cv2065
06-08-2025, 04:16 PM
I was reading up on some MSD documentation for the 6AL Digital box, and came across their recommended starting table for spark plug gaps. When I pulled the Roush plugs to change them out, they had Autolite 605s and were gapped between .028 and .032. I bought some 605s, but they were pre-gapped at .060 out of the box. I thought that was odd and quite a ways from the stock Roush gapping, plus, they were a tapered fit and not with washer, so I went with the 3924s that AFR recommends with those 205 heads and gapped at .035.
MSD recommends the following:
Compression
Up to 10:5:1 (Gap 0.050" - 0.060")
10.5:1 - 13.0:1 (Gap 0.040" - 0.050")
Above 13.0:1 (Gap 0.035" - 0.045")
Do MSD's recommendations make sense?
BUDFIVE
06-08-2025, 06:49 PM
CV,
Below is a cut and paste from the instructions for my Progression Ignition Bluetooth connected smart distributor—
“Correct spark plug gap for
most engines is 0.035”-0.045”. If the engine's compression ratio is above 10.5:1 , spark
plug gap must decrease. Too much spark plug gap increases the chance of crossfire in
small diameter cap distributors. If crossfire is suspected, decrease the gap.”
I should point out that in my 347 with about 10.7:1 compression, I had a high rpm miss on the Dyno (described in post 2 of my build thread). We ran out of time on the Dyno and couldn’t root cause the miss-but 3 things we changed/mitigated in the car since included fuel delivery (Dyno setup was a dead head regulator really meant for EFI), a suspect coil 12V wire crimp, and reducing spark plug gap to .035. I have not had any misses since but haven’t put the hammer down, yet.
I read a lot on the subject and it seems the higher the compression the more difficult it is for the spark to jump the gap at the plug and the more likely it will jump elsewhere. With high performance motors (good spark, and high compression, etc), we don’t need the wide gaps of the smog motors (low compression) of the 80’s. Gaps in the 60’s and 70’s were commonly about .035.
I’m curious what others will say.