Packard Forum Index » Post-War (1946-54)

Good morning,

Are there any factory spec callouts for what the cylinder compression should be on the 359 CID engine? I’ve viewed various threads regarding this in the forum, but I don’t seem to find anything that nails down what it should be.

Here is my situation:
1. All the work on the block was completed, e.g., cleaned, checked for cracks, sleeves in 3 cylinders, valve job, etc.
2. Pistons deemed to be good, so new rings installed, new bearings installed, etc.
3. Aluminum head cleaned/inspected. It was not milled.

With all the above completed, the engine is ‘buttoned up’, but not installed back into the car. Installed an Autolite starter and the engine cranked Cylinder compression measured and is consistent in each cylinder at about 75 psi. I think we were expecting it to be higher, but pleased it was consistent.

I’ve seen threads that indicate compression in running engines to be in the 130’s and 140’s and one that indicated even lower compression in the 90’s, but these were not for the 359 engine. One responder even stated that as long as the engine is running good, no smoke, etc., then the lower compression should be “fine for now”.

Whereas I’m encouraged that the consistent compression from cylinder to cylinder is a good indicator, is it enough? My thinking is that it will increase with lubrication once it’s running and rings settle in, etc.

So . . . .

1. Is there a factory callout for cylinder compression?

2. If not, it there a consensus among ‘the Packard community’ what a reasonable number would be for an engine at the stage mine is?

Thanks again for any input/guidance!

As I understand things, if you just rebuilt the engine, it needs some time to break in and let the pistons, rings, and cylinders wear together before it will reach peak mechanical performance. So presumably the compression will read higher 500mi from now.

I can't find a list of normal compression numbers, however I did see that it says to make sure to have all the plugs out when testing compression. I would assume that's how you did it but I thought I'd mention.

Thanks for the quick reply!

You and I are thinking the same, so I feel better that I’m not out in left field somewhere!

When compression was checked, yes, all plugs were out. We considered putting some oil in each cylinder, but decided against that at this time, so the only lubrication the rings have is residual from when the pistons were installed.

I don't have a 359, I have a 327, which has an 8.0 compression ratio. I get about 120 psi compression when running a compression test after having rebuilt my engine, though I didn't bore the cylinders, just new pistons and rings. I only tested the compression after having got the new engine started. Folks say you don't want to run the engine purely at idle for break-in, you want to rev it up and down the rev range some.

I have seen this rule of thumb used - multiply the pressure at your elevation (here I use sea level) by the compression ratio to get an approximate number:

14.7 psi*8 = 120

The 359 has an 8.1:1 compression ratio, IIRC, so you should get about 130 psi.

Compression and Leakdown test

75 psi seems pretty low to me, but the rings do seat with break-in, and I would put a little bit of oil in the cylinders if you are doing this test before running the engine at all.

Other folks should weigh in with more experience than myself, having only done this once.

more than a rule of thumb - pretty much the definition of compression ratio. Check your math - 8.1 * 14.7 = 119 psi

I'd blame unseated rings.


First I've seen of the "rule of thumb."


I spend very little time at sea level.


Air pressure drops about an inch of Mercury for every 1000' of altitude. So if I did this correctly, air pressure at Fred's house is about 14.21 PSI. That times an 8.1:1 CR yields 115.1 pounds. For what it's worth . . . .

Seems there is at least a consensus on how to calculate - use pressure at altitude expressed in pounds per square inch multiplied by the compression rating. Because I’m at just a skosh above sea level and the compression ratio per spec for the 359 used in the 5431’s is 8.7:1, I should expect to see ABOUT 127 psi if all the planets are in alignment! LOL

I think we’ll move forward by putting a bit of oil in the cylinders (maybe one to start) to see if there is any difference. Then, go from there.

Thanks again!

Hi There,

In terms of my math, I was rounding up:

8.0*14.7 = 117.6

I rounded up because the formula is not exact for all kinds of reasons, the biggest (three) are that 14.7 is atmospheric pressure, so it would read 0 on the gage.

So one might want to calculate

8.0*14.7 - 14.7 = 102.9

But, when you are doing a compression test dynamically (e.g. using the starter or perhaps with the engine at idle) the air is being compressed largely adiabatically, so it heats up and that increases the pressure reading.

And finally, how long the intake valve stays open will also effect this value.

Here's a chart that "Owen Dyneto" posted, don't know the provenance, but it has higher values than the "rule of thumb", and it doesn't go up to 8.0 values of compression ratio or above, so from an earlier era.

Compression reading versus compression ratio and altitude

Here is what Isky Cams says about the effect of cam duration:

Isky Cams: Compression Gage Readings and Cam Duration

Here is a nice summary of how to do initial startup and break in of an engine, from Engine Builder magazine:

Engine Builder Magazine: How to break in your piston rings

The engine should be at operating temperature also. You're good to go with even unwet compression at this point. Unless something was done wrong, like the ring gap, valves, etc.
I generally recommend to run rebuilds on the dyno or engine stand before installing, because if there's a problem, you don't want to discover it on the car and have to yank it out again. A common malady - cheap insurance. I had them dyno the Cad motor and break it in that way. That way I didn't have to worry about if it was right.
ps: you should subtract 14.7 (at sea level) from the total to get it to "gauge" pressure. 14.7 psia = 0 psig(auge) Don't over think it.

Extrapolating Owen Dyneto's chart, the values at sea level would be:
8.0: 141
9.0: 162
10.0: 183

with in-between values linearly interpolated, and 1% should be subtracted for each 200 ft above sea level, up to 5000ft.