I just replaced all plugs, after noticing a bot of a miss at certain RPM range.
Also decided to check compression reading and ended up with:

8 cylinders ~180psi
2 cylinders ~170
1 cylinder ~175
1 cylinder ~168

Are we ok with this spread?

 

Rule of thumb is no more than 10% difference between highest and lowest. Using that standard you're OK.

Overall, though, they're a bit on the low side. You should be in the 200 psi range.

Cheers
DD

 

I agree, a tad low on a good day.

We run 240+ on ours, and since we have 12.5:1 is about right. I think your market is 11.5:1?

Missing at certain rev ranges, mainly higher than 3000, is generally one/both ign coils, but you probably knew that I reckon.

Plug (HT) leads take a pounding on the V12, so anything over 6 years old, would be getting tired by now.

 

Had you got WOT when you checked the figures ?

 

Engine is original, from 1982, never rebuilt and seems to pull like bad out of hell.
I did measure with WOT, using my gear reduction starter.

 

Does the gear reduction gadgy(no idea what that is !!) crank more slowly than the starter ?

 

According to this chart, I am very close to factory 11.5:1 ratio, for a 31 year old motor:

Compression Ratio Estimated Cylinder Pressure @ TDC
given 60% volumetric efficiency at
cranking speed
Percent
Change from
8:1 CR
Nominal Dynamic*
PTDC = 1atmosphere · volumetric efficiency · nominal CR1.3[corrected air-gasoline specific
heat factor]=
for 10:1 CR & 60% VE, then 14.7 psi · 0.6 · 25.119 = 176 psi – 14.7 psi = 161
psi gauge reading
7:1 4.2:1 96 psi gauge reading -18%
8:1 4.8:1 117 psi gauge reading 0%
9:1 5.4:1 139 psi gauge reading 19%
10:1 6.0:1 161 psi gauge reading 38%
11:1 6.6:1 184 psi gauge reading 58%
12:1 7.2:1 208 psi gauge reading 78%