Cams and Compression

[digger]

overlap has almost nothing to do with cranking compression as the engine isnt being fired and is only running at a few hundred rpm , it is the inlet valve closing (IVC which is in degrees after BDC) late that reduces compression. there are some online calculators that will tell you the compression with a cam with IVC 'X' and then you can see what happens if IVC is chnaged to 'Y'

Cams will drop compression all else equal, a 24* duration increase will mean about 12* later IVC (same centreline) which is easily a 20psi or more reduction on a cranking test

i wouldnt worry about cranking compressoin except for diagnostic purposes as the s54 is not a fixed timed engine, so more doesnt mean much

[STAATS]

Here is some background theory that has prompted me to start asking the question based on the experience I am having:

https://www.crankshaftcoalition.com/wiki/Cam_and_compression_ratio_compatibility

Now obviously we have variable valve timing (VANOS) so it gets a tad more complicated and that why we can have smooth idle and still get power up high BUT I think think that when the spark plugs and fuel pump are disconnected the VANOS won't act and as a result shouldn't change a compression test so hotter cams should drop compression test figures... I am just not sure by how much and/or if all my above VANOS assumptions are correct... hence its hard for me to glean any practical take home from the theory linked other than I need more knowledge.

[STAATS]

Yer I am surprised I couldn't find anything within the E46 M3 community after scouring the internet. There is many articles on how cams impact compression but in our community the only comp numbers I can find seem to be stock numbers. Cams for this engine have become and extremely standard mod in this community in the last 10yrs so it's interesting that this topic hasn't come out sooner.

[maupineda]

The spec per ISTA+ says 12 to 14 bar, which is 168 to 196. A bit of a large range, but maybe that is dependant on what temp the engine is and the position of the rings at the time of measure, so I understand why the wide range.

If the aftermarket cams affect the overlap much then yes, the compression reading may go lower, you would need the datasheet of both cams to do some analysis. Your question is very interesting indeed, I hope someone knows based on the engineering spec of the listed cams.

[STAATS]

I know a healthy stock S54 should be 160-175psi compression per cylinder (with spread tolerance) but given the VANOS shouldn't do anything when doing a compression test I can only imagine that the static compression of cammed engines is lower. Assuming stock CR build does anyone know the target / healthy range compression numbers for the various sets of Schrick cams? Ie 280/272, 288/280 etc... specifically I am interested in the 280/272 but figured I'd open the floor in the topic it all...