Model A compression

marty in Ohio

marty in Ohio

Well-known member
There has been a lot of discussion about compression ratios on a Model A. What was the ratio of a new car back in the day, and what can we expect on our cars today with original equipment?
Marty
 
Let me reflect this question back to you - if there’s been a lot of discussion about it, and you’ve read the discussion, put forward what you think the consensus is. No one will be mad if you’re wrong.
 
I'll start ---- when it first rolled off the line the compression ratio was 4/1 which would put your compression pressure at about 70 psi.

Sorry about my math: Not 70 psi but more like 60 psi
 
Last edited:
marty in Ohio said:
There has been a lot of discussion about compression ratios on a Model A. What was the ratio of a new car back in the day, and what can we expect on our cars today with original equipment?
Marty
Click to expand...

"Theoretical" Compression at sea level would be 61.74 psi, 4.2 (stock compression ratio) X 14.7 (atmospheric pressure at sea level).

But you asked "What was the ratio of a new car back in the day and what can we expect on our cars today with original equipment?" Which would be 4.2:1. The compression ratio would be the same today if nothing has changed in the engine, i.e. overbore cylinders, milling head, etc.
 
Last edited:
So, in order to get a more accurate number, what would the formula be for overbore?

Does anyone know?

14.7 X compression ratio x (overbore)?
 
old31 said:
So, in order to get a more accurate number, what would the formula be for overbore?

Does anyone know?

14.7 X compression ratio x (overbore)?
Click to expand...

You would have to calculate the new compression ratio after overbore (cylinder volume+combustion chamber volume/combustion chamber volume) X 14.7.
 
old31 said:
Call me slow.

So, what is the formula for a 6.1 head with a .060 overbore?
Click to expand...

See post number 10. The formula is the same. The trick is knowing the cylinder volume and combustion chamber volume. Use the atmospheric pressure for your altitude.

You should treat the result of the calculation as a theoretical compression ratio and a theoretical compression pressure calculation. Your compression pressure measurement will most likely never be the theoretical compression pressure. There are just too many unknowns in the pressure measuring process such as leakage past the rings/valves, the calibration of your pressure measuring gage, sealing your gage in the plug hole, carbon build up on the piston and in the head, and ---- on and on.
 
The compression ratio, as has been pointed out, is how much the total fuel/air mixture volume is compressed. If we make some wide simplifications, the initial volume to be compressed is the displacement volume of the cylinder plus the combustion chamber volume. The final volume is that same combustion chamber volume.

Model A stock published displacement is 200.5 cubic inches for four cylinders, or 50.13 cubic inches for each cylinder. For a 6.1:1 compression ratio, the formula would be

cylinder displacement (i.e., 50.13) + combustion chamber volume / combustion chamber volume = 6.1

Solving, this would give a combustion chamber volume of 9.83 cubic inches.

A 0.060" overbore increases the cylinder displacement by 1.55%, to 50.90 cubic inches. That volume, plus the combustion chamber volume of 9.83 cubic inches, is being compressed into the same 9.83 cubic inches, so the theoretical compression ratio will be 50.90 + 9.83 / 9.83 = 6.18:1. Not much difference. And, as Bruce pointed out, it's all theoretical anyhow based upon a whole bunch of assumptions (perfect seals, equal pressures in all locations at all times, etc.).

JayJay
 
Y-Blockhead said:
"Theoretical" Compression at sea level would be 61.74 psi, 4.2 (stock compression ratio) X 14.7 (atmospheric pressure at sea level).
Click to expand...

JayJay said:
Not much difference. And, as Bruce pointed out, it's all theoretical anyhow based upon a whole bunch of assumptions (perfect seals, equal pressures in all locations at all times, etc.).

JayJay
Click to expand...

And was mentioned in post #6.
 
So a 5.9 head would be about 85psi; would there be more blow by potential and/ or need for better breathing like in later motors ?
 
Addendum: decking of the block or cylinder head reduces the combustion chamber volume. And thickness of the head gasket also influences the combustion chamber volume. Both of these affect the "apparent" compression ratio, again another uncertainty.

AzBob, your 105-ish psi on compression sounds a bit high to me - theoretical at sea level would put it in the 80's-90's. OTOH, at 5200 feet elevation your ambient pressure is not 14.7 psi, but more like 12.1 psi. If you're reading 105 psi then this would make your apparent compression ratio at about 8.7! You may have a combination of decked block, decked head, and thin head gasket. Or a compression tester that is wildly out of calibration (it happens).

Regardless, an equally valid issue with compression is (as I suspect most of us recognize) not the absolute value per se, but the difference between cylinders. On a used engine, anything more than about 10% between cylinders may indicate an issue to be looked into. Low across all cylinders may indicate rings, which can be checked with a couple squirts of oil into the cylinder and retesting.

But I'm not intending to treat this as a discourse on compression testing. There are far wiser folks than I out there.

JayJay
 
JayJay said:
You may have a combination of decked block, decked head, and thin head gasket. Or a compression tester that is wildly out of calibration (it happens).

JayJay
Click to expand...


Block was decked .010". Compression Gauge is accurate.
 
Intake valve closing also has an effect on compression readings. Early closing more
psi, later closing lower psi. To a lessor degree connecting rod length plays a role also.

I have a 140lb reading with 8-1 and a Model B type cam grind.

Go figure......
 
You must log in or register to reply here.
Back
Top