‘on’ or ‘off’ for me.
 
depends....how bad were the old ones?o_O


but, doubtful if the old ones were working.
 
I just changed out my Champion REM 38E massive electrode plugs for the Tempest equivalent, at 330 Hobbs hours of use. Very noticeable improvement in ease of starting. Checked magneto timing, and right mag was off only 1 degree from 25 BTDC.

I didn't bench-test the old plugs, but I noticed the Champions were the old design (slotted contact on top; new, improved design is smooth) that have a history of developing high resistance.
 
Doug Reid said:
Noticeable improvement going from rem40e to rem37by in my Jacobs engine. Less prone to fouling . Unless a plug was defective, I never noticed any difference when replacing an old rem40e with another new rem40e in my Bonanza.
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I used to replace all the REM38/40 plugs with the REM37BY (better: UREM37BY) plugs in any engine that had them listed as alternates. Those plugs are indeed fouling-resistant, and I think they actually fire more consistently because the electrodes are sticking a little farther into the cylinder and the air/fuel mixture. Sometimes I've wondered if the exhaust gases all get scavenged out of the deep wells in the 38/40 plugs.
 
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