Hey everyone. I took some compresion readings on my engine today and was wondering if someone could tell me what the comp. ratio is? Here are the cylinder readings.

1:131 2:140
3:135 4:138
5:125 6:146
7:130 8:141

Thanks in advance.

You can't tell the compression ratio by the cranking pressure as there are way too many variables, like intake valve closing point, cranking speed, leakage, altitude, etc. The static compression ratio is a purely mathematical entity that compares the minimum and maximum volumes of the cylinder.

The guys on here told me to take these readings and they would do the rest. Oh well.

No bigee

Yeah, you were set-up for an old time joke. Now you know.

You can get a "ball park" estimate from cranking compression. I would guess at 8:1. I would recheck your cylinder #5 again. All readings should be within 10% of each other. The #5 clinder seems a little low.

One thing the engine wasn't at was operating temp. The engine was completely cold so the readings should be higher than these I assume.

You can get a "ball park" estimate from cranking compression. I would guess at 8:1. I would recheck your cylinder #5 again. All readings should be within 10% of each other. The #5 clinder seems a little low.

Why would you guess 8:1? Is the 125# cylinder 8:1 or is it the 146# cylinder?

Why would someone set me up for this joke? What was the poiont?

No point. I just know it as locker-room humor. But maybe they just didn't know what they were talking about. If it wasn't a joke, then they didn't.

My ZL-1 motor has 12:1 static and the cranking is 145psi. Late intake valve closing of the ZL-1 camshaft is the reason. So you cannot tell CR by psi unless it is a stock motor-then you can look it up in Chevy spec manual.

Pressure is going to depend of the dynamic compression at a given rpm...which depends on the profile of the cam...valve overlap etc...
At cranking speeds a high rpm cam will give low pressure reading, if taken at high rpm they will be much higher.
The above explanation is very over simplified, to give basic concept.
Every cam profile will give individual charactoristics as to crank compession and power range compression.
At crank mine is about to 130lbs increasing to 165 to 170 at higher rpm.

I was just stateing what I have been told over the years. I was assumeing an about 140 PSI average. I know there are alot of factors that you need for exact calculations. I understand about what cam you have changes it completely. I was just giving him a guesstimate. I assumed he was running a close to stock engine.

a 1969 350SS motor is 180PSI....and its never been apart....bone stock.

You guys are forgetting that these readings are on a cold motor. It wasn't even so much as fired up before the test. I came home last night and 9:00am this morning I did the test. The readings would be higher had I warmed the engine up. According to my GM manual a stock 350 in 67 had 10.25:1 comp. and the 327 had 3 different ratios. 210hp: 8.75:1, 275hp/300hp: 10:1, 325hp/350hp: 11:1

You guys are forgetting that these readings are on a cold motor. It wasn't even so much as fired up before the test. I came home last night and 9:00am this morning I did the test. The readings would be higher had I warmed the engine up. According to my GM manual a stock 350 in 67 had 10.25:1 comp. and the 327 had 3 different ratios. 210hp: 8.75:1, 275hp/300hp: 10:1, 325hp/350hp: 11:1


I recently done a "cold" compression check on my 327/300hp and was a little disappointed as 2 of the cylinders registered 135# and the rest were either 145# and as high as 155#....According to the books, this motor should read 160# with a 10:1 compression ratio so perhaps these dudes are correct about the other factors such as cam profile and such

The compression test has to be done with the engine at operating temperature. With it being cold it doesn't have oil sealing or the right ring seat so the test gives false readings if it's done cold. I primarally wanted to do a consistancy test. With the engine at operating temp I know the readings would have been higher.

I just did a warm compression test on my new DZ spec motor last week. Compression was 192-200 psi in all cylinders. I have 11:1 pistons. Just a point of reference.

Pressure is going to depend of the dynamic compression at a given rpm...which depends on the profile of the cam...valve overlap etc...

What does overlap have to do with compression?

Isn't overlap the point at which both valves are momentarilly open??.....If so then I can see this giving a lower compression reading

Isn't overlap the point at which both valves are momentarilly open??.....If so then I can see this giving a lower compression reading

When does overlap occur?

When does overlap occur?

On some cams... never. On others depending on the number of degrees separating the exhaust and intake lobes then you can have a duration of overlap.

LT1 engines had overlap so vettes in 1993 never had a EGR valve because exhaust and intake were open at the same time briefly allowing mixing of exhaust gas into intake... so I imagine that overlap would briefly occur just as the intake valve opens...

anyone ...?

Back to the original Thread....

Compression testing is a relative health check of the engine.. done dry (no oil added to cylinder). Then if a cylinder is much lower ie more than 10% you can squirt oil into each cylinder and re-compare. If the compression does not rise in the low cylinder then it is likely a valve problem... if it goes up to normal you have a ring problem..

Comparing compression from one engine to another is like bench racing... even a weak battery effects engine rotation. And how long you crank before you decide that it enough.... it can build beyone even three or four revolutions...

So... what 69 Camaro RSSS really wants to know is his static compression ratio...

So we can help him... if the engine is not modified all we need to ask him is the engine stamping no?

;)

On some cams... never. On others depending on the number of degrees separating the exhaust and intake lobes then you can have a duration of overlap.

LT1 engines had overlap so vettes in 1993 never had a EGR valve because exhaust and intake were open at the same time briefly allowing mixing of exhaust gas into intake... so I imagine that overlap would briefly occur just as the intake valve opens...

anyone ...?

Can you name one modern automotive cam that has no overlap?

I guess that some event that occurs when the intake valve is just opening is not going to have an effect on compression.

I thought it was the LT4's that increased the overlap to delete the EGR...

There in lays the problem. I can give you the engine stamp number, but I can't tell you what mod's the engine has. I know cam, intake, carb, but not if it has been punched out or what heads it has on it. The engine was already in the car, but it was rebuilt before it was put in. So, I'm sure the ratio is going to be off. Besides the fact that I conducted this test on a cold engine, I have no idea what internals I am dealing with.

I know cam, intake, carb, but not if it has been punched out or what heads it has on it.

So, tell us what cam(or did I miss it?). It does make a difference. We won't be able to tell you much without the other info, but we could tell you a little.

And I think your compression test is probly OK. No gaping wounds anyhow. Even a cold test with a closed butterfly spots wounds.

FYI OVERLAP
overlap is 1 revolution away from TDC compression/power stroke. Remember 4 strokes or 2 revolutions.

Typically an intake valve opens shortly before TDC of intake stroke(in other words it starts to open as exhaust stroke is finishing) and stays open all the way down the hole and does not usually begin to close until the piston is going back up the hole. How long it stays open after the piston continues back up is what affects dynamic compression and also cranking compression but not compression ratio. 3 different critters Charlie. Overlap therefore has no affect on cranking compression, since it is 1 rev away from peak cyl pressure.

Leery of saying all cams have overlap. I have no qualms with stating that I've touched tons of cams and been in extremely close proximity to at least thousands that have overlap and evry stinking one of em overlap is 1 revolution(crank) from TDC Compression/Power

my 2cents, hope I am not coming across as condescending or anything else. Just trying to clarify, hope I didn't make it clear as mud in my effort:D

My cam is the Edelbrock RPM Performer #7102. The test was done with the fly open, but no warmth at all. Completely cold. Sat overnight for a good 9-10 hours. No oil to seal rings.

My cam is the Edelbrock RPM Performer #7102. The test was done with the fly open, but no warmth at all. Completely cold. Sat overnight for a good 9-10 hours. No oil to seal rings.

Charlie, I'm OK with a cold test. I do em alladamtime. They still tell you something. A compression test is just an overall health check. Not the panacea to troubleshooting. You are probly ok. Your original post was regarding compression RATIO, via compression TEST. I'm only trying to splain why the cam won't let you do that. We cool?

your cam page 3
http://www.edelbrock.com/automotive_new/mc/valvetrain/pdf/cams_lifter_kits.pdf

specs of 107icl 308 adv and 234 at .050

fed into this calc, on a 350 that is not what you have but probly within a half mile:D. Try it with different cams and different static CR's, you'll see:)

http://www.hotroddersauctions.com/DynamicCompressionRatioCalculator.php


spit out this info, and down towards the bottom you see it knocks avout 2CR points and 3/4" stroke off static, dealing with an unbored 350 and 10.5:1 CR . That's what we're talking bout. Intake valve closing affects compression
-------------------------------------------
copy/paste from dcr calc







Your corrected Compression Ratio with boost and altitude is 10.50:1.
Your camshaft specifications call for an inlet valve closing of 44 degrees ABDC measured at .050.
At 6500 RPM your intake valve is considered closed at 64.27 degrees ABDC.
Your chamber volume is 68.25 cc's. With this camshaft and RPM your dynamic, or effective stroke is 2.72 inches. Your dynamic compression ratio is 8.21 :1 corrected for cam timing, rpm, and rod length. Your dynamic cylinder pressure, corrected for cam timing, rpm and rod length is 189.59 PSI. Your dynamic volumetric compression ratio, reflecting static c.r., cam timing, rpm and 94 % VE is 7.72 :1. at 6500 RPM

EA3.2 requires a static compression of 9.0:1 in a 355 in order to get 145# of cranking pressure with the Edelbrock 7102 set in as ground.

So I am now officially lost. A 350 is exactly what I have. So do I have 9:1 or 10.5:1? I am confused.

9ish

sorry I muddied it for you Charlie

I was trying to help

So my compression sucks ass right? 8.21:1 or 7.72:1 sucks.

please do a re-read

you are not 8ish

I wouldn't worry a whole lot about the static compression, there's nothing you can do about it unless you do some mechanical work to the engine, at which time you can measure and calculate your actual compression ratio and then you'll know what it is. If you want to increase the cranking compression, get a cam that closes the intake valve sooner. On my 406 I use a 224°/224° cam with 10.3:1 static compression, cranking compression is about 195#.

Is it the 10.5:1? There were 3 ratios. I don't know which one it is.

On my 406 I use a 224°/224° cam with 10.3:1 static compression, cranking compression is about 195#.

I have the same cam in my 383 with 9.87:1 static compression and cranking compression was right around 200lbs if I remember correctly...

onova - you're only about an hr and a half from me, I believe you do dyno tuning, are you interested in dialing in my engine? It's very close to your 406 with spII heads, etc. I really think there is a lot being left on the table the way it's setup now.

The dyno shop general partner died last October and the shop is under new management. I basically worked there for the general partner, a good friend, so I don't work there any more.

So, tell us what cam(or did I miss it?). It does make a difference. We won't be able to tell you much without the other info, but we could tell you a little.

And I think your compression test is probly OK. No gaping wounds anyhow. Even a cold test with a closed butterfly spots wounds.

FYI OVERLAP
overlap is 1 revolution away from TDC compression/power stroke. Remember 4 strokes or 2 revolutions.

Typically an intake valve opens shortly before TDC of intake stroke(in other words it starts to open as exhaust stroke is finishing) and stays open all the way down the hole and does not usually begin to close until the piston is going back up the hole. How long it stays open after the piston continues back up is what affects dynamic compression and also cranking compression but not compression ratio. 3 different critters Charlie. Overlap therefore has no affect on cranking compression, since it is 1 rev away from peak cyl pressure.

Leery of saying all cams have overlap. I have no qualms with stating that I've touched tons of cams and been in extremely close proximity to at least thousands that have overlap and evry stinking one of em overlap is 1 revolution(crank) from TDC Compression/Power

my 2cents, hope I am not coming across as condescending or anything else. Just trying to clarify, hope I didn't make it clear as mud in my effort:D
HEY TIM!!! How about coming out here and standing next to my car?!:thumbsup:

Is it the 10.5:1? There were 3 ratios. I don't know which one it is.

NONE of them

Please re-read my post 28 again. I evidently didn't do a good job of explaining that I input 10.5 because if you don't then the calc is missing a variable. Got to have all the variables. I used 10.5 as a guinea pig to show you the affects of intake valve closing with your cam. Then I copy/pasted below the dashed line what the dcr calc generates with your cam and 10.5. But we don't know you are 10.5. You are getting lots of different kinds of ratios confused. It's OK, it is confusing. It will continue to be confusing until you devote some time to understanding all of them. Don't expect clarity overnight on understanding what a cam is doing for a living:D

Here is how a site says to calculate.

1. Determine the displacement of your engine. Displacement formula is:

BORE X BORE X STROKE X .0031416 = DISPLACEMENT

Example: 92 x 92 x 82 x .0031416 = 2180cc

2. Determine the swept volume of one cylinder:

Example: 2180cc divided by 4 = 545cc

3. Determine the deck volume of each cylinder. The deck volume is the distance from the top center of the piston to the top of the cylinder when the piston is at top dead center. Measurement is made in thousandths.

Example: on cylinder #1 you measure and find you have .020" deck height.

BORE X BORE X DECK HEIGHT X .01996 = CC"s

Example: 92 x 92 x .020" x .01996 = 3.378cc's

Measure each cylinder.

4. Measure the volume in each cylinder head. To do this, use a piece of plexiglass cut to fit in the cylinder head to cylinder mating area. Drill a 1/4" hole in the center of the plexiglass. Lightly grease the edge of the plexiglass and install in the head. (Spark plug and valves have to be installed) with a syringe graduated in cc's fill the cylinder head chamber up with a light weight oil. Record the measurements. Repeat for other three chambers. Average cc volume of a new head chamber is between 47 and 51 cc's.

5. You now have all the measurements to determine your compression ratio.

one cylinder swept volume + deck cc + head cc

deck cc + head cc

Example: 545 + 3.378 + 48 = 596.378

3.378 + 48 = 51.378 = 11.6:1

In this example, 11.6:1 is the actual unadjusted compression ratio for one cylinder. Add cylinder shim thickness which will increase the deck cc volume and refigure compression ratio. This must be done for all cylinders. Average deck height shim for use on a 2180cc engine will be between .150" and .185". We use a .177" shim for AVGAS and a .216" or .256" shim for unleaded premium auto fuel. Lower is better for increased engine life and fewer valve related problems.

I have no idea about any of this. What I thought was going to be an easy computation turned into a nightmare. At least I know that I don't have a blown head gasket or anything like that.

The cranking compression is what it is. You can't do anything about it without actually working on the engine, which means you will have a chance to determine what the static compression actually is. I wouldn't worry about it.

The cranking compression is what it is. You can't do anything about it without actually working on the engine, which means you will have a chance to determine what the static compression actually is. I wouldn't worry about it.

:hurray:Neither would I

I have no idea about any of this. What I thought was going to be an easy computation turned into a nightmare. At least I know that I don't have a blown head gasket or anything like that.


Like I said, I wish I had never performed that compression test because I was expecting all cylinders to read 160 psi!.....I guess the leak down test will tell all!!.....Don't worry about it and just be glad you don't have a blown head gasket, worn piston rings or leaky/burnt valves!!:thumbsup: