
6.2L/6.5L Tech Tech forum containing the best technical information about these engines. Please post in the related 6.2L & 6.5L forums. We'll transfer the best topic threads to this forum. 

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#1




Boost/Compression Ratio Calculator
http://rbracingrsr.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.
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93 K1500 Ext. Cab, 6.2L D, 4L60E, 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, 4L80E, 3.73, 170K, 2.25" Crossover, 3.0" Down Pipe and Exhaust (sold to brother) 
#2




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.
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93 K1500 Ext. Cab, 6.2L D, 4L60E, 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, 4L80E, 3.73, 170K, 2.25" Crossover, 3.0" Down Pipe and Exhaust (sold to brother) 
#3




Those who run their stock CR of 21.3:1 at 15 psi:
Quote:
Jim 
#4




So why lower the static compression ratio and run more boost when the end result is the same?
Randy 
#5




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 181 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




Ennybody compare that to 3000psi combustion pressures?
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jd '96 Dodge 3500HD cc 2wd drw............'89 GMC 3500 cc 4wd drw 5.9 12v #10TST 6sp SBC131.375.......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




Quote:
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.
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'94 Barth 28' Breakaway M/H ("StaRV II") diesel pusher: Spartan chassis, aluminum birdcage construction. Peninsular/AMG 6.5L TD (230HP), 18:1, Phazer, nonwastgated turbo, hipop injectors, 4L80E (Sun Coast TC & rebuild, MH Pan), Dana 80 (MH Cover), Fluidampr, EGT, trans temp, boost gage. Honda EV4010 gaso genset, furnace, roof air, stove, microwave/convection, 2dr. 3way reefer. KVH R5SL Satellite. Cruises 2, sleeps 4, carries 6, and parties 8 (parties 12  tested). Standins are an '02 Cadillac Escalade AWD 6.0L and an '06 Toyota Sienna Limited. 
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