340 Static Compression Numbers and Test Procedure

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340inabbody

340inabbody

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Hi, I just did a static compression test for helping sort out a bunch of other things. So I just would like to focus this thread on static compression testing procedure (not sure if I did it correctly) and to share my numbers because they were higher than I anticipated and not sure what it means. So any comments on the numbers also would be appreciated.

Procedure: All plugs in and measured cold cranked until no changes seen typically 3-4,5 times
Motor:1973 340stock steel heads high rise Mopar Manifold Holley 650 double pumper lots of unknowns CR? CAM? Pistons? Deck height etc?
Data
Cy/PSI
1 210
8 202
4 204
3 210
6 207
5 215
7 197
2 197

All readings are within 10% of the minimum and maximum range.
I assume the numbers would rise if done on a warm motor right? How do these numbers relate to the compression ratio?
Thanks in advance for the reply......
 
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That is a good procedure, though propping the throttle wide open may have given a few more psi. Were these taken with no extra oil being put in the cylinders?

Numbers are a good spread, so valves and rings seem to be sealing well at cranking speeds.

Before anything else, what gauge do you have and where did you get it?
 
nm9stheham said:
That is a good procedure, though propping the throttle wide open may have given a few more psi.

Numbers are a good spread.

Before anything else, what gauge do you have and where did you get it?
Click to expand...

HF, US General 0-300 PSI Gauge, I suppose it could be off?
 
You're measuring cylinder pressure there, not static compression ratio. What cam do you have?
 
Off the top of my head, those numbers suggest to me decent compression ratio.
Are those flat top pistons? (you may be able to see in the spark plug hole, even read the number sometimes)
My guess is the cam has a relatively short duration.
 
340inabbody said:
Hi, I just did a static compression test for helping sort out a bunch of other things. So I just would like to focus this thread on static compression testing procedure (not sure if I did it correctly) and to share my numbers because they were higher than I anticipated and not sure what it means. So any comments on the numbers also would be appreciated.

Procedure: All plugs in and measured cold cranked until no changes seen typically 3-45 times
Motor:1973 340stock steel heads high rise Mopar Manifold Holley 650 double pumper lots of unknowns CR? CAM? Pistons? Deck height etc?
Data
Cy/PSI
1 210
8 202
4 204
3 210
6 207
5 215
7 197
2 197

All readings are within 10% of the minimum and maximum range.
I assume the numbers would rise if done on a warm motor right? How do these numbers relate to the compression ratio?
Thanks in advance for the reply......
Click to expand...
Those numbers are pretty high, that is no stock engine.
 
RustyRatRod said:
You're measuring cylinder pressure there, not static compression ratio. What cam do you have?
Click to expand...
Understood. Again I don't know the details on the CAM. This is a rebuilt hot 340 with an aggressive cam and CR. Ultimately I would like to back into a CR if possible.
 
Mattax said:
Off the top of my head, those numbers suggest to me decent compression ratio.
Are those flat top pistons? (you may be able to see in the spark plug hole, even read the number sometimes)
My guess is the cam has a relatively short duration.
Click to expand...
Don't know anything about the internals of the motor. I will see if I can look into the spark plug hold but I doubt I will see any thing.
 
340inabbody said:
Understood. Again I don't know the details on the CAM. This is a rebuilt hot 340 with an aggressive cam and CR. Ultimately I would like to back into a CR if possible.
Click to expand...

Have you HEARD the engine run?
 
Point is, that's pretty high compression. That suggests a relatively high CR and/or valves opening and shutting to capture a good amount of volume.
In the other thread you mentioned pinging. Not suprised.
 
Mattax said:
Point is, that's pretty high compression. That suggests a relatively high CR and/or valves opening and shutting to capture a good amount of volume.
In the other thread you mentioned pinging. Not suprised.
Click to expand...
Bingo if my CR is too high it would explain the detonation possibly but also I don't know what cam is in there. Ultimately I want to try and estimate the CR but I also don't know the swept volume etc so I don't think I can do that.
 
340inabbody said:
Don't know anything about the internals of the motor. I will see if I can look into the spark plug hold but I doubt I will see any thing.
Click to expand...
thumbs_up-gif.gif
Success depends on the light and where the piston is located. I've had best luck with a single LED flashlight with a flexible connection to the battery.
Might see the piston number, the overbore, or at least the shape and the number of notches.

This one
upload_2020-1-18_21-20-27.jpeg
 
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That is race fuel cranking compression territory.

My 340 is about 9.8:1 static compression (iron heads), and with the throttle held open and all the plugs out it cranks at ~175 psi. It runs on 91 octane, but barely. I pulled a couple degrees of timing to make it happy on the street all the time and with the seasonal fuel changes.
 
340inabbody said:
Bingo if my CR is too high it would explain the detonation possibly but also I don't know what cam is in there. Ultimately I want to try and estimate the CR but I also don't know the swept volume etc so I don't think I can do that.
Click to expand...

I feel for you, having been through engines where I didnt know.
But unless you're going to pull the heads, just knowing its relatively high is good info.
The cam you could get a sense of with a degree wheel and dial indicator.

As far as the pinging, its not the CR's fault per se. It's that the 'typical' hot rodder thinking of more and quicker timing adv is wrong for those conditions.
Conditions are favorable for a good hot combustion right off idle so long lead isn't needed. You'll have to experiment and make decisions based on performance. If you change fuel, you may have to adjust again.
 
340inabbody said:
Bingo if my CR is too high it would explain the detonation possibly but also I don't know what cam is in there. Ultimately I want to try and estimate the CR but I also don't know the swept volume etc so I don't think I can do that.
Click to expand...

Ok, so it is detonating? Then those numbers I believe are correct. Dump some 114 octane race gas in it and see how it runs. It's probably a 12.5:1 engine.
 
OP needs to stop where he is, pull the engine and take it apart for a good look around. Guesswork is crazy.

It could be a 10:1 engine with the cam advanced or something.

Who knows? That's my point. The OP doesn't know and he NEEDS to know.
 
yellow rose said:
OP needs to stop where he is, pull the engine and take it apart for a good look around. Guesswork is crazy.

It could be a 10:1 engine with the cam advanced or something.

Who knows? That's my point. The OP doesn't know and he NEEDS to know.
Click to expand...

Correct.
 
RustyRatRod said:
Ok, so it is detonating? Then those numbers I believe are correct. Dump some 114 octane race gas in it and see how it runs. It's probably a 12.5:1 engine.
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I have the detonation under control with my advance curve. the 2.71 does not help and neither do the steel heads. I still have to check the AF. So is measuring the cylinder pressures of this compression test valid cold? Seems like one way or the other there should be a standard method since we are always comparing......Lastly is there anyway to determine the CR with this data?
 
340inabbody said:
I have the detonation under control with my advance curve. the 2.71 does not help and neither do the steel heads. I still have to check the AF. So is measuring the cylinder pressures of this compression test valid cold? Seems like one way or the other there should be a standard method since we are always comparing......Lastly is there anyway to determine the CR with this data?
Click to expand...

No. The only way to figure static compression is to disassemble and measure for it.
 
340inabbody said:
Understood. Again I don't know the details on the CAM. This is a rebuilt hot 340 with an aggressive cam and CR. Ultimately I would like to back into a CR if possible.
Click to expand...
I do not want to deflate your balloon but there is no accurate rule of thumb that you can "back into" the compression ratio. The only way you are going to find out for sure is to take it apart and take some measurements.
 
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race gas, propane, e-85 could all work without pulling the engine
if you look in the spark plug hole with a bore scope and see domes post back
btw what rockers? if rollers can you see the ratio
and you can measure valve lift without dissambly
he who posted that the cam may be too advanced could be right
something you can fix by pulling the timing cover
back off on the timing till you get it sorted out
yellow rose may be the most experienced help
 
Well, if the engine was out, You could measure how much fluid it takes to fill one chamber with the piston at TDCcompression and the spark-plug hole as the high point. It's kindof hard tho after the engine is installed,lol.

But I call bs on the tester.
To get to an average of say 205psi with a typical big-cam Ica of say 67* would require a 12.5Scr.
By 60* the requirement is down to 11.6;and by 53* down to 11.1
The typical street cam will fall into the Ica range of 53* to 67*, which is just about 4 cam sizes; which covers from say 288 to 260. So my guess is the tester is off or the cam is just too small for the Static compression ratio. This can happen when a guy decams a race-motor for street.
If you redo the test, just keep cranking with the throttle at WOT, until you get two consecutive shots at about the same pressure, irregardless of how many shots it takes. Cranking speed will affect the readings, so when the last one is done, repeat the first one to verify it's still cranking right.
But like the guys say, this will be impossible to cure, perhaps even with thicker head gaskets and a different cam. So since it has to come apart anyway, I would just bite the bullet and take a head off. That ,or measure the cam, which means the front has to come off. Sooner or later the cam will have to be measured anyway,so it makes sense to me to just start there.

unless maybe it has Rhoads-type lifters in it.........
 
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