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Thread: Boost/Compression Ratio Calculator

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  1. #1
    Join Date
    Aug 2006
    Location
    Fayetteville, NC
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    126

    Default Boost/Compression Ratio Calculator

    http://rbracing-rsr.com/compression.htm

    The link above is for a boost psi to compression ratio calculator. I ran a couple and was quite surprised at what I got!

    1) A typical stock 6.5L
    You are running 8 PSI of boost at an altitude of 1000 feet. Your motor's static compression is 21.2 :1. At this boost level and altitude your effective compression ratio is 32.54 :1, and without altitude correction your compression ratio would be 32.74 :1.

    2) Reduced 18:1 CR and 15 PSI boost
    You are running 15 PSI of boost at an altitude of 1000 feet. Your motor's static compression is 18 :1. At this boost level and altitude your effective compression ratio is 36.17 :1, and without altitude correction your compression ratio would be 36.37 :1.

    Do these seem correct!? Can our 6.5s (or any diesel) handle this CR!? I was a bit shocked at the effective CR. I came upon this because I was wondering what is the effect of boost to CR. I knew it had to go up but, still I am very shocked.
    93 K1500 Ext. Cab, 6.2L D, 4L60-E, 3.73, 174K, 3" exhaust, air cleaner mod, EGR mod, EPR mod (my toy)
    04 Dodge Dakota Sport 4x4, 4.7L (gaser), 545rfe, 3.92, 95K (wife's ride now)
    98 K1500 Z71, 6.5L TD, 4L80-E, 3.73, 170K, 2.25" Crossover, 3.0" Down Pipe and Exhaust (sold to brother)

  2. #2
    Join Date
    Aug 2006
    Location
    Fayetteville, NC
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    126

    Default Update

    Another way of looking at this by "first level approximation" is:
    15psi = 2 atmospheres
    8psi ~= 1.5 atmospheres

    18 CR x 2 atmospheres = 36 CR
    21.2 CR x 1.5 atmoshperes = 31.8 CR

    So, I believe the calculator is accurate! I would never have thought that any engine would be able to handle a 32 much less a 36 CR. This is why it is necessary to reduce the typical 6.5L CR from 21.2 to ~18 to run 15psi of boost.
    93 K1500 Ext. Cab, 6.2L D, 4L60-E, 3.73, 174K, 3" exhaust, air cleaner mod, EGR mod, EPR mod (my toy)
    04 Dodge Dakota Sport 4x4, 4.7L (gaser), 545rfe, 3.92, 95K (wife's ride now)
    98 K1500 Z71, 6.5L TD, 4L80-E, 3.73, 170K, 2.25" Crossover, 3.0" Down Pipe and Exhaust (sold to brother)

  3. #3
    Join Date
    Feb 2000
    Location
    Montana
    Posts
    11,382

    Default

    Those who run their stock CR of 21.3:1 at 15 psi:

    You are running 15 PSI of boost at an altitude of 1000 feet. Your motor's static compression is 21.3 :1. At this boost level and altitude your effective compression ratio is 42.83 :1, and without altitude correction your compression ratio would be 43.03 :1.
    That's why lowering the 6.5 CR to 18:1 improves durability..... Now, imagine what would happen if you raised CR above stock...

    Jim

  4. #4
    Join Date
    Jul 2007
    Location
    Maryland
    Posts
    10

    Default

    So why lower the static compression ratio and run more boost when the end result is the same?

    Randy

  5. #5

    Thumbs up Same dynamic CR, Same A/F density, More A/F Volume

    more boost = more combustion O2 avilable. Add a corresponding amount of additional fuel and you have a more powerful combustion event. Even though the dynamic compression ratio remains the same +/- , the lower height of the 18-1 piston crown creates a larger effective combustion chamber volume, which is now packed with the same density of air/fuel as the smaller area that was provided by the stock piston compression height.

    Same DCR + Same A/F Density+ more A/F Volume = bigger bang per power stroke

  6. #6
    Join Date
    Sep 2000
    Location
    usa
    Posts
    2,646

    Default

    Ennybody compare that to 3000psi combustion pressures?
    jd
    '96 Dodge 3500HD cc 2wd drw............'89 GMC 3500 cc 4wd drw
    5.9 12v #10TST 6sp SBC13-1.375.......6.5TD EFI maxEtorq v2.0 DSG
    DODGE makes it CUMMINS shakes it.....4L80E 205 4.10 Dana60\70HD
    6 in a row makes it go.......................Grandpa's big truck

  7. #7
    Join Date
    Sep 2006
    Location
    Brooker, FL
    Posts
    1,217

    Default

    Quote Originally Posted by robscarab View Post
    more boost = more combustion O2 avilable. Add a corresponding amount of additional fuel and you have a more powerful combustion event. Even though the dynamic compression ratio remains the same +/- , the lower height of the 18-1 piston crown creates a larger effective combustion chamber volume, which is now packed with the same density of air/fuel as the smaller area that was provided by the stock piston compression height.

    Same DCR + Same A/F Density+ more A/F Volume = bigger bang per power stroke
    Nicely said! Although the fuel-air "density" isn't the same, but the fuel-air ratio is - unless as you mentioned, more fuel and more air is introduced. Without more fuel/air, with the larger volume, the density is decreased. The fuel-air mixture has, however, the same energy content, but it will be slightly less efficient than at the higher compression ratio. But this will decrease BMEP (Brake Maximum Effective Pressure) in the cylinders, which reduces piston head pressures and does, in fact contribute to longevity at the cost of a slight decrease in efficiency. With a bit more fuel/air, the power will be nearly the same, but still with less stress on piston crowns and skirts.

    Compression ratio is a compromise - higher CR results in higher theoretical efficiency, but at the cost of longevity - and sometimes it's too high, such that decreasing it raises efficiency. Which is why I elected to get my new engine with 18:1 pistons. And fuel economy is up 15% over the original GM 6.5L TD.
    '94 Barth 28' Breakaway M/H ("StaRV II") diesel pusher: Spartan chassis, aluminum birdcage construction. Peninsular/AMG 6.5L TD (230HP), 18:1, Phazer, non-wastgated turbo, hi-pop injectors, 4L80E (Sun Coast TC & rebuild, M-H Pan), Dana 80 (M-H Cover), Fluidampr, EGT, trans temp, boost gage. Honda EV-4010 gaso genset, furnace, roof air, stove, microwave/convection, 2-dr. 3-way reefer. KVH R5SL Satellite. Cruises 2, sleeps 4, carries 6, and parties 8 (parties 12 - tested).

    Stand-ins are an '02 Cadillac Escalade AWD 6.0L and an '06 Toyota Sienna Limited.

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