RonD, questions about compression ratio calculation

[pjtoledo]

4.0l OHV engine runs 9.0:1 compression ratio, multiplier for sea level compression test is 18
18 X 9 = 162
So 162 PSI on compression test should be expected on lower mile COLD engine
18 is from 15 psi air pressure at sea level(14.7psi), + 3 for mechanical action of engine compressing the air

I pulled this from a recent thread. my question is mostly about the "multiplier".
I'm fully versed in using the volumes to calculate CR, I want to do it backwards.
I'm trying to get a handle on determining the compression ratio from the compression test results.
theoretical 175 PSI cranking divided by "multiplier" should get me close to CR,,,right?
getting the correct value for the multiplier is what I want. in the above example it's just local pressure plus 3.
I realize that atmospheric pressure will affect the results, a check of the local weather should get me close on that number.
yes, I'll convert the Hg to PSI. (if I can find my old physics book)
the +3 for mechanical action has me wondering,, is that a "standard" for a healthy engine?
any other variables?

if I use 175 psi test results, and 14.5psi local pressure, I'm at 650' above sea level,
175 PSI /(14.5 + 3) = 10 CR

I realize the accuracy may be a bit off, just trying to get close.
comments anybody?

 

[RonD]

The 18 was actually 18.3 cold cranking at 200rpm with newer engine, under 50k miles
Warm engine means warm air which means higher pressure

I just use 18 as an example, and because it's easier and closer to what you will see in our Rangers, older engines lol

Not sure you can do the calculation backwards, i.e. 175 / 18 = 9.7 ratio

Even 18 x 9.7 = 175 is a guesstimate there is no real math to support it except for the air pressure at sea level, 14.7
The mechanical compression of 3 is totally made up number but works for gasoline engines

 

[rusty ol ranger]

Uniformity among cylinders is more important then overall PSI. IIRC a gas engine only needs like 90psi to run properly and you wont even notice "weakness" till you drop dorn in the 115 range.

 

[alwaysFlOoReD]

This intrigues me. I have bought a built 2.3l engine. The builder has passed and there is no notes. Without tearing down the motor I'd like to get a rough idea of CR so I know what fuel I can use.

 

[pjtoledo]

Uniformity among cylinders is more important then overall PSI. IIRC a gas engine only needs like 90psi to run properly and you wont even notice "weakness" till you drop dorn in the 115 range.

this is just hypothetical, I'm not measuring an actual engine,,,,,,yet

thanks Ron

 

[pjtoledo]

This intrigues me. I have bought a built 2.3l engine. The builder has passed and there is no notes. Without tearing down the motor I'd like to get a rough idea of CR so I know what fuel I can use.

that's kind of what I'm looking into.

is there a correlation of cylinder PSI vs octane needed?

example,, 145 PSI could get away with 87, 170 PSI needs 89, 205 needs 93?

 

[RonD]

You can get heat pinging with almost any compression ratio, but generally speaking 9.4:1 is about the upper limit for 87octane, which is about 170psi COLD engine compression

At 9.5:1 and up engine makers start putting in Knock sensors to allow buyers to run regular gas, i.e. the 4.0l SOHC runs 9.7:1 and uses Knock sensor, but the 2.9l OHV ran with knock sensor for a few years using 9.0:1 ratio

Pinging can be from other reasons not related to compression temperature

 

[alwaysFlOoReD]

I will eventually tear the motor down, I just don't have a place right now. This 2.3 came out of a Ranger that was racing at some salt flats. Story is he held the record in his class for a few hours. I couldn't find any evidence of it. It should be interesting to tear down. It has custom mechanical fuel injection so I suspect it was run in the 1980's to maybe mid 90's.