Boost/Compression Ratio Calculator

[Randy H]

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

Randy

[robscarab]

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

[gmctd]

Ennybody compare that to 3000psi combustion pressures?

[robscarab]

No problem, as long as that pressure rise occurs at the right time!

[rustyk]

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.

[More Power]

For an IDI (InDirect Injected) diesel to produce clean cold starts with short glow cycle times, and to produce fewer cold start emissions, the CR has to be somewhere at/above 20:1. The precups/prechambers cool the charge on a cold engine. A DI (Direct Injected) diesel (like the current Duramax, Cummins, PSD) starts well cold at a lower CR. So, a higher CR was an IDI cold start compromise.

I did a short tech piece on the Isuzu C240 I-4 2.4L diesel recently. It too is an IDI engine that has 20:1 CR. Its 3.0L DI siblings are, on the other hand, just over 18:1 CR.

Diesel farm tractors with DI diesels (of those I've seen the data for) run at anywhere between 15 and 18:1 CR. The Duramax/PSD/Cummins ran at 17.5-18:1 (the LMM Duramax is now at 16.8:1). The marine 5.9L Cummins runs with 15:1 CR. And so on...

Tractor, industrial and marine diesels are usually run at high load for extended periods, and tend to have a lower CR for improved durability. Certainly, tractors, OTR diesels, marine diesels and so on are also designed with efficiency in mind. If a higher CR was a better compromise (for efficiency), they'd all have a higher CR. They don't because of increasing durability issues with increasing CR.

Jim

PS - Here's a question for the thinkers out there.... What effect does CR have on crankshaft harmonics?

[robscarab]

The reason I selected the 18-1 pistons is so I could safely run higher boost while towing. I have increased the amount of fuel injected per power stroke by "turning up" my DB2 and installing marine injectors. The only time I will benefit from these mods is when I am accellerating a load or pulling it up a grade at cruise. I chose the comprimise of less efficient low boost performance and harder starting. If I was not using this rig exclusively for towing than I would have stayed with the stock compression ratio. If I had a half ton used for running around town empty,I personally would not go with lower compression. I live in Canada where we have periods of very cold temps and starting a 6.5 at -35C can be a challange at the best of times!

[tomtaylor]

Please tell me can you make up for the loss of efficiency with the lower CR by introducing propane injection? Thanks.

[DmaxMaverick]

Please tell me can you make up for the loss of efficiency with the lower CR by introducing propane injection? Thanks.

No. Propane is just another fuel. Propane injection only adds to volume of fuel(s) already in the combustion. Adding more/other fuels until the combustion process uses all the available O2 available can be advantageous from an economic standpoint (if the alternative fuel is significantly less expensive than the primary fuel), but offers little more than that. Also, consider that almost all fuels (capable of being used in an internal combustion engine) other than #2 Diesel have a significantly lower BTU potential (translates to less "efficient"). Mixing alternative fuels into any engine may have an economic advantage, but you'll have to consider all the costs involved. Most often, the ROI (Return On Investment) is far beyond what you will actually ever achieve.

None of this will change the "efficiency" of a charge-air system. The principals, function and science are very simple. X amount of exhaust gas energy will produce X amount of charge-air energy. On the charge-air side, you will hit a wall on compressor efficiency, at which point, it will fall off exponentially (the value of the fuel input compared to the output power depart very quickly). None of this takes into consideration the mechanical limitations of the platform involved (the point an efficiency increase start breaking things).

[86 CUCV]

For an IDI (InDirect Injected) diesel to produce clean cold starts with short glow cycle times, and to produce fewer cold start emissions, the CR has to be somewhere at/above 20:1. The precups/prechambers cool the charge on a cold engine. A DI (Direct Injected) diesel (like the current Duramax, Cummins, PSD) starts well cold at a lower CR. So, a higher CR was an IDI cold start compromise.

I did a short tech piece on the Isuzu C240 I-4 2.4L diesel recently. It too is an IDI engine that has 20:1 CR. Its 3.0L DI siblings are, on the other hand, just over 18:1 CR.

Diesel farm tractors with DI diesels (of those I've seen the data for) run at anywhere between 15 and 18:1 CR. The Duramax/PSD/Cummins ran at 17.5-18:1 (the LMM Duramax is now at 16.8:1). The marine 5.9L Cummins runs with 15:1 CR. And so on...

Tractor, industrial and marine diesels are usually run at high load for extended periods, and tend to have a lower CR for improved durability. Certainly, tractors, OTR diesels, marine diesels and so on are also designed with efficiency in mind. If a higher CR was a better compromise (for efficiency), they'd all have a higher CR. They don't because of increasing durability issues with increasing CR.

Jim

PS - Here's a question for the thinkers out there.... What effect does CR have on crankshaft harmonics?

Pretty sure changing CR will affect the frequency in some if not all critical vibration orders.
Most of the engines I worked with in my Navy career were good sized like 16 ton that's where a great deal of my diesel education comes from.
In relation to crankshaft torsional vibrations one of my favorite engines used a crankshaft torsional vibration absorber (Fairbanks Morse 38 ND 8-1/8 OP) engine.
The absorber insisted of a hub with plates and free floating pins of different diameters and weights. Great component. Better to absorb than dampen.
I know there is a similar unit for gassers called the Rattler.
Is one available for the 6.2/6.5 to your knowledge and what is your opinion on dampen vs absorb.
New here and seeing that your knowledge and ability to express it make you well worth following
Thank you

[More Power]

Pretty sure changing CR will affect the frequency in some if not all critical vibration orders.
Most of the engines I worked with in my Navy career were good sized like 16 ton that's where a great deal of my diesel education comes from.
In relation to crankshaft torsional vibrations one of my favorite engines used a crankshaft torsional vibration absorber (Fairbanks Morse 38 ND 8-1/8 OP) engine.
The absorber insisted of a hub with plates and free floating pins of different diameters and weights. Great component. Better to absorb than dampen.
I know there is a similar unit for gassers called the Rattler.
Is one available for the 6.2/6.5 to your knowledge and what is your opinion on dampen vs absorb.
New here and seeing that your knowledge and ability to express it make you well worth following
Thank you

Thanks for your comments.

Generally, a damper recommendation depends on the owner.

For example, if the owner plans to own the same vehicle for some number of years, then I'd recommend the Fluidampr. It's a little pricey, but it does a better job of damping, and it never-ever wears out or deteriorates over time. Plus, you could sell the damper if you needed to, to recoup some of the cost.

Most owners are best served by using the original equipment GM/AMG damper, if a replacement becomes necessary. Avoid the cheap imports like the plague. A genuine piece will cost ~$100 and an import will cost about half that.

I don't know of any other aftermarket type dampers for these engines - a "Rattler", for example. Jim