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Thread: Effect of Compression Ratio on HP?

  1. #1
    Registered User Big T's Avatar
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    Default Effect of Compression Ratio on HP?

    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?
    1999 Chevy Suburban K2500 4x4 w/ 6.5TD 190K miles. 4L80E and 3.73 rear gears. HD lift pump, ATT, Heath HO injectors, 4" Diamond Eye Turbo Back exhaust, Warpspeed x-over, deleted soot trap , D-Tech pmd behind bumper, Heath programming for ATT, Pillar mounted gauges: EGT, Boost, Tranny Temp. Replaced IP at 130K. HID light conversion with All-4-On-Hi. Forged Alcoa wheels.

    My son's 1994 GMC Suburban K2500 4x4 w/ 6.5TD 243K miles. 4L80E and 4.10 rear gears. Buddy 4 position tune. Rebuilt Bosch injectors and new Duraterm glows. HO water pump with Dmax fan, HD fan clutch and new AC Delco 195 T-stats. Diamond Eye 3" Downpipe and 4" exhaust, deleted soot trap. Warp speed 2.5" mandrel bent crossover. D-Tech PMD mounted behind the bumper. Pillar mounted gauges: EGT, Boost, Tranny Temp. Rancho 9000 shocks. Rebuilt front end and steering box tightened. Forged Alcoa wheels.

  2. #2
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    Default

    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.
    WHO SAYS YOU CAN'T HAVE IT ALL, JUST BUILD IT

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    DIAMONDEYE 4" exhaust, CORSA muffler, AFE stage 1 dry filter, custom tuning by me, KENNEDY single pump and pump rub kit.


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    Registered User 6.2 turbo's Avatar
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    I've been struggling with this thought for quite some time.
    92 Chevy truck modified 6.2 diesel, Has spun bearings 97 Chevy truck solid axle and 5.9 cummins no fuel plate, 370 marine inj,valair twin disc clutch. 96 4 door Tahoe getting a 6.5 diesel swap

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    Registered User ak diesel driver's Avatar
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    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.
    96 gmc k2500 4" exhaust, marine inj, #9 resistor, GL4, turbomaster, duraterms,-1.94 TDCO,FTB, remote pmd, isspro pillar gauges, lift pump relay added ,141 block, 2 1000 w block heaters, 2 150w glue on oilpan heaters, 2 150w glue on transpan heaters, 97 K47 air filter , IP replaced at 229k ,gear drive set awaiting install, Parts for intercooler waiting to be installed

    94 k1500 excab sb 6.5 plow truck. Fisher plow w/engine driven pump, F intake, h/m turbomaster, plowing chip by Buddy, remote PMD w#9 resistor, Duraterms

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    Registered User buddy's Avatar
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    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.
    94 6.5TD C1500 ECLB w/ 50 gallon aux tank/box; 235/75R17 tires on Alloy wheels; 3.42 gears; A-Team Turbo; 4" Aero Turbine 4040 muffler to 5" Aero Turbine 5050XL muffler to 6" tip before rear axle; FSD heatsink, marine injectors; "F" intake; SynBlend 10W-30 oil; Walbro FRB-5 w/ 12 micron prefilter; Feed The Beast; WMI and Propane fumigation; OPS extension & relay; 110K original miles; Engh GMTDScanTech
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    Registered User Big T's Avatar
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    Quote Originally Posted by buddy View Post
    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.
    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.
    1999 Chevy Suburban K2500 4x4 w/ 6.5TD 190K miles. 4L80E and 3.73 rear gears. HD lift pump, ATT, Heath HO injectors, 4" Diamond Eye Turbo Back exhaust, Warpspeed x-over, deleted soot trap , D-Tech pmd behind bumper, Heath programming for ATT, Pillar mounted gauges: EGT, Boost, Tranny Temp. Replaced IP at 130K. HID light conversion with All-4-On-Hi. Forged Alcoa wheels.

    My son's 1994 GMC Suburban K2500 4x4 w/ 6.5TD 243K miles. 4L80E and 4.10 rear gears. Buddy 4 position tune. Rebuilt Bosch injectors and new Duraterm glows. HO water pump with Dmax fan, HD fan clutch and new AC Delco 195 T-stats. Diamond Eye 3" Downpipe and 4" exhaust, deleted soot trap. Warp speed 2.5" mandrel bent crossover. D-Tech PMD mounted behind the bumper. Pillar mounted gauges: EGT, Boost, Tranny Temp. Rancho 9000 shocks. Rebuilt front end and steering box tightened. Forged Alcoa wheels.

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