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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  
Old 07-20-2007, 05:13 PM
afgunn afgunn is offline
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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.
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Old 07-21-2007, 12:45 PM
afgunn afgunn is offline
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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.
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Old 07-21-2007, 01:29 PM
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More Power More Power is offline
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Those who run their stock CR of 21.3:1 at 15 psi:

Quote:
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
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Old 07-29-2007, 07:42 PM
Randy H Randy H is offline
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So why lower the static compression ratio and run more boost when the end result is the same?

Randy
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  #5  
Old 08-07-2007, 01:31 AM
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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
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Old 08-09-2007, 03:48 PM
gmctd gmctd is offline
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Ennybody compare that to 3000psi combustion pressures?
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Old 08-13-2007, 10:21 PM
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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.
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