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Effect of Compression Ratio on HP?

Discussion in 'All Other Diesels' started by Big T, Dec 5, 2010.

  1. Big T

    Big T Well-Known Member

    Sep 25, 2009
    Fullerton, CA
    I have a Volvo 31A series diesel in my 20' center console fishing boat. Reportedly, the A-series has a 16.5:1.0 compression ratio. The B-series and later had 17.6:1.0 compression ratios. The factory specs report the same hp output, but all the boaters conclude that the latter series engines have a bit more horsepower due to the additional compression.

    Does compression ratio affect hp output in a diesel as it does in a gasser? Or is it more a function of turbo boost?


    May 9, 2008
    diesel engines react to cylinder pressure more so than compression ratio, but CR plays a role in cylinder pressure. Higher CR means it takes less boost to reach X amount of cylinder pressure than a lower CR engine. BUT a lower CR will have more air in the combustion chamber to be burned with the fuel than a high CR engine will have at the same cylinder pressure. Higher CR normally give better off idle throttle response though as they don't need as much boost to reach the needed cylinder pressure to achieve maximum power, but are limited as to how much boost they can run up higher in the RPM range. Also higher CR engines tend to start better with less glow plug or intake heater required to keep the smoke down when cold.
  3. 6.2 turbo

    6.2 turbo Active Member

    Jan 3, 2010
    I've been struggling with this thought for quite some time.
  4. ak diesel driver

    ak diesel driver 6.5 driver

    Feb 7, 2009
    If everything else stays the same more comp will give more hp. With lower comp it allows more boost (oxygen). With high comp you can't add as much boost.
  5. buddy

    buddy Active Member

    With higher compression you can achieve more power with less fuel, because you can create a higher potential energy at TDC. But with lower compression you can throw more fuel and boost at it safely, and run higher RPMs to end up with more power than you could achieve with high compression. So NA diesel, high compression is desirable.

    A NA engine can only pull into the cylinder the amount of air it displaces, so changing CR doesnt matter on airflow, is all about displacement bore and stroke, the only amount of fresh air entering is the displacement of the piston. The piston moving down is the vacuum and once it reaches BDC its not sucking any more and it cant suck onces the whole space is filled to atmospheric pressure.
  6. Big T

    Big T Well-Known Member

    Sep 25, 2009
    Fullerton, CA
    Good stuff there Buddy. The Volvo 41 series block saw hp ratings (as 40 series) from a NA 165 hp all the way to a sequential compressor/turbo charged 285 hp in the KAD 300 version. Same basic block, obviously a lot done to address heat. When everything is in tune and running fine, no problems. But if one little thing happens to any part of the cooling system, watch out. I've have my 31A go from normal operating temp of 190 to 250 in a blink of an eye, just by sucking a piece of kelp onto the inlets of the outdrive. Fortunately the fix is easy: throw it in reverse to blow the obstruction off and leave it on to run cool water through the system. Boaters without "diesel knowledge" have grenaded many a marine diesel by continuing to run them in overheating conditions. What was supposed to be a 10,000 hour motor, becomes a 500 to 1000 hour motor.

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