Spark plug gap for high(er) compression head

philcobill

philcobill

Senior Member
The engine on our '29 Special Coupe has high compression aluminum head. I remember reading that the spark plug gap for higher compression heads should be narrower (0.030 in) vice 0.035 in for stock heads. (I wish I could find where I read that.)



Is this something that I should be concerned about? What difference would a narrower spark plug gap make?

Bill
 
6.5 compression aprox, 3X plugs, and I set the gap to 25 thou. Works for me. It is fun to experiment, try it.
 
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J Franklin said:
6.5 compression aprox and I set the gap to 25 thou. Works for me. It is fun to experiment, try it.
Click to expand...


I think my head is 5.5-6.0 to one. My engine is stuttering on acceleration and higher speeds, but don't know if it is due to a wider spark plug gap, or something else.

The plugs are currently 35 thou, so I imagine I will start with 30 thousandths.

Bill
Walworth, NY
 
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My engine with a 6.0 iron head didn't like 3X plugs. I went with W18's reduced gap to .032 to .030, also mine likes more fuel at higher rpm's.
 
Philcobill I was part of the last thread where you had the no start problem...not trying to get back into all that but did you manually set the timing with a test light per Les Andrews red book or did you use the Nu Rex wrench? That wrench is great on the roadside but I too was running poorly until I manually set the timing according to the book, including the turning of the cam (I believe Counter Clockwise to remove slack) and getting the gap in the points correct. If you don't have the red book please get that. The timing could be causing your stutter.
 
High pressure will restrict the spark. Air works like a resistor. The higher the pressure the higher the resistance. Years ago there were plug and ignition testers made that tested the spark in a chamber with compressed air.
 
Normally one would reduce the gap on a higher compression head. But, 5.5 or 6 isn't really isn't too high. Therefore it would mainly have to do with available voltage. The old coils were 20-25,000v coils, new ones such as most epoxy coils are in the 40-45,000v range so plug gap with them isn't much of an issue.
 
After reading this thread I reset my plugs from .035 to .030, I have 5.5 heads, and I thought my car ran great with the plugs set @ .035. It now runs even better, smoother
idle, faster response on the gas pedal.
 
Well, I first poo hooded this but you guys have something here! In my pickup I'm using Autolite 3076 set at 35. I tried 32 and it really made a difference in engine smoothness and acceleration. Immediately noticeable. I also have a coupe with the repro 3X plugs that always had less performance than the pickup. I put the Autolite, set at 30 and put them in the coupe. Another immediate difference with both heads 5.5 from Snyders. On another more "scientific" Model a trial I have a test hill down the street from me. Always thought if I could reach 35mph at a certain point the car was running as it should. So using the 30 plug gap, the truck has gained 3 mph and the coupe 5. I'm sold! You can learn so much from others on this site!
I've had my coupe from before high school and like Henry didn't want to change ...sure glad I did. Performs like a different car!!
And yes, the top brass spark plug nuts can be found at Snyder's Model T section in metric.
 
I have a high compression head as well and have been reading this thread with interest.
I average 5000 miles a year on my A so am quite familiar with all of its groans and moans etc. I set my plugs at.30 gap, after test driving, it did run smoother and seemed to have a bit more acceleration. I am scared to tell my Model A buds as many will think I am committing heresy by setting my plug gaps at 30.
Shhhh, lets keep this our secret :cool:
Gerry Birch Bay WA
 
My 28 Roadster just has a common 5.5 high compression head, plugs set at .035. So, the other day I re-gapped the plugs to .030 and off I went. Now “This dog wants to run”, thanks for the nugget.

Mike
 
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