Low compression on #6

Car is a (recently) built 5.8L, stock cams, 3.2 Kenne Bell. All the goodies. 878/832 at the wheels.

Noticed the car sounded like it was lightly missing when the engine was under load... just did a compression test and #6 is especially low at 100psi.

complete compression test results are as follows...
1 = 172
2 = 168
3 = 168
4 = 165
5 = 155
6 = 100
7 = 140
8 = 153

I'm not exactly sure what could be the issue. Maybe valves???

It's low enough to where I'd probably pull the engine and the driver's side head to check it out. It could be a head gasket. #7 seems to be an outlier too.

Pull the head and inspect cylinder walls. Hopefully just a rebuild on the head,

I'd do a leak-down test before pulling it apart. At least to know exactly where to look when you get it apart.

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If it was a head gasket wouldn’t I be seeing oil in the coolant?

Im wondering if this isn’t valve related.

What year cams?
How has it been driven since rebuilt? Datalogging?
That #6 cylinder definitely needs to be evaluated.
Bore scope?
-J

It’s a 2011.
Car hasn’t been driven very hard. Never taken to the track. In fact when the issue presented itself I was on a road trip just doing highway speeds. Of course when I passed people I geared down and had some fun but I wasn’t anywhere near redline.

What did the plugs look like?

Who built your motor?

The plugs are mint and the motor was built by Performance Plus in Saskatoon

you dont always get water in the oil or oil in the water with a head gasket, hell my lightning was leaking coolant out the side of the motor when it let go

Leak down test will really tell you more, fingers crossed it could be as simple as a leaking valve seat.

Edit:
Looked at your compression numbers again and really your entire driver side bank is questionable, its 11% to 72% down compared to cylinder #1.

1 =172 = 0%
2 = 168 = 2%
3 = 168 = 2%
4 = 165 = 4%
5 = 155 = 11%
6 = 100 = 72%
7 = 140 = 23%
8 = 153 = 12%

Hopefully it could be confined to the head and its something like a head gasket, cam timing off 1 tooth or bad valve seats.