Mercury 500 HP Cranking Compression

[SC288]

Has anyone completed a compression test on a healthy Mercury 500HP (Carb) engine? If so what is the average. Thanks in advance!

[DollaBill]

pretty sure the spec is online but remember it's more about consistency

[MILD THUNDER]

I think you want consistancy. Doing it on a warm engine is more accurate. For example, if you have 145-155 on all cylinders and 1 or 2 are only at 110, you have something going on. Oh, and dont forget to keep the throttle open while doing it.

[SC288]

I just do not know if the engines in question were rebuilt to stock or if the compression ratio was bumped up....the cranking compression should explain this.

[PatriYacht]

Hard to tell without opening up an engine. A cam with longer than stock duration will cause the engine to have lower cranking compression while higher comp. pistons will cause it to have higher. Add these two together and you might have the same cranking compression as stock. Can't you ask the original owner or mechanic?

[Chris]

Compression tests tell little. Do a leakdown.

[WETTE VETTE]

To re-phrase the question. Should a healthy broken in but not worn out HP 500 carb have a cranking compression of 150 average or 180 average? No reason to do a leakdown as the compression is consistent across the board and a leakdown does nothing to predict dynamic compression. The motor was rebuilt and getting info from the builder may be difficult since it has been a few years since the re-fresh and the original owner is not known. Just trying to use this as a tool to decide if it was rebuilt to OEM spec. The lake test among other things will also be done, but for now this is what we have available. I realize its possible for the compression / cam to be altered from OEM and still net the same cranking compression. Just trying to get a baseline to compare against.

Thanks,

Craig

[Airpacker]

To re-phrase the question. Should a healthy broken in but not worn out HP 500 carb have a cranking compression of 150 average or 180 average? No reason to do a leakdown as the compression is consistent across the board and a leakdown does nothing to predict dynamic compression. The motor was rebuilt and getting info from the builder may be difficult since it has been a few years since the re-fresh and the original owner is not known. Just trying to use this as a tool to decide if it was rebuilt to OEM spec. The lake test among other things will also be done, but for now this is what we have available. I realize its possible for the compression / cam to be altered from OEM and still net the same cranking compression. Just trying to get a baseline to compare against.

Thanks,

Craig

You could always pop a pair of rocker arms off and cc the cylinders with oil.
Divide BDC volume by TDC volume and presto, static comp ratio.

[WETTE VETTE]

You could always pop a pair of rocker arms off and cc the cylinders with oil.
Divide BDC volume by TDC volume and presto, static comp ratio.

Very true and that may need to be done. It still would be nice to hear from someone who has checked the cranking compression on one of these so as to establish a baseline. I cannot imagine the cranking compression being much more than 140 PSI with 8.75:1 compression.

[SC288]

pretty sure the spec is online but remember it's more about consistency

I have looked for the spec. but have been unsuccessful....any ideas?

[obsessed1]

I checked mine with the throttle wfo and it was between 142 and 147 on all cyls. And it had some hours on it.

[PatriYacht]

Stock they are around 150.