Compresion ratio | The Z Club of Great Britain
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Compresion ratio

Discussion in 'Engine' started by andrew muir, Sep 28, 2016.

  1. andrew muir

    andrew muir
    Z Club Member

    Hi going to do a comp test on a 280zx engine I might buy!
    Wwhat would be the approx correct compression ratio lb/ft reading I should be looking at for a P90 head 280zx motor? Will be done dry!

    Ta Andrew :thumbs:
  2. Moriarty

    Moriarty Well-Known Forum User

    To measure compression ratio you need to dismantle the engine, CC the chamber etc etc so I presume
    you're trying to measure compression (not compression ratio) which is measured in psi via a pressure gauge at the spark plug hole.

    Your looking more for consistency across the bores rather than a specific number...having said that look for something in excess of 140 +/- a couple of psi

    If you get a low reading on 1 or more cylinders drop a teaspoon of oil down the bores and try again to see what happens, the result will tell you most of what you need to know.
  3. Huw

    Z Club Member

    Hi Andrew

    From memory I think its:

    8.3:1 (1979-1980) 8.8:1(1981-1983)

  4. SeanDezart

    Z Club Member
    Official Trader

    If this is for a performance rebuild....I wouldn't worry too much - just buy it cheap. :)
  5. Rob Gaskin

    Rob Gaskin Treasurer Staff Member Moderator

    My standard 240 (9:1) is 160 psi.

    My old track car (2.8, CR ? ) was 200 psi.

    My DJ car (2.8) ? I must test it sometime, CR is 11:1 so I'm expecting it to be high.

    Andrew, the most important thing is that they are all similar (10 psi tolerance ? ) and around 150 IMO. That should indicate an engine without any major issues.
  6. johnymd

    Z Club Member

    I don't think a compression test is in any way connected to the mathematical compression ratio of an engine.

    The compression test will give you a reading in psi and will indicate the general condition of the engine to hold pressure and show up any problems between cylinders.

    The compression ratio is a ratio of the difference in volume with the piston at the bottom of the cylinder to the volume with the piston at the top.
  7. tel240z

    Z Club Member

  8. Rob Gaskin

    Rob Gaskin Treasurer Staff Member Moderator

    John, surely an engine running 8:1 compresses the gas less than one running 10:1 i.e. 8 parts into 1 instead of 10. Therefore the pressure will be greater. Things like valve timing will affect it also.
  9. johnymd

    Z Club Member

    It makes no difference Rob. To give you an example. On my turbo engine with a compression ratio of 8:1 I am getting compression test readings of 210-220. On my 260z with higher compression I get roughly the same, around 210 on all cylinders and that should be running over 10:1. Cam will make a difference but only on the compression stroke and the point at which the inlet valve closes. Not the inlet/exhaust overlap as that's on the induction stroke.
  10. johnymd

    Z Club Member

    Very interesting discussion though and well worth talking through.
  11. jonbills

    ZClub Administrator
    Staff Member Moderator

  12. johnymd

    Z Club Member

    Not really sure how they can be related Jon. The CR is mathematically calculated and will never change during the life of an engine, yes? The compression under cranking will change during the life of the engine and under fault conditions. How can they be related if one can vary independent of the other?

    Jon - I'm not saying you are wrong, I would just like to know how they are related.
  13. johnymd

    Z Club Member

    Interesting wiki that makes great reading Jon.
  14. Rob Gaskin

    Rob Gaskin Treasurer Staff Member Moderator

    John, when you pump your bike tyres up try it with only half a stroke. Can you generate enough pressure to overcome the valve?

    Of course as your pump wears the pressure will be a bit less than when it was new but.......
  15. johnymd

    Z Club Member

    Please elaborate Rob. I'm not sure what you are getting at? Is this with regard to the relationship of Cranking compression and compression ratio?
  16. Moriarty

    Moriarty Well-Known Forum User

    1. Don't confuse static comp with dynamic comp

    2. Compression ratio has nothing to do with the "condition" of the engine.......

    3. Compression ratio can be theoretically calculated from cam spec and other engine parameters. but it would require a proper understanding of ICE theory, not just forum guessing games.

    4. Compression testing is not a reliable indication or power output or performance

    As Dezart said, if your going to rebuild it then that should include measuring the bores, reboring to suit, new pistons and rings, re cutting of valves etc so current compression is a moot point really
  17. Rob Gaskin

    Rob Gaskin Treasurer Staff Member Moderator

    John, yes Compression Ratio is a figure that can be calculated however Compression Pressure although directly related to it is affected by other factors e.g. gas seal (piston rings, valve seating), valve timing, throttle opening, turbo-charging etc.

    However my bicycle pump analogy was trying to explain the affect of reduced CR on pressure. If you limit pump travel you are effectively lowering the CR i.e. you are only trying to compress half the gas into a given space and so the pressure will be less.
    Last edited by a moderator: Sep 29, 2016
  18. Rob Gaskin

    Rob Gaskin Treasurer Staff Member Moderator

    Why does a powerful engine need a high CR?

    To get a high Compression and a better explosion.

    A diesel wouldn't work at a low CR because it needs the Compression pressure to generate heat (like the bike pump does in your hand) to ignite the fuel.
  19. Rob Gaskin

    Rob Gaskin Treasurer Staff Member Moderator

    See above.
    Last edited by a moderator: Sep 29, 2016
  20. Moriarty

    Moriarty Well-Known Forum User

    "The cam doesn't come into the equation"
    Dynamic compression is all about the cam

    As I said there is static and dynamic compression

    If we were only talking about static you would be correct

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