Compression PSI

[BILL WILLIAMSON]

Dog here, if I memorize ALL this RED letter stuff & recite it to Vermin, will he act "funny" or run differently??? I hope none of this hoopey applies to the BBQ grille, or we'll never get to EAT! "Let's eat, I'm hungry, please don't think I'm rude, you're sweet & lovely, but I'm in the mood for food, your kiss is thrilling, but LOVE can be renewed, right now, my darling, I'm in the mood for FOOD"!
Buster T.

[MikeK]

I suddenly realized the world is flat, the sun orbits around us, earth is only 6,320 years old, and evolution is a crock. I take back all that nonsense about adiabatic compression. Physics is just plain wrong. What was I thinking?

[Tom Wesenberg]

After all this I need to visit the decompression chamber.

[van Dyck]

Also, a fully warmed up engine causes the ring end gaps to diminish thus providing a more accurate pressure reading.

[Pinstripe]

I agree with MikeK.

[PC/SR]

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The physics is right MikeK, any compression of a gas will increase its temperature/pressure. I just doubt that the heat rise of a 4 or 5 to 1 compression of 50 cubic inches (or less) of air/fuel is going to make a measurable difference on a $35 compression gauge. But I can be persuaded. What do the calculations of the formulas presented come out to?

[Mike V. Florida]

Originally Posted by MikeK

I suddenly realized the world is flat, the sun orbits around us, earth is only 6,320 years old, and evolution is a crock. I take back all that nonsense about adiabatic compression. Physics is just plain wrong. What was I thinking?

Now you got it. Is not the world a friendlier place without all the "noise" about physics?

Since one is compairing each cylinder to the other do we really care about a difference caused by warm gas? Is a pound or three difference from an exact reading change the way the engine works and runs?

Don't worry be happy

[MikeK]

Originally Posted by Mike V. Florida

. . .Since one is compairing each cylinder to the other do we really care about a difference caused by warm gas? Is a pound or three difference from an exact reading change the way the engine works and runs? Don't worry be happy

I think you missed the point here, Mike. The OP asked if his readings were "within specification" and then all that nonsense pile of wrong simplistic figuring to determine a cranking pressure began.

I totally agree cylinder balance (your 'comparing') is a more important factor than a specific numerical goal, but that's not what was asked!

I was simply pointing out in an engineering way the folly of using some simple arithmetic to circumvent finding an actual manufacturer's spec. The posted dismissive replies were not atypical on this forum.

[Tom Wesenberg]

" The posted dismissive replies were not atypical on this forum. "

I thought the same thing about some of the replies. In fact I was also going to mention the heat of compression raising the numbers. Now I'm glad you did instead of me. LOL

[Duffy1]

Keep up the good work MikeK .

[Marco Tahtaras]

Originally Posted by MikeK

I think you missed the point here, Mike. The OP asked if his readings were "within specification" and then all that nonsense pile of wrong simplistic figuring to determine a cranking pressure began.

I totally agree cylinder balance (your 'comparing') is a more important factor than a specific numerical goal, but that's not what was asked!

I was simply pointing out in an engineering way the folly of using some simple arithmetic to circumvent finding an actual manufacturer's spec. The posted dismissive replies were not atypical on this forum.

Thanks Mike. I've attempted to explain this several times over the years on Fordbarn. Unfortunately it never seems to "stick". I suppose it's not the easiest thing to visualize.

Originally Posted by PC/SR

The physics is right MikeK, any compression of a gas will increase its temperature/pressure. I just doubt that the heat rise of a 4 or 5 to 1 compression of 50 cubic inches (or less) of air/fuel is going to make a measurable difference on a $35 compression gauge. But I can be persuaded. What do the calculations of the formulas presented come out to?

As an alternative I've also put out here Ford's simplified estimate which they instructed their service departments to use as "suitable for Ford cars" in the early 1930's. Their recommendation was:

(Atmospheric pressure x Compression ratio) + Atmospheric pressure

So starting with a stock Model A and using the average pressure at sea level of 14.7 where I am would be as follows:

(14.7 x 4.22) + 14.7=76.73 psi

Using the average atmospheric pressure of Denver, CO of about 12.2 we have:

(12.2 x 4.22) + 12.2=63.68 psi

Now, there are two things to keep in mind when looking at Ford's recommendation.

1. This is an estimate.
2. These cars were all relatively low compression engines.

As Mike K noted, compression ratio vs. compression pressure is far from linear. The more the "gasses" are compressed the more they heat. The more they heat, the greater the thermal expansion of those gasses which further increases the pressure. The bottom line is that the more the compression ratio is raised, the greater the PERCENTAGE thermal expansion of the gasses plays in the pressure increase.

Finally, here is an example to illustrate how far off Ford's crude method becomes at higher compression ratios using the Jaguar XKE Roadster I had for a short time as a kid. The compression ratio on the Jag was 9:1 so using Ford's equation we get:

(14.7 x 9) + 14.7=147 psi

However the actual compression pressure by the book was 190 psi and close to that on this particular car. Ford's use of the additional "atmospheric pressure x1" was simply to approximate the the added effects of the gasses. This quickly becomes insufficient as we begin raising the compression ratio.

[Terry, NJ]

Marco and Mikek, Please understand, I was not being dismissive of you. Believe me, anyone who can use such technical language and formulae has my respect. Because there is no linear connection between the compression and the CR.and the variables such as altitude, humidity, cranking speed etc, I used the terms about and approximate to describe the results. But offhand, for a mod A, 60lbs is good compression. In fact, I've never heard of anything better on a stock engine. Who knows? Maybe I should get around more. I'll concede that possibility. All I've ever said was that I think that for all those factors to be included, one would need a higher compression Ratio.I never denied their existance or effect, As I said I have two Diesels and one medium sized compressor. As Mike said, the tube carrying the air to the tank gets hot before the head and cyl. I don't know the displacement of the compressor, but the CR of one diesel is 19-1. With a CR like that, I expect things to happen and they do. But this effect shrinks to almost insignificance with a cold block, low RPMs, few revolutions, and a low CR. As far as this subject is concerned, That's all I'm going to say. I never wanted an argument with Mike. or anyone else, especially over such a mundane subject. All I was trying to do was give Rocaas some very basic rules of thumb to go by.
Terry

Originally Posted by Marco Tahtaras

Thanks Mike. I've attempted to explain this several times over the years on Fordbarn. Unfortunately it never seems to "stick". I suppose it's not the easiest thing to visualize.



As an alternative I've also put out here Ford's simplified estimate which they instructed their service departments to use as "suitable for Ford cars" in the early 1930's. Their recommendation was:

(Atmospheric pressure x Compression ratio) + Atmospheric pressure

So starting with a stock Model A and using the average pressure at sea level of 14.7 where I am would be as follows:

(14.7 x 4.22) + 14.7=76.73 psi

Using the average atmospheric pressure of Denver, CO of about 12.2 we have:

(12.2 x 4.22) + 12.2=63.68 psi

Now, there are two things to keep in mind when looking at Ford's recommendation.

1. This is an estimate.
2. These cars were all relatively low compression engines.

As Mike K noted, compression ratio vs. compression pressure is far from linear. The more the "gasses" are compressed the more they heat. The more they heat, the greater the thermal expansion of those gasses which further increases the pressure. The bottom line is that the more the compression ratio is raised, the greater the PERCENTAGE thermal expansion of the gasses plays in the pressure increase.

Finally, here is an example to illustrate how far off Ford's crude method becomes at higher compression ratios using the Jaguar XKE Roadster I had for a short time as a kid. The compression ratio on the Jag was 9:1 so using Ford's equation we get:

(14.7 x 9) + 14.7=147 psi

However the actual compression pressure by the book was 190 psi and close to that on this particular car. Ford's use of the additional "atmospheric pressure x1" was simply to approximate the the added effects of the gasses. This quickly becomes insufficient as we begin raising the compression ratio.

[BILL WILLIAMSON]

Welcome back, Marco, we hope you haven't been ill or something. Bill W. & Dog.

[jiffy]

Car runs fine? Do not fix what is not broke. John

[Mike V. Florida]

Originally Posted by MikeK

I think you missed the point here, Mike. The OP asked if his readings were "within specification" and then all that nonsense pile of wrong simplistic figuring to determine a cranking pressure began.

I totally agree cylinder balance (your 'comparing') is a more important factor than a specific numerical goal, but that's not what was asked!

I was simply pointing out in an engineering way the folly of using some simple arithmetic to circumvent finding an actual manufacturer's spec. The posted dismissive replies were not atypical on this forum.

Are they truly dimissive or just rules of thumb that work in this case?

If calibrated instruments are used and measurements of the atmospheric pressure and humidity are figured in along with your formulas, this would give a "true" indication of the pressure. Since the question was "Is this within Specification??" and we now have the actual method and formula for the perfect reading, have we posted anywhere here what the low limit for the engine is? Is 60 lbs within spec? Is the pressure really 60lbs?

[30Tudor]

MikeK and Marco. Thanks for the great explanations. It's always great to learn something new! Please continue your great contributions to this forum.

[BILL WILLIAMSON]

Originally Posted by jiffy

Car runs fine? Do not fix what is not broke. John

Man after my own heart, Jifffy! Dog here, bet you got a Dog!
If your Model A's runnin' great & makin' 'em Good sounds, why you even messin' with it? If there was a specified, proper compression spec, how do you know if your gage is correct? If yo' head's been surfaced 3 times or it's bored .100" over, like Vermin, a spec. # don't mean much. Like why do folks always "reading" their plugs when car's runnin' PRIMO? Ol' Glen's about wore out his threads takin' plugs out to "READ" them, when it's runnin' GOOD! I can read our plugs in the car, it reads, "CHAMPION-C16C" Buster T.

[Bearing1]

what are the acceptable lower limits?
My cold engine (not running as yet) has 50psi in all 4 cylinders. reachs 50 on first rev and goes no higher. Only went 3-4 revs. Old engine with new valves and guides. Not run for 25 years. Hope to start soon as gas tank cleaned.

[Tom Wesenberg]

Originally Posted by Bearing1

what are the acceptable lower limits?
My cold engine (not running as yet) has 50psi in all 4 cylinders. reachs 50 on first rev and goes no higher. Only went 3-4 revs. Old engine with new valves and guides. Not run for 25 years. Hope to start soon as gas tank cleaned.

My 28 also has 50 on all 4 cylinders, has good power and doesn't burn oil, so I think that's fine. If all 4 are equal and the engine doesn't burn oil, I'd even settle for as low as 45 lbs. before working it over.

It does have a noise I am going to check out though, so I'll be pulling it for that reason, and use another good short block in the meantime. I think it's piston slap. but won't know for sure until I take it apart.

[Patrick L.]

Originally Posted by Bearing1

what are the acceptable lower limits?
My cold engine (not running as yet) has 50psi in all 4 cylinders. reachs 50 on first rev and goes no higher. Only went 3-4 revs. Old engine with new valves and guides. Not run for 25 years. Hope to start soon as gas tank cleaned.

My 53 year old tired rebuild shows 45#. It runs good, doesn't leak, doesn't use or lose any oil. Even has good power, enough power for me anyway. I've replaced both timing gears and tightened the mains [ rods were fine] a few years ago.