What compression at idle..?

What would the compression (pressure) be in an engine idling at, say..20 " Vacuum in the intake manifold..?
If you assume it has 7:1 Compression..??

Must not be much pressure, yet they idle well..

Would that be equivalent to maybe 2:1 ..??

Certainly much lower than is considered to be a good "Starting Compression"...

Sounds just plain too low to be efficient...!!

Ron..

Seems to me that "compression is compression"- assuming no leakage, should be 7:1 regardless of speed of the pistons- the calculation is the volume of air in relation to the size of the combustion chamber, and has nothing to do with how fast you are compressing it.

But I'm always willing to be shown the error of my ways.

I agree with Mike each cylinder will only build X amount of compression be it turning over with the starter or be it running at idle or wide open

old and Mike... with the throttle closed, airflow is restricted, the air in the intake manifold is not at normal atmospheric pressure, making it less dense, so less air is drawn into the cylinder than if the throttle were wide-open.

So, as hog wrote, effective compression ratio is reduced.

Remember, you are "pumping" a GAS, not a liquid.

Don't have any hard numbers, though!

the compression ratio is not a ratio of pressure. it is a volume ratio;specifically its the cylinder+combustion chamber volume at BDC/combustion chanber+cylinder volume at TDC. The compression gauge pressure for typical gas engines on tractors is ~ C x pambient x[(compression ratio)^1.4-1] wwhere C is between 0.65 and 0.75. ).67 seems to work well on the Ferguson Continentals and the N seried Fords
The compression gauge pressure should always be measured with the throttle plate fully open.

So for your 7:1 compression ratio, if the ambient pressure is 14.7 psia , your compression pressure should be ~ 140 psi.

Having said that if the manifold pressure is 20 inches Hg vacuum, that"s about 4.9 psia so the compression pressure at idle would be ~ 48 psi.

I agree with Bob... to a point. Closing the throttle will restrict air flow, but the valve is also open for a longer time...
Compression -ratio- (ie, 7:1)is determined by the dimensions of the cylinder with the piston at BDC vs. TDC. Compression -pressure- is determined by a lot of things, two of which are RPM and volumetric efficiency.

running compression is usually about half the reading you get cranking. Use it occasionally to check for sticking valve/broken valvespring.

(quoted from post at 10:52:04 07/05/11)
What would the compression (pressure) be in an engine idling at, say..20 " Vacuum in the intake manifold..?
If you assume it has 7:1 Compression..??

Must not be much pressure, yet they idle well..

Would that be equivalent to maybe 2:1 ..??

Certainly much lower than is considered to be a good "Starting Compression"...

Sounds just plain too low to be efficient...!!

Ron..

Click to expand...

f you are running the engine at idle with a compression gauge in one cylinder, expect about half of the reading you got while cranking with throttle wide open.

That is the mechanical ratio.
The effective compression ratio depends on how much air flow reverses back up the intake. As the intake valve is still when the piston starts up the bore on the compression stroke.
Something that few people talk about is expansion on the power stroke. That is what we are working towards.

(quoted from post at 15:52:04 07/05/11)
What would the compression (pressure) be in an engine idling at, say..20 " Vacuum in the intake manifold..?
If you assume it has 7:1 Compression..??

Must not be much pressure, yet they idle well..

Would that be equivalent to maybe 2:1 ..??

Certainly much lower than is considered to be a good "Starting Compression"...

Sounds just plain too low to be efficient...!!

Ron..

Click to expand...

Ran across this years ago and it has served me well

The volumetric efficiency test or running compression test
is a very good test to use any time you have a engine miss
that can't be traced to ignition or other basic systems or
when there is a lack of power.

Step 1. Perform a cranking compression test with all plugs
out and the throttle open.

Step 2. Record your cranking compression readings. Make

sure the charging system was up to snuff so all cylinders
were tested under the same conditions.

Step 3. Install all plugs but one. Install a standard
compression gauge in the remaining hole with the shrader
valve in place.

Step 4. Start the engine. Yes, it will run but will be
missing on the cylinder with the compression gauge. Burp
the gauge and allow the reading to stabilize. It will be
bouncing around at about 50 to 60 psi. (the throttle is
causing a restriction and the piston is moving fast
compared to cranking speed causing only a partial fill of
the combustion chamber).

Step 5. Snap the throttle wide open and return to idle. The
gauge will hold at the peak reading. This reading is higher
than idle because the peak comes at the instant the
throttle restriction is removed and the piston speed is
relatively slow.

Step 6. Record your readings for running snap compression
for all cylinders. The running compression reading should
be approx. 80% of cranking compression. (example: cranking
compression = 150psi running compression should = about
120psi)

If your running reading is below 80% of cranking
compression, the intake system is causing a restriction. If
the running reading is above 80% of cranking compression,
the exhaust system is causing a restriction. If the problem
is in one cylinder you either have a problem with a worn
camshaft, broken spring/rocker, carbon build up, etc. If
the problem is on all cylinders look for a restricted
intake system or clogged cat/muffler. The 80% spec is a
ballpark spec and should be used as such. Remember that
cranking compression is a measure of cylinder seal while
running compression is a measure of volumetric efficiency
or the cylinders ability to draw the air/fuel mixture into
the combustion chamber.

This is the readings from a Flathead ford V8

cranking

Test Numbers, first hit/ fifth hit on gauge

1) 80/120
2) 50/125
3) 75/120
4) 75/110
5) 75/115
6) 65/100
7) 85/125
8) 75/120

Run'n

1st number running compression 2nd # blip throttle

1) 50/115
2) 50/115
2) 50/100
4) 50/100
5) 50/100
6) 50/100
7) 50-115
8) 50/115

No need to go farther with any cylinder efficient test,,, problem was (dual carb set up) one barrel was stopped up.. I would have bet money it was a cam are valve issue before the test...

That is very informative. I have never seen a test performed like that. Think you have done it all and then something else pops up. So many people don't realize in a engine that is throttle controlled air flow that a very small amount of air is being compressed under no load and light load conditions there for low compression pressures. Also , very little horsepower is being developed as hp is work and you are not doing much work.

Thanks Guys..you gave a wide variety of opinions..!!

If the pressure is 48 psi at idle with 20 " Vacuum, they do idle surprisingly well..
I wonder why we ween to think anything under 60 psi is not enough for an engine to operate..??
I know there would not be much "power", seems that is where we seem to draw the line..!!

Fuel would certainly vaporize better in a cylinder with a moderately high vacuum...

Then, we get to a Diesel that runs at FULL compression all the time and has metered fuel, to idle...exactly the opposite of a gas engine (more or less)...!!

Interesting topic...!

Ron.

Hobo is on it. Volumetric efficiency gets lower as engine speed increases due to having less time to fill the cylinder and other issues. That's why turbos work so well. If compression is 120 psi cranking idle should be 60-90 psi and running(2000 rpm) should be 30-60.

Depends on cam duration, rpm, port volume, port velocity etc.
That is why higher mechanical compression ratio's are used with longer duration cams for higher rpm operation.
Otherwise combustion chamber pressure pressure and expansion ratio drops. Due to the valves opening sooner and closing later.