cylinder compression?

[mihunter]

does anyone know what the cylinder compression should be on the 258? thanks

[mudkicker715]

Assuming stock 160 to165 psi.

[ammachine390]

For an 81 258, the compression should 120-150 psi with 30 psi max variation between cylinders.

[BenM]

The earlier engines will be less, they went to higher compression heads it 82. Very early AMC engines were about 8.0:1, by the early Eagles it was 8.2:1, and the post-81 Eagles were 8.6:1.

[Jurjen]

So considering that both inlet and exhaust valves are already closed in BDC, you can multiply the ratio by the ambient air pressure: 15psi.
Ratio 1:8 should give you 120psi.
Note that ambient is much lower when your living in the hills.

[tougeagle]

I'm running 127-135 in mine, at 5000 ft... I wonder what that works out to compressionwise....

[Jurjen]

That 's easy enough:

http://www.engineeringtoolbox.com/air-altitude-density-volume-d_195.html

Scroll down a bit to the table and you will find ambient pressure at 5000ft is 12.23 psi.
So your compression is well above 10:1 (130/12.23).
That is very well possible with the 4.0 head and EFI with knock sensor.

[tougeagle]

That 's easy enough:

http://www.engineeringtoolbox.com/air-altitude-density-volume-d_195.html

Scroll down a bit to the table and you will find ambient pressure at 5000ft is 12.23 psi.
So your compression is well above 10:1 (130/12.23).
That is very well possible with the 4.0 head and EFI with knock sensor.

I'm actually kind of surprised that it's above 10... I wonder if they zeroed the deck on the block when they rebuilt the motor. Though, the 4.0 harness up until at least 1996 did not include a knock sensor...

[mudkicker715]

Jurjen I would never doubt math.   But I have seenon a couple old motors 150psi tops.lowest cyl on them was iirc 120psi.

Gotta go with my 160 to 165psi.

[Jurjen]

There can be other factors too, like doing a compression test on a warm engine.

[mudkicker715]

It was cold.  carbon?

[BenM]

Perhaps dynamic vs static compression figures. Air can only move so fast, so a cam can bleed pressure off or help build pressure up. That may be affecting readings.