Register now to get rid of these ads!

Technical Straight 8 compression question

Discussion in 'The Hokey Ass Message Board' started by fyrffytr1, Aug 30, 2020.

  1. fyrffytr1
    Joined: Dec 20, 2016
    Posts: 834

    fyrffytr1
    Member

    I picked up a 50 Pontiac straight 8 recently as a spare. I have a 39 straight 8 but the compression on it is low and I thought about replacing it with the one I just got so I did a compression test on it which brings up my question. If my compression gauge is working right the recently acquired 8 showed between 125 and 130 psi per cylinder. If I divide that by 14.7 it gives me a CR range from 8.4 to 8.8 to 1
    Research showed me that the 50 straight 8 with a manual trans had a CR of 6.8 to 1 and the hydramatic had a 7.7 to 1 CR.
    6 of the 8 cylinders tested at 130psi and two were right at 125psi. So, is this something common or could it be that the head was milled, maybe during a rebuild, to give the higher compression?
     
  2. Torana68
    Joined: Jan 28, 2008
    Posts: 1,415

    Torana68
    ALLIANCE MEMBER
    from Australia

    Don’t think that division thing works. Find out what the manufacturer says compression test should show.
    Were those results dry or wet?
     
  3. seb fontana
    Joined: Sep 1, 2005
    Posts: 8,444

    seb fontana
    Member
    from ct

    125/130 is plenty good. I don't think the 14.7 divided into the pressure is written in stone..Just gets you a reference.
     
    VANDENPLAS and dirty old man like this.
  4. fyrffytr1
    Joined: Dec 20, 2016
    Posts: 834

    fyrffytr1
    Member

    Tests were done dry. As stated in my original post manual trans equipped Pontiacs had a 6.5 to 1 compression ratio and those that came with the hydramatic had a 7.7 to 1 CR. The motor I got is the hydramatic version.
     

  5. I've never heard of that equation before. :confused:

    Does anybody know where the 14.7 factor comes from? o_O
     
  6. That is not how to determine static compression. To do that you have to actually measure the volume of the cylinder head, dish or dome of the piston, size of the bore, and thickness of the head gasket. Then the math begins. The numbers you showed are good for an engine replacement if yours are currently lower.
     
    VANDENPLAS and Hombre like this.
  7. trey32
    Joined: Jul 27, 2014
    Posts: 326

    trey32

    Pretty sure 14.7 is atmospheric pressure at sea level, but I could be wrong...
     
  8. fyrffytr1
    Joined: Dec 20, 2016
    Posts: 834

    fyrffytr1
    Member

    That's it. I was told that in another thread I posted on another forum.
     
    ClayMart likes this.
  9. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,657

    Rusty O'Toole
    Member

    Sounds like that engine is in really good shape with good valves and rings. If the oil pressure is good, which indicates tight bearings, it should last you for years if you take decent care of it.

    It is possible the head was milled or, that a buildup of carbon in the combustion chamber raised the compression a bit. It does seem on the high side. Could also be compressing the air raises the temperature and makes it read higher. I have heard of similar flathead engines having that much compression pressure if they were in perfect condition.
     
    ClayMart and Truck64 like this.
  10. THE FRENCHTOWN FLYER
    Joined: Jun 6, 2007
    Posts: 5,367

    THE FRENCHTOWN FLYER
    Member
    from FRENCHTOWN

    A couple of points:
    Compressing a gas will increase its temperature. Increased temperature will raise the pressure of the confined gas in the cylinders. PV =nRT. Put your hand on a running air compressor for example.

    Dynamometer tests I have done have shown that cranking compression numbers vary considerably with the cranking speed. For example if the starter motor was spinning the engine at 150 RPM the compression will show about 10 psi less than if it were cranking at 210 RPM.
     
    VANDENPLAS, Hombre and Truck64 like this.
  11. Hmmm . . . Now that you mention it, that 14.7 number does look a bit familiar. Sounds like a handy "rule-of-thumb" type of calculation though. :rolleyes:

    But as Rusty O'Toole and Frenchtown Flyer mentioned above, cranking speed alone can account for a noticeable variation in cranking compression readings. On an older flathead engine I wonder how much of a difference in readings there might be between cranking it over with a 6 volt or a 12 volt battery.
     
    warhorseracing likes this.
  12. seb fontana
    Joined: Sep 1, 2005
    Posts: 8,444

    seb fontana
    Member
    from ct

    Ya otta see how fast my Crosley cranks on 12v!:eek:
     
  13. greybeard360
    Joined: Feb 28, 2008
    Posts: 2,074

    greybeard360
    Member

    Camshaft can greatly change cranking compression. Cranking compression isn't a true measure for determining static compression. It is used to determine equal pressure in all of the cylinders for diagnostic purposes. If all cylinders are equal pressure, then the rings and valves are all equal in wear/sealing. If a wet compression test shows a big increase in pressure there is a problem with the rings sealing. If one cylinder is lower than the rest during dry test but equal to the others after wet test... bad rings. You can test the same engine with 3 different cams (spec wise) and get 3 different cranking pressures.

    My old Motors manual doesn't go back tto 1950, only to 1955. A 1955 287 V8 had 8.0:1 compression. Cranking pressure shows 120 psi +- 20 on a good engine.
     
    warhorseracing and RidgeRunner like this.
  14. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,657

    Rusty O'Toole
    Member

    If you want to get technical, my Canadian Service Data Book (1954 Edition) gives a compression ratio of 6.5:1 and a cranking pressure of 127 PSI for a 1950 Pontiac Eight, model 2700.

    A car with a very mild cam is going to have better cylinder filling at low RPM than one with a "hot" cam. So a detuned flathead could have higher compression readings than a more modern high compression engine with a higher static compression ratio.
     
    Last edited: Aug 31, 2020
    warhorseracing and RidgeRunner like this.
  15. fyrffytr1
    Joined: Dec 20, 2016
    Posts: 834

    fyrffytr1
    Member

    Thanks for all the replies. I did a dry and wet test on the third straight 8 I have and it was on the order of the 39 motor.
    Dry test wet test
    #1 35psi 50psi
    #2 60psi 75psi
    #3 115psi 130 psi
    #4 110 psi 120psi
    #5 100psi 110psi
    #6 60 psi 90psi
    #7 100psi 110psi
    #8 100psi 130psi

    I put some Marvel Mystery Oil in all cylinders and will check compression again in about a week.
     

Share This Page

Register now to get rid of these ads!

Archive

Copyright © 1995-2021 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.