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Old 03-21-2017, 09:10 PM   #31
55 Tony
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I just thought of something that makes a LOT of sense. It's really simple. As the air is compressed, it gets hot, (David touched on this) and it expands, or it *tries* to anyway. So besides the actual compression of the air, you need to factor in the temperature of the compressed air and the volume that hot air would take if not under pressure. Then somehow cram that volume of air when hot number into the calculations and the numbers will make sense. I think I got that right, what do you think David?

Edit:Giving David more credit where credit is due, he much more than "touched" on the temperature increase, just not the pressure increase due to the temp increase.
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Old 03-22-2017, 01:03 AM   #32
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I just thought of something that makes a LOT of sense. It's really simple. As the air is compressed, it gets hot, (David touched on this) and it expands, or it *tries* to anyway. So besides the actual compression of the air, you need to factor in the temperature of the compressed air and the volume that hot air would take if not under pressure. Then somehow cram that volume of air when hot number into the calculations and the numbers will make sense. I think I got that right, what do you think David?

Edit:Giving David more credit where credit is due, he much more than "touched" on the temperature increase, just not the pressure increase due to the temp increase.
I did think of that but since the cylinder is a closed system I couldn't see how the air could expand due to its temoerature. It still occupies the same volume regardless of its temperature. And since we're concerned with relative volumes, temperatures aren't part of the equation.

Thinking this all over again I think I may have erred on one thing. Let's look at an ideal system.

If we have a cylinder with a movable piston and the piston has a perfect fit and we put a pressure gauge in the top of the cylinder and the piston is at the bottom, the gauge will read zero, which is the ambient air pressure inside the cylinder (14.7 psi) minus the ambient air pressure outside the cylinder, (also 14.7 psi)

Now, if we move the piston halfway up the cylinder, we have cut the volume in half but with the same amount of air, thus doubling the pressure which will now be 29.4 psi The gauge, however will now read 14.7 which is the pressure inside the cylinder less the air pressure outside the cylinder.

Likewise, if the piston is moved up to the 2/3 point, we have tripled the pressure inside the cylinder (1/3 the volume) to 44.1 psi with the gauge reading that amount less the outside air pressure, or a reading of 29.4 psi.

If we move the piston to where the air volume is 1/10 what it was originally the gauge will read 10 times 14.7, minus 14.7 or 132.3 psi.

Now, working backwards, if you obtain a compression reading of, for example, 200 psi, you first add 14.7 (to get 214.7) and then divide by 14.7 to get a compression ratio of 14.6

A pressure reading of 110 psi, plus 14.7 and then divided by 14.7 gives a cr of 8.2

The 14.7 psi in the calculations should be adjusted to whatever your local atmospheric air pressure is. Normal pressure is 14.7 psi at sea level and drops 1.85 psi for every 1000 feet altitude. For instance, the air pressure in Denver is 12 psi.

This is all assuming a perfect seal with the rings and valves, and that your cam allows the valves to completely close from BDC to TDC on the compression stroke. If you lose pressure through an open valve (or through the rings), your compression ratio will be the same as what you may calculate with cylinder volume, piston stroke length and combustion chamber size but the maximum cylinder pressure will be less and the gauge reading will also be less.
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Old 03-22-2017, 09:03 AM   #33
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I don't know David, I'll have to read that later when I'm awake. I still feel the temperature and the air *trying* to expand is the key. No matter if it's a closed system, if the air (or any gas) is heated by any means, the pressure will increase. When heated by compression, you have two laws of science building pressure.

I get lost in all the mumbo jumbo, check out "ideal gas law" and see if that makes any sense.
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Old 03-25-2017, 09:54 AM   #34
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I've used compression gauges a lot on engines, the readings you get have a lot to do with cam timing. If you have a stock engine with a very small cam with very little overlap, 9:1 is about the most you can do on pump gas, but there is a lot of factors involved, like how efficient the chamber is with swirl and tumble how small and efficient your intake runners are at keeping a atomized fuel mixture. Etc.
if you take a measured or static compression of 12:1 on an engine and put a small stock cam on it, you would have trouble running the engine with any advance on the timing to make maximum power on street gas your best timing numbers if you had high octane gas would be much higher than the number you could run on the pump gas without detonating. So say with the stock cam and 12:1 cranking psi you got 220 psi that is pretty high and would require race gas or backing of your timing way back less than optimal to avoid detonation. But if you installed a bigger camshaft with more overlap your cranking psi test would show a lower number maybe 180-200. That's why bigger cams will say 10:1-11 compression reccomended or 12;1 etc.. because they bleed of so much cylinder psi out the exhaust that if you don't have enough initial compression you'll give up tons of low end torque.

I look for cranking psi in the 180-200 range so at 185 your looking good. Big blocks usually need a lot of timing all in, try setting your timing ar 3500 rpm when your distributor stops advancing and start at 30 degrees. Then keep increasing it up to about 38 degrees. Most big blocks like 36-38 degrees. Should be able to run that on pump gas

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Old 03-25-2017, 09:00 PM   #35
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I've used compression gauges a lot on engines, the readings you get have a lot to do with cam timing.
Here is the cam I have, advanced 2, which that alone increases compression. Heads are hardly anything special, stock 781's, good for street and that's where I drive it. Since the first post in this thread, two people said the heads look like they have been milled considerably. I used to have the timing like you say but with the rebuild with domed pistons and the cam below I had to back it off about 2 or it would kick back or stop dead when trying to start it.
http://www.compcams.com/Company/CC/c...?csid=436&sb=0
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Old 03-30-2017, 08:40 PM   #36
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I had a car like that had so much cranking compression it made it really hard to turn over. So I installed an ignition kill switch so I could crank it without ignition then after a few revolutions I hit switch and it fired right up
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Old 03-30-2017, 10:09 PM   #37
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I had a car like that had so much cranking compression it made it really hard to turn over. So I installed an ignition kill switch so I could crank it without ignition then after a few revolutions I hit switch and it fired right up
Yes, I've heard of that trick before, I think I'd rather play it safe with only 7 initial timing.
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