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Compression Ratios, what do they mean?

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To anyone searching this thread,

I've deleted all of my original content on this older thread.   Sorry for any inconvenience this may cause you.

It's now the time for others to take over this thread.

Dean Los Angeles:
Ah, if we ever get to just below the cheddar curtain my wife and Mrs. Fordboy could compare notes.

I see the words static and dynamic have reared their ugly heads again. Boys, it's a dynamic world inside that engine that is far, far different than your static thinking.
Very good description. The dynamic view of compression ratio is the only one that matters.

One of the terms mentioned is BMEP, Brake Mean Effective Pressure. It is the singular measurement that allows us to compare equally the performance of the small block Chevy to that Briggs and Stratton.

Read here:

I did a write up about fuel a few years ago:,2308.0.html

It is also effective to talk about what exactly we are compressing and why.
The combustion cycle

Air + fuel + ignition = heat = pressure = hp

More air + more fuel + ignition = more heat = more pressure = more hp

More air + more fuel + ignition + compression = even more heat = even more pressure = even more hp

On a nice summer night you wander outside and gaze up at the stars. That clear view is through 75 miles of air. There is more nothing than there is air.
At 62 miles you have passed through 99.99997% of the atmosphere. 75 miles effectively puts you in outer space.

Let’s put some of that in our theoretical single cylinder 1000cc engine. Remove the head and rotate the piston from TDC 180 degrees to BDC. You now have 1000cc of air.

But air is a highly flexible substance. It is 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases. It’s the 21% oxygen that we work with, the rest is just in the way. And don’t forget the 0.0000325% of nitrous oxide.
At sea level that 75+ miles of air in a 1 sq in column weighs 14.7 pounds. That 14.7 pounds per square inch is what is pushing the air into the cylinder.

But that 14.7 is a variable. At 6,000 ft it’s 11.8 pounds. 50% of the atmosphere is below 18,000 ft so gaining altitude cuts the amount of available air to the engine in a hurry.

Heat is another variable. To simplify thinking without going into the science, think of an atom as vibrating and pushing the other atoms away the more it vibrates. As the heat increases there is less air in the same volume.

But wait! There’s more! Air also contains a variable amount of water vapor, on average around 1%. Humidity is the term, and the amount of water vapor in the air displaces the air and makes less available to the engine.

High, hot and humid = poor hp
Low, cool, and dry = very nice hp

Either through on-site calculations from your weather station, or internal calculations from the air/fuel ratio sensors you have to adjust the fuel to match the air.

Let’s put the head back on our engine. We are going to replace the intake with a soda straw. Now when we rotate the piston 180 degrees we still get 1000 cc’s . . . eventually. That is the problem you face when you are trying to stuff the maximum volume of air into a space in a fraction of a second through the restrictions of the intake  and exhaust system. The faster you try and do it, the less air actually ends up in the engine. Air is the limiting factor on producing horsepower - until you reach the end of the mechanical limits of the engine. The bulk of the work on a racing engine is to provide more air.

In addition to porting, cams, valves, etc. additional air can be inserted through intake and exhaust length tuning to take advantage of standing pressure waves in the system. More air can be obtained by a ram air system. At 200mph 0.7 psi can assist in pushing the air in the engine. The ultimate is to put a supercharger or turbocharger to push pressurized air into the engine. All of these are discussions for another topic.

Ok, now we have air. The next step is fuel. Gasoline, methanol, nitro methane, diesel, etc. For this discussion the type of fuel doesn’t matter. What you do with it does.

Gasoline requires approximately a 14.7:1 air/fuel ratio by mass. Methanol requires exactly a 6.4:1 air/fuel ratio by mass. Nitromethane requires exactly a 1.7:1 air/fuel ratio by mass.
This is the stoichiometric ratio. In a laboratory setting all of the fuel will consume all of the air and there will be no excess fuel or air left.

Notice the word “approximately” for gasoline. For gasoline fuel, the stoichiometric air–fuel mixture is approximately 14.7; i.e. for every one gram of fuel, 14.7 grams of air are required (the fuel oxydation reaction is: 25/2 O2 + C8H18 -> 8 CO2 + 9 H2O). Any mixture less than 14.7 to 1 is considered to be a rich mixture; any more than 14.7 to 1 is a lean mixture – given perfect (ideal) "test" fuel (gasoline consisting of solely n-heptane and iso-octane). In reality, most fuels consist of a combination of heptane, octane, a handful of other alkanes, plus additives including detergents, and possibly oxygenators such as MTBE (methyl tert-butyl ether) or ethanol/methanol. And you have no idea what you are actually getting . . . ever.

But we are not in a laboratory setting. You have thousands of a second to get the fuel and air into the cylinder and ignite it. Kiss the laboratory goodbye and welcome to the dynamic world inside the cylinder.

The head is back off and the piston is at bdc. In the 1000cc’s of air we are going to put 1 oz of gasoline in the bottom. 100% of the fuel is at the bottom, 100% of the air on top. Neither one burns by itself. At the interface between the two is a very small area of intermixing that we will ignite. You can do this in a tall jar if you want to experiment. Gasoline is flammable. Do it outside with a fire extinguisher. The ignition will produce a small flame and then it will turn sooty black and go out, long before any amount of fuel is consumed. As the air is consumed it is replaced by exhaust products that displace the air. Just having the two in the same space isn't enough.

To get maximum effects you have to combine 100% of the fuel with 100% of the oxygen molecules in the air. This is never possible, there isn’t enough time. The liquid fuel has to be atomized and then vaporized so that the gaseous vapor combines with the oxygen molecules at a molecular level. Everything happens to individual molecules, not big clumps of them.

On a float carbureted  engine the venturi in the carburetor produces a partial pressure that sucks the fuel out of the bowl and spits big drops into the air that is rushing into the intake manifold. Turbulence and heating help to atomize and vaporize those drops prior to ignition. There are still large quantities of fuel that are not attached to any oxygen molecules and go out the exhaust unburnt.

Fuel injection certainly helps this problem. The fuel is pressurized and injected through a nozzle and the reduction in pressure when it is sprayed assists atomization. Turbulence in the design of the head is a major factor.

This is why you are always running richer than stoichiometric. That oxygen sensor in the exhaust is telling you about all of the wasted air you didn't ignite.

Ok, on our test engine the head is back on, the piston is at bdc and the cylinder has the optimal ratio and mixing of fuel and air. If we ignite the fuel at this point we will get 100% of the heat of combustion. Now refill and then rotate the piston 180 degrees. At a 10:1 compression ratio we have compressed the mixture to 100cc’s. If we ignite the fuel at this point we will get >100% of the uncompressed mixture. How can that be? What did we gain? Compression is all about time and distance. At bdc the molecules are far apart and the advancing flame front is cooling rapidly between molecules. When the mixture is compressed the molecules are compressed together and the flame front travels faster. The more compression the better. There is no end to the benefits of more compression . . . until you find the limits of detonation.

And you wonder why there are very big books written on engines.

WOW!!! This is some great info.!!! Thanks guys!!!    :cheers:

Peter Jack:
This is for next week when I have some time and the inclination to absorb the information supplied. Great work guys!   :-D :-D :-D :cheers: :cheers:


Milwaukee Midget:
One of the key things that Fordboy has done for my engine development is to take all of these engine theories and practices, things I’ve been reading about since high school in publications like Hot Rod, Popular Mechanics and David Vizard’s book, “Tuning the A-series Engine”, and put them in their proper order and sequence.  It’s his experience that has set the proper order of things, and has dispersed many of the misconceptions I’ve had for years.

There’s no way I’d be as far along as I am without his insight on this.

I’ve said it before, I’m at best an amateur builder, but in our numerous conversations over the last year, Fordboy has turned my scattered knowledge into acute insight, and that has been crucial to my build.
Professor Fordboy, I’ve said it before, and I’ll say it again – Thank you.



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