Hoffman900 wrote: ↑12 Feb 2026, 19:35
Pressure sensors aren’t going to show compression RATIO like you some of uou imagine it will. This has been spelled out several times in this thread.
How many of you actually have experience with these sensors and what they can show you? I do.
Nor will the gas displacement method you are advocating necessarily measure it, which presumably measures gas displaced by the piston between TDC and IVC and/or EVO, or maybe it's comparing compression through that same region. If the latter, it's doing what you say can't be done. Those methods will resolve a certain definition of 'compression ratio,' but that's not necessarily what the FIA aims to inspect. They aim to inspect the GCR which is, I would say, independent from valve timing--a geometric figure, not always derivable from a comparison of gas volumes, gas flow, nor a comparison of pressure changes. How they define GCR is important, and we don't know what their definition is.
Martin Keene wrote: ↑13 Feb 2026, 11:01
No it can't. If you advance ignition timing it will increase PCP, but the compression ratio has not changed.
He's talking about cold cranking the ICE. Ignition is irrelevant. A certain definition of CR can be deduced by comparing pressure changes between valve closing and TDC. That's unavoidable.
AR3-GP wrote: ↑13 Feb 2026, 10:58
There are gas laws that relate pressure to cylinder volume. That the connection between cylinder pressure and compression ratio. There are some minor considerations like valve openings and the TJI to be mindful of, but it can be done.
This is fundamentally correct although it won't necessarily provide the GCR owing to variables such as valve timing (as you say), among others. Ideally the valves can be kept closed through 180* either side of TDC, but if the cams can't supply that, you need some other method. Disconnecting the cam gears would be too time consuming. If they're measuring at the track it's presumably an in-situ type of measurement, not an engine teardown.
So what's left?
They could be defining GCR as "compression/expansion between TDC and valve opening" ie "observed compression regardless of valve timing" in which case they are ignoring GCR in the sense of a true comparison of the total volume at BDC with the total volume at TDC. In which case it becomes easier to measure.
If the engine is disassembled then piston travel and the CC can be inspected with probes and optics. This could be cross-referenced to CAD. CAD might be necessary if there are inaccessible voids, as is being suggested. Such voids, including something like a TJI, would make measurement with fluids difficult. Invert head, fill, wait for air bubbles to escape... seems unlikely.
Using a band saw to inspect inaccessible voids would be a non-starter. The concept of an "unmeasurable CC" is interesting to consider in this context.
