tools for a compression check ?

[Highbanked Hauler]

 I am seriously considering taking a head off of my " racin "  440 to do a compression check to document what the ratio is. I was told it was 12.0 when I bought it 2 years ago and it was going in my 68 and run it on E-85 so that wasn't an issue.  Now I am batting around the idea of putting it in the 500 to run it for a while and shelve the original engine.

   So what do I need for measuring tools ??

[BSB67]

Depends on heads/pistons and how accurate you want to be. Best case, feeler gauge.  Worst case, burette, dial indicator, and indicator mount..

I would probably pull the heads first, then decide what you might need.

[Highbanked Hauler]

Depends on heads/pistons and how accurate you want to be. Best case, feeler gauge.  Worst case, burette, dial indicator, and indicator mount..

I would probably pull the heads first, then decide what you might need.

    I don't do machine work so I ask.   Don't you need something to measure the depth of the piston and the diameter of the bore ? also what about the combustion chamber of the head ?

[A383Wing]

here is a couple...I can post others if ya want

http://www.wikihow.com/Calculate-Compression-Ratio

https://youtu.be/bWze92nt9OU

[A383Wing]

here is another

http://www.csgnetwork.com/compcalc.html

[Highbanked Hauler]

here is a couple...I can post others if ya want

http://www.wikihow.com/Calculate-Compression-Ratio

https://youtu.be/bWze92nt9OU

Ah,there is more to it than I thought.  I thought  ALL you did the CC measurement on the head and a depth and dia. measurement on one cylinder..

[BSB67]

here is a couple...I can post others if ya want

http://www.wikihow.com/Calculate-Compression-Ratio

https://youtu.be/bWze92nt9OU

Ah,there is more to it than I thought.  I thought  ALL you did the CC measurement on the head and a depth and dia. measurement on one cylinder..

Right, but don't over complicate.  If you know the brand of head, you can likely make a close enough guess on its volume.  If it is a flat top piston,  a straight edge and feeler gauge is all you need.  If you have a part number, or sometime you can tell by visual observation which piston it is, whether it is dished, domed, and has valve reliefs, again, all you need is a feeler gauge, as the net volume for the reliefs/dome/dish is available on line for about every piston.  Finally, 0.030" over verses 0.060" over makes very little difference in the compression ratio.

[Highbanked Hauler]

It has 906 heads that I am sure of and it is 60 over but I don't know what the pistons are and I am 99% sure its a stock crank. so I guess the next thing is pull a head off to check the rest. I got a run of the mill mini starter for this motor and  with a battery and a jumper box on it the starter has to work to turn it over but I know other factors are involved there .

[A383Wing]

that sounds like cable resistance to me

[Highbanked Hauler]

that sounds like cable resistance to me

       Its an original cable and I  think  that is part of it as it acts like it hits a high spot on each revolution. I was thinking of getting arc welding cable for it.

[BSB67]

that sounds like cable resistance to me

       Its an original cable and I  think  that is part of it as it acts like it hits a high spot on each revolution. I was thinking of getting arc welding cable for it.

A compression test will provide some insight.

[Highbanked Hauler]

  A compression test doesn't have anything to do with ratio does it ?

[A383Wing]

not really...if engine is in good condition, a higher compression ratio will lead to higher compression numbers on the gauge....but if engine is in poor shape...a high compression engine can have low numbers and it will run poorly

did that make sense?

[BSB67]

 A compression test doesn't have anything to do with ratio does it ?

Not directly by itself without knowing something about the camshaft.   But it is not clear what you are actually looking for, or needing to know.  Here are the things you can determine from a compression test 1) will the engine run on pump gas, 2) Will the motor be a turd, 3) will the motor have relatively good power, or power potential. 4) Is the compression ratio 12-ish, 11-ish, 10-ish, 9-ish.......

Also, if you know something about the camshaft, you can get pretty close to determining the actual CR from a compression test.

At the end of the day, I don't know if "knowing" that it is an actual 11:1 compression motor is really any more valuable than knowing that it blows 210 psi on a compression gauge.

When you are building a motor, you usually target a compression ratio for part selection purposes. Once the motor is together, it kinda doesn't matter as much anymore.

What are you planning to do with the compression ratio number?  Are you going to buy new pistons, or cylinder heads?

Finally, looking into my crystal ball, unless you paid a lot of money for that motor, or the guy you bought it off of gave you details on specifically what is in the motor, it will likely have the 2295, 2355, or low compression piston in it.

[Highbanked Hauler]

   The guy I bought  this engine  from was going to put it in a 72 Charger (real street-pro street-hot rod- drag strip, smoke the tires down the road and piss off the neighbors type car ) and my idea was along the same line so for $2600. on a just broken in  25 mile motor (I don't think it had track time other than some burn outs in front of his house) I figured it would do the same for me. He did tell me it was a 12.0 comp ratio AND he ran it on pump gas which I questioned him on about detonation and he said there wasn't any.  I did hear the engine run before I got it and I have had it running in the yard here. It idles fine with instant throttle response but I haven't had it under load like a launch would do.
    I do have cam specs somewhere and it is hydraulic with 1-6 Crane roller rockers  on 906 heads and for anymore I 'd have to find the papers he gave me on it.
         My reason for knowing the comp.ratio is so I don't damage  the motor from running it on lower octane fuel than it needs. 
    Is a comp. test accurate on a cold motor ?
        My goal NOW is to put it in my 500 and put the numbers motor on a shelf.

     Yep 383Wing it sure does.

[BSB67]

  The guy I bought  this engine  from was going to put it in a 72 Charger (real street-pro street-hot rod- drag strip, smoke the tires down the road and piss off the neighbors type car ) and my idea was along the same line so for $2600. on a just broken in  25 mile motor (I don't think it had track time other than some burn outs in front of his house) I figured it would do the same for me. He did tell me it was a 12.0 comp ratio AND he ran it on pump gas which I questioned him on about detonation and he said there wasn't any.  I did hear the engine run before I got it and I have had it running in the yard here. It idles fine with instant throttle response but I haven't had it under load like a launch would do.
   I do have cam specs somewhere and it is hydraulic with 1-6 Crane roller rockers  on 906 heads and for anymore I 'd have to find the papers he gave me on it.
        My reason for knowing the comp.ratio is so I don't damage  the motor from running it on lower octane fuel than it needs.  
   Is a comp. test accurate on a cold motor ?
       My goal NOW is to put it in my 500 and put the numbers motor on a shelf.

    Yep 383Wing it sure does.

Compression test is better than compression ratio for determining if it will run on pump gas.  Iron head, no quench, 165 psi is about the cut off point.  You might be able to go a little higher with good tune, cool plugs, 160° thermostat, heat cross over blocked, right timing curve and a short gear, and/or a four speed tranny.

Yes you can do it on a cold motor.  It might make a 5 psi difference, if any.

Here is what I can tell you right now for certain, it is not 12:1, it is not 11:1, and it is probably not 10:1, unless it has a pretty good size cam in it.

$2,600, my money is on 8:1.  Just guessing on that one, but I have never seen a motor in that price range that wasn't.

[Highbanked Hauler]

 I should have added he had started out at 4K about a year earlier and of course no interest. They were headed for a divorce and he kept dropping the price. It was advertized on here for 35 hundred and by the time I contacted him it was 30 and he was getting desperate to sell it and I offered him 26  and think I got it for 27.

[BSB67]

Yea, I could be completely wrong in my last statement.  I tend to be a little skeptical.  Okay, maybe a lot....