Compression ratio & PSI

[Fahlin Racing]

Is there a great difference of PSI reading of a comp ratio of 8:1 - 8.9:1 compared to a psi reading in the area of 9:1 - 9.9:1 CR. What would the PSI reading go to if there was a change like this... 15 more? 20 more psi after CR is changed.I know there is a difference in general from just doing compression tests of any engine.

But in example.... say a 460 Ford with 9:1 before the CR is raised to say..... 10:1

Is there a substantial PSI level rise, or is there just a smaller rise in the PSI reading seen on the gauge?

[#84Dave]

It's tough to tell what the delta of cranking compression would be. Depends to some degree on the cam in the engine. Engine cranking speed is so low that the PSI results may not reflect much as to what happens in the running engine. It wouldn't surprise me at all if you observed a greater PSI increase by advancing the cam 4-6 degrees, than you'd observe going from 9:1 to 10:1 on the static compression ratio. Depending on the cam duration. -Dave-

[David Redszus]

Not knowing the exact details of the engine design such as inlet valve close angle, stroke, conrod length and ambient pressure, the following data can only be considered a close approximation. Piston ring leakage has been ignored.

CR 8.0 = TCR = 6.32 = 170.5 psi
CR 8.9 = TCR = 7.01 = 195.6 psi
CR 9.0 = TCR = 7.08 = 198.5 psi
CR 9.9 = TCR = 7.77 = 224.6 psi
where CR = geometric compression, TCR = trapped comression ratio

At cranking speeds, pulse tuning pressure can be disregarded.
In a running engine, pulse tuning pressure must be included.

Perhaps more important than the pressure produced by compression is the compression temperature and squish velocity.

[Warpspeed]

Yes, there are so many variables, like leak down rate, valve timing, and cranking speed that small changes do not really mean much, especially after the engine has been rebuilt, or changed in some way.

[PackardV8]

As David's chart shows, in theory, an engine might show 25-30 more PSI of cranking compression when going from 8:1 to 9:1. However, cranking compression is dependent upon intake valve closing. Less cam duration/early closing cams will be more straight line with the theoretical equation. Longer cams with later closing lose so much cranking compression, what he calls trapped compression ratio or otherwise known as dynamic compression ratio, they often do not increase linearly.

[Fahlin Racing]

So if the longer duration & lobe lift the less cranking compression or dynamic compression occurs?

Lets say leak down rate is at the minimum (whichever you have seen I do not know)
AND
Connecting rod length of all FOMOCO factory 460s is 6.605
AND
This info I am using below is from Bill Carroll's Fooorrd V8 performance guide The "stockers" bible that I bought a while back.

-Intake seat angle for the 460 is 45-o
-Cylinder head part # D0VZ-D uses a 2.080 intake valve
-1.85" by 2.56" Intake port dimension
-The compression of these heads fed by a 2v is 10.5 & if it is fed by a 4v the CR is 11.0 (does not state the CCs of the chamber itself)
-CAM specs : Part #C85SZ-A
*Intake valves*
-Opens at 24-o BTDC
-Closes at 86-o ABDC
-Duration 290-o
-.442 lift
*Exhaust valves*
-Opens at 78-o BBDC
-Closes at 46-o ATDC
-Duration 304-o
-.486 lift
Since this engine is pre-1972 460 the timing is straight up at Zero. And is also a internally balanced 460 model.
-Rocker ratio is 1.75
-Piston pin offset is .0625 right
-Compression height, I have no clue what was used in 1971
Piston dish, if there was any dished style pistons in the first couple years of the 1970s, I havent found the CC rating of the pistons if they are not flat top style. (maybe somebody knows that here)
All this listed is from the years 1969 - 1971. Though I do not know if the camshaft was used in the base 460, which I doubt since the duration is pretty large. It might have been a PI or other Hi-po model. I just chose it for an example. As for if it were to be overbored, that can range tremendously with these cylinder blocks being a suggested .030 w/ NO sonic testing to be on the safe side, to as much as .140 overbore (that doesnt happen very often).

As for cranking speed -> chose any speed you would like that seems normal or "in range". I have not thought about the cranking speed thorough enough until now when I read all of your replies.

For the static CR of these heads etc lets use the 2V model w/ 10.5:1 with this cam profile & cyliner head I listed.

If we need the CC rating to help out, tell me, I will try and find it if possible. Thanks you guys.

[Fahlin Racing]

I found this in my notes laying around. In 1972, Ford went from 75cc closed chamber to 100+ cc open chamber heads. The 1970 & '71 heads, regardless of part number might be 75cc rated chambers, but not 100% sure. I will have to research them sometime soon to verify.

[Warpspeed]

Cranking speed is important, because it changes the final temperature rise of the air during compression.

Assuming a stone cold engine, and zero leakage past rings and valves, the air compresses, and the temperature of the air always rises due to that compression.
But that hot compressed air does not stay hot for long, when all the surrounding metal parts in the engine are completely stone cold.

If the engine spins over very fast, the air cooling effect will be less, because there is a far shorter time for the air to cool, and you will measure a higher compression pressure.

If the (leak free) engine is turned over very slowly, the compressed air has time to cool, and shrink, and the measured compression pressure will be much lower.

It is the difference between adiabatic and isothermal compression, and the difference between the two, is the amount of heat that escapes into the surroundings while compressing a gas.

[Stan Weiss]

When I run your event numbers I come up with the same duration as you posted.
I also get LSA of 113.5 degrees with an ICL 0f 121 and an ECL 106.
Using Bore 4.362" Stroke 3.85" Rod Legth 6.605" CR 10.5:1
Wrist Pin Offset 0.0625 DCR 6.800315
Wrist Pin Offset 0.0000 DCR 6.785365
Wrist Pin Offset -0.0625 DCR 6.770165

[Fahlin Racing]

Cranking speed is important, because it changes the final temperature rise of the air during compression.......

..........If the engine spins over very fast, the air cooling effect will be less, because there is a far shorter time for the air to cool, and you will measure a higher compression pressure.

............If the (leak free) engine is turned over very slowly, the compressed air has time to cool, and shrink, and the measured compression pressure will be much lower................

Since the cranking speed affects the temperature rise in compressed air. A weak starting system can be one reason why sluggish start would occur then woudlnt it? Also this would have to pertain to the environment of the place the vehicle is in too. Colder climate, the engine material will "pull" the heat out of the air into the iron causing the faster cooling compared to a warmer climate, with less heat transfer to the engine parts.

Wouldn't a warm climate with high humidity aid in a higher compression reading also? Unless high or low humidity at any elevation does not affect this enough to see a noticeable rise.

[Warpspeed]

In America the Rankine and Fahrenheit scales are appropriate.
Everywhere else in the world it is Kelvin and Celsius.

On a freezing cold day the temperature may be about 492 degrees Rankine (32F), on a nice warm day 542 degrees Rankine (82F) say fifty degrees hotter or very roughly ten percent.
http://en.wikipedia.org/wiki/Rankine_scale
So the ambient temperature does not make as big a difference as you might expect to compression readings. But it will make some difference.

The problem with all of this is that you cannot reliably predict from theory, what all the variables and unknowns are going to produce in the way of an actual cranking pressure reading. Some of the cheaper gauges are not that accurate either.

You just need to know from experience if what you are reading on your compression gauge is fairly normal for that particular type of engine, tested in the way you normally do compression testing..

In a way it is a bit like leak down testing.

You measure a fresh built engine. You measure again once the engine has been fired and broken in. And you measure again after some high mileage. The actual figures don't matter that much, but what you are looking for are changes, and cylinder to cylinder variations that can indicate problems.

[Fahlin Racing]

Since the temperature differences in ambient air affects this process very little but there is the general heat transfer at both levels (cold to warm), one quicker & the other slower with each cranking speed. Humidity level has no effect on the compression reading itself.

A recap: the PSI reading on a gauge will not rise in a linear direction as compression rises or lowers. All the variables involved will make it very difficult to "predict" what will be happening inside the engine which is more understandably known (to an extent) by experience like you mentioned engine to engine testing.
AND
....The Compression test is mainly meant for determining what repairs might be needed in the future (IE rings).

If the temperature & humidity differences affect it a very small amount (if nothing at all). Would ELEVATION have more of an affect in the cranking pressure. I know higher elevations mess with A/F ratios but does that mean it affects the cranking pressure to an extent also?

[Warpspeed]

I have not really thought about the effects of elevation. Where I am, it is dead flat.

But I suppose, less dense air will compress into less dense compressed air, everything else remaining equal.
It keeps coming back to using these tests to get a known bench mark cranking pressure, and keeping that as a reference for future testing on the same (or similar) engine..

[Fahlin Racing]

Alright, thank you for all your input and thank you to everyone elses replies!

[Stan Weiss]

Even at the same elevation, temperature, and humidity different Barometric Pressure will change the readings lets say the Barometric Pressure is 29.92 and the cranking Pressure is 186.3 at a Barometric Pressure of 28.92 the cranking Pressure is 180.1. If you do a search this has been talked about here before.